University of Groningen

The reactivity of rare-earth metallocenes towards alkynesQuiroga Norambuena, Victor

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Citation for published version (APA):Quiroga Norambuena, V. (2006). The reactivity of rare-earth metallocenes towards alkynes: mechanismand synthetic applications. [Groningen]: s.n.

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The organolanthanide-catalyzed polymerization of diynes

207

6. The organolanthanide-catalyzed polymerization of diynes

6.1. Introduction During the last two decades the research area of π-conjugated polymers has experienced a

tremendous growth.1 With an ever increasing interest from both academia and industry these polymers have been developed as advanced materials for electronic and photonic applications. The immense interest in π-conjugated polymers started in 1977, when Shirakawa, MacDiarmid and Heeger discovered that oxidation with chlorine, bromine or iodide vapor made polyacetylene 109 times more conductive.2 The study of π-conjugated polymers has since advanced rapidly in various directions. More recently, this field has witnessed not only a plethora of commercialized applications of conductive plastics (e.g. corrosion inhibitors, compact capacitors, antistatic coatings, electromagnetic shielding of computers, “smart” windows that can vary the amount of light they allow to pass, etc.), but the development of electroluminescent polymers as well. It is mainly their inherent synthetic flexibility, their potential ease of processing and the possibility of tailoring characteristic properties to accomplish a desired function that makes them promising candidates for a variety of applications in material science. Thus, they are used as laser dyes,3 scintillators,3 light-emitting diodes (LEDs),4 piezoelectric and pyroelectric materials,5 photoconductors,6 photovoltaic cells7 and are investigated for optical data storage,8 optical switching and signal processing9 as well as in nonlinear optical applications.10

The class of conjugated polymers which has found the most attention in the last years are undoubtedly the poly(p-phenylenevinylene)s (PPVs) which were applied as an organic polymeric LED in 1990.11 Other well-established classes of conjugated polymers are the poly(diacetylene)s (PDAs),12 poly(phenylene)s (PPP)13 and the aforementioned poly(acetylene)s (Scheme 6-1).14 Poly(aryleneethynylene)s (PAEs) have attracted considerable attention only during the last decade, as applications, such as molecularly wired sensors,15 polarizers for liquid crystalline displays16 and LEDs, were developed in the late 1990s.17

Among the numerous described conjugated polymers containing heterocyclic units in the backbone those containing 2,5-thienyl units represent a particularly interesting class.1 For example, poly(thiophene)s led to materials showing an important environmental and thermal stability with good conductivity when compared to other poly(heteroarylene)s.18 Poly(3-alkylthiophene)s are, moreover, soluble in common organic solvents which facilitates their processability and has widened their field of application. Substituted poly(2,5-thienylvinylene)s19 display high nonlinear optical responses, moderate charge mobilities and good electroluminescent properties and substituted poly(2,5-thienylethynylene)s20 were found to exhibit strong photoluminescence and third-order optical nonlinearity. As a consequence, thiophene-based oligomers and polymers are widely used in active organic electronic and photonic devices, such as organic field effect transistors (OFETs),21 LEDs22 or solar cells.23

Each class of conjugated polymer has its own methodology of synthesis which is characterized by its versatility in modifying the molecular structure of the material, its compatibility with various functional groups

Scheme 6-1. Well-established classes of π-conjugated polymers.

nnn

nnn

Carbyne Poly(acetylene) Poly(diacetylene)

Poly(phenylene) Poly(phenylenevinylene) Poly(phenyleneethynylene)

PA PDA

PP PPV PPE

Chapter 6

208

and its propensity to introduce structural defects into the polymers. The use of organometallic methodologies, widely exploited in the synthesis of well-defined molecules, has proven to be of great importance in the preparation of polydisperse conjugated materials, especially when a high regio- and stereoselectivity is required in building the polymeric backbone.24 For example, polyacetylenes have been prepared from 1-alkynes by Ziegler-Natta type polymerization with MoCl6, WCl6 or TaCl5 based catalysts25 or by using organorhodium(I) complexes,26 the palladium-catalyzed coupling of 1-alkynes with halides (the Cassar-Heck-Sonogashira reaction) constitutes the classic method to prepare PAEs,17b the palladium-catalyzed coupling of halides with alkenes (the Heck reaction),27 boron derivatives (the Suzuki-Miyaura coupling),28 tin derivatives (the Stille coupling),29 organomagnesium (the Kumada-Corriu coupling),30 organozinc (the Negishi coupling)31 has been used successfully in the preparation of PPVs, PAEs and PPPs, the homocoupling of aromatic halides catalyzed by Ni(0) has been employed in the synthesis of many polyarylenes and their copolymers,32 the acyclic diene metathesis (ADMET)33 and acyclic diyne metathesis (ADIMET)34 processes catalyzed by complexes of molybdenum and tungsten were applied in the synthesis of double- and triple-bond containing conjugated polymers and the ring opening metathesis polymerization (ROMP) process catalyzed by organomolybdenum and -tungsten complexes has been exploited as a precursor route to polyacetylenes and PPVs.35 Considering the well-recognized fascinating electrical and optical properties of poydiacetylenes that also contain enyne scaffolds (-CR=CRC≡ C-),12 it is remarkable that only a few reports on the synthesis and properties of poly(aryleneethynylenevinylene)s (PAEVs) exist in literature. To the best of our knowledge, the first report of a PAEV dates back to 1992, when Kane et al. described the synthesis of a series of conjugated poly(enyne)s I via the palladium-catalyzed coupling of aromatic diynes to vinylic bromides (Scheme 6-2).36 In 1996, Ueda et al. reported the analogous palladium-catalyzed preparation of II.37 The single other report involving PAEVs is by Endo et al. in 2000 who prepared insoluble poly(1,4-phenyleneethylenevinylene)s of unknown stereochemistry via a palladium-catalyzed coupling, followed by a Retro-Diels-Alder reaction.38

The metal-catalyzed dimerization of substituted 1-alkynes constitutes a highly attractive and atom efficient method to effect C-C bond formation, but its applications have been limited by the lack of control of

Scheme 6-3. Well-established thienyl-containing π-conjugated polymers.

Poly(thiophene) Poly(2,5-thienylenevinylene) P oly(2,5-thienyleneethynylene) PT PTV PTE

n

S

n

S

n

S

Scheme 6-2. The synthesis of poly(aryleneethynylenevinylene)s.

R = C4H9, C10H21, C16H35

OR

ROBr

Br+

OR

RO n

[Pd]

II

SBr Br

R R+Ar

[Pd]

Ar = , ,

R

R

R = OC10H21

I

n

S

R R

Ar

The organolanthanide-catalyzed polymerization of diynes

209

stereo-, regio- and chemoselectivity. Catalytic systems that combine both a high activity and a high selectivity have been developed only recently.39 Among these systems, the catalyst precursor Cp*2LaCH(SiMe3)2 was found to be an active and selective catalyst for the trans-head-to-head dimerization of a variety of substituted (hetero)aromatic 1-alkynes (Chapters 4 and 5). Motivated by the potentially interesting properties of PAEVs that represent a relatively unexplored class of conjugated polymers, a study was initiated to investigate the application of the lanthanocene-catalyzed linear dimerization of bifunctional (hetero)aromatic 1-alkynes to the preparation of PAEVs (Scheme 6-4).40 In the course of this study, three independent studies and one patent appeared, describing a similar approach towards the preparation of PAEVs.41 The details of these processes and comparisons with the present study are discussed in a later section. Moreover, an analogous approach towards poly(arylenebutadiynylene)s (PABs) via the copper-catalyzed oxidative dimerization of bifunctional 1-alkyne is well-established (Scheme 6-4) and has led to materials with photochemical and optoelectronic applications, such as nonlinear optical materials42 and photoresists,43 and as precursors for high-carbon materials.44

In this chapter, the lanthanocene-catalyzed linear dimerization of substituted (hetero)aromatic 1-alkynes is investigated as a means to prepare well-defined poly(aryleneethynylenevinylene)s. Four types of diynes, with and without solubilizing substituents, were used to probe the scope and feasibility of this synthetic approach. The formed polymers were characterized spectroscopically and their optical properties were studied as well.

6.2. Synthesis of the (hetero)aromatic diynes. Introduction

The preparation of a large variety of (hetero)aromatic diynes is reported in literature. In most cases halide (hetero)aryls were subjected to standard Hagihara-Sonogashira coupling reactions with trimethylsilylacetylene, followed by desilylation.45 This method of synthesis was adapted successfully in the present study after the incorporation of some minor modifications and additional purification steps. Attempts to prepared the desired monomers via palladium-catalyzed coupling reactions between iodo (hetero)arenes and zinc reagents (the Negishi coupling) failed, since solvent removal (i.e. THF) was accompanied with significant decomposition of the diethynyl monomers. It is believed that the present modifications of the reported syntheses and the instability of the diethynyl monomers justify a brief discussion of their synthesis and use.

Performing catalytic reactions with organolanthanides requires not only the rigorous exclusion of air and moisture, but substrates of high purity as well, since coordination of heteroatoms (e.g. oxygen, nitrogen) to these electrophilic metal complexes is in general both kinetically and thermodynamically favored.46 Coordination of heteroatoms may lead to a decrease in catalytic activity47 or even catalyst deactivation.48 In addition, halides are known to undergo metal-halogen exchange reactions with organolanthanide complexes.49 Obviously, traces of compounds containing acidic protons, halide precursors and heteroatom-containing side-products or solvents can have detrimental effects on the observed catalytic activity. 1,4-Diethynylbenzene (1)

The synthesis of 1,4-diethynylbenzene (1) was straightforward and performed analogously to literature procedures.50 The palladium-catalyzed coupling of 1,4-diiodobenzene with trimethylsilylacetylene

Scheme 6-4. The preparation of conjugated polymers via metal-catalyzed linear 1-alkyne dimerization.

Ar = , , , etc.S N

Ar[Cu]

nAr

Poly(arylenebutadiynylene)

ArAr

n

catalyst

Poly(aryleneethynylenvinylene)

Chapter 6

210

(TMSA) afforded white crystals in high yield after work-up which were indefinitely stable at room temperature. Subsequent basic hydrolysis afforded the desired diyne as white needles in high yield.

Upon standing at room temperature, the needles of 1 became dark within several hours. The instability of terminal diynes in general is well-recognized51 and 1,4-diethynyl aromatics, in particular, are known to be highly light and heat sensitive, forming cross-linked poly(ene)s presumably via a radical-initiated mechanism.52 As a consequence, only small portions of the silyl-protected precursor were desilylated prior to polymerization. Drying the diyne with CaH2 as a pentane solution and subsequent storage of the crystals or oils at -30 °C under nitrogen in the dark proved to be the most convenient method to use the diyne monomers in this study. 2,5-Di-n-hexyl-1,4-diethynylbenzene (2)

The synthesis of 2,5-di-n-hexyl-1,4-diethynylbenzene (2) started with 1,4-dichlorobenzene.53 As reported by others, iodination of 1,4-dichlorobenzene with KIO3 gave a mixture of mono- and diiodo compounds which was difficult to purify.54c Iodination with KIO4 was found to give the desired diiodide in reasonable yield and high purity as colorless needles after repeated crystallizations from ethanol.54 Nonetheless, careful monitoring of the reaction progress is required to avoid the formation of relatively large amounts of the mono- and triiodides. Palladium-catalyzed coupling with TMSA gave a white crystalline powder after purification by means of column chromatography and subsequent crystallization. Application of several desilylation protocols (e.g. KOH in MeOH/THF, K2CO3 in MeOH/THF, nBu4NF in cold THF) afforded in all cases a colorless oil which became yellow upon complete solvent removal. Subsequent column chromatography using pentane and neutral alumina furnished a pale yellow liquid which turned orange on standing for several hours in the dark in air.55

When the product was allowed to stand at room temperature (either in a pure form or in solution), a darkening of the color was observed, ultimately giving rise to a viscous blood-red liquid. The formed viscous dark-red oil gave poorly reproducible results in the polymerization reactions. When the yellow liquid was stored at -30 °C under nitrogen in the dark, it remained seemingly indefinitely stable and gave good results in the organolanthanide-catalyzed polymerization reactions.56 2,5-Di-n-hexyloxy-1,4-diethynylbenzene (3)

1,4-Diethynyl-2,5-di-n-hexyloxybenzene (3) was prepared from 1,4-dihydroxybenzene (hydroquinone). Iodination of 1,4-dihexyloxybenzene57 with KIO4 gave the diiodo compound after repeated

Scheme 6-5. The synthetic route towards the aromatic diynes 1-3.

55% 93%

92%

81%

86%

1

2

3

87% 97%

73%

40% 83%

R = nC6H13

R = nC6H13

SiMe3Me3Si

R

R

R

R

SiMe3Me3Si

OR

RO

OR

RO

ClCl(iii) (iv)

RR II

R

R

(i)

II SiMe3Me3Si(i) (ii)

OHHO(v) (iv)

ORRO II

OR

RO

(i)

(ii)

(ii)

Reagents: (i): Me3SiCCH (2.2 equiv), THF/HNiPr2, Pd(PPh3)2Cl2, PPh3, CuI; (ii): KOH, MeOH/THF; (iii): RMgBr, diethyl ether, reflux. (iv): I2, KIO4, HOAc/H2SO4/CHCl3, reflux.

The organolanthanide-catalyzed polymerization of diynes

211

Scheme 6-6. The synthetic scheme of 2,5-diethynyl-3-n-hexylthiophene (4).

4

S

R

S

R

II S

R

SiMe3Me3Si

S

R

S

R

(ii)

73%

(iii)

(iv)

95%

R = nC6H13

(i)

89%

Reagents: (i): RMgBr, diethyl ether, reflux. (ii): I2, KIO4, HOAc/H2SO4/CHCl3, reflux. (iii): Me3SiCCH (2.2 equiv), THF/HNiPr2, Pd(PPh3)2Cl2, PPh3, CuI. (iv): KOH, MeOH/THF.

crystallizations from diethyl ether as white crystals in moderate yield.54c The palladium-catalyzed ethynylation with TMSA afforded white crystals after work-up in good yield. Protodesilylation furnished the desired diyne as light-yellow crystals which seemed more stable than its n-hexyl analogue. Even so, crystals of 3 were stored at -30 °C under nitrogen and in the dark after drying with CaH2. 2,5-Diethynyl-3-n-hexylthiophene (4)

The syntheses of several 2,5-diethynyl-3-alkylthiophenes were reported concurrently by the groups of Whitesides and Stille via the palladium-catalyzed coupling of the corresponding 2,5-dibromothiophenes with TMSA, followed by deprotection.58 In view of the higher reactivity of iodides in the palladium-catalyzed cross-coupling reactions and the concomitant advantages (e.g. lower catalyst loadings, lower reaction temperatures and times, less side-reactions),45 a similar procedure involving the corresponding diiodides was adapted in the present study. The iodination method of Barker et al.59 was modified to prepare 2,5-diiodo-3-n-hexylthiophene from 3-n-hexylthiophene.60 Again, a careful monitoring of the progress of the reaction was necessary to avoid the formation of relatively large amounts of mono- and triiodides. After work-up the desired diiodo compound was obtained as a dark oil and appeared to be contaminated by small amounts of the corresponding monoiodide (<1 % by 1H NMR). Complete removal by column chromatography proved to be difficult and attempts to crystallize the desired product failed.

Palladium-catalyzed coupling of this diiodide with TMSA furnished 2,5-bis[(trimethylsilyl)ethynyl]-3-n-hexylthiophene as an oil in good yield. The monoacetylenic by-product and n-dodecane could be separated from the protected diyne by Kügelrohr distillation (170 °C, 5 mmHg). Similarly to 1,4-diethynyl-2,5-di-n-hexylbenzene (2), a colorless oil was obtained after performing different desilylation protocols (i.e. KOH in MeOH/THF, K2CO3 in MeOH/THF, nBu4NF in cold THF), but it darkened upon complete solvent removal. Flash column chromatography (neutral alumina, pentane) afforded a yellowish oil in good yield and purity (>99.5% according to 1H NMR, the only impurities being unidentified silanes, presumably side-products originating from the desilylation reaction). Drying this diyne with CaH2 as a pentane solution, evaporation and storage at -30 °C under nitrogen and in the dark was found to be a procedure that provided good and reproducible results in subsequent organolanthanide-catalyzed polymerization reactions. The oil darkened within minutes at room temperature both in air and under nitrogen and formed an insoluble solid within days, but in air considerably faster. Properties of the diyne monomers

As mentioned above, a vast majority of terminal diynes are highly thermo- and photosensitive molecules prone to rapid decomposition. The formation of insoluble, highly colored solids upon storage, both in the solid state and in solution, is well-documented for oligoethynyl aromatics.52 It is believed that photopolymerization takes place in the solid state, while a radical mechanism has been put forward to explain the cross-linking reactions of terminal ethynyl groups and the concomitant formation of poly(ene) structures in solution. In addition, oligoethynyl (hetero)aromatics should be treated as potentially explosive materials.4e Many of such terminal diynes, triynes, etc. are reported to decompose exothermically upon heating.

The high instability of 2,5-diethynyl-3-n-hexylthiophene (4) at ambient conditions was striking as compared to 1,4-diethynyl-2,5-di-n-hexylbenzene (2) in the present study. The rapid decomposition of 2,5-diethynyl-3-n-alkylthiophenes and 2,5-diethynylthiophene has been noted previously in literature.43a,58,61 Neat 2,5-diethynylthiophene is reported to undergo a gradual polymerization to insoluble dark brown resins within 5

Chapter 6

212

days at room temperature.61a Decomposition of dilute pentane solutions was apparent by a color change within a few days when stored at 0 °C.43a IR and solid 13C NMR of the solid indicated that most acetylene carbons had been converted to olefin carbons.

6.3. Polymerization reactions with (hetero)aromatic diynes.

6.3.1. Introduction The reaction of Cp*2LaCH(SiMe3)2 with bifunctional 1-alkynes can be regarded as a polymerization

reaction taking place by a step growth polymerization mechanism.62 In principle, polymerization reactions taking place by a step growth mechanism should give rise to one single macromolecule. In practice, however, the average length of the polymer chain is limited by the purity of the reactants, the presence of side reactions and the viscosity of the reaction medium. High molecular weight polymers are formed only at high conversion of the reactive functionality. As a consequence, step growth polymerization reactions place generally stringent requirements on any reaction to be used for polymerization, such as very high conversions and selectivities. It seems that these requirements are well-met in the present Cp*2LaCH(SiMe3)2-catalyzed polymerization reactions of diynes, because complete substrate conversion was found to be quite rapid in the analogous oligomerization reactions of monoynes and the by-products formed are not likely to result in chain termination.

In the absence of side-reactions, trans-head-to-head dimerization is expected to yield a regioregular polymer consisting only of (E)-vinylic linkages. When Cp*2LaCH(SiMe3)2 is allowed to react with 50-fold molar excess of phenylacetylene, a mixture of 2,4-diphenylbut-1-en-3-yne (0.1%), trans-1,4-diphenylbut-1-en-3-yne (97.8%), 1,3,6-triphenylhexa-1,4-diyne (1.6%) and 1,3,6-triphenylhexa-1,2-dien-4-yne (0.5%) is formed (Scheme 6-8). Hence, the reaction of Cp*2LaCH(SiMe3)2 with a bifunctional analogue of phenylacetylene will plausibly afford a polymer containing other types of linkages as well. Linkages originating from head-to-tail dimerization and trimerization are considered to be structural defects, as they interrupt the conjugation63 and, as a result, will impair the development of properties related to conductivity and nonlinear optics.64 Carbon-carbon bond formation via trimerization of ethynyl moieties in the present catalytic process leads also to chain branching.

Substitution of the phenyl ring and the presence of heteroaromatic moieties has been shown to influence both the activity and the selectivity of the Cp*2LaCH(SiMe3)2-catalyzed oligomerization reaction of phenylacetylene. It will be demonstrated that the polymers prepared in this study contain predominantly C-C

Scheme 6-8. The oligomerization of phenylacetylene by Cp*2LaCH(SiMe3)2.

PhPh

Ph

+ + +Ph

Ph

Ph

Ph

Ph

Ph

.

Ph

Ph

0.1% 97.8% 1.6% 0.5%

Cp*2LaCH(SiMe3)22 mol%

C6D625 °C

Scheme 6-7. The possible structure of the polymer resulting from the lanthanocene-catalyzed polymerization of (hetero)aromatic diynes.

Cp*2LaCH(SiMe3)2Ar

x

y

ArAr

Ar

The organolanthanide-catalyzed polymerization of diynes

213

linkages originating from trans-head-to-head dimerization, depending on the type of monomer used. The only defects as detected by 1H NMR and IR spectroscopy are α,α-ethylene linkages resulting from head-to-tail dimerization (Scheme 6-7).

6.3.2. Polymerization reactions Introduction

The reactions of Cp*2LaCH(SiMe3)2 with an excess of diyne were initially studied in Teflon-sealed NMR tubes with benzene-d6 as the solvent (~500 µL) and followed in time by normalized, in situ 1H NMR spectroscopy. In some cases, substrate conversion could be monitored by normalization of the aromatic proton resonance against that of an internal standard. The internal standard that were used are hexamethyldisiloxane (HMDSOO) and CH2(SiMe3)2. The latter forms upon rapid and quantitative protonolysis of the catalyst precursor, Cp2LaCH(SiMe3)2, by the substrate. Quantitative 1H NMR spectroscopy required the use of long pulse delays to avoid signal saturation under the present anaerobic conditions. 1,4-Diethynylbenzene (1)

Reactions of Cp*2LaCH(SiMe3)2 with excess of 1,4-diethynylbenzene (1) (10-50 equiv.) in benzene-d6 led to the formation of a yellow-colored material. The colorless catalyst solution turned instantaneously into an orange suspension upon substrate addition, followed by the precipitate of a yellow solid after ~5 min at room temperature. The solid was found to be completely insoluble in common organic solvents (e.g. toluene, chloroform, THF), even after prolonged heating and sonication. The polymerization of 1 by Cp*2LaCH(SiMe3)2 is rapid, consistent with the analogous oligomerization of its monofunctional congener (e.g. a 55-fold molar excess of substrate relative to Cp*2LaCH(SiMe3)2 is completely converted within 10 min at room temperature in benzene-d6, see Chapter 4). The observed precipitation of the product suggests that the preparation of high molecular weight polymer is limited by the solubility of the growing polymer.

Although the insolubility of the polymer hampered the determination of its microstructure by means of NMR spectroscopy, infrared spectroscopy confirmed the view that terminal acetylenic groups have been

5001000150020002500300025

50

75

100

ν (cm-1)

80

90

100

Tran

smitt

ance

(%)

100

(E)-1,4-diphenylbut-1-en-3-yne (KBr, air)

poly(1) (KBr, air)

1 (KBr, air)

Figure 6-1. Infrared spectra of the PAEV of 1,4-diethynylbenzene (poly(1), upper spectrum), 1,4-diethynylbenzene (1, middle spectrum) and trans-1,4-diphenylbut-1-en-3-yne (lower spectrum).

Chapter 6

214

converted so as to form predominantly (E)-CH=CH linkages. The presence of new peaks at 2183 and 941 cm-1 in the polymer relative to the monomer (Figure 6-1) are, in particular, structurally diagnostic and assigned to the stretching vibration of a carbon-carbon triple bond and a C-H bending vibration of a (E)-CH=CH group, respectively.65 These assignements agree well with values previously observed for PAEVs36-38,92 and other polymers containing enyne units.66 No bending vibrations corresponding to C=CH2 groups were observed (895-885 cm-1). It can also be seen that the acetylenic C-H and C-C stretch vibrations at 3260 and 2100 cm-1 are less intense in the polymer than in the monomer. No conclusive information could be obtained with respect to the possible presence of allene moieties, however. 2,5-Di-n-hexyl-1,4-diethynylbenzene (2)

In order to facilitate the characterization and investigations of the polymer properties, it was decided to increase the solubility of the polymer by substituting 1,4-diethynylbenzene with flexible aliphatic side chains.67 Substituents of different structure and length are known to affect not only the solubility, but also the solid-state structure, the phase behavior and the electronic properties of the polymers.68 The incorporation of substituents onto the diyne monomer are, furthermore, anticipated to influence both the reactivity of the monomer and the regioselectivity of the reaction, in analogy to the observed substrate effects on the rate and selectivity of the Cp*2LaCH(SiMe3)2-catalyzed 1-alkyne oligomerization reactions (Chapter 4). As a result, the choice of the substituent is critical. On the one hand, it can be expected that both solubility and the degree of polymerization will increase with the length of the side-chain.69 On the other hand, the steric size of the substituent will inevitably block metal coordination to a certain extent, thereby diminishing the observed reactivity. The n-hexyl substituent was chosen based on reports that the degree of polymerization and the solubility of 2,5-disubstituted poly(p-phenyleneethynylene)s34d did not increase significantly upon side-chain elongation. It was also shown that the presence of an ortho-methyl group in phenylacetylene resulted in an increased selectivity for dimerization in the reaction catalyzed by Cp*2LaCH(SiMe3)2 (Chapter 4).

When Cp*2LaCH(SiMe3)2 was added to a benzene-d6 solution of 10 or 20 equiv of 1,4-diethynyl-2,5-di-n-hexylbenzene (2), an insoluble black solid precipitated immediately and the complete reaction mixture solidified into a dark rubber-like solid within 45 min at room temperature. The formed polymer did not dissolve upon addition of benzene or THF, not even after heating overnight or sonication. Infrared spectra of the solid detected no absorbance at ~3300 cm-1 (acetylenic C-H stretch) which is moderately strong in the monomer. The addition of chloroform formed a faint yellow solution containing practically the same amount of dark solid.

In order to confirm that the reaction is catalyzed by Cp*2LaCH(SiMe3)2, the same experiment was conducted in the absence of catalyst. The color of the reaction mixture darkened after several hours (suggestive

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm

HHArH

C CHArCH2

2*

Figure 6-2. 500 MHz 1H NMR spectra of the mixture of Cp*2LaCH(SiMe3)2 and a 10-fold excess of monomer 2 in benzene-d6 at 25 °C. The top spectrum is before catalyst addition and the consecutive lower ones are taken each 10, 15, 30, 60 and 60 min later, respectively. The solvent signal is indicated by an asterisk (*).

The organolanthanide-catalyzed polymerization of diynes

215

of thermal decomposition, Section 6.2), but no significant monomer conversion was observed after several days with 1H NMR spectroscopy.

In situ monitoring of the reaction with 10 equiv of 2 by 1H NMR spectroscopy in the presence of hexamethyldisiloxane (HMDSO) as internal standard revealed the consumption of 2 and the formation of CH2(SiMe3)2 and two presumed Cp* signals (Figure 6-2). As the reaction progressed, the newly formed 1H resonances became increasingly broad and ultimately disappeared. These observations are consistent with the formation of oligomers and a gradual precipitation of the product as the molecular weight increases at higher substrate conversion. In addition, broad vinylic 1H NMR resonances were observed around δ 6.5 and 5.5 ppm (vide infra) which are associated with the vinylic protons of the trans-head-to-head and head-to-tail linkages. The observation of the vinylic protons of the trans-head-to-head dimers (δ 6.29, 6.99) and head-to-tail dimers (δ 5.56, 5.60) of phenylacetylene in this region of the 1H NMR spectrum supports this view (Chapter 4).

Complete solidification of the reaction mixture could be retarded, accompanied by an increase of the chloroform-soluble fraction, upon increasing the molar excess of diyne monomer relative to the catalyst. For example, performing the analogous reaction with 50 equiv of substrate 2 resulted in complete solidification of the reaction mixture after 2 h. Addition of CHCl3 yielded a yellow, chloroform-soluble solid in 21% yield after filtration and washing with methanol. A dark-brown solid was obtained as a residue in 74% yield which was found to be insoluble in organic solvents (e.g. THF, toluene, acetone), even after prolonged heating or sonication. Attempts to dissolve the chloroform-insoluble portion of the formed polymer under anaerobic conditions at temperatures of 100-150 ºC (in tetrachloroethane, nitrobenzene, chlorobenzene, o-dichloro-benzene, 1,2,4-trichlorobenzene) were unsuccessful.

A kinetic plot for the reaction with 20 equiv of 2 was obtained by monitoring the intensity of the aromatic signal relative to the internal standard (HMDSOO) with in situ 1H NMR spectroscopy. First-order rate dependence on substrate concentration (kobs = 2.33(4) M-1·min-1, R2 = 0.9989) was observed for the first 45 min in which 53% of 2 was converted. Deviation from first-order rate dependence on substrate concentration at a higher conversion is probably the result of the lowered solubility of the growing polymer chain, competing reactivity from dimers, trimers, etc. and the increasing viscosity of the reaction medium (Figure 6-3).

1H NMR spectroscopy of the chloroform-soluble fraction revealed resonances similar to those of the monomer. The ratio of the intensities corresponding to the acetylenic C≡ CH 1H end-groups and the α-alkyl-aryl groups ArCH2 is an indication of the number-average molecular weight Mn and in this particular example the soluble fraction contains oligomers having an average of 10(1) repeating units.70 The number-average degree of polymerization (Pn) thus obtained can be verified by integrating the combined vinylic signals or aliphatic signals versus the acetylenic signals or dissolving the sample in CD2Cl2 or THF-d8 and integrating the aromatic signals

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

00 30 60 90 120 150 180 210 240 270 300

t(min)

ln([S

]0/[S

]t )

2 3

Figure 6-3. Integrated rate plot of the concentration of 2 and 3 vs time for the diyne polymerization catalyzed by Cp*2LaCH(SiMe3)2. Lines represent fitted linear plots.

Chapter 6

216

versus the acetylenic signals. A Pn of 10(1) implies that oligomers of higher molecular-weight are insoluble in the reaction mixture and confirms the notion that solubility is the limiting factor in obtaining polymers of high molecular weight. As the α-alkyl groups having an ortho-ethynyl and -ethenyl substituent resonate at different chemical shifts (also observed for the linear dimers of 2-methyl- and 2-methoxyphenylacetylene, Chapter 4), two sets of 1H NMR resonances are observed for the present α-alkyl groups. By integrating the vinylic 1H NMR resonances corresponding to the head-to-tail (HT) and trans-head-to-head (HH) coupling the relative amount of HT linkages can be determined. In this particular example, the polymer consists for 89(1)% of HH linkages and for 11(1)% of HT linkages.70 The chloroform solution of the soluble polymer also exhibited a strong blue fluorescence upon irradiation (λ = 366 nm). The formation of poly(2,5-di-n-hexylphenylethynylenevinylene)s was furthermore supported by 13C NMR and IR spectroscopy (vide infra).

It should be noted that a precipitate formed in the chloroform solutions of the soluble polymers upon standing at room temperature. This precipitate was a red solid which was found to be insoluble in common organic solvents, even after prolonged heating or sonication. When this solid was analyzed by IR spectroscopy, the spectra of the insoluble solid and the soluble fraction were practically identical, except that the vibrations corresponding to acetylenic end-groups were less intense in the precipitate than in the soluble fraction. This finding may point to acetylenic cross-linking, as discussed previously for the diethynyl (hetero)aromatic monomers (Section 6.2). Precipitation of the chloroform-soluble fraction was found to be accelerated by heat and light and also took place under an inert nitrogen atmosphere. The instability of these acetylenic end-capped polymers is not unreasonable, considering the instability of the monomers and reports of oligo(enyne)s that were found to become increasingly reactive with increasing oligo(enyne) length.71 The polymer solutions could not be filtered through 0.5-1.0 µm filters and as a result the soluble polymer fraction could not be analyzed by gel permeation chromatography (GPC). Whether this is the result of the acetylenic cross-linking or microgel formation is not known at present.

Concentrated polymer solutions that were prepared for 13C NMR spectroscopy underwent gelation and the increased viscosity led to difficulties in obtaining a spectrum of good quality. It was attempted to prepare concentrated polymer solutions suitable for 13C NMR analysis by preparing low-molecular weight polymers, as oligo(enyne) instability is known to increase with chain length.71 The synthesis of low-molecular weight oligomers was achieved by quenching reaction mixtures before complete solidification. When the reaction of Cp*2LaCH(SiMe3)2 with 2 (20 equiv) was quenched with methanol after 1.5 h a dark-red, viscous oil was obtained nearly quantitatively which was completely soluble in chloroform. 1H NMR analysis indicated Pn =

HH

C CH

H

H

x

y

7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm

2

*

CCH2

CH=CH

CH

C CH

CCH2

CHCH=CH

=C(H)H=C(H)H

Figure 6-4. 500 MHz 1H NMR spectra in CDCl3 of monomer 2 (lower spectrum) and the chloroform-soluble fraction of its polymer after work-up (upper spectrum). * denotes the solvent signal.

The organolanthanide-catalyzed polymerization of diynes

217

3.9(4) and HH:HT = 94(1):6(1), thereby suggesting that polymerization of 2 proceeds in the gel-state. It is important to note that the yield of these thermo- and photolabile polymers is strongly dependent on the isolation and purification procedure.

The 13C{1H} NMR spectrum of the oligomers can readily be interpreted by comparison with the monomer and the model enyne compound, (E)-1,4-diphenylbut-1-en-3-yne (Figure 6-5). For each type of aromatic resonance several sets are observed depending on the nature of substitution (Scheme 6-9). Also multiple sets are found for each type of vinylic, acetylenic and aliphatic resonance, probably as a result of the different neighboring groups in the oligomeric chain.

The exact nature of the solidification of the reaction mixture during reaction is unknown at present. It is generally accepted that gelation takes place via molecular aggregation and that the structures intertwine to create thermo-reversible three-dimensional networks.72 In the present study, thermo-reversible gelation was not observed. When the rubber-like material obtained after complete solidification of the reaction mixture was dissolved in chloroform or THF (dry and under nitrogen in the dark), only a small portion of the solid dissolved (<1-5 wt%), even after prolonged stirring, mild heating and sonication. The possibility that the insoluble polymer fraction consists of polymers having a molecular weight too high to be soluble could neither be discarded nor confirmed by means of IR analysis.

Several types of chemical cross-linking processes may, in addition, proposed to take place during

150 140 130 120 110 100 90 80 ppm

R

R 2

CR CH CC C CC C

CCH CH CCH CHCC C

CC C CC C

x

y

R

R

R

R

H

H

CH

CC CH

CCH CH

a

a a'

a'

b

b

c

c

cd

d

b'b'

e

ef

f

g

h

h

i j

g

i

j

Figure 6-5. 100 MHz 13C{1H} NMR spectra in CDCl3 of monomer 2 (lower spectrum), 1,4-diphenylbut-1-en-3-yne (middle spectrum) and the chloroform-soluble fraction of the polymer derived from 2 (upper spectrum).

Scheme 6-9. Different possible types of substitution in the 2,5-dialkyl-substituted poly(p-phenyleneethynylenevinylene)s.

H

R

R

R

R

R

R

R

R

H

R

R

Chapter 6

218

reaction or during work-up. Examples of photo- or thermooxidation of terminal ethynyl groups73,74 and internal carbon-carbon double and triple bonds75 are well-known. The infrared spectrum of the insoluble solid after aerobic isolation indicated the presence of acetylenic groups, but the possibility that a small degree of the acetylenic end-groups is responsible for chemical cross-linking cannot be ruled out based on IR spectroscopy alone. Oxidative chemical cross-linking seems unlikely, because irreversible gelation was also observed after work-up under anaerobic conditions. In addition, IR spectra of solids exposed to air for prolonged times did not exhibit the presence of carbonyl or hydroxyl groups. Another possibility is the thermal and light-induced cross-linking via the carbon triple bond which is well-documented for poly(arylenebutadiynylene)s.76 IR spectra of the insoluble solids could neither confirm nor discard this type of chemical cross-linking. Experimentally, more light can be shed on this issue by performing in situ IR and UV-vis spectroscopy during the diyne polymerization reaction or solid-state 13C NMR spectroscopy of the insoluble solid. 2,5-Di-n-hexyloxy-1,4-diethynylbenzene (3)

Motivated by the interesting properties exhibited by many diortho-substituted alkoxy derivatives of phenyl containing π-conjugated polymers (e.g. PPs, PPVs, PPEs77) and the fact the Cp*2LaCH(SiMe3)2-catalyzed oligomerization of ortho-ethynylanisole was found to be highly selective for linear dimerization, it was decided to conduct polymerization reactions with 2,5-di-n-hexyloxy-1,4-diethynylbenzene (3).

When Cp*2LaCH(SiMe3)2 was added to a benzene-d6 solution containing 20 equiv of 3, the clear light-yellow solution turned brown instantaneously and a viscuous, red solution formed within 2 h. The reaction mixture solidified completely into a red solid overnight. Chloroform was added to the reaction mixture and filtration afforded a red solid both as residue (71% yield) and filtrate (22%). The residue did not dissolve in common organic solvents, even after prolonged heating or sonication. The filtrate was soluble in CHCl3 and THF and exhibited green fluorescence upon irradiation (λ = 366 nm).

A kinetic plot for the reaction with 20 equiv of 3 was obtained by monitoring the intensity of the aromatic signal relative to the internal standard (HMDSO) with in situ 1H NMR spectroscopy (Figure 6-3). First-order rate dependence on substrate concentration was observed (kobs = 0.694(9) M-1·min-1, R2 = 0.9988) for the first 188 min at which 63% of 3 was converted. The polymerization of 2 is 3.2 times faster than that of 3 which is consistent with the observation that the analogous oligomerization of 2-ethynyltoluene is faster that that of 2-ethynylanisole (Chapter 5). Similar to 2, deviation from first-order kinetics at a higher conversion is observed, probably due to the lowered solubility of the growing polymer chain, competing reactivity from higher oligomer and the increasing viscosity of the reaction medium.

1H NMR spectroscopy of the chloroform-soluble fraction indicated that the polymer consisted of 92(1)% of HH couplings and of 8(1)% of HT couplings. This result is surprising, as 1-alkyne oligomerization

ArH C CH

7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm

3

ArOCH2

*

*

O

O

O

O

O

O

H

H

x

y

Figure 6-6. 500 MHz 1H NMR spectra in CDCl3 of monomer 3 (lower spectrum) and the chloroform-soluble fraction of its polymer after work-up (upper spectrum).

The organolanthanide-catalyzed polymerization of diynes

219

reactions demonstrated that the selectivity for the trans-head-to-head dimer was lower for 2-ethynylanisole than for 2-ethynyltoluene at analogous substrate-to-catalyst molar ratios (Chapter 5). Acetylenic 1H resonances were observed in the chloroform-soluble fraction of 3 after work-up and integration indicated a number-average degree of polymerization of 16(1), thereby implying that n-hexyloxy groups confer a higher degree of solubility to the poly(phenylethynylenevinylene)s than n-hexyl groups. Also, 13C NMR and IR spectroscopy of the soluble fraction support the formation of dialkoxy-substituted poly(p-phenyleneethynylevinylene)s (Figure 6-8). Similar to the di-n-hexyl substituted derivatives, the formation of an insoluble solid was observed, when a solution of a chloroform-soluble polymer fraction was allowed to stand at ambient conditions. 2,5-Diethynyl-3-n-hexylthiophene (4)

Previous investigations directed to assess the variation in the aromatic moiety of the 1-alkyne in the Cp*2LaCH(SiMe3)2-catalyzed oligomerization reaction indicated that 2-thienylacetylene is converted faster and more selectively to the desired trans-head-to-head dimer than phenylacetylene. This finding, together with the demonstrated enhancement of several desired properties (e.g. solubility, thermal and environmental stability, etc.) upon incorporating a 2,5-thienyl moiety into the main chain of π-conjugated polymers (Section 5.1), encouraged us to perform polymerization reactions with diethynylthiophene analogues. Substitution of a n-hexyl substituent was based on reports that the solubility of 3-substituted poly(thiophene-2,5-diyls)s1a and poly(2,5-thienyleneethynylene)s43a did not increase significantly upon chain elongation beyond the n-hexyl substituent.

Addition of Cp*2LaCH(SiMe3)2 to a solution of 50 equiv of 4 caused the reaction mixture to solidify completely into a dark solid rubber-like material within 8 h at room temperature. The dark solid was found to be insoluble in common organic solvents such as toluene, THF and CHCl3. No 1H NMR resonances indicative of head-to-tail dimerization were observed during the course of reaction as monitored by 1H NMR spectroscopy. Analogous reactions with higher relative amounts of 4 (70-140 equiv) afforded in all cases brown solids which were practically insoluble in chloroform. When a similar reaction mixture containing 140 equiv of monomer was quenched with methanol prior to complete solidification, a dark red viscous oil was obtained which was only partially soluble in chloroform. Filtration yielded a red solid as filtrate (28% yield) and a shiny green solid as residue. 1H and 13C NMR analysis of the chloroform-soluble fraction supported the formation of the expected polymers. The intensities of the acetylenic 1H resonances are low (Figure 6-8), but indicate, nonetheless, that the chloroform-soluble fraction consists of polymers having a number-average degree of polymerization of 100(5). The regioregularity of the polymers with respect to the formed butenyl linkages is very high, as no vinylic

160 140 120 100 80 ppm

OR

RO

C CHC CH OCH2

CCCH

CO

3

O

O

CO

CCH CH

CCH CH CCH CHCH

C C C C

OR

RO

OR

RO

H

H

x

y

CC C

a b

ba

c

c

d

e a'

a'de

f f

g

g

h i

ih

j

j

k

l m

l mk

Figure 6-7. 100 MHz 13C{1H} NMR spectra in CDCl3 of monomer 3 (lower spectrum), 1,4-di(2-methoxyphenyl)but-1-en-3-yne (middle spectrum) and the chloroform-soluble fraction of the polymer derived from 3 (upper spectrum).

Chapter 6

220

signals of HT couplings were observed. The multitude of aromatic signal suggests that the position of alkyl substituent is regiorandom, however. The proposed structure is supported by 13C{1H} NMR and IR spectroscopy, even though the 13C{1H} NMR spectrum of the polymer is too complex to allow unambiguous assignements. The latter is undoubtedly related to the asymmetrical alkyl substitution of the thiophene moiety which gives rise to twice the amount of resonances as observed for 2 and 3.

It is important to note that the precipitate formed in the chloroform-soluble fraction upon standing in solution at ambient conditions took place at a considerably higher rate than observed previously for the di-n-hexyl and di-n-hexyloxy derivatives of poly(p-phenylene-ethynylenevinylene)s. After filtration of the reaction mixture and washing with chloroform, a non-negligable amount of the filtrate (5%) did not redissolve in chloroform upon standing for 4 days at ambient conditions. The solid formed was found to be insoluble in common organic solvents and no acetylenic vibrations were observed in its IR spectrum. These observations are suggestive of acetylenic cross-linking affording poly(ene)s, as encountered before. Cross-linking was found to be accelerated by heat and also took place under an inert nitrogen atmosphere.

The high reactivity of 4 impeded a kinetic analysis under similar reaction conditions as performed for

Table 6-1. The diyne polymerization reactions catalyzed by Cp*2LaCH(SiMe3)2.a Entry Catalyst

(mM) Monomer

(equiv) Yield soluble fraction (%)

t (h)

Pnb HH:HTb

1 8.4 1 (20) 0 0.5 - - 2 13 1 (50) 0 0.5 - - 3 32 2 (20) <5 12 - - 4 13 2 (50) 21 12 10(1) 89(1):11(1) 5 20 2 (20) 98 1.5 3.9(4) 94(1):6(1) 6 12 3 (20) 22 12 16(1) 92(1):8(1) 7 13 4 (50) <1 12 - - 8 16 4 (72) <1 12 - - 9 12 4 (120) <5 12 - - 10 11 4 (138) 28 12 100(5) 100:0

a Reaction conditions: C6D6 (0.50 mL), 25 °C. b Number-average degree of polymerization of the chloroform-soluble polymer fraction as determined by 1H NMR spectroscopy.

7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm

4

*

S C CH

CH

CCH2

*

3.80 3.60 3.40 ppm

S HH

n

Figure 6-8. 500 MHz 1H NMR spectra in CDCl3 of monomer 4 (lower spectrum) and the chloroform-soluble fraction of its polymer after work-up (upper spectrum). * denotes the solvent signal.

The organolanthanide-catalyzed polymerization of diynes

221

2 and 3. No 1H resonances of 4 were observed after 5 min, when 20 equiv of 4 were reacted with Cp*2LaCH(SiMe3)2. By means of line-shape analysis a conversion of 85% was found after 15 min, when Cp*2LaCH(SiMe3)2 was reacted with 72 equiv of 4. As the high reactivity of 4 relative to 2 and 3 was clear and in agreement with analogous 1-alkyne oligomerization reactions (i.e. the rate of 2-ethynylthiophene conversion was found to be ~10 times faster than that of 2-ethynyltoluene and ~15 times faster than that of 2-ethynylanisole under identical reaction conditions, Chapter 5), no further attempts were made to quantify this difference in reactivity.

6.3.3. Polymerization reactions with end-capping agents Introduction

The use of a monofunctional monomer to control the molecular weight of the polymer is well-established in step-growth polymerization reactions.28,78 In the present study, the application of a suitable end-capping agent could in principle provide a means to control the molecular weight of the polymers by adjusting the molar ratio of both substrates. A concomitant advantage of an end-capping group is its ability to infer stability upon conjugated oligomers containing yne-, ene- and enyne-scaffolds,79 as arylacetylenic (vide supra) and vinylacetylenic80 end-groups are known to be fairly reactive. As a result, the use of end-capping agents is expected to yield products that are more stable than those of analogous homopolymerization reactions. Another advantage of the application of end-capping groups is their use as a tool to determine the molecular weight by, for example, 1H NMR spectroscopy.

Obviously, the success of this method relies on the relative reactivity of both substrates. The reactivity of the monoyne as compared to that of the diyne depends not only on the structural properties of the 1-alkyne (such as electronic and steric effects of the 1-alkyne substituent), but also on its concentrations relative to that of the diyne. A detailed study of substituent effects on the catalytic oligomerization reaction mediated by Cp*2LaCH(SiMe3)2 demonstrated that the reactivity of 1-alkynes can be enhanced by substituents with an increasing σ-electron-withdrawing character or the presence of heteroatoms in proximity of the ethynyl group, while steric effects can be used to retard the reactivity (Chapter 5). Hence, the choice of an appropriate monoacetylenic end-capping group seems a priori restricted to 1-alkynes having (kinetic) acidities similar to those of the aromatic diynes.81 Phenylacetylene (5)

Phenylacetylene (5) seemed the most obvious first choice in the pursuit of a suitable end-capping agent for the reactions of Cp*2LaCH(SiMe3)2 with aromatic diynes. Phenylacetylene was found to be more reactive than 1,4-diethynyl-2,5-di-n-hexylbenzene (2), as is clearly demonstrated by the reaction of Cp*2LaCH(SiMe3)2 with 24 equiv of 2 and 25 equiv of phenylacetylene.82 Under these reaction conditions, phenylacetylene was converted completely within several minutes (1H NMR), while the reaction mixture solidified after several hours. When Cp*2LaCH(SiMe3)2 was allowed to react with 44 equiv of 2 and 7 equiv. of phenylacetylene in benzene-d6, no precipitate was observed. Instead, a dark brown solution formed, thereby demonstrating the successful application of phenylacetylene as an end-capping agent to prepare soluble oligo(phenylene-ethynylenevinylene)s. Washing with methanol, filtration and dissolving in CHCl3 yielded a brown solid in practically quantitative yield. Overlapping 1H NMR resonances hampered the determination of the relative rate of consumption of phenylacetylene and 2 by in situ 1H NMR spectroscopy.

Based on the observation that the solutions of the phenylacetylene end-capped poly(2) did not yield any observable precipitate after standing for several weeks at room temperature, it can be concluded that the end-capping phenylacetylene groups also confer some stability on the polymer. However, the formation of an

Scheme 6-10. The use of a monoacetylenic end-capping group to prepare soluble and more stable poly(aryleneethynylenevinylene)s.

R ArR

n

Cp*2LaCH(SiMe3)2x + yAr R

Chapter 6

222

insoluble solid was observed, when end-capped poly(2) was heated to 80 °C or higher temperatures. Infrared spectroscopy of this solid revealed some minor changes, but did not provide conclusive evidence for a distinct chemical change. Unfortunately, the solutions of the end-capped polymer of 2 could not be filtered through 0.5-1.0 µm filters and GPC analysis of the end-capped poly(2) was therefore not possible. Trimethylsilylacetylene (6) Another advantage of the application of end-capping groups is their use as a tool to determine the molecular weight by, for example, 1H NMR spectroscopy. As the 1H NMR resonances of a phenylacetylenic end-group overlap with those of the polymer, phenylacetylene cannot be used in this respect. Thus, other 1-alkynes were investigated to satisfy this objective. In view of its kinetic acidity which is comparable to that of phenylacetylene83 and its relatively high 1H NMR sensitivity, trimethylsilylacetylene (6) was investigated. Surprisingly however, trimethylsilyl-acetylene was found to be substantially more reactive than 2. For example, when Cp*2LaCH(SiMe3)2 was allowed to react with 55 equiv of 2 and 5 equiv of trimethylsilylacetylene, the monoacetylene was completely converted to oligomers before consumption of 2, as monitored by in situ 1H NMR spectroscopy, and solidification of the reaction mixture took place after several hours. (2,6-Dimethyl)phenylacetylene (7)

As monoacetylenes applicable both as end-capping agents and tools for quantitative 1H NMR end-group analysis are quite severely limited by their acidity and availability, alkyl-substituted phenylacetylenes were subsequently explored. The study of (2,6-dimethylphenyl)acetylene (7) as end-capping agent was motivated by the need to mimic the ortho-substituted bifunctional substrates (as phenylacetylene was found to be significantly more reactive than 2 and the addition of small quantities was not considered practical in these small-scale reactions) and its relatively high 1H NMR sensitivity. Unfortunately, the reactivity of (2,6-dimethylphenyl)acetylene (7) was found to be much lower than that of the (hetero)aromatic diynes 2 and 3. For example, following the reaction of Cp*2LaCH(SiMe3)2 with 1,4-diethynyl-2,5-di-n-hexyloxybenzene 3 (25 equiv.) and 7 (26 equiv.) by in situ 1H NMR spectroscopy revealed that the amount of 7 did not change significantly during consumption of 3. A dark precipitate formed after several hours, while unreacted 7 was observed in considerable amounts in the soluble polymer fraction after work-up. 2-Ethynyltoluene (8)

2-Ethynyltoluene (8) represents another alkyl-substituted phenylacetylene that was investigated as an end-capping agent capable of controlling the molecular weight of the polymer, possibly providing these oligomers with stability and concomitantly serving as a tool for quantitative end-group analysis by 1H NMR spectroscopy. When Cp*2LaCH(SiMe3)2 was allowed to react with 2 (25 equiv) and 2-methylphenylacetylene (26 equiv) in a 1:1 ratio, a brown solution formed and no precipitate was observed upon standing for several

7.0 6.0 5.0 4.0 3.0 2.0 1.0 ppm

x

y

ArCH3

Figure 6-9. 500 MHz 1H NMR spectra of the substrate mixture of 2 and 8 (1:1) in benzene-d6 before (lower spectrum) and 4 h after addition of Cp*2LaCH(SiMe3)2 (middle spectrum). The upper spectrum corresponds to the end-capped polymer in CDCl3 after work-up.

The organolanthanide-catalyzed polymerization of diynes

223

hours. In situ 1H NMR spectroscopy indicated the consumption of both substrates at approximately similar rates. By estimating the decrease in intensity of the aromatic ArH 1H resonance of 2 by means of line-shape analysis and integrating the ArCH3 1H resonance of 2-ethynyltoluene versus the internal standard (HMDSO), it was found that 2 and 2-ethynyltoluene were converted for 64% and 73%, respectively, 33 min after addition of the catalyst solution to the substrate mixture. After 63 min 2 and 2-ethynyltoluene were converted for 86% and 92%, respectively, and after 93 min for 91% and 96%, respectively. After 2 h the 1H NMR spectrum of the reaction mixture did not change significantly. The newly formed 1H signals corresponding to ArCH3 of the end-capping group are similar to those of the linear dimers of 2-ethynyltoluene (Chapter 4). Thus, two sets corresponding to vinylic (δ 2.06 ppm) and acetylenic (δ 2.00 ppm) aromatic groups were observed for the terminal ArCH3 groups.

The reaction mixture was quenched with methanol and evaporated to dryness. The polymer was dissolved completely in chloroform, filtered, washed with copious amounts of methanol (filtrate was analyzed and did not contain linear dimers of 2-ethynyl-toluene) and isolated as a brown-yellow powder in a virtually quantitative yield. Subsequent 1H and 13C NMR analysis of the polymer was indicative of oligo(2,5-di-n-hexylphenyleneethynylenevinylene)s end-capped with 2-ethynyltoluene (Figure 6-9). No acetylenic 1H NMR resonances were observed. The intensities of the end-capping ArCH3 and internal ArCH2 1H NMR resonances were estimated by line-shape analysis and indicated a number-average degree of polymerization of 4.7(4). The oligomers contained 6(1)% HT linkages as determined by integration of the vinylic 1H NMR resonances.

In an attempt to prepare 2-ethynyltoluene end-capped poly(2) of lower molecular weight, the analogous reaction was performed with 2 (10 equiv) and 2-ethynyltoluene (40 equiv) in a 1:4 ratio, respectively. Upon standing a reaction mixture formed that solidified overnight. 1H NMR and GC-MS analysis of the soluble fraction after work-up revealed the presence of head-to-tail and trans-head-to-tail dimers of 2-ethynyltoluene. It appears therefore that the control over the molecular weight of the polymer by adjusting the molar ratio between the di- and monoyne is a rather delicate balance. At both a high and low concentration of monoyne relative to diyne, consumption of the monoyne was found to be more rapid than the consumption of the diyne, thereby allowing the diyne polymerization to take place in the absence of end-capping agent. Obviously, an optimal molar ratio can be found experimentally, but it will most likely be highly dependent on the nature of the monomer and the end-capping agent, the amount of solvent and the reaction temperature.

After several attempts a five-fold molar excess of diyne relative to the end-capping agent was found to provide a reaction mixture which eventually did not solidify in the Cp*2LaCH(SiMe3)2-catalyzed polymerization reactions of 1,4-diethynyl-2,5-di-n-hexyloxybenzene 3 (25 equiv) in the presence of 2-ethynyltoluene (5 equiv). This five-fold excess instead of a stoichiometric amount as observed for the polymerization reactions of 2 reflects the lower reactivity of the di-n-hexyloxy monomer 3 relative to its di-n-hexyl congener 2.

Monitoring this reaction with in situ 1H NMR spectroscopy indicated that 2-ethynyltoluene (8) was consumed after conversion of 3. This finding indicates that the rate of conversion of 8 can only compete with that of 3 at relatively high conversion of 3. A red viscous reaction mixture was obtained after 16 h and 1H NMR spectroscopy indicated that both monomers were completely consumed. Addition of chloroform and precipitate with methanol afforded a red suspension which was filtered and washed with methanol and pentane. After

Table 6-2. The diyne polymerization reactions catalyzed by Cp*2LaCH(SiMe3)2 in the presence of an end-capping agent.a Entry Catalyst

(mM) Diyne (equiv)

Monoyne (equiv)

Yield soluble fraction (%)

Pnb HH:HTb

1 12 2 (20) 5 (7) 93

2 13 2 (55) 6 (5) <1 - -

3 13 3 (25) 7 (26) <4 - -

4 13 2 (50) 8 (26) 96 4.7(4) 94(1):6(1)

5 13 3 (25) 8 (5) 95 11(1) 90(1):10(1)

6 11 4 (92) 8 (80) 88 3.9(4) 100:0 a Reaction conditions: C6D6 (0.50 mL), 25 °C. b Number-average degree of polymerization of the soluble polymer fraction as determined by 1H NMR spectroscopy.

Chapter 6

224

rotatory evaporation the residue was obtained as a bright red viscous oil (68% yield) and the filtrate as a yellow oil (20%). 1H and 13C{1H} NMR analysis of the red oil indicated the presence of the expected resonances for the end-capped poly(2,5-di-n-hexyloxyphenyleneethynylenevinylene)s having a number-averaged degree of polymerization of 11(1) and consisting of 90(1)% of HH couplings and 10(1)% of HT couplings. 1H NMR and GC-MS analysis of the filtrate revealed the presence of linear dimers of 2-ethynyltoluene and some low-molecular weight oligomers.

When a catalyst solution was added to a nearly equimolar amount of 1,4-diethynyl-3-n-hexylthiophene 4 (92 equiv) and 2-ethynyltoluene (88 equiv), no precipitate was observed after 1 day at room temperature. Monitoring the reaction in situ with 1H NMR spectroscopy revealed that 4 was consumed nearly completely within 40 min and that the amount of 2-ethynyltoluene had not changed significantly at this point. As observed above, 1H NMR spectroscopy indicated that 2-ethynyltoluene is converted after complete consumption of 4. After 2 h significant conversion of 2-ethynyltoluene is observed concomitant with the formation of its linear dimers. After 8 h no 1H NMR resonances of the monomers were observed and the no changes were observed in the 1H NMR spectrum of the reaction mixture.

The oligomers were isolated and purified by dissolving the reaction mixture in chloroform, precipitate by addition of methanol and filtration followed by washing the residue with methanol and pentane. The washings were evaporated to dryness forming an orange solid containing mainly linear dimers of 2-ethynyltoluene and low-molecular weight oligomers (1H NMR, GC-MS). The residue is redissolved in chloroform and was cast as a thin film upon rotatory evaporation (88% yield). 1H and 13C NMR analysis support the formation of 2-ethynyltoluene end-capped oligo(3-n-hexyl-2,5-thienylethynylene-vinylene)s. No vinylic 1H NMR resonances attributable to HT couplings were observed. Unfortunately, the resonance of the terminal methylphenyl groups overlapped with those of the α-alkylthienyl groups in CDCl3, but in THF-d8 the overlap was smaller and an averaged-number degree of polymerization of 3.9(4) could be estimated by line-shape analysis.

6.3.4. The effect of reaction temperature Introduction The effect of reaction temperature on the present polymerization reactions and the formed products is manifold and difficult to predict a priori. Higher reaction temperatures are likely to decrease both the reaction time and selectivity of the coupling reaction, in analogy to Cp*2LaCH(SiMe3)2-catalyzed 1-alkyne oligomerization reactions (Chapter 5). The solubility of the growing polymer and the viscosity of the reaction medium may also be affected by the reaction temperature. Furthermore, it can be expected that the reaction temperature will influence the relative rate of mono- and diyne conversion in the polymerization reactions with end-capping reagents. To determine the precise effect of reaction temperature on the present polymerization reactions and the properties of the formed polymer, several reactions were performed at higher reaction temperature. Polymerization reactions

When the reaction of Cp*2LaCH(SiMe3)2 and 1,4-diethynyl-2,5-di-n-hexylbenzene 2 (50 equiv) was performed at 50 °C, a decrease in the selectivity for the trans-head-to-head dimerization (i.e. HH:HT = 82(1):18(1) as compared to 89:11 for the analogous reaction at 25 °C) was observed. Concomitantly, the amount of the chloroform-soluble polymer fraction (5% rather 21%) was also smaller. In addition, solidification of the reaction mixture took place after 2 h rather than 12 h as observed at 25 °C. Remarkably, no decrease in selectivity was observed by increasing the reaction temperature in the reaction with 1,4-diethynyl-3-di-n-hexylthiophene (4). For example, when the reaction of Cp*2LaCH(SiMe3)2 and 4 (100 equiv) was performed at 50 °C, the reaction mixture solidified within 4 h. The polymer was isolated as a shiny green, insoluble solid (77%) and a red, chloroform-soluble solid (10%). In situ 1H NMR spectroscopy indicated that 4 was consumed completely within 10 min and that no vinylic 1H NMR resonances, due to HT couplings, formed throughout the course of reaction. 1H NMR analysis of the chloroform-soluble polymer fraction confirmed the absence of HT couplings and indicated a number-averaged degree of polymerization of 72(5).

The organolanthanide-catalyzed polymerization of diynes

225

Polymerization reactions with end-capping groups As discussed above, the successful application of 2-ethynyltoluene as an end-capping agent in the present diyne polymerization reactions depends on the relative reactivity of both substrates. It was found that the use of molar ratios that provided soluble oligomers at ambient temperatures afforded reaction mixtures that solidified after several hours, when the analogous reactions were conducted at higher reaction temperatures. Experiments revealed that a five-fold molar excess of 1,4-diethynyl-2,5-di-n-hexyloxybenzene 3 (25 equiv.) relative to 2-ethynyltoluene (5 equiv.) was necessary to impede the formation of insoluble, high molecular weight oligomers at 80 °C. In situ 1H NMR spectroscopy revealed that 3 is consumed completely within 10 min at 80 °C and 5 within 26 min and that the reaction mixture did not change after complete conversion of these substrates. The reaction mixture was completely soluble and oligomers formed exhibited an averaged-number degree of polymerization of 5.7(5) and consisted of 86(1)% of HH couplings and 14(1)% of HT couplings (determined by 1H NMR). Interestingly, no linear dimers of 2-ethynyltoluene were found. The results obtained for reactions of 1,4-diethynyl-2,5-di-n-hexylbenzene (2) and 1,4-diethynyl-2,5-di-n-hexyloxybenzene (3) indicate that increasing the reaction temperature leads both to lower reaction times and selectivities for trans-head-to-head dimerization. The monomer 2,5-diethynyl-3-n-hexylthiophene (4) represents a remarkable exception on this general observation, however. It is believed that the higher activity and selectivity of 4 can plausibly be attributed to a combination of favorable electronic substituent effects and the heteratom-directed nature of 1-alkyne oligomerization, as proposed for analogous Cp*2LaCH(SiMe3)2-catalyzed 1-alkyne oligomerization reactions (Chapter 4).

6.3.5. The effect of monomer concentration Introduction

The scope and limitations of the present organolanthanide-catalyzed diyne polymerization reactions were mostly identified by monitoring reaction mixtures with NMR spectroscopy. As a result, these exploratory polymerization reactions were performed at a relatively high monomer concentration. In order to investigate the effects of relatively low monomer concentration on the polymerization reaction, experiments were carried out in a larger amount of solvent. It seems reasonable to expect that the molecular weight of the formed polymers will increase under more dilute reaction conditions, because both the precipitation of a growing polymer chain and the gelation of the polymer solution are promoted by high polymer concentrations. When the experiments are conducted at very low monomer concentration, however, cyclic polymers may form, as is well-documented for the oxidative copper-catalyzed reactions of diynes.84 Typical polymerization experiments at relatively high monomer concentration were performed with 50-100 mg in 0.5 mL of benzene-d6, corresponding to polymer concentrations of 100-200 g/L or 11-21 wt%. Typical polymerization experiments at relatively low monomer concentration, on the other hand, were conducted with 50-100 mg of monomer in 2.0 mL of toluene, corresponding to polymer concentrations of 1-6 wt%.

Table 6-3. The diyne polymerization reactions catalyzed by Cp*2LaCH(SiMe3)2 at different temperatures.a Entry Catalyst

(mM) Diyne

(equiv.) Monoyne (equiv.)

soluble fraction

(%)

t (h)

T (°C)

Pnb HH:HTb

1 13 2 (50) - 21 12 25 89(1):11(1)

2 13 2 (50) - 5 8 50 82(1):11(1)

3 13 3 (25) 8 (5) 95 12 25 11(1) 90(1):10(1)

4 12 3 (25) 8 (5) 92 2 80 5.7(5) 86(1):14(1)

5 11 4 (138) - 28 12 25 100(5) 100:0

6 15 4 (100) - 10 4 50 72(5) 100:0 a Reaction conditions: C6D6 (0.50 mL), 25 °C. b Number-average degree of polymerization of the soluble polymer fraction as determined by 1H NMR spectroscopy.

Chapter 6

226

1,4-Diethynyl-2,5-di-n-hexylbenzene (2) When Cp*2LaCH(SiMe3)2 was allowed to react under stirring with 40 equiv. of 1,4-diethynyl-2,5-di-

n-hexylbenzene (2) in toluene (2.0 mL) at 25 °C, the brown solution solidified completely within 4 h, yielding a brown solid. The reaction mixture was opened to air after 12 h and subjected to three cycles of dissolution in chloroform, precipitation with methanol, filtration and washing with methanol. The polymer was isolated as a yellow solid in reasonable yield (78%). 1H NMR analysis of the chloroform-soluble polymer fraction indicated an average degree of polymerization of 38(2) and the presence of 96(1)% HH couplings and 4(1)% HT couplings. When the same reaction was quenched after 4 h, the chloroform-soluble polymer fraction was isolated in a high yield (87%) and was found to have an number-average degree of polymerization Pn of 32(1) and a HH:HT ratio of 94:6 (1H NMR). 1,4-Diethynyl-2,5-di-n-hexyloxybenzene (3)

Remarkably, the analogous reaction with 1,4-diethynyl-2,5-di-n-hexyloxybenzene (3) did not solidify completely after 2 days. Instead, a red viscous solution was formed containing a red solid. The reaction mixture was opened to air and quenched with chloroform. Heating under stirring afforded a clear solution from which the polymer was precipitated by addition of methanol. The work-up procedure as described above produced a red solid in good yield (79%). A number-average degree of polymerization Pn of 13(1) was found and the oligomers contained 92% HH couplings and 8% HT couplings (1H NMR analysis). 2,5-Diethynyl-3-n-hexylthiophene (4)

The reaction of Cp*2LaCH(SiMe3)2 with 110 equiv of 2,5-diethynyl-3-n-hexylthiophene (4) in toluene afforded a reaction mixture which solidified completely into a red-brown solid in 1.3 h at 50 °C. The above described work-up procedure afforded a green solid as the insoluble polymer fraction and a red solid (33%) as the soluble polymer fraction. 1H NMR analysis of the chloroform-soluble polymer fraction indicated a number-average degree of polymerization of 200(10) and a very high regioregularity (HH couplings >99.8(1)%).

6.4. Structural characterization and properties of the polymer. General remarks

As discussed above, 1H and 13C{1H} NMR and IR spectroscopy support the identity of the expected poly(aryleneethynylenevinylene)s. Most of the 1H and 13C NMR resonances could be assigned by comparison with the respective monomers and the model compounds 1,4-diphenylbut-1-en-3-yne (9), 1,4-di(2-methoxyphenyl)but-1-en-3-yne (10) and 1,4-di(2-thienyl)but-1-en-3-yne (11). All polymers exhibited a C≡ C stretching band at ~2160 cm-1 and a C-H bending vibration of the (E)-CH=CH group at ~945 cm-1 in the IR spectra. Unfortunately, GPC analysis was not possible, as polymer solutions could not be filtered through 0.5-1.0 µm filters. The exact reason for this behavior is unknown at present. Optical properties

The optical properties of the polymers were studied by UV-Vis absorption and fluorescence (FL) spectroscopy. The results are summarized in Table 6-5. It can be seen that all polymers are fluorescent at excitation wavelengths that correspond to their maximum absorption, whereas the monomers are not. UV-Vis

Table 6-4. The diyne polymerization reactions catalyzed by Cp*2LaCH(SiMe3).a Entry Catalyst

(mM) Diyne

Substrate (mM)

Soluble fraction

(%)

t (h)

T (°C)

Pnb HH:HT

1 2.3 2 75.4 78 12 25 38(1) 96(1):4(1)

2 2.3 2 75.4 87 4 25 32(1) 96(1):4(1)

3 1.7 3 69.8 79 48 25 13(1) 92(1):8(1)

4 2.0 4 224 33 1.3 50 200(10) 100:0 a Reaction conditions: toluene (2.0 mL). b Number-averaged degree of polymerization determined by 1H NMR spectroscopy.

The organolanthanide-catalyzed polymerization of diynes

227

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

3 poly(3) 10

Wavelength (nm)

Em

issi

on in

tens

ity (n

orm

aliz

ed)

0.0

0.2

0.4

0.6

0.8

1.0

1.2A

bsorption intensity (normalized)

Figure 6-10. UV-vis absorption and fluorescence spectra (CHCl3) of 1,4-diethynyl-2,5-di-n-hexyloxybenzene (3), (E)-1,4-diphenylbut-1-en-3-yne (10) and poly(3) (see for details, Table 6-5).

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

poly(2+8) 2 poly(3+8)

Wavelength (nm)

Emis

sion

inte

nsity

(nor

mal

ized

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorption intensity (norm

alized)

Figure 6-11. UV-vis absorption and fluorescence spectra (CHCl3) of 1,4-diethynyl-2,5-di-n-hexylbenzene (2) and polymers end-capped with 2-ethynyltoluene (8), poly(2+8) and poly(3+8) (see for details, Table 6-5).

Chapter 6

228

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

Wavelength (nm)

Emis

sion

inte

nsity

(nor

mal

ized

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

poly(4)a 11 poly(4)b A

bsorption intensity (normalized)

Figure 6-12. UV-vis absorption and fluorescence spectra (CHCl3) of (E)-di(2-thienyl)but-1-en-3-yne (11), poly(4)a and poly(4)b (see for details, Table 6-5).

spectroscopy offers a qualitative measure of the π-orbital overlap of the conjugated polymer. The maximum UV-Vis absorption λmax is attributed to the π-π* transition of the conjugated polymer backbone. The higher the λmax in the UV-Vis spectrum, the higher the degree of conjugation present in the polymer backbone. It is well-established that the band gap increases with the length of the oligomers until a convergence limit is reached which is termed the effective conjugation length.85 The considerable red-shift of the polymers relative to their corresponding monomers is indicative of extended π-conjugation.

It is difficult to make fair comparisons between the different types of polymers in the present study, since λmax is also influenced by the oligomeric chain length and the regioregularity (i.e. (E)-head-to-head (HH)

Table 6-5. Optical data of the polymers in solutions.a Entry Compound Absorpion

λmax (nm) Emission λmax (nm)

1 3 275, 337 -

2 10 318 352, 754

3 poly(3) [Pn 16, 92% HH] 450 514

4 2 300, 310, 336 -

5 poly(2+8) [Pn 7, 94% HH] 397 472, 502

6 poly(3+8) [Pn 11, 92% HH] 462 467

7 11 348 385, 403

8 poly(4)a [Pn 10, 100% HH] 420,454 515

9 poly(4)b [Pn 100, 100% HH] 480 565 a UV-vis absorption and fluorescence spectra were recorded in a dilute chloroform solution at room temperature. The wavelength of the absorption maximum was chosen as the excitation wavelength.

The organolanthanide-catalyzed polymerization of diynes

229

and head-to-tail (HT) couplings) of the polymer. Based on the difference of λmax between polymer and monomer, the present results suggest that the polymers derived from 1,4-diethynyl-2,5-di-n-hexylbenzene, poly(2), exhibit a lower degree of conjugation than those derived from 1,4-diethynyl-2,5-di-n-hexyloxybenzene, poly(3) (Entries 3,5 and 6, Table 6-5), while polymers based on 2,5-diethynyl-3-n-hexylthiophene, poly(4), display a higher degree of conjugation than poly(3) (Entries 3 and 8, Table 6-5).

A blue-shift in the absorption maximum is observed for poly(2+8) relative to poly(3+8) ) (Entries 5 and 6, Table 6-5). This behavior has also been observed for 2,5-dialkyl-substituted poly(p-phenyleneethynylene)s as compared to their dialkoxy congeners and has been attributed to the less electron-donating nature of the alkyl groups.17b,86 The influence of the oligomeric chain length can be seen clearly by comparing poly(4)a and poly(4)b (Table 6-5, entries 8 and 9). A considerable red shift is observed in both the absorption and fluorescence spectrum, indicative of a higher effective conjugation length. MALDI-TOF spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has gained increasing importance in the characterization of synthetic polymers.87 Both the sample preparation and the nature of the polymer determine the success and the quality of the MALDI mass spectrometric analysis. The type of matrix and the relative ratio of matrix to sample are of considerable importance and their selection and optimization is still often a trial-and-error process. It is generally believed that polymers which are not amenable to MALDI analysis lack suitable ionization sites making the creation of intact gas- phase macromolecules difficult. Experimental experience has shown that polar polymers containing heteroatoms show cationization after being mixed with sodium or potassium salts, while unsaturated polymers without heteroatoms (e.g. polystyrene, polybutadiene, polyisoprene) can be successfully ionized after the addition of silver or copper salts which interact with the π-electrons of these ion-binding sites.

Unfortunately, MALDI-TOF mass spectra of good quality could not be obtained for the polymers derived from 1,4-diethynylbenzene (1). The use of chloroform as solvent and α-cyano-4-hydroxycinnaminic acid as matrix produced a MALDI-TOF mass spectrum of good quality for poly(2). However, even though the signals were 294.3 Da apart corresponding exactly to the molecular mass of the repeating unit, the signals were consistently 120 Da too high relative to the masses of the expected oligo(2) species. The cause of this discrepancy is unknown presently. Variation of matrix, solvent and the addition of metal salts gave similar results.

Figure 6-13 shows a MALDI-TOF mass spectrum of poly(3) (Mn 16(1) and 92% HH, as determined by 1H NMR spectroscopy) using ditranol as a matrix and chloroform as a solvent. No cationization salt solutions

Figure 6-13. MALDI-TOF MS for poly(3).

Chapter 6

230

were needed to obtain ionization and the addition of copper(I) and silver(I) salts did not improve the quality of the spectrum. The repeat unit from this spectrum is found to be 326 Da, which corresponds to the monomer repeat unit formula weight exactly. However, the distribution of molecular weights does not correspond to the polymer having a number-average molecular weight of 16(1). The underrepesentation of high-mass components with respect to the lower mass components in MALDI-TOF mass spectra is commonly observed and is believed to be caused by several factors, including sample preparation, mass-dependent desorption/ionization, ion focusing/transmission and mass-dependent ion detection.88 The origin of the second series of peaks in Figure 6-13 corresponding to masses which are 71-85 Da lower than those of poly(3) is unknown at present.

Attempts to obtain a MALDI-TOF mass spectrum of poly(3) end-capped with 2-methylphenylacetylene (8) produced a high quality spectrum (dithranol, chloroform) exhibiting signals that were 36 Da lower than those expected for poly(3+8) (Pn = 16(1) and 92% HH, as determined by 1H NMR spectroscopy). This observation can be rationalized by the addition of HCl to the double or triple carbon bond either during the MALDI desorption/ionization process or during polymer dissolution in chloroform under heating, as hydrogen chloride is known to be formed from chloroform upon heating.89 No attempts were undertaken to study this phenomenon more carefully.

A MALDI-TOF mass spectrum of moderate quality was obtained of poly(4) (Pn = 10(1) and 100% HH, as determined by 1H NMR spectroscopy) using dithranol and chloroform. Similar results were obtained upon changing the matrix and the amount of solvent and after addition of metal salts (Figure 6-14). The main peaks are equally spaced and 216 Da apart, representing the expected oligomers which differ in mass by the molecular weight of the repeating unit. The lower intensity signals (including the signal at 347.4 Da) cannot be accounted for at present.

6.5. Discussion

After initiation of this study three independent reports by different research groups appeared in literature describing a similar synthetic approach to the preparation of PAEVs by means of the metal-catalyzed dimerization of terminal acetylenes. Katayama et al. studied the polymerization of 2,7-diethynyl-9,9-dioctylfluorene by three different catalysts which were highy regioselective for (E)-head-to-head, (Z)-head-to-head and head-to-tail dimerization.40a Based on the palladium catalyst developed by Nolan et al.,90 a highly regioregular polymer (99% HH couplings, as indicated by 1H NMR) of moderate molecular weight and high

Figure 6-14. MALDI-TOF MS for poly(4).

The organolanthanide-catalyzed polymerization of diynes

231

polydispersity was obtained in good yield. It is interesting to note that GPC analysis in THF based on polystyrene standard could be performed. However, comparison of the number-average molecular weight values obtained from 1H NMR end-group analysis (Pn = 14.1) with those from GPC (Pn, GPC = 31.9, Mw/Mn = 2.35) indicated that GPC overestimates the molecular weight of the PAEVs by a factor of 2.3, probably due to the rigid-rod structure of the polymers.91

Nishiura, Hou and co-workers reported the polymerization of 1,4-diethynyl-2,5-dioctylbenzene by three different organolanthanide catalysts which were regioselective for (E)-head-to-head, (Z)-head-to-head and head-to-tail acetylene dimerization (Scheme 6-11).40b,c The lanthanidocene catalyst Cp*2PrCH(SiMe3)2 was used to prepare the corresponding (E)-vinylene rich poly(arylene-ethynylenevinylene) in high yield and regioselectivity (98% HH as determined by 1H NMR). GPC analysis (based on polystyrene) indicated a high molecular weight and polydispersity (Pn, GPC = 39.0, Mw/Mn 2.35), but no end-group analysis by means of 1H NMR spectroscopy was reported. The actual degree of polymerization is thus unknown. Assuming that the correction factor found by Katayama et al. is also valid for this system, this result points to a polymer of Pn ≈ 17. It was shown that the molecular weight could be controlled by varying the polymerization time (i.e. Mn = 2400 after 15 min, Mn = 5400 after 30 min, Mn = 16000 after 1 h at room temperature), but the occurrence of gelation is not mentioned. Comparison with the present results is not possible, because important experimental conditions such as monomer and catalyst concentration and spectral data including NMR and IR spectroscopy were not reported. The (Z)-rich polyenyne was studied with UV-vis spectroscopy and no relationship between Mn and λmax was observed. This process was later patented by Nishiura et al.40e and both aromatic and heteroaromatic diyne monomers were claimed, but only the preparation of a polymer derived from 1,4-diethynyl-2,5-di-n-octyloxybenzene was described.

Ueda and Tomita used a low-valent titanocene catalyst generated from Cp*2TiCl2 and i-PrMgBr to

Scheme 6-11. The regio- and stereoselective synthesis of poly(p-phenylene-ethynylenevinylene)s containing (E)-, (Z)- and gem-vinylene units.40b,c

Mn 16000, Mw/Mn 3.7299% yield, 98% selectivity

Mn 5300, Mw/Mn 5.4895% yield, 96% selectivity

Mn 10000, Mw/Mn 4.4499% yield, 100% selectivity

C7H8, 2 h, r.t.

C7H8, 7 h, 110 °C

C7H8, 159 h, 110 °C

OC8H17

H17C8O

Cp*2PrCH(SiMe3)2

Cp*2Lu(THF)CH2(SiMe3)

Lu(THF)CH2(SiMe3)2SiN

H

OC8H17

H17C8OH

n

H

OC8H17

H17C8OH

n

H

OC8H17

H17C8O

H

n

Scheme 6-12. Model compound III and enyne-containing polymers IV and V.

O

O

O

R

nVIV

n

S

H21C10O OC10H21

Ph Ph

III

Chapter 6

232

polymerize 1,7-octadiyne and 1,4-bis(2-propynyloxy)benzene to gem-vinylene rich PAEVs.40d The reaction of 1,4-diethynylbenzene gave rise to low-molecular weight oligomers, while 1,4-diethynyl-2,5-dialkoxybenzenes provided insoluble cross-linked products. Low conversions of 1,4-bis(2-propynyloxy)benzene were found and the authors believed that this was the result of metal oxygen coordination.

When comparing the present results with those of poly(aryleneethynylenevinylene)s prepared either by metal-catalyzed 1-alkyne dimerization or palladium-catalyzed cross-coupling between vinylic halides and diacetylenes, several dissimilarities are apparent. Firstly, the occurence of gelation is not entioned in these studies. This is remarkable, as gelation and aggregation phenomena are common in concentrated solutions (typically 5-20 wt%) of many conjugated polymers having extensive back bone conjugation, including rigid-rod polymers such as polyarylenes72q, poly(p-phenyleneethynylene)s72h-k, poly(p-phenylenebutadiynylene)s43f and poly(thiophene)s72e-g having linear alkyl or alkoxy groups. Only Venkatasan et al. noted that poly(aryleneethynylenevinylene)s II (Scheme 6-2) swelled in a variety of solvents (e.g. N-methylpyrrolidine, dimethylformamide, dimethylsulfoxide, chloroform, THF).37 Secondly, the formation of insoluble material in PAEV solutions upon standing at room temperature has little precedent. The single similar observation is by Kane et al. who reported that the prepared poly(aryleneethynylenevinylene) I (Scheme 6-2) did not redissolve after drying.36 The same authors also prepared the model compound III and found that it decomposed upon standing at room temperature (Scheme 6-12). The instability of I seems not to be heat-induced, as DSC indicated a single exotherm with onsets ranging from 300 to 380°C. The only other DSC study performed on PAEVs involves II which was found to undergo acetylenic cross-linking at temperatures above 250 °C.37 However, thermal cross-linking of the triple carbon bond of the repeating enyne unit has also been observed at lower temperatures in other enyne-containing polymers such as IV (~100 °C)92, V (~80 °C)40d and a variety of polyimides93 (200-300 °C) (Scheme 6-12). As noted before and in analogy to poly(arylenebutadiynylene)s, both photochemical and thermal acetylene cross-

Table 6-6. Comparison of the optical properties of poly(2+8) in solution with similar polymers of other classes.a

Entry Polymer Properties

λmax (nm) [solvent]

Ref.

1 C6H13

H13C6n

Pn = 7 94% HH

397b [CHCl3] 472, 502c [CHCl3]

present work

2

nH

C6H13

H13C6

H

Pn, GPC = 13 309b [cC6H13] 432c [cC6H13]

53

3

I I

C6H13

H13C6

n

Pn, GPC = 58 Pn, EA = 31

388b [cC6H13] 428c [cC6H13]

53

4

RMe2Si SiMe2R

C8H17

H17C8

n

Pn = 22-26

384b [C7H8] 426, 448c [C7H8]

91f

5

n

C6H13

H13C6

CH3 CH3

d 384b [CHCl3] 425, 450c [CHCl3]

17b

a Abbreviations: R = CH2CH(CH3)CO2CH2CH2OH. Pn, Pn, GPC, Pn, EA are the number-average degrees of polymerization based on 1H NMR, GPC and elemental analysis, respectively. b UV-vis absorption maximum. c Emission maximum upon excitation. d Not reported.

The organolanthanide-catalyzed polymerization of diynes

233

linking affording insoluble solids are plausible processes which rationalize the observed instability of the present PAEVs.

Another difference between the present PAEVs and those reported in literature is that GPC analysis could be performed on II37, poly(2,5-dioctylphenyl-ethynylenevinylene)s,40b,c and poly(9,9-dioctyl fluorenylethynylenevinylene)s.40a This difference in behavior may possibly be attributed to the following rationales: (i) polymer II is most likely to have iodine end-groups94, thereby affecting both stability and gelation behavior, (ii) the fluorenyl moiety might confer an increased stability or a decreased tendency for microgelation relative to the present phenyl or thienyl moieties. Unfortunately, Nishiura, Hou and co-workers did not report experimental details of their GPC analyses, such as solvent or temperature, rendering any attempt to rationalize this difference in behavior as mere speculation.

Nishiura, Hou and co-workers also used a lanthanidocene catalyst to prepare (E)-vinylene-rich dialkoxy-substituted PAEVs. The present choice of Cp*2LaCH(SiMe3)2 was based on a systematic study of the substrate effects in the Cp*2LnR-catalyzed 1-alkyne oligomerization reaction (Chapter 4). It was found that the lanthanidocene catalysts have an electronic preference for trans-head-to-head dimerization, but that this preference is counterbalanced by unfavorable steric effects between the ancillary ligand system and the coordinated substrate. Increasing the steric requirements of the substrate by means of ortho-substitution or decreasing the metal ion radius gave rise to larger amounts of the head-to-tail dimers. The observation that the catalyst Cp*2PrCH(SiMe3)2 is more selective for trans-head-to-head dimerization than Cp*2LaCH(SiMe3)2, while Pr is considerably smaller than La,95 is thus surpising and not understood at present.

The chemical structure of PAEVs combines those of the well-established conjugated polymer classes of poly(aryleneethynylene)s and poly(arylenevinylene)s, but relationships with polydiacetylenes and poly(arylenebutadiynylene)s are also apparent. It seems therefore interesting to compare the optical properties of

Table 6-7. Comparison of the optical properties of poly(3) and poly(3+8) in solution with similar polymers of other classes.a

Entry Polymer Properties

λmax (nm) [solvent]

Ref.

1

nHH

OC6H13

H13C6O

Pn = 16 92% HH

450b [CHCl3] 514c [CHCl3]

present work

2

nHH

OC8H17

H17C8O

Pn, GPC = 14 98% HH

(Pn ≈ 6.2)d

451b

40b,c

3 OC6H13

H13C6On

Pn = 11 92% HH

462b [CHCl3] 467c [CHCl3]

present work

494

I

OC10H21

H21C10O

I

n

Pn, GPC = 22-26

430-440b [CHCl3] 490, 520c [CHCl3]

37

5

n

OC8H17

H17C8O

Pn = 23 449b [CHCl3] 475, 505c [CHCl3]

97

6

HH

OC3H7

H7C3On

Pn = 11 481b [CHCl3] 98

a Pn and Pn, GPC are the number-average degrees of polymerization based on 1H NMR and GPC analysis, respectively. b UV-vis absorption maximum. c Emission maximum upon excitation. d Estimated, see text for details.

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234

the present PAEVs with those of analogous polymers of the well-established classes. The absorption and emission maxima of poly(2+8) and those of similar polymers belonging to the classes of PAEs and PABs are tabulated in Table 6-6. It can be seen that poly(2+8) exhibits both a higher absorption and emission maximum relative to analogous PAEs (Entries 3-5) and PABs (Entry 2), indicative of a higher degree of conjugation length. It is interesting to note that the nature of the end-group and the solvent seem to have little effect on the absorption and emission maxima in PAEs (Entries 3-5).96 Assuming similar behavior in PAEVs, the higher absorption and emission maximum wavelengths may plausibly arise from an intrinsically higher effective conjugation length.

The absorption and emission maxima of the hexyloxy derivatives poly(3) and poly(3+8) and those of analogous PAEVs, PPEs and PAVs are shown in Table 6-6. Poly(3) exhibits a significantly lower absorption maximum relative to the other conjugated polymers, while its emission maximum is higher (Entry 1). It seems unlikely that this difference can be attributed to the relatively low regioregularity, because the regioregular gem-vinylic dioctyloxy analogue (Pn, GPC = 12.8, 100% HT) showed an absorption maximum of 435 nm.40c Poly(3+8), on the other hand, displays a higher absorption and lower emission maximum relative to its non end-capped dioctyloxy PAEV derivative (Entry 2) and phenyl end-capped dioctyloxy PPE analogue97 (Entry 5), but a lower absorption maximum than its dipropyloxy PPV analogue98 (Entry 6). Assuming that the convergence limit of poly(3+8) is reached at the undecamer stage, these results indicate that the effective conjugation length of the present end-capped PAEVs is higher than that of similar PABs and PAEs, but lower than that of similar PAVs. It

Table 6-8. Comparison of the optical properties of poly(4) in solution with similar polymers of other well-established classes.a

Entry Polymer Properties

λmax (nm) [solvent]

Ref.

1 S

HH

C6H13n

Pn = 100, 100% HH 480b [CHCl3] 565c [CHCl3]

present work

2 3 4 5

S

C6H13

n

50% HTT 70% HTT 98-99% HTT Pn, GPC = 152, 98-99% HTT

428b 436b 442b 456b [CHCl3] 570c [CHCl3]

64a 100 60a 64a

6 7

S

C12H25n

Pn, GPC = 43, regioirregular/ cross-conjugated defects Pn, GPC = 29, 100% HTH-HTT

550b [CH2Cl2] 577b [CH2Cl2]

88c

88c

8 SH

C6H13

SiMe3

n

Pn = 16, 100% HTT

440b [CHCl3]

101a

9

10

S

C6H13n

Pw, GPC = 105, 100% HTT Pw, GPC = 605, 100% HTH

441b [CHCl3], 440b [THF] 506, 536c [THF] 437b [CHCl3], 436b [THF] 505, 535c [THF]

101b

101b

11 SMe3Si

C6H13

SiMe3

n

Pn, GPC = 19 430b

102a

12 S

n

Pn, GPC = 1200-18000 410b [THF] 450c [THF]

102b

a Pn and Pn, GPC are the number-average degrees of polymerization based on 1H NMR and GPC analysis, respectively. Pw, GPC is the weight-average degree of polymerization based on GPC analysis b UV-vis absorption maximum. c Emission maximum upon excitation.

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235

Scheme 6-13. Regioregularity in asymmetrically substituted thienyl polymers.

S

SR

R

S

S

R

R

head-to-head(HTH)

head-to-tail(HTT)

is interesting to note in this respect that a systematic study revealed that the absorption maximum of oligo(2,5-propyloxy-p-phenylenevinylene) did not increase beyond the undecamer stage. The observation99 that the didecyloxy derivative of PPE exhibits an absorption maximum of 429 in THF suggests, moreover, that the length of the alkoxy side group and the nature of the solvent and end-capping group affect the absorption maximum of the PPEs only to a small extent.

The optical properties of poly(4) are compared with those of poly(3-n-hexyl-2,5-thienylene)s60a,64a,100 (P3HTs), poly(3-n-dodecyl-2,5-thienylenevinylene)s88c (P3DTVs), poly(3-n-hexyl-2,5-thienylene-ethynylene)s101 (P3HTEs) and poly(3-n-hexyl-2,5-thienylenebutadiynylene)s102 (P3HTBs) in Table 6-8. It is well-recognized that irregularly substituted poly(thiophene)s have structures where unfavorable head-to-head couplings (HTH) cause a sterically driven twist of thiophene rings, resulting in loss of conjugation (Scheme 6-13).18 Regioregular, head-to-tail (HTT) PTs can easily access a low energy planar conformation, leading to highly conjugated polymers. The effect of this type of regioregularity on the effective conjugtion length is still present in PTVs (Entries 9-10) and PTEs, albeit less pronounced than in PTs (Entries 2-5). It can be anticipitated that the effect of this type of regioregularity plays only a minor role in polymers where the distance between the thienyl moieties is larger, such as PABs and PAEVs, but no systematic studies are known that confirm this hypothesis.

Pearson et al. found that a near saturation of the optical properties of HTT oligo(3-n-ethyl-2,5-thienyleneethynylene)s occurred at the octamer stage, based on the observation that doubling the conjugation length to the 16-mer caused little change in the absorbance maximum (Entry 8) of the octamer.101a Wenz et al. reported that poly(diacetylene)s display an effective conjugation length of approximately 5-7 repeating units, as indicated by UV-vis, resonance-Raman and 13C NMR spectroscopy.103 These results suggest that the present poly(4) having Pn = 100(1) must certainly have reached its convergence limit with respect to its absorption and emission maximum. Comparison of the optical properties of poly(4) with similar polymers of other well-established classes such as PAs, PAVs, PAEs and PABs confirms previous notion that PAEVs have effective conjugation lengths in solution that are longer than PAs, PAEs and PABs, but shorther than PAVs.

Further studies are needed to evaluate the full potential of the present polymers as components in advanced materials. Even though the present synthetic study established that highly regioregular conjugated polymers with relatively large effective conjugation lengths can be prepared, investigations with respect to their solid state properties and the exact nature of their instability, in particular, are required.

6.6. Conclusions

Soluble conjugated poly(aryleneethynylenevinylene)s have been prepared in good yields by step growth polymerization of (hetero)aromatic diynes using the organolanthanide-catalyzed dimerization of 1-alkynes. The molecular weight and the yield of the soluble fraction of the polymers can be controlled by variation of monomer structure, monomer concentration and the ratio between the diyne and an end-capping monoyne. The polymers based on 1,4-diethynyl-2,5-di-n-hexylbenzene (2), 1,4-diethynyl-2,5-di-n-hexyloxybenzene (3) and 2,5-diethynyl-3-n-hexylthiophene (4) have been characterized by 1H NMR, 13C NMR, IR spectroscopy and MALDI-TOF mass spectrometry. They are highly fluorescent and their photophysical properties suggest that they represent promising materials for electronic and photonic applications.

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236

6.7. Experimental Section

General experimental procedures. For general remarks and physical and analytic measurements, see Sections 2.7 and 5.7. The compounds 1,4-bis(trimethylsilylethyne)benzene,50 1,4-di-n-hexylbenzene,104 1,4-di-n-hexyloxy-benzene,105 3-n-hexylthiophene,60 2-ethynyltoluene106 and 2,6-dimethylethynylbenzene106 were prepared according to literature procedures. N,N-Diisopropylamine (KOH) and phenylacetylene (CaH2) was purchased from Aldrich and dried as recommended.107 The UV-vis absorption spectra were recorded on a Hewlett-Packard HP 8453 diiode array spectrophotometer and the fluorescence spectra were recorded on a SPF-500C spectrofluorometer (SLM Aminco). All emission studies were performed at room temperature in optically dilute solutions. The solutions were freshly prepared by weighing out the polymer (typically 2.00 mg) and dissolution in spectroscopic grade chloroform (typically 100.0 mL). The solutions were diluted until absorption maxima less than 0.1 were obtained to avoid the inner filter effect.

General purification procedure for aromatic diynes. The oils or solids obtained after synthesis were dissolved in pentane containing freshly ground CaH2 and stored at 4 °C under nitrogen for at least 24 h after several freeze-thaw-pump degassing cycles. Filtration, followed by solvent evaporation, or crystallization afforded the dried monomers which were stored under nitrogen, in the dark and at -30 °C as soon as possible.

1,4-Diethynylbenzene. 1,4-Bis(trimethylsilylethynyl)benzene (1.5 g, 5.55 mmol) was dissolved in MeOH (50 mL) and THF (30 mL) in a 200-mL Erlenmeyer equipped with a magnetic stir bar. After addition of 2.0 mL (24 mmol) of an aqueous KOH solution (12 M), the colorless solution was stirred at room temperature for 1 h during which it turned light-yellow. After addition of water (100 mL) and pentane (80 mL), the organic layer was separated, washed with water and rotatory evaporated to yield a white solid with a distinct smell. Cooling a concentrated pentane solution gave white needles. Yield: 0.68 g (97%).

1H NMR (400 MHz, CDCl3, 18 °C): δ 7.42 (s, CH, 4 H), 3.15 (s, CCH, 2 H). 13C-{1H} NMR (100 MHz, CDCl3, 18 °C): δ 131.98 (CH), 122.52 (CCCH), 82.98 (CCH), 79.06 (CCH). IR (KBr, [cm-1]): 3298 (m), 3260 (s, C-H stretching, ≡ C-H), 2956 (m), 2921 (m, C-H stretching, aromatic =C-H), 2851 (m), 2101 (w, C≡ C stretching), 1916 (m), 1628 (m), 1492 (m), 1401 (m), 1249 (m), 1103 (m), 1015 (m), 962 (w), 834 (s), 706 (m), 674 (s), 639 (s), 619 (s), 545 (s).

2,5-Diiodo-1,4-bis(n-hexyl)benzene. The following procedure is a modification of a reported one.54c 1,4-Di-n-hexylbenzene (22.9 g, 92.8 mmol) was brought in a round-bottomed, three-neck flask (500 mL), equipped with a cooler and stir bar, and dissolved in a mixture containing acetic acid (140 mL), CCl4 (38 mL) and concentrated sulfuric acid (24 mL). After addition of iodine (26.30 g, 103.6 mmol) and KIO4 (7.50 g, 32.6 mmol) the reaction mixture was stirred overnight at 90 °C, forming a deep-purple colored suspension. Then, the reaction mixture was cooled, neutralized with an aqueous solution of NaHCO3 and an aqueous solution of Na2SO3 to remove the excess of iodine. The crude product was extracted with petroleum ether and rotatory evaporation afforded a brown, viscous oil. Repeated crystallization from ethanol yielded off-white needles. Yield: 25.4 g (55%).

1H NMR (300 MHz, CDCl3, 25 °C): δ 7.55 (s, CH, 2 H), 2.55 (t, 3JHH = 7.9 Hz, CCH2, 4 H), 1.50 (m, CH2, 4 H), 1.30 (m, CH2, 12 H), 0.86 (t, 3JHH = 6.6 Hz, CH3, 6 H). 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 144.83 (CCH2), 139.28 (CH), 100.32 (CI), 39.83(CH2), 31.59 (CH2), 30.15 (CH2), 28.98 (CH2), 22.57 (CH2), 14.07 (CH3). Anal. Calcd. for C18H28I2 (498.23): C, 43.39%; H, 5.66%. Found: C, 43.47%; H, 5.61%.

2,5-Bis(trimethylsilylethynyl)-1,4-di-n-hexylbenzene.53 N,N-Diisopropylamine (50 mL) was brought into a three-neck, round-bottom, 250-mL flask equipped with a magnetic stir bar and condenser. Addition of 2,5-diiodo-1,4-bis(n-hexyl)benzene (4 g, 8.03 mmol), Pd(PPh3)2Cl2 (0.30 g, 0.43 mmol) and PPh3 (0.22 g, 0.84 mmol). The yellow suspension was heated for 1 h at 60°C under stirring. In a Schlenk flask equipped with a magnetic stir bar CuI (0.08 g, 0.42 mmol) is added to THF (50 mL) forming a suspension which was subsequently stirred under heating for 1 h. Then trimethylsilylacetylene (2.70 mL, 19.1 mmol) and the CuI suspension are added to the yellow suspension. The reaction mixture was heated to 90 °C and stirred for 12 h. After allowing the mixture to cool to room temperature the suspension was filtered over a glass filter in vacuo. The volatiles were removed from the brown filtrate by rotatory evaporation to yield a brown oil. The crude product was purified by column chromatography (neutral alumina, petroleum ether 40-60 °C) to afford a yellow oil. White crystalline material was obtained by cooling a concentrated solution of the yellow oil in aqueous ethanol. Yield: 3.61 g (93%).

1H NMR (400 MHz, CDCl3, 18 °C): δ 7.22 (s, CH, 4 H), 2.65 (t, 3JHH = 7.7 Hz, CCH2, 4 H), 1.57 (m, CH2, 4 H), 1.30 (m, CH2, 12 H), 0.87 (t, 3JHH = 7.7 Hz, CH2CH3, 6 H), 0.23 (t, 3JHH = 6.8 Hz, SiCH3, 18 H). 13C-{1H} NMR (100 MHz, CDCl3, 18 °C): δ 142.66 (CCH2), 132.45 (CH), 122.53 (CC≡ C), 103.93 (CC≡ C), 98.88

The organolanthanide-catalyzed polymerization of diynes

237

(C≡ CSi), 34.11 (CCH2), 31.72 (CH2), 30.57, 29.26, 22.62, 14.11 (CH2CH3), -0.04 (SiCH3). Anal. Calcd. for C28H46Si2 (438.84): C, 76.63%; H, 10.57%. Found: C, 76.49%; H, 10.65%.

2,5-Diethynyl-1,4-bis(n-hexyl)benzene.53 2,5-Bis(trimethylsilylethynyl)-1,4-di-n-hexyl-benzene (4.38 g, 9.05 mmol) is dissolved in MeOH (85 mL) and THF (15 mL) in a 200-mL Erlenmeyer equipped with a magnetic stir bar. After addition of 2.0 mL (24 mmol) of an aqueous KOH solution (12 M), the colorless solution was stirred at room temperature for 1 h during which it turned light-yellow. After addition of water (100 mL) and pentane (80 mL), the organic layer was separated, washed with water, and rotatory evaporated to yield a light-yellow oil. Yield: 2.46 g (92%).

1H NMR (300 MHz, CDCl3, 25 °C): δ 7.27 (s, CH, 2 H), 3.26 (s, CCH, 2 H), 2.69 (t, 3JHH = 7.7 Hz, CCH2, 4 H), 1.58 (m, CH2, 4 H), 1.30 (m, CH2, 12 H), 0.87 (t, 3JHH = 6.8 Hz, CH3, 6 H). 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 142.70 (CCH2), 132.94 (CH), 121.90 (CCCH), 82.25 (CCH), 81.05 (CCH), 33.76 (CCH2), 31.62 (CH2), 30.41 (CH2), 29.08 (CH2), 22.57 (CH2CH3), 14.08 (CH3). IR (neat, [cm-1]): 3310 (m), 3295 (m), 2955 (m), 2925 (s), 2855 (s), 2105 (w), 1490 (m), 1460 (m), 1395 (w), 1380 (w), 1212 (w), 900 (m), 720 (m).

2,5-Diiodo-1,4-bis(n-hexyloxy)benzene. The following procedure is a modification of a published one.105 1,4-Di-n-hexyloxybenzene (20.1 g, 72.2 mmol), iodine (25.1 g, 98.9 mmol), KIO4 (20.4 g, 88.7 mmol), acetic acid (130 mL), water (10 mL), and H2SO4 (7 mL) were brought in a 500-mL three-necked flask equipped with magnetic stir bar and condenser. The reaction mixture was stirred for 4 h at 70 °C. The flask was placed in an ice-water bath and Na2S2O4·2H2O (20 g, 95 mmol) was added under stirring to remove the excess of iodine upon which the mixture turned first brown and then yellow. The mixture is filtered. The filtrate was washed with water and extracted with ether and dried over MgSO4. The crude product in the residue and extracted filtrate were purified by repeated crystallizations in diethyl ether at low temperature (-30 °C) to afford white crystals. Yield: 15.23 g (40%).

1H NMR (300 MHz, CDCl3, 25 °C): δ 7.13(s, CH, 1 H), 3.88 (t, OCH2, 4 H), 1.76 (m, CH2, 4 H), 1.5-1.3 (m, CH2, 12 H), 0.87 (t, CH3, 6 H). 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 152.86 (CO), 122.79 (CH), 86.29 (CI), 70.35 (OCH2), 31.46 (CH2), 29.10 (CH2), 25.70 (CH2), 22.58 (CH2), 14.01(CH3). Anal. Calcd. for C18H28I2O2 (530.23): C, 40.77%; H, 5.32%. Found: C, 40.86%; H, 5.40%.

2,5-Bis(trimethylsilylethynyl)-1,4-bis(n-hexyloxy)benzene. N,N-Diisopropylamine (100 mL) is brought into a three-neck, round-bottomed flask equipped with a magnetic stir bar and condenser. Addition of 2,5-diiodo-1,4-bis(n-hexyloxy)benzene (4.02 g, 7.58 mmol), Pd(PPh3)2Cl2 (0.34 g, 0.49 mmol) and CuI (0.13 g, 0.66 mmol). Trimethylsilylacetylene (2.65 mL, 18.8 mmol) was added to the yellow suspension under stirring. The reaction mixture was heated to 70 °C and stirred for 2 h. After allowing the mixture to cool to room temperature toluene (50 mL) was added. The suspension was filtered in vacuo over a glass filter and the brown filtrate was rotatory evaporated to dryness yielding a brown oil which solidified upon cooling to room temperature. The brown solid was purified by column chromatography (silica, 230-400 mesh, 60 Å, petroleum ether 40-60 °C) affording a brown solid after rotatory evaporation of the solvent. When a concentrated solution of this solid was cooled -40 °C, off-white crystals formed. Yield: 2.95 g (83%).

1H NMR (400 MHz, CDCl3, 25 °C): δ 6.85 (s, CH, 2 H), 3.90 (t, 3JHH = 6.5 Hz, OCH2, 4 H), 1.74 (m, CH2, 4 H), 1.46 (m, CH2, 4 H), 1.30 (m, CH2, 8 H), 0.86 (t, 3JHH = 6.7 Hz, CH3, 6 H,). 13C-{1H} NMR (100 MHz, CDCl3, 25 °C): δ 154.00 (CO), 117.23 (CH), 113.97 (CC), 101.07 (C≡ C), 100.03 (C≡ C), 69.44 (OCH2), 31.59 (CH2), 29.28 (CH2), 25.67 (CH2), 22.61 (CH2), 14.03 (CH3), -0.08 (SiCH3). Anal. Calcd. for C28H46O2Si2 (470.84): C, 71.43%; H, 9.85%. Found: C, 71.49%; H, 10.01%.

2,5-Diethynyl-1,4-bis(n-hexyloxy)benzene. 2,5-Bis(trimethylsilylethy-nyl)-1,4-bis(n-hexyloxy)-benzene (1.52 g, 3.23 mmol) was dissolved in THF (50 mL) and MeOH (50 mL) in a 250-mL Erlenmeyer equipped with magnetic stir bar. Addition of 15 mL (75 mmol) of an aqueous NaOH solution (5 M) upon which the colorless solution changed to light-yellow. After stirring for 2 h at room temperature the reaction mixture was extracted with pentane. The organic layer was rotatory evaporated to dryness to yield a yellow oil. When a concentrated ether solution of this oil was cooled, yellow crystals formed. Yield: 0.91 g (86%).

1H NMR (500 MHz, CDCl3, 25 °C): δ 6.91 (s, CH, 2 H), 3.93 (t, 3JHH = 6.6 Hz, OCH2, 4 H), 3.29 (s, CCH, 2H), 1.75 (m, CH2, 4 H), 1.42 (m, CH2, 4 H), 1.29 (m, CH2, 8 H), 0.86 (t, 3JHH = 7.0 Hz, CH3, 6 H). 13C-{1H} NMR (125 MHz, CDCl3, 25 °C): δ 153.97 (CO), 117.75 (CH), 113.27 (CCCH), 82.37 (CCH), 79.77 (CCH), 69.65 (OCH2), 31.49 (CH2), 29.07 (CH2), 25.55 (CH2), 22.56 (CH2), 13.98 (CH3). IR (KBr, [cm-1]): 3271 (s, C-H stretching, ≡ C-H), 2969 (m), 2952 (m), 2922 (m, C-H stretching, aromatic =C-H), 2852 (m), 2103 (w, C≡ C stretching), 1710 (m), 1497 (s), 1464 (m), 1399 (m), 1384 (s), 1271 (m), 1220 (s), 1196 (m), 1048 (m), 996 (m), 943 (m), 861 (m), 692 (m), 659 (m).

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238

2,5-Diiodo-3-n-hexylthiophene.60a 3-n-Hexylthiophene (5.50 g, 32.7 mmol) was dissolved in dry CH2Cl2 in a 250-mL, three-neck, round-bottom flask, equipped with stir bar, cooler and drop funnel. After addition of finely powdered iodine (8.30 g, 32.7 mmol) the reaction mixture was heated to 65 °C under reflux. A mixture of concentrated HNO3 (5 mL) and water (5 mL) was added dropwise within 30 min. After 3 h at 65 °C the reaction mixture was allowed to cool to room temperature. The excess of iodine was removed by the addition of an aqueous solution of Na2S2O4. The organic layer was extracted with petroleum ether (40-60 °C) and dried over MgSO4. Rotatory evaporation afforded a dark-brown, viscous oil. The crude product was purified by column chromatography (silica, petroleum ether) to yield a yellow liquid. Yield: 9.95g (73%).

1H NMR (300 MHz, CDCl3, 25 °C): δ 6.87 (s, CH, 1 H), 2.49 (t, 3JHH = 7.8 Hz, CCH2, 2 H), 1.53 (m, CH2, 2 H), 1.29 (m, CH2, 6 H), 0.87 (t, 3JHH = 6.9 Hz, 3 H). 13C-{1H} NMR (75 MHz, CDCl3, 25.0 °C): δ 149.49 (CCH2), 137.76 (CH), 130.25, 127.93 (CI), 32.90 (CCH2), 31.57 (CH2), 29.90, 38.87, 22.55, 14.03 (CH3). 2,5-Bis(trimethylsilylethynyl)-3-n-hexylthiophene.43a In a 250-mL, round-bottom flask equipped with a cooler and stir bar dry N,N-diisopropylamine (50 mL) was brought. After addition of 2,5-diiodo-3-n-hexylthiophene (4.95 g, 11.8 mmol), PPh3 (0.30 g, 1.14 mmol), Pd(PPh3)2Cl2 (0.43 g, 0.61 mmol) the yellow suspension was heated to 60 °C under stirring until the solids were dissolved. Then, a suspension of CuI (0.11 g, 0.58 mmol) in dry THF (25 mL) and trimethylsilylacetylene (4.50 mL, 31.8 mmol) were added successively. The reaction mixture was heated overnight to 90 °C on reflux. After allowing the mixture to cool to room temperature toluene (50 mL) was added. The suspension was filtered over a glass filter in vacuo and the brown filtrate was rotatory evaporated to dryness yielding a viscous yellow oil. The oil was purified by column chromatography (silica, 230-400 mesh, 60 Å, petroleum ether 40-60 °C) affording a light-yellow oil after rotatory evaporation of the solvent. The product was found to be contaminated with traces of a mono(trimethylsilylethynyl) analogue (<1%) which was removed completely by Kügelrohr distillation (~250 °C, ~1 mmHg). Yield: 4.10 g (95%).

1H NMR (400 MHz, CDCl3, 25 °C): δ 6.93 (s, CH, 1 H), 2.58 (t, 3JHH = 7.7 Hz, CCH2, 2 H), 1.55 (m, CH2, 2 H), 1.27 (m, CH2, 6 H), 0.86 (t, 3JHH = 6.6 Hz, CH3, 3 H). 13C-{1H} NMR (100 MHz, CDCl3, 25 °C): δ 148.39 (CCH2), 133.47 (CH), 122.76, 119.86 (CC≡ C), 101.80, 99.40, 97.40, 96.86 (C≡ C), 31.54 (CCH2), 29.87, 29.34, 28.77, 22.56, 14.06 (CH3), -0.12, -0.20 (SiCH3). Anal. Calcd. for C20H32SSi2 (360.71): C, 66.60%; H, 8.94%. Found: C, 66.78%; H, 9.11%. 2,5-Diethynyl-3-n-hexylthiophene.43a,58a 2,5-Bis[(trimethylsilyl)ethynyl]-3-n-hexyl-thiophene (3.73 g, 10.3 mmol) was dissolved in MeOH (50 mL) and THF (50 mL) in an erlenmeyer under stirring. After addition of KOH (0.62 g, 11.0 mmol) the mixture was allowed to stir in the dark for 1 h at room temperature. Then, the mixture was washed with water, extracted with petroleum ether (40-60 °C) and dried over MgSO4. After rotatory evaporation, the crude product was purified by flash chromatography (neutral alumina, pentane) to afford a light-yellow oil. Yield: 2.12 g (95%).

1H NMR (400 MHz, CDCl3, 25 °C): δ 6.99 (s, CH, 1 H), 3.42 (s, CCH, 1 H), 3.30 (s, CCH, 1 H), 2.62 (t, 3JHH = 7.7 Hz, CCH2, 2 H), 1.54 (m, CH2, 2 H), 1.27 (m, CH2, 6 H), 0.86 (t, 3JHH = 7.7 Hz, CH3, 3 H). 13C-{1H} NMR (75 MHz, C6D6, 25 °C): δ 148.83 (CCH2), 134.00 (CH), 122.72, 119.44 (CCCH), 84.46, 82.29 (CCH), 76.77, 76.11 (CCH), 31.81 (CH2), 30.16, 29.51, 29.07, 22.84, 14.22 (CH3). IR (neat, [cm-1]): 3308 (s, C-H stretching, ≡ C-H), 2956 (m), 2928 (m, C-H stretching, aromatic =C-H), 2858 (m), 2103 (m, C≡ C stretching), 1692 (w), 1529 (w), 1465 (m), 1379 (w), 1199 (m), 1118 (m), 1017 (m), 847 (m), 723 (m), 662 (m), 594 (m).

Representative polymerization reaction of diyne catalyzed by Cp*2LaCH(SiMe3)2 at relatively high monomer concentration (NMR scale). To a NMR tube containing a solution of 1,4-diethynyl-2,5-di-n-hexyloxybenzene (41.3 mg, 127 µmol) in benzene-d6 (400 µL) was added a 100 µL (6.13 µmol) of a stock solution of Cp*2LaCH(SiMe3)2 (61.3 mM) in benzene-d6 with a microsyringe. The mixture was stirred manually during which the light-yellow solution turned brown in color. The NMR tube was brought in a sonicator bath and the reaction mixture competely solified after 12 h at room temperature into a red solid. The product mixture was quenched by opening the tube to air and the addition of chloroform (~2 mL). Addition of chloroform (5 mL) and prolonged stirring led to the formation of a red suspension. The insoluble polymer fraction was separated by filtration and washed with chloroform. Yield: 29.2 mg (71%). The filtrate was precipitated with methanol and collected by filtration. After washing with hexanes and methanol, the soluble polymer fraction was dried in vacuo and obtained as a red solid. Yield: 9.1 mg (22%).

Soluble polymer fraction of poly(3): 1H NMR (400 MHz, CDCl3, 25 °C): δ 6.92 (s, Ar), 6.51 (d, J = 16.4 Hz, CH=CH), 5.91 (br. s, =CH2), 3.97 (m, CCH2), 3.32 (s, CCH), 3.31 (s, CCH), 1.8-0.8 (m, CH2 and CH3). Integration revealed HH:HT = 92:8 and Pn = 16. 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 153.6 (CO), 151.0

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(CO), 136.0 (CCH=CH), 126.7 (CCC), 118.5-116.7 (CCH=CH), 101.6 (CC), 95.6 (CC), 89.0 (CCH), 82.2-82.0 (CCH), 31.8 (CH2), 29.7 (CH2), 29.2 (CH2), 25.9 (CH2), 22.9 (CH2), 14.1 (CH3). Only characteristic resonances are reported due to the complexity of the spectrum. IR (neat, [cm-1]): 3286 (s, C-H stretching, ≡ C-H), 2941 (m, C-H stretching, aromatic =C-H), 2854 (m), 2104 (m, C≡ C stretching), 1496 (s), 1389 (s), 1211 (s), 1029 (s), 860 (m).

Representative polymerization reaction of diyne catalyzed by Cp*2LaCH(SiMe3)2. at relatively low monomer concentration. To a Schlenk vessel (50 mL) containing a stirred solution of 2,5-diethynyl-3-n-hexylthiophene (96.8 mg, 447 µmol) in toluene (2.0 mL) was added 100.0 µL (4.00 µmol) of a stock solution of Cp*2LaCH(SiMe3)2 (40.0 mM) in toluene. The reaction mixture was heated under stirring to 50 ºC and the dark yellow solution gradually darkened. After 1.5 h, the reaction mixture solidified completely and the product mixture was quenched by opening to air and the addition of chloroform (~10 mL). Addition of more chloroform (10 mL) led to the formation of a red suspension. The insoluble polymer fraction was separated by filtration and washed with chloroform to afford a dark red material that turned shiny green upon drying in vacuo. Yield: 51.3 mg (53%). The filtrate was precipitated with methanol and collected by filtration. After washing with hexanes and methanol, the soluble polymer fraction was dried in vacuo and obtained as a dark red solid. Yield: 31.5 mg (33%).

Soluble polymer fraction of poly(4): 1H NMR (500 MHz, CDCl3, 25 °C): δ 6.94 (br. s, Ar), 6.78 (br. s, CH), 6.13 (m, CH=CH), 3.47 (s, CCH), 3.45 (s, CCH), 2.62 (m, CCH2), 2.56 (m, CCH2), 1.58 (m, CH2), 1.30 (m, CH2), 0.88 (m, CH3). Integration revealed HH:HT = 100:0 and Pn = 200. 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 148.7 (CCH2), 147.9 (CCH2), 141.4 (CCH=CH), 123.3 (CCC), 107.8-106.1 (CCH=CH), 97.1 (CC), 94.1 (CC), 89.0 (CCH), 31.6 (CH2), 20.8 (CH2), 30.0 (CH2), 28.9 (CH2), 22.6 (CH2), 14.2 (CH3). Only characteristic resonances are reported due to the complexity of the spectrum. IR (neat, [cm-1]): 2921 (m, C-H stretching, aromatic =C-H), 2851 (m), 2167 (m, C≡ C stretching), 1464 (m), 1259 (s), 1019 (s), 807 (m).

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45 (a) Heck, R. F. Palladium Reagents in Organic Synthesis, Academic Press: London, 1985. (b) Modern Cross-Coupling Reactions, Stang, P. J., Diederich, F., Eds.;VCH: Weinheim, 1997. (c) Brandsma, L., Vasilevsky, S. F., Verkruijsse, H. D. In Applications of Transition Metal Catalysts in Organic Synthesis; Springer: Berlin, 1998; p 179. (d) Sonogashira,, K. in Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press, Oxford; Vol. 3, p. 521. (e) Metal-Catalyzed Cross-Coupling Reactions, Stang, P. J.; Diederich, F, Eds.; Wiley-VCH, Weinheim, 1998, p. 203.

46 For recent organolanthanide reviews, see: (a) Aspinall, H. C. Chem. Rev. 2002, 102, 1807-1850. (b) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H, Chem. Rev. 2002, 102, 1851-1896. (c) Arndt,

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S.; Okuda, J. Chem. Rev. 2002, 102, 1953-1976 (d) Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187-2210. (e) Inanaga, J.; Furuno, H.; Hayano, T. Chem. Rev. 2002, 102, 2211-2226. (f) Molander, G. A. Chemtracts: Org. Chem. 1998, 18, 237-263. (g) Edelmann, F. T. Top. Curr. Chem. 1996, 179, 247-276. (h) Edelmann, F. T. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 4, Chapter 2. (i) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865-986. (j) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283-362. (k) Evans, W. J. Adv. Organomet. Chem. 1985, 24, 131-177. (l) Marks, T. J.; Ernst, R. D. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 21.

47 For examples, see: (a) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111. (b) Haar, C. M.; Stern, C. L.; Marks, T. J. Organometallics 1996, 15, 1765. (c) Roesky, P. W.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 4705. (d) Molander, G. A.; Dowdy, E. D.; Schumann, H. J. Org. Chem. 1998, 63, 3386. (e) Kretschmer, W. P.; Troyanov, S. I.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 1998, 17, 284. (f) Molander, G. A.; Dowdy, E. D.; Schumann, H. J. Org. Chem. 1999, 64, 9697.

48 The permethyllanthanidocene hydrides [Cp*2Ln(µ-H)]2 are known to affect C-O cleavage of ethers, see: (a) Watson, P. L. J. Chem. Soc., Chem. Commun., 1983, 276. (b) Booij, M. Ph. D. thesis, University of Groningen, 1989. (c) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10, 134. (d) Deelman, B.-J., Ph. D. thesis, University of Groningen, 1994. (e) Deelman, B.-J.; Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1995, 14, 2306.

49 For examples, see: (a) Finke, R. G.; Keenan, S. R.; Schiraldi, D. A.; Watson, P. L. Organometallics 1986, 5, 598. (b) Finke, R. G.; Keenan, S. R.; Schiraldi, D. A.; Watson, P. L. Organometallics 1987, 6, 1356. (c) Finke, R. G.; Keenan, S. R.; Watson, P. L. Organometallics 1989, 8, 263. (d) Ref. 14a. (e) Booij, M. ; Deelman, B.-J. ; Duchateau, R. ; Postma, D. S.; Meetsma, A. ; Teuben, J. H. Organometallics 1993, 12, 3531. (f) Qian, C.; Zhu, C.; Zhu, D. Appl. Organomet. Chem. 1995, 9, 457.

50 (a) Whitall, I. R.; Cifuentes, M. P.; Humphrey, M. G.; Luther-Davies, B.; Samoc, M.; Houbrechts, S.; Persoons, A.; Heath, G. A.; Hockless, D. C. R. J.Organomet.Chem. 1997, 549, 127. (b) Bodwell, G. J.; Miller, D. O.; Vermeij, R. J. Org.Lett. 2001, 3, 2093. (c) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627.

51 Brandsma, L. Preparative Acetylenic Chemistry, Elsevier: Amsterdam, 1988 52 (a) Ried, W. Angew. Chem. 1964, 76, 933. (b) Ried, W. Angew. Chem. 1964, 76, 973. (c) Ried, W. In

Neuere Methoden der präparativen organischen Chemie; Verlag Chemie: Weinheim, 1966; Vol. 4. (d) Ruthledge, T. F. Acetylenic Compounds. Preparation and Substitution Reactions, Reinhold: New York, 1968. (e) Rohde, O.; Wegner, G. Makromol. Chem. 1978, 179, 1999. (f) Bohlmann, F. In Chemistry of Acetylene; Viehe, H. G., Ed.; Marcel Dekker: New York, 1969; Chapter. 14. (g) Hart, H.; Shamouillian, S.; Takehira, Y. J. Org. Chem. 1981, 46, 4427. (h) Hagihara, M.; Yamamoto, Y.; Takahashi, S.; Hayashi, K. Int. J. Radiat. Appl Instrum., Part C 1986, 28, 165. (i) Bleicher, L.; Cosford, N. D. P. Synlett 1995, 1115. (j) Khan, M. S.; Al-Suti, M. K.; Al-Mandhary, M. R. A.; Ahrens, B.; Bjernemose, J. K.; Mahon, M. F.; Male, L.; Raithby, P. R.; Friend, R. H.; Köhler, A.; Wilson, J. S. J. Chem. Soc., Dalton Trans. 2003, 65.

53 Mangel, T.; Eberhardt, A.; Scherf, U.; Bunz, U. H. F.; Müllen, K. Macromol. Rapid Commun. 1995, 16, 571.

54 (a) Scherf, U.; Müllen, K. Synthesis 1992, 23. (b) Bao, Z.; Chan, W.; Yu, L. Chem. Mater. 1993, 5, 2. (c) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31, 52. (d) Kloppenburg, L.; Jones, D.; Bunz, U. H. F. Macromolecules 1999, 32, 4194. (e) Zhang, W.; Moore, J. S. Macromolecules 2004, 37, 3973. (f) Kloppenburg, L.; Song, D.; Bunz, U. H. F. J. Am. Chem. Soc. 1998, 120, 7973.

55 Similar observations are reported for the di-n-butyl analogue, see: Pelter, A.; Jones, D. E. J. Chem. Soc., Perkin Trans. 1 2000, 2289.

56 It is remarkable that Müllen et al. who reported the preparation of this monomer as a colorless oil do not comment on its instability.53

57 (a) Castro, R.; Nixon, K. R.; Evanseck, J. D.; Kaifer, A. E. J. Org. Chem. 1996, 61, 7298. (b) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.; Bubeck, C. Eur. J. Org. Chem. 2001, 4431.

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(c) Lowe, J. L.; Peak, D. A.; Watkins, T. I. J. Org. Chem. 1951, 3286. (d) Ruiz, J. P.; Dharia, J. R.; Reynolds, J. R. Macromolecules 1992, 25, 849.

58 (a) Rutherford, D. R.; Stille, J. K. Macromolecules 1988, 21, 3530. (b) Callstrom, M. R.; Neenan, T. X.; Whitesides, G. M. Macromolecules 1988, 21, 3528. (c) Ref. 43a.

59 (a) Barker, J. M.; Huddleston, P. R.; Wood, M. L. Synth. Commun. 1975, 5, 59. (b) Mao, H.; Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 1163. (c) Antonelli, E.; Rosi, P.; Lo Sterzo, C.; Viola, E. J. Organomet. Chem. 1999, 578, 210.

60 (a) Mao, H.; Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 1163. (b) Li, W.; Maddux, T.; Yu, L. Macromolecules 1996, 29, 7334. (c) Chaloner, P. A.; Gunatunga, S. R.; Hitchcock, P. B. J. Chem. Soc., Perkin Trans. 2 1997, 1597.

61 (a) Li, J.; Pang, Y. Macromolecules 1997, 30, 7487. (b) Williams, V. E.; Swager, T. M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4669. (c) Krömer, J.; Rios-Carrera, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.; Bäuerle, P. Angew. Chem. Int. Ed. 2000, 39, 3481. (d) Higuchi, H.; Ishikura, T.; Mori, K.; Takayama, Y.; Yamamoto, K.; Tani, K.; Miyabayashi, K.; Miyake, M. Bull. Chem. Soc. Jpn. 2001, 74, 889. (e) Altamura, P.; Giardina, G.; Lo Sterzo, C.; Vittoria Russo, M. Organometallics, 2001, 20, 4360.

62 (a) Odian, G. Principles of Polymerization, Wiley: New York, 2004; 4rd ed. (b) Synthetic Methods in Step-Growth Polymers, Rogers, M. E.; Long, T. E. (Eds.), Wiley-Interscience: New Jersey, 2003.

63 Polymer containing linkages that originate from the allenic trimerization reaction will be conjugated, but are anticipated to be unstable in analogy to the trimers (Chapter 4) and may reasonably give rise to other types of structural defects.

64 Generation of structurally homogeneous, regioregular materials allows for efficient solid-state packing, a necessary criterion for optimizing electronic and photonic properties of such materials. For example, fully regioregular, head-to-tail coupled, poly(3-alkylthiophene)s have higher electrical conductivities, nonlinear optical responses, higher charge mobilities for field-effect transistors, and more pronounced chemical sensory responses than regiorandom/regio-irregular analogues, see: (a) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233. (b) McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992, 70. (c) Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 4457. (d) Bouman, M.; Meijer, E. W. Adv. Mater. 1995, 7, 385. (e) McCullough, R. D.; Tristram-Nagle, S.; Wiliams, S. P.; Lowe, R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910. (f) McCullough, R. D.; Williams, S. P. J. Am. Chem. Soc. 1993, 115, 11608. (g) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58, 904. (h) McCullough, R. D.; Jayaraman, M. J. Chem. Soc., Chem. Commun. 1995, 135. (i) McCullough, R. D., Ewbank, P. C. In Handbook of Conducting Polymers; Skotheim, T. A., Reynolds, John R., Eds.; Marcel Dekker: New York, 1998; p 225. (j) Wu, X.; Chen, T.-A.; Rieke, R. D. Macromolecules 1995, 28, 2101.

65 (a) Lin-Vien, D.; Colthup, N. B.; Fately, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: London, 1991. (b) Silverstein, R. M.; Bassler, G. C. ; Morrill, T. C. Spectrometric Identification of Organic Compounds, Wiley: New York, 1981. (c) Siesler, H. W., Holland-Moritz, K. Infrared and Raman Spectroscopy of Polymers, Marcel Dekker: New York, 1980.

66 Carbon-carbon triple bond stretching vibrations due to enyne units in polymers typically are in the range of 2130-2220 cm-1, see: (a) Wudl, F.; Bitler, S. P. iJ. Am. Chem. Soc. 1986, 108, 4685. (b) Carre, F.; Devylder, N.; Dutremez, S. G.; Guerin, C.; Henner, B. J. L.; Jolivet, A.; Tomberli, V.; Dahan, F. Organometallics 2003, 22, 2014. (c) Corriu, R. J. P.; Gerbier, P. Guerin, C.; Henner, B. J. L.; Jean, A.; Mutin, P. H. Organometallics 1992, 11, 2507.

67 Typical examples of this approach include poly(thiophene)s, poly(arylene-vinylene)s and poly(aryleneethynylene)s.17b

68 For examples, see: (a) Rehahn, M.; Schlüter, A. D.; Wegner, G. Makromol. Chem. 1990, 191, 1991. (b) Vahlenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994, 195, 1933. (c) Witteler, H.; Wegner, G.; Schulze, M. Macrmol. Rapid Commun. 1993, 14, 471.

69 Examples in which the degree of polymerization and the solubility of the polymer increased with side-chain elongation include poly(p-phenylene-ethynylene)s34d and poly(thiophene)s.1a

70 The average number and the experimental error were obtained by integration of the same spectrum for at least three times.

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71 For examples, see: (a) Anthony, J.; Boudon, C.; Diederich, F.; Gisselbrecht, J.-P.; Gralich, V.; Gross,

M.; Hobi, M.; Seiler, P. Angew. Chem. Int. Ed. 1994, 33, 763. (b) Polhuis, M.; Hendrikx, C. C. J.; Zuilhof, H.; Sudhölter, E. J. R. Tetrahedron Lett. 2003, 44, 899. (c) Walker, J. A.; Bitler, S. P.; Wudl, F. J. Org. Chem. 1984, 49, 4733. (d) Boldi, A. M.; Anthony, J.; Knobler, C. B.; Diederich, F. Angew. Chem. Int. Ed. Engl. 1992, 31, 1240.

72 For reviews, see: (a) te Nijenhuis, K. Adv. Polym. Sci. 1997, 130, 1. (b) Guenet, J.-M. Thermoreversible Gelation of Polymers and Biopolymers, Academic Press: London, 1992. For examples, see: (c) Semenov, A. N.; Rubinstein, M. Macromolecules 1998, 31, 1373. (d) Cotts, P. M.; Swager, T. M.; Zhou, Q. Macromolecules 1996, 29, 7323. (e) Malik, S.; Nandi, A. K. J. Phys. Chem. B. 2004, 108, 597. (f) Yue, S.; Berry, G. C.; McCullough, R. D. Macromolecules 1996, 29, 933. (g) Malik, S.; Jana, T.; Nandi, A. K. Macromolecules 2001, 34, 275. (h) Huang, W. Y.; Matsuoka, S.; Kwei, T. K.; Okamoto, Y. Macromolecules 2001, 34, 7166. (i) Perahia, D.; Traiphol, R.; Bunz, U. H. F. J. Chem. Phys. 2002, 117, 1827. (j) Chu, Q.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2002, 35, 7569. (k) Perahia, D.; Jiao, X.; Traiphol, R. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3165. (l) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem. Int. Ed. 2003, 42, 332. (m) Hsieh, B. R.; Yu, Y.; VanLaeken, A. C.; Lee, H. Macromolecules 1997, 30, 8094. (n) Yin, C.; Yang, C.-Z. Synth. Met. 2001, 118, 75. (o) Nakaoki, T.; Tashiro, K.; Kobayashi, M. Macromolecules 2000, 33, 4299. (p) George, M.; Weiss, R. G. Chem. Mater. 2003, 15, 2879. (q) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbaskaran, M.; Woo, E. P.; Soliman Acta Polymer. 1998, 49, 439.

73 The polymer with acetylenic end groups obtained by homopolycyclotrimerization of 1,8-nonadiyne became partially insoluble upon storage under ambient conditions, see: (a) Xu, K.; Peng, H.; Sun, Q.; Dong, Y.; Salhi, F.; Luo, J.; Chen, J.; Huang, Y.; Zhang, D.; Xu, Z.; Tang, B. Z. Macromolecules, 2002, 35, 5821.

74 Development of (pre)polymers with triple bond functional ends (acetylene-terminated (pre)polymers) as thermally curable resins for industrial application has been an active area of research. For reviews, see: (a) Hergenrother, P. M. In Concise Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New York, 1990. (b) Sergeev, V. A.; Chernomordik, Yu. A.; Kurapov, A. S. Rus. Chem. Rev. 1984, 53, 307. For examples, see: (c) Gandon, S.; Mison, P.; Silion, B. Polymer 1997, 38, 1449. (d) McQuilkin, R. M.; Garratt, P. J.; Sondheimer, F. J. Am. Chem. Soc. 1970, 92, 6682. (e) Nakamura, K.; Ando, S.; Takeichi, T. Polymer 2001, 42, 4045. (f) Reghunadhan Nair, C. P.; Bindu, R. L.; Ninan, K. N. Polymer 2002, 43, 2609. (g) Rohde, O.; Wegner, G. Makromol. Chem. 1978, 179, 1999. (h) Badarau, C.; Wang, Z. Y. Macromolecules 2003, 36, 6959 and references therein.

75 (a) Ranby, B.; Rabek, J. F. Photodegradation, Photo-oxidation and Photostabilization of Polymers, Wiley: London, 1000. (b) Handbook of Polymer Degradation, 2nd ed.; Hamid, H. S., Ed.; Marcel Dekker: New York, 2000. (c) Becker, H.; Spreitzer, H.; Ibrom, K.; Kreuder, W. Macromolecules 1999, 32, 4925. (d) Wan, W. C.; Antoniadis, H.; Choong, V. E.; Razafitrimo, H.; Gao, Y.; Feld, W. A.; Hsieh, B. R. Macromolecules 1997, 30, 6567. (e) Hay, A. S. J.Org.Chem. 1960, 637. (f) Hay, A. S. J. Polym. Sci., Part A: Polym. Chem. 1969, 7, 1625. (g) Choi, S.-K.; Gal, Y.-S.; Jin, S.-H.; Kim, H. K. Chem. Rev. 2000, 100, 1645. (h) Ruthledge, T. F. In Acetylenes and Allenes; Reinhold: New York, 1969; Chapter 5. (i) Ginsburg, E. J., Gorman, C. B., Grubbs, Robert H. In Modern Acetylene Chemistry; Stang, P. J., Diederich, François, Eds.; VCH: New York, 1995.

76 For examples, see: (a) Kijima, M.; Tanimoto, A.; Oya, A.; Liang, T.-T.; Yamada, Y. Carbon 2001, 39, 297. (b) Kijima, M.; Tanimoto, H.; Shirakawa, H. Synth. Met. 2001, 119, 353. (c) Hay, A. S.; Bolon, D. A.; Leimer, K. R.; Clark, R. F. Polymer Lett. 1970, 8, 97. (d) Liang, R.-C.; Reiser, A. J. Polym. Sci.: Polym. Chem. Ed. 1987, 25, 451. (e) Nallicheri, R. A.; Rubner, M. F. Macromolecules 1991, 24, 517. (f) Stanford, J. L.; Young, R. J.; Day, R. Polymer 1991, 32, 1713. (g) Chance, R. R. Polym. Prepr. 1987, 28, 445. (h) Enkelmann, V. Adv. Polym. Sci. 1984, 63, 91. (i) Ref. 43b. (j) Ref. 44a,s. (k) Ref. 78c.

77 The 2,5-dialkoxy-substituted PPEs are a special class of π-conjugated polymers that have exhibited high photoluminescence, some electroluminescence and liquid crystalline properties.17b

78 For examples, see: (a) Anthony, J.; Boudon, C.; Diederich, F.; Gisselbrecht, J.-P.; Gralich, V.; Gross, M.; Hobi, M.; Seiler, P. Angew. Chem. Int. Ed. 1994, 33, 763. (b) Ref. 44a. (c) Miller, T. M.; Kwock, E. W.; Baird, T., Jr.; Hale, A. Chem. Mater. 1994, 6, 1569. (d) Ref. 58b. (e) Ref. 44s. (f) Diederich, F.

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Chem. Commun. 2001, 219. (g) Wudl, F.; Bitler, S. P. J. Am. Chem. Soc. 1986, 108, 4685. (h) Lindsell, W. E.; Preston, P. N.; Tomb, P. J. J. Organomet. Chem. 1992, 439, 201.

79 The use of a tert-butyl, methyl and phenyl group has been crucial in the preparation of isolable and characterizable oligo(yne)s, oligo(enyne)s and oligo(enediyne)s. Their main function seems to be the steric inhibition of polymerization at the end groups. Hydrogen-capped oligo(yne)s and oligo(enyne)s78 are normally stable only below room temperature in the dark. For examples of oligo(yne) chemistry, see: (a) Raphael, R. A. Acetylenic Compounds in Organic Synthesis, Academic Press: New York, 1955. (b) Lagow, R. J.; Kampa, J. J.; Wei, H.-C.; Battle, S. L.; Genge, J. W.; Laude, D. A.; Harper, C. J.; Bau, R.; Stevens, R. C.; Haw, J. F.; Munson, E. Science 1995, 267, 362. (c) Kijima, M.; Kinoshita, I.; Hattori, T.; Shirakawa, H. J. Mater. Chem. 1998, 8, 2165. (d) Diederich, F. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, 1995; p 443. (e) Grösser, T.; Hirsch, A. Angew. Chem. Int. Ed. 1993, 32, 1340. (f) Schermann, G.; Grösser, T.; Hampel, F.; Hirsch, A. Chem. Eur. J. 1997, 3, 1105. (g) Jones, E. R. H.; Lee, H. H.; Whiting, M. C. J. Chem. Soc. 1960, 3483. (h) Bohlmann, F. Chem. Ber. 1953, 86, 657. (i) Johnson, T. R.; Walton, D. R. M. Tetrahedron 1972, 28, 5221. (j) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am. Chem. Soc. 2000, 122, 810.

80 Vinylacetylene is readily polymerized by heat to form viscous oils and hard resinous solids, see: Nieuwland, J. A.; Calcott, W. S.; Downing, F. B.; Carter, S. J. Am. Chem. Soc. 1931, 53, 4197.

81 The kinetic acidities of several acetylenes have been estimated based on hydrogen-exchange rates. For recent examples, see: (a) Kresge, A. J.; Powell, M. F. J. Org. Chem., 1986, 51, 819. (b) Kresge, A. J.; Pruszynski, P.; Stang, P. J.; Williamson, B. L. J. Org. Chem. 1991, 56, 4808.

82 This difference in reactivity is likely to be mainly steric in origin, as the kinetic acidity of 1,4-diethynylbenzene has been found 14 times higher than that of phenylacetylene, based on hydrogen-exchange rates. See: Dessy, R. E.; Okuzumi, Y.; Chen, A. J. Am. Chem. Soc. 1962, 84, 2899.

83 The estimates of the pKa values for trimethylsilylacetylene and phenylacetylene are 21.1 and 19.1, respectively, based on detritiation rates in aqueous solution, see: Kresge, A. J.; Pruszynski, P.; Stang, P. J.; Wiliamson, B. L. J. Org. Chem. Soc. 1991, 56, 4808.

84 For reviews, see: (a) Chemistry of Acetylenes, Viehe, H. G. (Ed.), Marcel Dekker: New York, 1969. (b) Cyclic Polymers, Semlyen, J. A. (Ed.). Kluwer Academic Press: New York, 2000. (c) Ref. 42. For recent examples, see: (d) Zhou, Q.; Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59, 1294. (e) Rubin,Y.; Kahr, M.; Knobler, C. B.; Diederich, F.; Wilkins, C. L. J. Am. Chem. Soc. 1991, 113, 495. (f) Pak, J. J.; Weakley, T. J. R.; Haley, M. M. J. Am. Chem. Soc. 1999, 121, 8182.

85 (a) Electronic Materials: The Oligomer Approach, Müllen, K.; Wegner, K. (Eds.), Wiley-VCH, Weinheim, 1998. (b) Martin, R. E.; Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1350.

86 Yoshino, K.; Tada, K.; Onoda, M. Jpn. J. Appl. Phys. 1994, 33, L1785. 87 For reviews, see: (a) Raeder, H. J.; Schrepp, W. Acta Polym. 1998, 49, 272. (b) Nielen, M. W. F.

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The organolanthanide-catalyzed polymerization of diynes

247

91 GPC experiments based on polystyrene calibration are well-known to lead to a significant

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94 The nature of the end-group analysis was not reported, but polycondensation based on palladium-catalyzed cross-coupling reactions between halides and terminal acetylenes are well-known to produce polymers (e.g. poly(aryleneethynylene)s17) having halide end-groups.

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