Diene polymerisation by lanthanide catalysts, concerted

vs. pseudoanionic mechanism

An answer deduced from microwave experiments

Philippe Zinck1, Denise Barbier Baudry*

1, André Loupy

2

1Laboratoire de Synthèse et d’Electrosynthèse Organométalliques, LSEO, UMR 5632,

Université de Bourgogne, Bâtiment Mirande, 9 Avenue Alain Savary, BP 47870 Dijon Cedex

France

Fax : 33 (3) 80 39 60 84 ; E-mail : Denise.Baudry@u-bourgogne.fr

2Laboratoire des Réactions Sélectives sur Supports, ICMMO, UMR 8615, Bât. 410,

Université Paris-Sud, 91405 Orsay Cedex, France

Keywords: borohydride; catalysis; lanthanide; microwave; polyisoprene; stereospecific

polymers

Summary

The pseudoanionic character of the polymerisation of dienes by lanthanide catalysts has been

assessed from microwave experiments. The microwave activation of isoprene polymerisation

with Nd(BH4)3(THF)3 / Mg(Bu)2 and Nd(BH4)3(THF)3 / Al(Et)3 leads to a modification of the

reactivity, the selectivity being only slightly modified. An explanation of the observed effect

is proposed based on our current knowledge of the catalytic mechanism and by considering

the alkylated complex as a ion pair. The bimetallic active species are thought to evolute

toward a transition state where ion pairing is much more loose due to negative charge

delocalisation, resulting in an enhancement in polarity during the reaction progress. The

formation of a highly stabilising polymetallic specie involving tetraaluminate groups is

advanced in order to explain the difference in reactivity observed between both catalytic

systems. Finally, a “depolymerisation” effect of microwave irradiation in course of reaction is

observed at high temperature.

Introduction

Lanthanide catalysed polymerisation and copolymerisation of dienes is now a field

widely underscored. Nevertheless, most of the studies concern the obtaining of well defined

materials, and the exact mechanism of the polymerisation act is not clearly defined.

Lanthanides are hard elements, their chemistry is generally ionic and it is currently admitted

that the polymerisation of oxygenated monomers catalysed by organolanthanide complexes

occurs via a pseudoanionic mechanism[1]

. The situation is less clear when only metal carbon

bonds are involved. In this case, concerted transition states with an orbital control instead of a

charge control are postulated. This point is of great interest since the knowledge of the

mechanism allows to act on the rate and on the stereocontrol of the reaction. Pseudoanionic

and coordination polymerisation are controlled by two distinct and antagonist mechanisms,

the monomer coordination and the successive chain migration on the coordinated monomer.

An electron-poor and non-crowded complex favours the coordination of the monomer, the

migration being hindered. The polarisability of the metal-carbon bond in an electron-rich

complex will in turn improve the migration of the growing chain, but the coordination of the

monomer is hindered. In fact, the chemistry of lanthanides is governed by steric requirements,

especially in the early series, and the role of electronic factors in the reactivity of the

lanthanides complexes remains largely in debate[2]

. Since one of the key point lies in

polarisability, the use of microwave activation can be envisaged as a tool for obtaining

information about the active species involved in the catalytic mechanism. Indeed, reactivity

and selectivity are affected by microwave activation as soon as one of the reaction

intermediaries shows a polar character[3]

, and it was recently shown for polymer synthesis that

microwave activation allows enhancement in reactivity and selectivity[4]

.

Recent work in our group were devoted on the controlled polymerisation of 1,3 diene using

Nd(BH4)3(THF)3 (THF is tetrahydrofurane) with Mg(Bu)2 or Al(Et)3 cocatalysts, the former

providing stereo-regular 1,4-trans polyisoprene[5]

. From 1H RMN experiments, a molecular

structure was postulated based on bimetallic active species. We present here, on the basis of

microwave experiments, further investigations on the nature of the catalytic mechanisms and

the active species involved in the polymerisation of dienes by lanthanide catalysts.

Results and discussion

Isoprene polymerisation with Nd(BH4)3(THF)3 / Mg(Bu)2

The polymerisation of isoprene using a bisalkylmagnesium cocatalyst reported Table 1

leads to a highly trans-stereoregular structure. The reaction was first conducted at a

temperature of 60°C in a sealed tube, in order to limit the boiling of isoprene (Teb = 34°C at

760 mm Hg). Yields are higher under microwave radiation : 47% and 69% for one and two

hours, respectively, vs. 35% and 45% for conventional heating (runs 1-4). It must be noticed

there that the microwave power regulation needed to fix the temperature of the reactants in

toluene at 60°C is not easy and corresponds to the lower limit of the apparatus. Figure 1

shows that an average power of 15W is required. It is therefore preferred to conduct

experiments at a temperature of 80°C. The reaction is very fast at this temperature; yields of

37% are observed after 5 minutes microwave irradiation. After 15 minutes, the polymerisation

is quasi finished, with yields up to 70% which do not exceed 75% after 30 minutes. As

represented Figure 2, the thermal reaction runs slower, several percents in 5 minutes, 53% in

30 minutes and 73% in one hour. The slowdown observed for higher reaction times may be

due to the lower quantity of monomer present in the solution at the end of the reaction. If the

reaction under microwave heating is initially accelerated, its selectivity is only slightly

modified. The ratio of 1,4-cis polyisoprene is doubled, from 2-3% under conventional heating

to 4-6% under microwave heating. Experimental molecular weights are in good agreement

with theoretical values, although the distributions are larger for the polyisoprenes formed

under microwave irradiation. The polydispersity indexes ranges from 2 to 2.5 vs. 1.5 to 2 for

conventional heating.

In order to observe more important yields changes or modifications of the

microstructure after microwave irradiation, reactions were further conducted at higher

temperatures, corresponding to higher applied powers : 55 W in order to regulate at 100°C

and 95 W at 120°C vs. 32W at 80°C. The power is moreover higher in the early stages of the

reaction, but the real temperature for these steps remains uncertain. The reaction time was

increased in order to account for the evaporation of the monomer. The thermal reaction at

120°C leads to a yield of 58% vs. 65% under microwave irradiation. From the ratio of 1,4-cis

polyisoprene obtained, the stereochemistry of the reaction is less controlled, as it is often the

case for an increase in temperature. An attempt to improve yields was realised by increasing

the quantity of solvent, with the aim to maintain a greater quantity of monomer in the liquid

phase. Under these dilution conditions, yields of 88% and 73% were observed under

conventional and microwave heating, respectively (runs 20-21). A degradation of the polymer

was also observed under microwave irradiation from the decrease of the yield from 73% to

59% after 30 and 60 minutes at 120°C, respectively (runs 21 and 23). If we assume that low

molecular weight chains are lost from the precipitation of the polymer in ethanol, runs

performed for shorter times are thought to be significant. From the experimental results

reported there, microwave heating does not significantly affect the stereochemistry of the

polymerisation of isoprene with Nd(BH4)3(THF)3 and Mg(Bu)2 as cocatalyst, but the reaction

is two to three times accelerated. Microwave irradiation does not significantly influence the

coordination mode of the monomer on the lanthanide.

Isoprene polymerisation with Nd(BH4)3(THF)3 - Al(Et)3

The nature of the alkylating agent as a great influence on the microstructure of polyisoprene,

and we further focussed on a second catalytic system. Polyisoprenes containing a higher ratio

of 1,4-cis units were synthesised previously in the presence of Al(Et)3[4]

. The modification of

the selectivity of the reaction was attributed to the formation of an under-coordinated active

specie involving 4 coordination of the monomer.

From Table 2, trisalkylaluminium based systems react slower than bisalkylmagnesium based

system, but a yield of 50% is obtained at 120°C under microwave irradiation vs. 9% for

conventional heating (runs 27-28). The obtained molecular weights are smaller. This can be

attributed to (i) two or three simultaneous growing chains as a consequence of an excess of

alkylating agent (this will further be discussed hereafter), or (ii) to a chain transfer to

aluminium. The polydispersity indexes are smaller under conventional heating as reported for

the bisalkylmagnesium cocatalyst, and similar conclusions can be drawn. Microwave

irradiation does not significantly affect the stereochemistry of the polymerisation of isoprene

with Nd(BH4)3(THF)3 and Al(Et)3 as cocatalyst, and does not significantly influence the

coordination mode of the monomer on the lanthanide. The reaction is accelerated, the effect

being more important as compared to the bisalkylmagnesium cocatalyst. Starting from our

current knowledge of the catalytic mechanism, we attempt now to deduce information which

may explain the experimental results observed here.

Alkylation step

RMgX magnesium derivatives are not alkylating agents, since they react with a metallocene

[Ln] to give a bimetallic specie as represented Scheme 1.

Scheme 1.

Reaction between Ln-X and RMgX

Ln X + RMgX Ln

X

X

MgR

The R group has to present an allyl functionality in order to be transferred to the lanthanide,

and great difficulties are encountered to eliminate MgX2. MgR2 like compounds are therefore

preferred for the alkylation (Scheme 2).

Scheme 2.

Lanthanide complexes alkylation by MgR2

Ln X + MgR2 Ln

X

R

R

Ln

X

R

RMg

Mg

The formation of a bridged bimetallic complex between Nd(BH4)3(THF)3 and MgR2 was

proposed by Monakov and co-workers[7]

. The presence of a X bridging group rather than a X

terminal group has been proposed the on the basis of 1H RMN experiments

[5]. The reactivity

of a bridging R group is generally considered as weak, and a terminal R group is needed in

order to achieve polymerisation A stoichiometric amount of bisalkylmagnesium is sufficient

to generate an active catalytic system.

The situation differs in the case of Al(Et)3. Aluminium is characterised by a good Lewis acid

behaviour, but is not known as a powerful alkylating agent. An excess of cocatalyst is

therefore needed to achieve alkylation, from 5 to 20 equivalents. The active bond formation

represented Scheme 3 involves tetraaluminate species, as it has been proposed to explain the

inactivity of binary chlorolanthanidocene/Al(Et)3 catalytic systems against ethylene[7]

. The

excess of Al(Et)3 introduced may lead to the formation of a polymetallic specie, showing a

tridipolar structure. The resulting electron-poor lanthanide will favour an η4 coordination of

the monomer, leading to a higher 1,4-cis ratio in the microstructure.

Scheme 3.

Lanthanide complexes alkylation by Al(Et)3

LnX3 + nR3Al Ln3+

, [AlXR3]3-

Ion pair

Coordination – Migration step

Considering the active bond in the course of coordination – migration steps as an ion pair, the

monomer can act as a solvent and lead to a looser ion pair. The action of microwave

irradiation can be similar to a dilution effect on the dipole. As proposed for explaining the

effect of microwave irradiation on alkylating reactions[8]

, an increase in the system polarity

can be advanced in the course of coordination – migration mechanisms. The ground state of

the reaction, corresponding to bimetallic active species represented Schemes 4 and 5, evolutes

toward a transition state where ion pairing is much more loose due to negative charge

delocalisation. It results in an enhancement in polarity during the reaction progress and

consequently, to an increase in microwave – catalytic system interactions magnitude

responsible for the observed acceleration.

Scheme 4.

Possible Nd/Mg active initiating specie and polarised transition state

Ln

X

R

RMg

Ln

X

R

RMg

+

-

+

Ln+

X

R-

RMg

Ln

X

R-

RMg

+ R' group

Ln

X

R

RMg

R' group

Ln+

X

R-

RMg

Considering our systems in the transition state, the migrating R group is close to the fourth

carbon of the diene, resulting in a charge delocalisation. It can be considered as the formation

of a new crowded R´ group, R´ = R + isoprene. The bond to the lanthanide is now less strong

in comparison with a carbon bearing charge linked to the metal, and the microwave

absorption is more important for this polar specie as compared to the initial complex. The

catalytic mechanism postulated here is presented Scheme 4 for the bisalkylmagnesium based

system, and is in accordance with a pseudo-anionic polymerisation similar to the

polymerisation of oxygenated monomers. The living character of the polymerisation has been

assessed previously[5]

, with the formation of a unique active specie. The trans stereochemistry

may result from a trans coordination of the monomer on the metal, rather than a cis

coordination followed by an isomerisation.

Scheme 5.

Possible Nd/Al active initiating specie

Nd R

XAl

R

R

-

+

Nd

AlR3 X

X AlR3

AlR3 X RAl

X

R

-

R

R

XAl

R

R

-

+

+

The polymerisation of isoprene with Nd(BH4)3(THF)3 - Al(Et)3 involves an allylic active

specie from a η4 coordination of the monomer on the metal, leading to 1,4-trans rich

polyisoprene[6]

. Phenomenons postulated for explaining the reactivity changes for Mg(Bu)2

based systems under microwave irradiation can also be advanced for Al(Et)3 as a cocatalyst.

The greater acceleration of the reaction observed with triethylaluminium as cocatalyst may be

due to the highly stabilising effect of the formation of the polymetallic Nd(XAlR3)3 involving

tetraaluminate groups (Scheme 5). It may result in a greater energetic threshold, as

represented in figure 3.

Practical potentialities of microwave activation

A practical tool against catalyst ageing

Trans polymerisation of isoprene has received much attention recently, due to the interest of

high performances rubbers and tyres industries in 1,4-trans polyisoprene [9]

. Industrial

production often suffers from time schedules, and a catalytic system keeping its efficiency

after several hours after its preparation presents potential interests. Runs 13 to 15 performed

with Mg(Bu)2 as alkylating agent after 2h rest at room temperature highlight the potentiality

of microwave activation in this frame. 44% yield is obtained after 4h for thermal heating,

indicating a degradation of active species at room temperature, whereas yields up to 85% are

obtained for microwave activation. The degradation of bimetallic active species into hydride

species may explain the higher yield obtained under microwave irradiation after 2 hours rest

at room temperature. Change of the selectivity of the reaction is noticed when the amount of

solvent is set to twice the amount of monomer (2.3 vs 13.8% 1,4-cis polyisoprene for

conventional and microwave heating, respectively). With a smaller quantity of toluene, the

percentage of 1,4-cis polyisoprene is quite similar to conventional heating (3.1%) for a yield

of 78%.

Depolymerisation under microwave irradiation

Due to the large excess of triethylaluminium introduced, the number of growing chains per

metal can be greater than one for the alkylating agent. Consequently, the measured molecular

weights can be smaller than that predicted with the assumption of one growing chain per

metal (see runs 25, 28 and 29). A chain transfer to aluminium can also be advanced. From

Table 2, a large excess of cocatalyst and a long reaction time are needed for increasing the

number of growing chains under conventional heating. On the opposite, microwave

irradiation favours the number of growing chains at shorter reaction times, highlighting a

specific effect on the selectivity. For longer reaction times, the number average molecular

weight and the yield decrease, from 50 to 36% for the latter for 30 and 60 minutes,

respectively. A similar reduction in yield is observed with the bisalkylmagnesium cocatalyst

under microwave irradiation at 120°C (runs 21 and 23). Such a depolymerisation

phenomenon under microwave irradiation has never been reported to our knowledge, and is of

particular interest for practical applications.

Conclusion

The pseudoanionic character of the polymerisation of dienes by lanthanide catalysts has been

assessed from microwave experiments. The microwave activation of isoprene polymerisation

with Nd(BH4)3(THF)3 / Mg(Bu)2 and Nd(BH4)3(THF)3 / Al(Et)3 leads to a modification of the

reactivity, the selectivity being only slightly modified. An explanation of the observed effect

is proposed based on our current knowledge of the catalytic mechanism and by considering

the alkylated complex as a ion pair. The bimetallic active species are thought to evolute

toward a transition state where ion pairing is much more loose due to negative charge

delocalisation, resulting in an enhancement in polarity during the reaction progress. The

formation of a highly stabilising polymetallic specie involving tetraaluminate groups is

advanced in order to explain the difference in reactivity observed between both catalytic

systems. Finally, a depolymerisation effect of microwave irradiation in course of reaction is

observed at high temperature.

Experimental part

Materials

Toluene was dried over sodium/benzophenone ketyl and deoxygenated by distillation

immediately before use. Isoprene (Aldrich) was dried first over calcium hydride, then over

molecular sieves (3Å) and distilled just before use. Al(Et)3 (1.9 M, toluene) and Mg(Bu)2 (1

M, heptane) were purchased from Aldrich. The complex Nd(BH4)3(THF)3 was synthesised

from NdCl3(THF)3 as reported in the literature[10]

.

Isoprene Polymerisation

In a glove box, the catalyst was weighted in a 10 mL vessel. Toluene, the cocatalyst and the

monomer were added to the flask in this order using syringes. The mixture was magnetically

stirred at the reaction temperature for a given time under thermal and microwave heating. The

viscous mixture was then diluted in toluene and the resulting solution was poured into

ethanol. The off-white polymer was filtered off and dried under vacuum for 48 h.

Microwave irradiation

Reactions were conducted using a Discover CEM reactor operating at 2450 MHz in sealed

vessels with magnetical stirring. Temperature is measured by IR detection and maintained

constant by emitted microwave power monitoring between 15 and 180W. The main power

values necessary to reach and maintain the temperature are indicated figure 1.

Measurements

1H NMR spectra were recorded on a Brüker Avance 300 at 300K in CDCl3 solutions. Steric

exclusion chromatography analyses were carried out in THF as eluent at 20°C (1 ml.min-1

)

using a Gynkotek P580A apparatus equipped with two JORDI divinylbenzene mixed B

columns and an IOTA2 refractive index detector. Corrections were not achieved because

Mark-Houwink coefficients are not available for pure 1,4-trans polyisoprene in the literature,

and we checked that for a low molecular weight trans polyisoprene synthesised with an

allylsamarium catalyst, the values obtained by 1H NMR and SEC are quite similar.

References

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Tortosa, C.R. Acad. Sci Ser. IIc 2000, 3, 631

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Chem. J. 2000, 24, 939 ; [2b] D. Barbier-Baudry, A. Dormond, M. Visseaux, J. Organomet.

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Comm 2004, 25, 873

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248, 397

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Liagre, K. Burle, M. Moneuse, Pure Appl. Chem. 2001, 73, 161; [8c] S. Chatti, M. Bortolussi,

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Table 1. Isoprene polymerisation with Nd(BH4)3(THF)3 / Mg(Bu)2

Runa)

[Mg]/[Nd] Heating Temperature Time Yield Microstructure b)

nM (th).10

-3c) nM .10

-3d) PDIe)

°C min % %

3,4- 1,4-cis 1,4-trans

1 1 TH 60 60 35 2.0 1.7 96.4 11700 36700 1.7

2 1 MW 60 60 47 1.9 2.7 95.4 17900 15700 2.3

3 1 TH 60 120 45 2.0 1.8 96.3 20600 - -

4 1 MW 60 120 69 2.3 4.3 93.3 26300 18600 2.3

5 1 MW 80 5 41 2.5 2.9 94.5 18500 22500 1.8

6 1 TH 80 15 46 2.4 2.2 95.4 17548 19900 3

7 1 MW 80 15 72 2.2 5.0 92.8 26200 25700 2.2

8 1 TH 80 30 53 2.0 2.0 96.0 20218 14400 2.4

9 1 MW 80 30 76 2.4 4.5 93.1 29400 17800 2.4

10 1 TH 80 60 71 2.1 3.0 95.0 24319 18100 1.5

11 1 MW 80 60 75 2.1 4.2 93.7 29900 23000 2.5

12 1 TH 80 240 100 2.2 3.3 94.5 39200 27300 2.3

13 1 MW 80 240g)

78 2.4 3.1 94.4 29800 19400 2.4

14f) 1 MW 80 240

g) 85 0.9 13.8 85.3 32400 6400

h) 2.5

15f)

1 TH 80 240g)

44 3.9 2.3 94.0 17000 17200 1.8

16 1 TH 100 30 57 2.2 4.6 93.2 20900 20800 1.6

17 1 MW 100 30 72 2.6 6.0 91.4 27600 15800 2.7

18 1 TH 120 30 58 2.8 4.9 92.4 26700 12100 3.0

19 1 MW 120 30 65 2.9 5.5 91.7 24200 13400 2.9

20f)

1 TH 120 30 88 1.8 6.8 91.3 33600 16300 2.8

21f) 1 MW 120 30 73 2.4 7.5 90.1 27900 20800 2.2

22f) 1 TH 120 60 83 2.4 6.4 91.2 20700 28400 2.2

23f) 1 MW 120 60 59 2.3 7.0 90.8 22300 24900 1.8

a) Polymerisation conditions : V(isoprene) = 1 ml; V(toluene) = 0.5 ml; isoprene / Nd(BH4)3(THF)3 = 560

b) On the basis of

1H NMR integration in CDCl3.

c) Expected molecular weights taking the yield into account : nM (th) = ([monomer]/[Nd])*yield.

d) Measured by SEC analysis using polystyrene standards for calibration.

e) Polydispersity index wM / nM .

f) V(toluene) = 2 ml.

g) 2h at room temperature before polymerisation.

h) Presence of insolubles in THF.

Table 2. Isoprene polymerisation with Nd(BH4)3(THF)3 / Al(Et)3

Runa)

[Al]/[Nd] Heating Temperature Time Yield Microstructure b)

nM (th).10

-3c) nM .10

-3d) PDIe)

°C min % %

3,4- 1,4-cis 1,4-trans

24f) 5 TH 80 480 45 3.2 26.0 70.8 4000 11400 2.8

25 20 TH 80 480 72 3.0 19.5 77.5 24400 8600 1.8

26 20 MW 80 480 20 3.7 21.9 74.5 7800 7100 2.1

27 20 TH 120 30 9 3.0 16.6 80.4 3400 5300 1.6

28 20 MW 120 30 50 3.7 17.9 78.4 19000 6500 2.1

29 20 MW 120 60 36 3.7 18.0 78.3 13900 6100 2.0

a)

Polymerisation conditions : V(isoprene) = 1 ml; V(toluene) = 0.5 ml; isoprene / Nd(BH4)3(THF)3 = 560

b) On the basis of

1H NMR integration in CDCl3.

c) Expected molecular weights taking the yield into account : nM (th) = ([monomer]/[Nd])*yield.

d) Measured by SEC analysis using polystyrene standards for calibration.

e) Polydispersity index wM / nM .

f) Isoprene / Nd(BH4)3(THF)3 = 135

Figure 1.

Power (a) and temperature (b) curves for runs 4, 9 17 and 19.

0

30

60

90

120

150

180

0 300 600 900 1200 1500 1800

Time (s)

Po

we

r (W

)

(a)

60°C

80°C

100°C

120°C

0

20

40

60

80

100

120

140

0 300 600 900 1200 1500 1800

Time (s)

Te

mp

era

ture

(°C

)

(b)

Figure 2.

Yields for the Nd(BH4)3(THF)3 / Mg(Bu)2 system as a function of reaction time at 80°C – (●)

microwave heating and (▲) conventional heating.

0%

20%

40%

60%

80%

100%

0 10 20 30 40 50 60

Time (min)

Yie

ld (

%)

Figure 3.

Postulated energetic profile.

GMg

GAl

En

erg

y

Time

GMg

GAl

En

erg

y

Time

Table of contents

The pseudoanionic character of the

polymerisation of dienes by lanthanide

catalysts has been assessed from

microwave experiments. The microwave

activation of isoprene polymerisation with

Nd(BH4)3(THF)3 / Mg(Bu)2 and

Nd(BH4)3(THF)3 / Al(Et)3 leads to a

modification of the reactivity, the

selectivity being only slightly modified.

An explanation of the observed effect is

proposed based on our current knowledge

of the catalytic mechanism and by

considering the alkylated complex as a ion

pair. The bimetallic active species are

thought to evolute toward a transition state

where ion pairing is much more loose due

to negative charge delocalisation, resulting

in an enhancement in polarity during the

reaction progress. The formation of a

highly stabilising polymetallic specie

involving tetraaluminate groups is

advanced in order to explain the difference

in reactivity observed between both

catalytic systems. Finally, a

“depolymerisation” effect of microwave

irradiation in course of reaction is observed

at high temperature.

.

Ln

X

R

RMg

Ln

X

R

RMg

+

-

+

Ln+

X

R-

RMg

Ln

X

R-

RMg

+

Postulated pseudoanionic mechanism for

Nd(BH4)3(THF)3 - Mg(Bu)2