1. NMR Character is at Ion of Macro Molecules in Solution

8/3/2019 1. NMR Character is at Ion of Macro Molecules in Solution

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1 NM R CHARACTERISATIONOF MACROMOLECULESIN SOLUTIONA. H. FAWCETT, J. G. HAMILTON AND J. J. ROONEYSchool of Chemistry, The Queens University of Belfast, Belfast BT9 5AG ,Northern Ireland, UK

1.1 INTRODUCTIONThe NM R method of studying the microstructure of macromolecules is the mosteffective available, provided that the materials can be obtained in solution. Themethod is now routinely employed to characterise and to identify the structurespresent in polymers, both those in common use and those created by the chemistwhen working with new monom ers o r new catalyst systems [ 1 - 6 ] . Derivatives ofpolymers and reactions on polymers are similarly accessible to study. The NM Rparameter that is sensitive to these structural issues is the chemical shift,commonly measured in ppm from an internal reference. It senses readily informa-tion on the framework of the polymer its connectivityby providing informa-tion on the number and type of atoms linked to each particular nucleus, and alsosenses such factors as the relative chirality of pairs of such centres and cis/transisomerism within double bonds.

The nucleus most often employed for both man-made and natural macro-molecules is 13C, despite its being rather dilute (only 1% of the carbons). This isbecause in the spectrum the dispersion of shifts is particularly large; much detailor fine structure is generally encountered tha t is directly related to the polymerstructure itself, and signal intensity is rarely a problem with modern high fieldinstruments. Many other NMR-active nuclei such as 1 9F and 3 1 P may be usedtoo when they are present in the macromolecule. Proton NMR spectra arecomplicated by the presence of coupling effects between the spins of the protons if,as is usual, the protons are present on directly bonded carbon atom s. In certaincases these coupling effects are of extreme value: as Bovey showed forpoly(methyl methacrylate) [2,7] , the tacticity of the polymers m ay be identifieddirectly, and the value of vicinal coupling constants provides information on theconformational properties of the bond [5,8] . However, frequently, as for examplewith polyolefins, they conceal the shift effects associated with the microstructurePolymer Spectroscopy. Edited by Allan H. Fawcett 1996 John W iley & Sons Ltd

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by creating a multiplicity of splittings, a complicating factor which may berelieved only by the use of a substantial proportion of selective deuteration, as hasbeen demonstrated for polypropylene [9 ,10 ] .We may note two rather special cases of proton NMR spectra: for highly

syndiotactic polystyrene the methylene pro tons, being equivalent, have a simplethree line 1:2:1 pattern that derives from the coupling effect of the two flankingmethine protons [ H ] . The highly isotactic polymer has a slightly more com plexbut still recognisable spectrum [12]. Features in the spectrum of the atacticpolymer are quite unrecognisable, as proton coupling effects intermingle withchirality effects, coupled with substantial chemical shift anisotropy from thephenyl ring [ 13 ]: each main chain carbon bears at least one proton , a situationthat is unfortunately more usual. We are familiar with only one case, involvingthe furfurol oligomer bis(5-furfuryl-2-furylmethane), in which the methyleneprotons are more sensitive to pos ition than is the carbon of the same group; this isprobably because the central methylene protons sam ple the anisotropic shieldingcone of the furan rings in a manner different from that for the protons of theflanking methylene groups, but the carbons, being in the plane of the rings,experience a constant effect [ 14 ].

Du ring the last 25 years the developm ent of the N M R m ethod, firstly in termsof the power of the magnet em ployed and secondly by turning to computer-basedoperating systems, has often been stimulated, if not driven, by the need tounderstand polymer microstructure. In 1971 the chemical companies Dow, ICIand D u P ont them selves com m issioned new magnets that increased the m agneticfield beyond 5 T in order to pursue their studies of polymers so vital to theirbusiness. This magnetic field, equivalent to m ore than 200 MHz in terms of theproton resonance frequency, was achieved by em ploying superconducting w ind-ings at cryogenic temperatures [ 1 5 ] . The stronger the magnetic field, the greaterthe sensitivity and the dispersion of shifts (and the closer the pro ton spectra com eto being first order). Initially m an-m ade polymers were the subjects of study, butmore recently biological polymers have been the targets. The last ten years hasseen field strengths in com m on use rise to 11.74 T (equivalent to 5 0 0 M H z forproton s and 125.7 M H z for carbons) by the adoption of superconducting m ag-nets, and similar technical improvements associated with versatile signal trans-mitter and receiver coil design have also come into common practice. Indeed,17.5 T instruments have recently been announced.

Just as important a s these developm ents in magnet design has been the intro-duction of pulsed Fourier transform methods, for these permit the performance ofnew types of experiment by the computerised systems that control the produc-tion, acquisition and processing of the experimental data. New pulse sequencesincreasingly made available by instrument manufacturers within their softwaresuites permit the routine performance of these new experiments: an early exam pleis the distortionless enhancement polarisation transfer, or DEPT, experiment toidentify the number of protons attached to a carbon by controlling the finalCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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proton pulse flip angle [16]. A later example is provided by 2-D and 3-Dexperiments, the introduction of which has made the connectivity of the carbonand protons m uch clearer [ 17 ,18 ] , has much reduced the problem of distinguish-ing couplin g effects from shift effects by providing extra dimensions for d isplayingthe NM R signal, and has even provided an extra structure-discriminating route[ 1 9 - 2 2 ] .

One development that exploits the storage of data on the computer base forsubsequent processing can be optimised for a particular purpose, such asresolution enhancement using the Lorentz-Gaussian transformation technique,in which the free induction decay data is multiplied by the product of a Lorent-zian and a Gaussian weighting function prior to the Fourier transformation [ 2 3 ] .Similarly, the computer base has been used for some time to control measure-ments within the time dom ain and to provide values for such parameters as T 1 ,the spin-lattice relaxation time, which is sensitive to the motions of the chains,such as those of polysulp hones, whose dynam ic response is dispersed on opp ositesides of the Larmor frequency when made from 1-olefins and 2-olefins [24]. Thenuclear Overhauser enhancement (N O E effect) is also sensitive to the mo tions ofthe polym er chains, and g oo d practice, when careful quantitative m easurementsof 1 3 C signals are required, is to use instrument settings that eliminate the NOE[25] , so preventing it from enhancing the signals of certain carbons relative tothose of others.

1.2 BRANCHED MOLECULES: POLYETHYLENEAND A POLYESTER SYSTEMWe choose to start our discussion of the 1 3C chemical shift effects in macro-m olecules w ith a mention of the substitution parameter schemes such as those ofGrant and Paul [2 6 ], which were introduced into polymer spectroscopy byBovey at an earlier conference in the series [ I ] . The rule that a carbon's chem icalshift increases by a fairly constant increment when a covalently attached hydro-gen atom is replaced by a methyl group, the alpha effect, has proved of value w henspectral a ssignments are mad e. Similar parameters associated w ith substitutionat progressively more remote sites, the beta, gamma and even delta effects, havebeen e stablished and found to diminish in magnitude (alpha = 11 to 2.5 ppm ,beta = 9 to 7pp m , ga m m a= 2.5ppm, delta = O to 0.5ppm). Although quiteprecise values are often given [2 , 3 ] , the values of these parameters are sensitive tothe exact structure of the site of supposed structural change , and the best practiceutilises mod el co m pou nds clo se to the target structures, as in R andal's studies onthe side chains of polyethylene [ 27 -29 ] . A development of this substitutionapproach, which is appropriate to m olecules containing heteroatom s, is to studythe effect on chemical shifts of replacing a C H 2 group with another atom orgroup. This has been used to predict shifts in m olecules and polymers containing



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Figure 1.1 100 MHz 13C NMR spectrum of a high density polyethylene sample insolution at 125 0C. The spectrum shows peaks from end groups (E) and methyl, ethyl andbutyl side chains. The sample had been irradiated at 423 K with 300 KGy of gamma rays[32] and shows minor features near 29 ,32 and 41 ppm from the H structures thus formed O , N H and S O 2 groups, the electronegativity of these groupscausing in general a down-field effect. Thus, the shifts of polymers containingheteroatoms may also be predicted from first principles, for assignm ent purposes,if the shift of the corresponding hydrocarbon is known [30].

For the high density polyethylene spectrum of Figure 1.1, the main feature isthe intense signal at 30 ppm from the lon g runs of methy lene un its. The shifts ofthe end groups (marked E 1 , E 2 , E 3 as we move inwards from the methyl signal)are the next feature, but a num ber of resonances from side chains are present. Themethyl group of a butyl side chain coincides w ith E 1 , but the second methylenegroup, E 2 , is distinguished at % 23.4 ppm . The m ethyl groups of a small prop or-tion of ethyl side chains (Et x) and methyl side chains (M e 1) are also seen at 20 and11 ppm respectively. The main chain carbons at the root of and next to thebranches are also seen, the assignments for those next to the butyl unit beingshow n in the first part of Scheme 1. M ethyl and ethyl side chains are probablyderived from traces of propene and but-1-ene within the ethylene feedstock. T hefeatures from these are clear, but are in very small prop ortion s com pared with theend group signals for this linear polyethy lene.Copyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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- C H 2 - C H 2 - C H - C H 2 - C H 2 -C H 2 - C H 2 - C H 2 - C H 3

Bu3 Bu2 BujButyl side chains to polyethylene C H 2 - C H 2 - C H - C H 2 - C H 2 - C H 2 - C H 2 - C H - C H 2 - C H 2 -- C H 2 - C H 2 - C H - C H 2 - C H 2 - C H 2 - C H 2 - C H 2 -

H crosslinks Y links / long branches (1)Scheme 1 Elementary structure in polyethylenes.

Application to the field of low density polyethylenes w as prom pted by the needto understand the high proportion of carbons in the form of methyl groups(perhaps as much as 8%), an early result from the IR spectra. The studies led tothe recognition of an elaborate branched structure, for the p rodu ction of whichthe mechanism of Roedel, backbiting by the propagating rad ical, was introduced.The normal process produces butyl side chains as a result of a cyclic transitionstate of five carbons-I-one hydrogen for the intramolecular hydrogen atomabstraction. Ethyl side chains (Et) may have formed by two consecutive backbit-ings. Randal has characterised low density polyethylene and related copolym ersby carbon-13 NM R spectroscopy: complex dendritic structures are revealed bythe analysis [30]. Long side chains form also by intermolecular abstractions ofhydrogen atomschain transfer to polymer. A study of linear low densitypolymers, the side chains of which, as they derive from a 1-olefin component ofknown structure and occurrence, are well-defined, allowed the derivation ofsubstitution parameters appropriate to the polyethylene problem itself, gavemuch security to this approach [ 2 7 , 2 8 ] , and so led to the full assignm ent of themethylene carbon shifts dispersed on each side of the m ain signal at 30 .0 ppmfrom the long runs of methylene groups. More assignments subtle were alsofound, such as a distinction between the methyl groups at the end of butyl sidechains (14.21 ppm) and those at the ends of lon ger chains (14.01 ppm) [2 9 ] .Besides the use of substitution parameters, assignments were also made usingspecial spectrometer settings: A PT (attached proton test) and D E P T techniquesallow the direct recognition of quaternary carb ons, of m ethylenes and of m ethylsand methines together [ 30 ]. A coherent view of the com plex dendritic structure offree radically-produced low density polyethylene is now available. The usualmicrostructural features of high density p olye thy lene , alkyl side chains , have alsobeen observed in ultra high m olecular weight po lyethy lene, but in much smallerproportions [3 1] .



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A related study has been the elucidation of the crosslink structures inducedwithin polyethylene by high energy radiation. The secondary carbon radicalsthus produced by CH bond scission may diffuse by hydrogen atom abstrac-tion. They have been show n to combine in pairs to form H type junctions, and tocreate Y type junctions by reactions with thevinyl end groups of the chains andwith primary carbon radicals produced by main chain scission. In each case theshifts characteristic of the new structure were identified [32]. The shifts of theH junctions are distinct, being 41.1, 31.9 and 28.7 ppm respectively at the (CH)junction and the first andsecond linked carbons, as is shown in Scheme 1, but theshifts of the Y junctions coincide with those at the roots of long branches, andtheir formation is recognised only when a careful comparison has been made ofthe areas of these shifts before and after irradiation.

In a similar area, that of the characterisation of branched and networkpolyesters from difunctional acids and tri- or tetra-functional alcohols, in systemsthat were first used about 150years agowhen there was no understanding of theirpolymeric nature, our studies have found a similar sensitivity in the NMRspectrum [33]within the55-75 ppmregion, where thecarbons ofthe alcohol andester functions are found; seeScheme 2. The shifts of the carbons of glycerol [33]or erythritol [34] during the progressive conversion of alcohol functions to estergroups by a reaction with succinic anhydride change after each step by a few ppmin a manner that is readily recognised, for the sequence in time and symmetry ofsubstitution of the molecules that form reflects the greater reactivity of theprimary alcohol sites. Thus, replacing the O H g r o u p of an alcohol with anOHiI CH2-CH-CH2-O-H II C H 2 - C H - C H 2 - O - H

O - S A - O H O-H O - S A - O HOH OH

III CH 2 - C H - C H 2 IV C H 2 - C H - C H 2 - O - S A - O HOSA-OH O SA-OH O SA-OH

OSA-OHV C H 2 - C H - C H 2 - O - S A - O Hl

O S A - O HScheme 2 Primary oligomers of glycerol andsuccinic acidCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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O su cc inate has at the first, second and third carbons alpha, beta and gamm aeffects of respectively +2.6, 3.1 and 0.4 ppm [3 3 ] . (These alpha, beta andgamma parameters correspond to the beta, gamma and delta parameters ofGrant and Paul [ 2 6 ] because of the intervening oxygen atom.) If the second site ofthe succinic acid residue subsequently forms an ester, the shifts of the previouslylinked glycerol residue appear in slightly different places. Thus, a glycerol residuelinked 1, 3 within a chain has different shifts from one linked 1, 3 at the end ofa chain and from the oligomeric 1,3-discuccinate (III). We have introduced theterm III" for such a chain-extending unit and III' for a unit at the end of a branch,the number of primes indicating how many of the second, and m ore remote, acidgroups have reacted. The shifts of the glyceryl residues of the oligomers of Schem e2 thus provide g ood guides to the shifts of glyceryl residues at branch points (V),in chain extenders (III and IV) and at chain ends (I and II) in the highly branchedor fractal polymers that may be made, thus allowing the assignments of Figure 1.2.The trisuccinate oligom er V can be readily obtained in pure form [33 ], unlikethe other oligomers. It may be polymerised in a single process by heating ina vacuum, where succinic acid is first lost as the anhydride to the vapour phase,and the vacated alcohol site (in a III or IV type residue, for which evidence ispresent in Figure 1.3) then forms an ester with an acid group of another oligom er.The consequence of this development of linkages is seen in the shifts of eachcarbon of the glycerol residue, where extra fine structure develops as the m oleculeevo lves towards a dendritic or fractal structure. The initial molecu le is a heptamer(XV of Scheme 3 [33]), but others emerge. The shifts are sensitive not only towhether the link at the rem ote site has formed an ester, but a lso (in the case of thecentral carbon) to whether that site was a primary or a secondary alcohol. Theshifts of the network node are sensitive to the structure of the immediately

Figure 1.2 13C NM R spectrum at 126 MHz of the mixture of oligomers formed by thereaction of glycerol with succinic anhydride [33]. Only the region of the glycerol residueshifts is shown. The oligomers are identified in Scheme 2; G refers to glycerolCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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Figure 1.3 13C NMR spectrum at 126MHz of the mixture of oligomers formed byheating oligomer V in a vacuum at 1800C. Parts (a) and (c) for 40 min, part (b) for 20 min,part (d) for 60 min. The labels refer to Scheme 3. The region of the glycerol residue shifts isshown in (a), and for the higher resolution plots (b-d) only the signal from the centralmethine carbon. The resolution of the latter parts was obtained with zero line broadening.Reproduced with permission from [3 3]

adjacent nodes when the link is succinic acid, but not if glutaric acid is used, forthe extra methylene g roup renders the linkage too remote. In Figure 1.3 the shiftsat three early stages may be seen, as the molecules evolve towards a polymericform of III: peaks z a nd y 3 we assign to the shifts of the primary and secondaryglycerol carbons when the primary carbon is linked to another glycerol residue;peaks yx and y 2 come from a secondary carbon of a glycerol which is linkedthrough a succinic acid residue to respectively a primary and a secondary site ofa glycerol residue, as sho w n in Schem e 3. These distinctions in the fine structureare relatively m inor, are best observed with a high field system [3 3 ] , and assist inthe development of the chemistry of the formation of fractal polyesters. Novelliquid crystalline forms, for exam ple, have been produced using such means, the



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XV[ V '- S A -VT3 y3 z yi 1H O S A - O C H 2 - C H - C H 2 - O S A - O C H - ( C H 2 - O - S A - O H ) 2

OSA-OH3 y3 zXVI H - O - S A - O - C H 2 - C H - C H 2 - O - S A[ V - S A - V " - S A -V l I IHOSA-O O

Y2 C H - C H 2 - O - S A - O HzCH 2

OH - O - S A - O - C H 2 - C H - C H 2 - O - S A

HOSA-Oxvn[ V 1 - S A - V ] 3 k c* 2 1H O S A - O C H 2 - C H - C H 2 - O S A - O C H - ( C H 2 - O - S A - O H ) 2O H

Scheme 3 Some higher oligomers of glycerol and succinic acid; the numbers are those ofref. [33]mesogenic units being present as pendent groups demonstrably in full comple-ment upon what was a poly(erythntolfractal glutarate [ O H ] 2 backbone [3 4] .

1 3 THE MICROSTRUCTURE OF LINEAR CH AINSThe first microstructural issue of linear hom opolym er chains that w e examine istacticity, which w e illustrate with spectra from tw o system s from our ow n work:the poly(alkyl cyanoacrylates) [ - C H 2 - C ( C N ) C O O R - ] , which constitutea vinylidene system the spectra of which are shown in Figure 1.4, and thepolyalkene sulphides and sulphones: [ C H 2 C H R S ] a n d [CH2C H R S O 2 ] , spectra of which are shown in Figure 1.5. We show meso orm dyad structures of tw o of these polymers in Scheme 4. N ote how the two chiralcentres of the first polymer appear to be equivalent, but for the second polym erthe equivalence is less immediately evident, for the residues contain three bon ds



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Figure 1.4 NMR spectra of poly(ethyl cyanoacrylate) samples. Part (a) has the mainchain methyleneproton signals at 400 MHz ofsamples prepared in acetone with sparteineas initiator (A2) and in THF with cinchonidine as initiator (A5). Part (b) shows the 13Cspectrum of the side chain methylene carbons of the samples A2 and A5, with triad andpentad assignments [38](a)CNH CN H C H 2 - CH 3I I I I I

CCC - S O 2 - C H 2 - C - S O 2 - C H 2 - C - S O 2 -COH CO CH 2 -CH 3 HI (b) IOEf ; OEtScheme 4 Mesostructures of poly(ethyl cyanoacrylate) and poly(but-l-ene sulphone).The projections have the backbones in a planar ziz-zag, and show the chain from aboveand in successive residues a particular atom is in turn in the "up" and the "down"position. Triad, tetrad, pentad and longer sequences may be obtained by thesuccessive inclusion of extra residues and may be recognised by NMR. Thestereochemical structure of these longer sequences are described in terms ofthe m or r relationships of the successive pairs of chiral centres [2,3]. In the case



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Figure 1.5 13C NMR spectra of the backbone methylene carbons of (a) a tactic poly(but-l-esulphone) made from it by oxidation, and (c) of an atactic polysulphone. The dispersion of shiftof the gamma-gfaucfie effect of the oxygens. The small peak at 6 = 49.2 ppm is from H H se

ppm ppm

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of the first polymer the residues, as they have just two backbone carbons, aresensitive to influences equally from each direction along the chain, and m r and rmheterotactic sequences are identical as far as the signals from carbons at orpendent to the central chiral centre are concerned. At high resolution theinfluence of the next two chiral centres may be expressed, so we may be able todistinguish the rmrr and mmrr pentads. For the polysulphides and polysul-phones the residues have three com ponen ts, so that the influence upon chem icalshifts of on e residue that derives from the chiral centres of the tw o neighbouringresidues is diflferent, and depends upon the direction: thus, an mr sequence willnot for symm etry reasons have the sam e shifts as an rm sequence. (The m echan-ism that generates shift multiplicity depends upon fine differences in bondrotation populations for different chiral sequences that are coupled to thegamm a-^auc/ie interactions, as Tonelli describes elsewhere [ 8 ]) . As in the relatedolefin ox ide and styrene oxide polymers [34 ,35] , the residues of the polysulph idepredominantly orientate in only one d irection, so that head to head junctions arealso encountered, and provide minor features in the spectrum, as we indicate inFigure 1.5. This type of enchainment has been termed positional isomerism,orienticity [ 3 ] or regioselectivity, the last term being used be low for ring-openingmetathesis polymerisation (ROMP) systems. Another consequence of the pres-ence of three distinct groups in each residue of the linear backbone is thepossibility of optical activity, a property that independently permits recognitionof isotacticity [37].

We first discuss the spectra of poly(ethyl cyanoacrylate), proto n spectra beingshown in Figure 1.4(a) and the corresponding 1 3C spectra in Figure 1.4(b) [ 3 8 ] .We use the classical route, first used by Bovey and Tiers for poly(methyl-methacrylate) [ 2 , 7 ] , PM M A , for determining the type of tacticity that predom i-nates. They recognised the four-line pattern of an AB quartet in the 60 MHzspectrum of a predominantly isotactic polymer in the signal from the main chainmethylene protons within a meso dyadthis was distinctly different from thesingle line from the methylene protons of a racemic dyad that was found ina polym er produced by a different mechanism (the absence of an effect from thecoupling constant deriving from the equivalence of the two protons). For ourassignment two polymers were available, poly(ethyl cyanoacrylate)s that hadbeen made in different so lvents and w ith different chiral initiators for the anionicpolymerization process (it transpired that the solvent was the important factor).In contrast to the case with PMMA, an AB quartet was not immediatelyapparent in the proton N M R spectrum, and a pair of clear lines (a and b in Figure1.4(a)) considered for part of such a system was found to be unsuitable: thesplitting between the lines was not 14 Hz (the value of a geminal cou pling) norwere there signals nearby at that splitting. Moreover, their relative intensitieschanged in a simple manner with the value of the tacticity parameter deducedfrom the 1 3 C NMR spectrum. They were thus assigned to rrr and rrm finestructure, and these assignments were confirmed by checking their relative



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intensities with values predicted with the aid of a single (Bernoullian) tacticityparameter obtained from the side chain methylene carbon spectrum. D iscrepan-cies betw een the B ernoullian and the experimental intensities were of the order of2% within both proton and carbon spectra. The direct recognition of an ABquartet in spectra such as those of Figure 1.4(a)was prevented by partial overlapof m dyad signals dispersed by tetrad effects and a coinc idence with the remainingr-centred tetrad, as a two-dimensional experiment has subsequently m ade clear[39] . The m ain com ponen ts of the AB structure lie near 2.6 and 2.8 ppm . In thecarbon spectrum pentad effects were resolved within the rr-centred triad of theside chain methylene carbons (Figure 1.4(b)). The two peaks of the mr-centredtriad may be assigned as indicated in the figure to mrmm and (mrmr + rrmm)sequences, of expected relative intensities of 0.100 and 0.096 respectively of A 2;the remaining sequence rrmr, of Bernoullian intensity 0.02, is apparently notresolved in the signal. This set of pentads may be more readily recognised on thebasis of more clearly different line intensities in the spectrum of A 5. They and theother peaks were assigned, once the chains were recognised as being predomi-nantly isotactic, on the manner in which their intensities varied with the va lue ofP1-, a practice which is widely adopted when samples of different tacticities areavailable.

In the case of polyacrylonitrile [ C H 2 C H (C N ) ], which gives an atacticpolymer when the free radical reaction is performed in solution, enhancement ofthe tacticity to P f values as high as 0.70-0.87 has been provided by performinga po lymerisation when the monom ers were constrained, or lined up, within a ureacanal complex. This allows the development within the 1 3 C NMR spectrum ofintense peaks from certain heptads [12 ], the emphasis providing clear indica tionof the origin of the signals from sequences of high isotactic content. The finestructure of the 1 3 C NMR signals from the methyl groups of polypropylenedisplays pentad and partial heptad fine structure, for the assignment of whicha number of methods were adopted, depending mainly upon the availability ofpolymers of known tacticity, as their crystal structures had previously beendetermined, but also using 13C-labelled model compounds of known stereosequence content [40]. Highly isotactic polystyrene has been produced usinga titanium trichloride-derived catalyst [ 4 1 ]. Once such a material is available thespectra may give an insight into the manner in which the process behaves:a catalyst for isotactic polypropylene som etimes allows errors in stereochemistry,but these are immediately corrected, as the presence of mrrm but not mrmmpentads testifies [ 2 ] . Such interesting evidence on the manner in which a catalystfunctions helps us to understand the mechanism; we conclude this review w ith anaccount of such effects discovered in our studies of ring-opening metathesispolymerisation, or ROMP, which likewise use metal-centred catalysts.The Bernoullian nature of the free radical or ionic propagation in a polymermay be ascertained from the relative intensities of the rr, mr -I- rm, and mmcomponents of the triad fine structure, as in our studies of the side chain



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methylene group in poly(ethyl cyanoacrylate)s. Provided that each new chiralcentre forms in a manner that depends on ly u pon the type of the previous chiralcentre, so that only o ne statistical parameter is invo lved for dyad occurrences, theweights of the triads are respectively [ 2 ,3 ] (1 - F 1)2 ,2P 1(I - P1) and (P,)2 . Usingin turn (from left to right) the first tw o areas, the secon d tw o, and then the first andthird of each part of Figure 1.4(b), we so lve for P 1 to obtain 0.63,0.7 2 and 0.68 forsample A2, values which are hardly significantly different from each other; andfor sample A5 we have correspondingly 0.52, 0.60 and 0.56, which are close. Atest for Markov behaviour is provided by the relationships involving two para-meters [ 2, 3 ]:

JV/m) = = (nn)/(2(m)) = (mr)/(2(mm) + (mr))and

P (m/r) = w = (rm)/(2(r)) = (mr)/(2(rr) + (mr)),where P (r /m) is the probability that an r dyad will follow an m dyad. Markovbehaviour has u + w < 1.00. For the cyanoacrylate spectra of Figure 1.4(b) thevalues of u and w are respectively 0.28 and 0.66 for polymer A2 and 0.46 and 0.54for polymer A5, indicating that both polymerisations are close to Bernoullian.Sample A 2, which deviates more from the ideal was made using as initiatorsparteine. As this comp oun d is a dinitrogen base, it may enh ance the formation ofa complex between the oppositely charged initiator and the propagating ends ofthe chain in a zwitterion. A clear case of M arkov behaviour is given below . Thestatistical index P = 4IS/H2 = (4(mm)(rr)/(mr)2] has been used to characterisethe isotactic acrylonitrile polymers prepared within the canal complexes [12].Tw o distinct mechanisms were identified from the dependen ce of this index uponthe isotactic content, a much stronger dependence being found for the polymersproduced at low temperatures after irradiation than for those produced duringirradiation at a moderately low temperature, for which canal coherence mighthave been upset by the evolution of heat and the irradiation itself.

The second aspect of linear polymers from our ow n field may be considered asa who le, for polysulph ones m ay be obtained by ox idation of polysulph ides as wellas by the free radical copolymerisation of SO 2 with an olefin. Indeed, thischemical change is beneficial to the spectroscopy, for fine structure develops asa result of oxidation, as may be seen in Figure 1.5, where the shifts, each at500 MH z, of the methylene carbo ns of an isotactic polysulph ide and the polysul-phone prepared from it are displayed in parts (a) and (b) respectively. As discussedelsewhere [2, 5, 8], fine structure may be the consequence of gamma-grawc/ieinteractions weighted according to the occupancy of the intervening bondconformational states. In this case the fine structure undoubtedly developsa larger dispersion and becom es more sensitive to the stereochemistry because wehave introduced oxygens gamma with respect to each main chain carbon; suchoxygens may cause a shift effect as large as 9.4 ppm , the particular value



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depending upon the conformation adopted by the intervening CS bond[ 30 ,42 ] .Poly(l-olefin sulphone)s have been found to be atactic when made from themonom ers by the free radical reaction; wh en first observed the back bone carbons

show ed incipient or clear triad fine structure [ 4 1 , 4 2 ] . The first carbon of the sidechain displays dyad stereochemical sensitivity at low resolution, the upfield halfof the signal being assigned to an m d yad when a com parison was m ade with anisotactic poly(propylene sulphone) made by oxidising an isotactic polysulphide[41] . The poly(but-l-ene sulphone)s prepared by free radical means showedsimilar spectra of the main chain methine when examined at high field (Figure1.5(c)), showing clearly mm, mr + rm, and rr triads, as labelled by comparisonwith the other spectrum, that of the optically active polymer prepared froma polysulphide. The test on the Markov nature finds M = 0.51 ( 0 .0 1) andw = 0.480 (0 .00 5), giving u + w = 0 .99 ( 0 .0 1 ), so the free radical polymerisa-tion process was clearly Bernoullian. For the polymer prepared by oxidation ofthe polysulphide the parameters are w = 0.25 ( 0 .0 1) and w = 0.51 (0 .01 ),giving M + W = 0.76 ( 0 .0 2) and indicating the Mark ov nature of the polymerisa-tion process the polysulphide precursor had experienced. (From the spectrum ofthe polysulphide itself we were able to obtain only one parameter, P 1 = 0.66,a number very close to w/(u + w) = 0.67, as expected.) It may well be that thepolysulphide formation was not Bernoullian, for the catalyst used was anoptically active zinc-centred species that favoured the R enantiomer of thesulphide, and the monom er itself contained an excess of the S enantiomer [4 4 ] .A second feature in the spectrum reflecting the polysu lphide formation mechan-ism is the presence of three minor features near 49.2 ppm in Figure 1.5(b) that weassociate with head to head structures. During propag ation, the sulphide anion atthe end of the chain may occasiona lly attack the meth ine carbon site as well as themethylene carbon site in the mon om er, and this remains when the polysulphoneis prepared.

W e note that the heterotactic triads signal of Figure 1.5(c) has more than threecomponents, consistent with the mr and the rm heterotactic sequences beingdistinguishable; as the relative intensities of the four not quite resolved lines forthe atactic polymer of Figure 1.5(c) are roughly in the propo rtion of 1:2:3:2, andthe four heterotactic-centred sequences m mrm , mmrr, rrmm and rrmr wou ld beexpected to have similar proportions (as P r = P1n) = 0.5), one of these pentadsmust be sensitive to an extra chiral centre. Our m ost recent work in this area hasshown that tactic main chains may be obtained in a free radical reaction if the1-olefin bears a chiral centre of a particular type (K 6r S) at the site next to theolefin group: the carbon N M R spectrum then d isplays from each atom within orclose to the backbone widely spaced pairs of peaks, the relative intensity w ithineach pair being 6:4 or 7:3 . This reflects within a residue a preferred relationsh ip ofthe two chiral centres [45], the one initially present within the olefin and thesecond created by the addition reaction.



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1.4 THE PARTICIPATION OF A CHARGE-TRANSFERCOMPLEX IN A FREE RADICAL POLYMERIZATIONREACTIONA long-standing issue in the formation of alternating copolymers, such as arefound when electron-rich and electron-deficient monomers polymerise by freeradical means, has been the question of the role of the charge-transfer com plex inthe polymerisation mechanism. For poly(olefin sulphone) feeds, many experi-mental techniques have demonstrated that the complex is present, but is thecomplex incidental or is it the reacting species? One p ossibility is that each type ofradical may react only with the other type of monomer; a second is thatthecharge-transfer adduct itself is the only reacting species [ 4 6 ] . In Schem e 5belowthese two possibilities areshown respectively as thevertical (c +d) and thehorizontal (a) propagation reaction paths. The rate-determining step for poly-merisation is apparently the reaction of an electron-deficient radical, presumablya sulphonyl radical, with an electron-rich m onom er, presumably either an olefin(d) or the olefin part of a charge-transfer com plex (a), for substitu tion to the olefingroup enhances the rate. AU the reactions are written as reversible in Scheme 5:there is a wealth ofexperimental evidence in support of this, for example, theolefins are known to isomerise at temperatures above and below the ceilingtemperature for polymer formation, and the ESR spectrum of the radicals presentindicates that this may be both C-based and S-based.

P - S O 2 - C - C *SO 2 J c S o 2

P-SO/ + C=C - =- P-SO2-C-C-SO*e dC=C

P-SO2-C-C"Scheme 5 The free radical formation of poly(but-2-ene sulphone) through charge-transfer complex reaction (horizontal route) or successive monomer addition (descendingroute)

If the precise alternation in the chain residues is the only criterion, there is noway of distinguishing between the two mechanisms. How ever, the stereochemis-try of the but-2-ene sulphone residues and their relationship to the cis or transnature of the olefin do es provide a guide [43 ,46] . Broadly speaking, two methylshifts are encountered: at high temperatures, whichever olefin is used, there isa single shift at 9 ppm , but at low temperatures, if the trans but not the cis olefinCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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Figure 1.6 13C NMR spectra at 101 MHz of the methyl groups ofs/B three samples ofpoly(b700C. The samples SCH/7, U27 and U23 were prepared at - 950C, - 63 0C and - 840C respefirst two from the trans olefin. Lowering the temperature has increased the intensity of the signfrom the trans olefin, but the signal from the polymer made from the cis olefin at an intermproportion of the racemic residues, with their methyl shift at 9 ppm [46]



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is used, there is a new peak at 13ppm; see Figure 1.6 for three examples. Theassignment of the order of the methyl carbon shifts to meso or to racemicbut-2-ene residues is not straightforward; since a gamma-gauche effect from theoxygen atom of the su lphone group (9.4 ppm) may well be larger than thegamma-gauche effect from a m ethyl group (6.4 ppm), the shift distinction may beassociated with the conform ations of the C S bonds, rather than with that of theC C bond as we first assumed [4 2 ]. W e now m ake the assignment of the mesoand racem ic structures on the basis of the similarity of the order of the shifts in thepolymers to models of known structure. The molecule alpha-2,3-bis(isopropyl-sulphonyl) butane has the structure shown in Figure 1.7(a), according to X-raymeasurements, m aking it the centrosymmetric meso form [4 6] . The central unitcorresponds exactly to a residue of a poly(but-2-ene sulphone) chain that isflanked by structures corresponding to a little over half the alkane component ofthe next residue. The carbon shift of the central m ethyls is at 13.7 ppm , comparedwith 10.0 ppm for the corresponding shift in the racemic molecule, shift differen-ces that are found in the polym ers, too . (The IR spectra show similar correspon-dences [46]). The fact that at low temperatures the trans olefin converts topolymer with partial retention of the configuration of the two prochiral centres

Figure 1.7 (a) The model bis(isopropylsulphonyl) butane in the crystal [ 4 6 ] , show ingits centrosymm etry and meso characteristic of the central portion; (b) plots of meso residuecontent against temperature of preparation for the series of poly(but-2-ene sulphone)sprepared from cis and trans olefin, curves (i) and (ii) respectively. That there are twodistinct curves indicates that the charge-transfer complex is a significant reacting species.The solid s ym bo ls record the results from the spectra of Figure 1.6

T /C



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on the olefin-C T comp lex and that the cis olefin converts similarly suggests thatthe reaction does proceed along path a of Scheme 5 at these low temperatures,when large proportions of the charge-transfer com plex are present. At the highertemperatures the polymer and the monomer structures are not related, bothyielding mainly racemic residues, consistent with alkyl radicals being presentlong enough during the polymerisation for radical inversions to eliminate thememory of the initial structure. Ch ain m icrostructure therefore indicates that thecomplex is a reacting species at low temperatures. We cannot tell whether thecomplex exclusively reacts, and the sulphonyl radical partly dissociates (path b),or whether paths a and c are alternatives, a being becoming favoured as thetemperature is lowered, to some extent reflecting the greater stability of thecharge-transfer com plex. T he rise in meso content when cis olefin is the precursorprobably indicates that path c is used even at low temperatures, and that then theradical intermediate favours less a m ode of reaction that y ields the racemic type ofproduct.

1.5 THE POLYMERISATION OF DIEN ESThe manner in which dienes become entrained within polymer chains dependsupon a number of factors, such as the type of mechanism (free radical, ionic orcoordination), the nature of the diene itself, and whether other monomers areinvolved. If one double bond reacts, a chiral centre is formed and the p olymersmay be tactic, if 1,4-addition (or 4,1-addition) takes place the main chainincorporates a double bond whose cis or trans nature may be important indetermining properties such as the glass transition temperature, and the reactionof a second double bond can cause crosslinks. The case of polychloroprene hasbeen described by Ebdon [4 7 ] , where proton shifts are sufficient to detect head totail (2.35 ppm ), head to head (2.5 ppm ) and tail to tail (2.2 ppm ) enchainm ents ofthis unsymm etrical mon om er [4 7 ]. For poly(butadiene)s, sequence triads involv-ing three different types of residue cis and trans 1,4-residues within the mainchain and 1,2-residues involving pendent vinyl groupsmay be distinguishedeven with a 270 M H z spectrometer in the region of the spectrum between 127 and133 ppm , where are found the reson ances of the 1,4-residues (see Tab le 1.1 andFigure 1.8). The assignm ents were obtained using a number of polym ers ofdistinctly different but recognisable microstructure. When the spectrum is ob-tained under conditions that avoid NOE enhancement of signal intensity, andlong delays between pulses reduce systematic errors in signal pro portions, fromthis region and that of the pendent vinyl group s (at 114 and 143 ppm) co m posi-tions accurate to better than 1% may be claimed [25]. In this study of poly-butadiene rubbers, when three different methods were compared, it was foundthat the microstructures as determined by the Raman and 1 3 C NMR methods



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Table 1.1 Triad sequences within the main chain olefinic region of thecarbon-13 NM R spectrum of poly(butadiene)s [25]. Reproduced from[25] with kind permission from Elsevier Science Ltd., The Boulevard,Langford Lane, Kidiington OX5 IGB, UKCarbon Atom Peak No. Triad assignment Shift (ppm)- C = C * - I vtv 13L82 ctv, ttv 131.43 w e 130.74 ctv, ttv 130.65 ccv, tcv 130.26 ctc,ctt 130.27 vcc, vet 130.18 ttc, ttt 130.1 *C = C 9 ctv, ttv 129.910 ccc, tec 129.711 cct, tct 129.512 ctv, ttv 129.313 vtc 128.514 vtt 128.415 vtc 128.216 vcc 128.117 vet 127.9

18 vcv 127.8*Note: v = vinyl, c =*cis, t = trans.

were in good agreement but that the IR m ethod w as much less consistent [ 2 5 ] , aspeaks were not very distinct and extinction coefficients were too variable.W e illustrate the reactions of dienes by our stud ies on furans a s m on om ers infree radical copolymerisations with acrylonitrile (AN), work undertaken to

deve lop the polymer chemistry of materials that may be ob tained from renew ableresources. We have found that a variety of structures may be entrained withina polyacrylonitrile chain; to some extent their proportions depend upon thepresence and the nature of substituents at the position alpha to the furan ring[ 4 8 - 5 0 ] . Only furan, the least aromatic of the heterocycles, seem s to behave inthis way. The five-membered furan ring remains intact. The differentiation ofstructures of types I and II was performed on the basis of the shifts of modelcom pounds obtained by reacting furan and methylfuran w ith the 2-cyan opropylradicals from decomposing 2,2'-azobis(isobutyronitrile), AIBN. The carbonshifts of the polymer residues were consistent with attack at the alpha or C 2position of furan and at the C 5 position of methylfuran by the acrylonitrileradical. The furan radical that forms then propagates in the manner of a dieneeither through the more remote alpha position or through the adjacent betaposition. A minor proportion of I residues from methylfuran in which thepolymer AN radical had attached to the C 2 were also detected from theappearance of minor shifts at 130 ppm (see Figure 1.9) from the beta carbon s,



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Figure 1.8 1 3C NMR spectrum of an anionically prepared polybutadiene at 25 0 C inC D C l3 at 60 M Hz. The labels correspond to the peak num bers and triad sequen ces ofTable 1.1. In this study [2 5] extreme care was taken in ob taining quantitative inform ation:avoidance of the nuclear overhauser enhancement was achieved by decoupling onlyduring the signal acquisition; pulse angle 90, 4 0 0 0 0 scans, 33 s pulse delay. R eproducedfrom [25] with kind permission from Elsevier Science Ltd., The Boulevard, LangfordLane, Kidlington OX5 IGB, UK

shifts that reflect the different arrangement in this residue of the methyl andnearest nitrile groups. The appearance at this place of the olef inic shifts is readilyration alised in terms of a beta effect of 3.7 p pm an d a ga m m a effect from th e nitrilegrou p o f 5.5 pp m .

Othe r peaks were found in bo th proton a nd 13C spectra in the region be low th eshif ts from the acrylonitrile res idues , and o ther p oss ib le s tructures we re sou gh t ,

i H in iv v viScheme 6 The structures of five residues derived from furan in acrylon itrile (AN)copoiymers, and the methylfuran radical



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Figure 1.9 13C NMR spectra of the low field region of (a)a dimethylfuran copolymer, (b)a methylfuran copolymer, and (c) a furan copolymer. Assignments of the nitrile carbon ofthe AN residues, of the olefinic carbons of the furan residues and of the bridgehead andother carbons next to an oxygen are indicatedbut a certain proof of a third type of structure w as more elusive. The characteristicfeature was a proton shift at 3.9 ppm [4 9 ] , a position appropriate to a pro tonon an ether carbon, and olefinic p rotons were though t to be lacking. We presenta relevant set of reactions in Scheme 7. It was eventually recognised [ 4 9 ] that theaddition of an excess of the furan monomer, which promoted II-AN-furansequences, had the effect of reducing the proportion of the unknown furanresidue, presumably by preventing the participation of the II structures ina second reaction (d) to give a structure of type III. On ce ~ I I - A N - A N ' radicalswere reduced in proportion by this means, the signals from the II structuresbecame clearly enhanced in the spectrum, as route (a) was then taken. This



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Scheme 7 Possible reactions of a II structure in a second manner during the acrylonitrilecopolymerisation [49]revealed the origin of the previously obscure third structure. A proof of theentrainment of AN -furan D iels- A lde r products was made by observation of theshifts of the residues formed by a direct copolymerisation involving an endoadduct of furan and a m ixture of endo and exo adducts: the carbon and the pro tonspectra together indicated that both adducts can become entrained within anacrylonitrile chain [48] to yield a structure of type IV, with carbon shifts at80ppm from the bridgehead sites and corresponding proton shifts at about4.7 ppm.

A careful inspection of the region near 130 ppm in the spectrum of eachpolymer (Figure 1.9) reveals that each carbon of the I residues has two shifts,a feature that we attributed to the influence of the chirality of the nearest C H C N chiral centre. N o feature that we could associate with the cis or transjunctions to the ring were identified, although for the I residues, if not the IIresidues, the structural variation seemed possible. Inspection of the spectra ata h igher field strength found a further set of peaks w hose intensities increased asthe furan content rose from 5 to 2 5% of the residues. This was attributed toa small sequence effect.

In an effort to clarify the sequence fine structure, both of the various furanresidues and of the acry lonitrile residue signals un-field, we added L ewis acids inthe hope of causing alternation of the residues by enhancing the electrondeficiency of the acrylonitrile radical through a coordination to the nitrile group .W hen the po lym erisation was performed in the presence of a mild Lewis acid such



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as ZnC l2, it was found instead that, although the yields were enhanced by anorder of magnitude, the furan proportion was increased only a little, but thepattern of the predominant II structures becam e modified considerably [5 0 ] . Forthe 2-methylfuran systems, the shift of the 5-proton was diminished relative tothe shifts of the other furan protons. The search for their new position in thespectrum, to provide structural evidence for the effect of the Lew is acid upon thereaction mode, was performed with deuterium NMR spectroscopy, the methyl-furan monom er having been deuterated at the single alpha position . In reactionsleaving a furan ring from radicals of structure V at the end of chains, thedeuteriums were found to have transferred from the furan radical to the Lewisacid-activated m onom er (creating D C H 2 C H C N ~ , shift at 1.5 ppm) and toacrylonitrile radicals (creating ~ C H 2 C H D C N , 2.2 ppm). This latter groupwas a lso identified at 14.3 ppm in the 1 3 C NMR spectrum, where it wasparticularly prominent if an independent source of hydrogen atoms, in the formof a chain-transfer agent, had been added [ 4 8 ] . Despite the transfer reaction andthe disproportionation promoted by the Lewis acids, processes which would beexpected to lower molecular weights, yields of the free radical reaction weregreatly enhanced and gels were produced, presumably through a crosslinkingsecond reaction of II residues, and the proton N M R signals conseq uently becam ebroader [50].

Isotopic enhancement may be also illustrated by B evington et al.'s exp lorationof the use of th e* 3C-enriched free radical initiators l,l'-azob is(pheny lethane) andAIBN in preparing butadiene polymers [51] and the use of dimethyl 2,2'-azobis(isobutyrate) to initiate the polymerisations of styrene, acrylonitrile,methyl methacrylate and m ethyl acrylate [ 5 2 ] . The signals from the ends are thusrendered more intense, and become observable in a standard 1 3 C NMR spec-trum, where they display information on the manner in which the initiatorradicals have attacked the first monom er t o b ecom e incorporated at the start ofthe polymer chain: one can thus compare initial and m ean tacticities. In a furtheruse of isotope enrichment, M oad and W illing found that sel ec tiv e 1 3C enrichmentof one monomer together with carbon-13-proton correlation NMR spectros-copy allowed the separation of tacticity and sequence effects; they used thisapproach for studying copolymers of butyl methacrylate with methyl metha-crylate [ 53 ].

1.6 RING-OPENING-METATHESISPOLYMERISATIONSPolymers formed by the ring-opening metathesis polymerisation (ROMP) reac-tion [54] exhibit a wide variety of microstructures which may be evaluated byspecctroscopic techniques. The first ROMP polymers were analysed by IRspectroscopy [55], but that can only determine the absolute stereochemistry of



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The ROMP reaction of Scheme 8 is catalysed by metallacarbenes [54] thathave been formed from a wide variety of transition metal salts, often but notexclusively in the presence of an organometallic co-catalyst in system s sim ilar tothe industrially important Ziegler-Natta catalysts. In addition, there are nowmany exam ples of metathesis of both cyclic and acyclic olefins using well-definedmetal carbene complexes [5 6 ]. In the former systems, which are considered here,the metallacarbene catalyst is formed from the various catalyst co m po nents andis very active, the concentration of active sites being extremely low but each sitehaving a very high turnover number [ 5 7 ] . As a result, observation of the w ork ingcatalyst by any spectroscopic or other means is not possible. We view thepolym ers, with their different microstructural features, as a "tape recording" ofevents at the catalyst site which may be "read" through the medium of 1 3 C N M Rspectroscopy. For highly strained monomers these events are the primary onesup to high conversion.

One may, by careful choice of monomer, study the potential of differentcatalysts to behave in a stereoselective or regioselective manner. Thus, witha symmetrical monomer such as norbornene [ 58 ], norbornadiene [5 9 ] or their5,6 [60] or 7-substituted derivatives [61,62] we have obtained polymers witha variety of cis main chain double bond contents and distributions. In a num berof the 7 -substituted exam ples, fine structure on certain 1 3 C NMR resonances isobserved which is attributable to tacticity effects. Conversely, one may use theunsymmetrical monomers such as 1-substituted derivatives [63] and delineatethe propensity of the different catalysts to regioselectivity, wh ich m anifests itselfas head-tail bias in the polymer.

the dou ble bon ds in the polmer, and provides no information on the sequ ences inwhich such microstructural variations might occur.This limitation is largely overcom e by 1 3C N M R spectroscopy, where sensitiv-ity to change in substitution and stereochemistry up to six carbon atom s rem ote

from the particular carbon under observation is regularly seen [54], In theremaining part of this article we deal a lmost exclusively with p olym ers formedfrom the bicyclic olefins norbornene, norbornadiene and their derivatives, butwill also discuss some work with oxygen-containing analogues, thus providinga comprehensive range of different microstructural types. These monomers havea substantial ring strain, so they are go od candidates for R O M P .

P * polymer chainScheme 8



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In the case of these substituted derivatives, the polymers formed with mostcatalyst systems exhibit fine structure in the spectra due to each of the possiblemicrostructural variations, leading to very complex spectra. We have made veryextensive use of chain transfer to acyclic olefin to obtain lower molecu lar weights,and consequent line narrowing in the spectra of the polymers, to optimiseresolution. Also, certain of the catalysts at our disposal may behave in a verystereospecific and regiospecific manner, allowing one to pinpoint certain lines inmore com plex spectra. These techniques, com bined with the excellent resolvingpower and sensitivity of modern high field NMR instruments, have allowedcomplete and unambiguous assignment of most spectra.

1 . 6 . 1 S T E R E O S E L E C T I V I T Y I N R O M PThere are two basic types of stereoselectivity observed in the ROMP of cyclicolefins, bo th of which m ay be observed in the 1 3 C N M R spectra of the polymers.The doub le bond s wh ich form part of the main chain may be either cis or trans,and in the case of the prochiral monomers norbornene, norbornadiene, theirsymmetrically substituted derivatives and their chiral unsymmetrically sub-stituted derivatives the residues may be enchained in such a way as to yieldtactic or, more commonly, atactic polymers [54]. A representation of atacticpoly(norbornene) is shown in Scheme 9, where cis and trans double bonds areassociated with r or m dyad units respectively.

Scheme 9Thus polymers w ith a given cis double bond content may be prepared with anappropriate catalyst, as is shown in Table 1.2. Resonances from the variousolefinic and cyclopentane ring carbon atom s are observed and fine structure due

to the effect of two or three neighbouring doub le bonds is resolved , Figure 1.10.One of the earliest observations to be made from these spectra was that therelative line intensities of the various cc, ct, tt and tc (etc.) resonances indicatedthat the distribution of cis and trans double bonds was non-random, and thatthere was an increasing tendency towards a blocky cis distribution as the cisdouble b ond content of the polymer increased [5 8 ]. An explanation was sugges-ted, based upon chain propagation involving different metallacarbenes whichhad been distinguished in terms of the stereochemistry of the last-formed doub leCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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Table 1.2 Fraction of cis double b onds in ring-opened polymers of norbornene, norbor-nadiene and derivatives obtained with different catalystsCatalyst

Monomer RuCl3 M o C l5 /B u 4Sn OsCl3 W C l6/M e4Sn ReCl5 Ref.

0 . 0 5 - 0.50 0.55 1.00 [54]

0 . 0 0 - 0.15 0.55 0.95 [61]

0 . 1 0 - - 0.55 0.95 [60]

0 . 3 7 0.90 0.51 0.82 [59]

0 . 2 0 0.97 0.42 - [62]

1.00 - 0.36 0.73 1.00 [66]

0 . 1 0 - 0.39 - 1.00 [66]

0 . 0 0 0.31 0.10 0.75 1.00 [63]

0 . 0 5 0.11 0.30 0.70 1.00 [65]

bonds. In essence this theory emphasised the importance of steric effects at thecatalyst site. Blocks of cis double bonds are obtained by propagation througha species P c (see Scheme 10) where the last-formed doub le bond is cis and wherethe next monomer unit reacts with the metallacarbene while the previouslyCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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Figure 1.10 * 3C NMR spectrum of poly(norbornene) with %60% cis, randomlyCopyright 1996 John Wiley & Sons Retrieved from: www


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Scheme 10formed cis double bond is still in the coord ination sphere of the m etal. The stericconstraint thus imposed aligns the incoming monomer unit in a cis orientation,leading to the formation of another cis double bond. The kinetically distinct P tspecies is believed to be too bulky sterically to be a chain carrier at all, and itrelaxes to a species P in which the last-formed double bond has left thecoord ination sphere of the metal; the mo nom er has then the opportunity to reactin either a cis or a trans orientation, with the trans orientation preferred on stericgrounds.

Th is phenom enon is also observed in the case of the stereospecific m etathesis ofacyclic olefins [68], where, in the pre-equilibrium stage of the reaction, cisprod ucts are often formed from cis substrates and trans from trans. Inspection ofTab le 1.2 shows that the cis content of polymers formed from bidentate chelatingolefins is significantly higher than that observed with the mono-olefin analogue.The highly stereospecific and rather unreactive RuC l3 catalyst exhibits extremebehaviour, as it is highly trans-directing with norbornene, and incidentally withmany other mono-olefin derivatives, but highly cis directing when using endo-dicyclopentadiene as m onom er [6 6 ] . It is significant that the catalytically activeresidual solution from RuCl3/endo-dicyclopentadiene polymerisation also pro-duces high cis polymers with norbornene derivatives, and that exo-dicyclopen-tadiene gives the "normal" high trans polymer. The link between steric crowdingof the catalyst site and cis stereospecificity is therefore well established, both byourselves [6 6] and by others [6 9 ].Copyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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1.6.2 DISTRIBUTION OF trans DOUBLE BO NDS IN HIGHcis POLY(NORBORNENE)The NM R spectra of both the cyclopentane ring and the olefinic carbon atoms inpoly(norbornene) are sensitive to the stereochemistry of the neighbouring doublebonds and, as seen above, this leads to cc/ct; tt/tc doublets for the cyclopentanering carbon atoms. There is , however, th e possibility of quartet fine structure forboth cis and trans olefinic carbon resonances, owing to the inequivalence of thecarbon atoms in a given double bond [6 4 ], as in Scheme 11.

C H = C Hmt / c uc * crtCftc/cScheme 11

In the spectrum of a poly(norbornene) of intermediate cis content, Figure 1.10,this fine structure is well resolved for the cis resonance, but overlapp of the etc andthe ttt components occurs in the trans resonance. In high cis polymer twodifferent types of non-random trans double bond distribution have been ob-served. Figure 1.11 shows the spectra of two polymers, one prepared using ReCl5,Figure 1.1 l(a), and the other using OsCl3, Figure 1.1 l(b) in the presence ofbenzoquinone, another chelating ligand which imposes a high a s directive effect[7O]. In these high cis polymers one would expect, statistically, that trans doublebonds would almost always be flanked by cis double bonds, leading to high tc/ttratios for the cyclopentane ring carbon atoms and a strong etc signal for theolefinic trans resonance. In fact, inspection of Figure l.ll(a) shows that thereverse is the case for the ReCl5-catalysed polymer; here the various ct and tt linesare of approximately equal intensity, and the centre component of the transolefinic resonance which arises from isolated trans, etc, or blocks of trans, ttt, hasbecome only a shoulder on the ttc line. This means that trans double bonds tendto occur in pairs in these predominantly cis chains. Mechanistically, this can beseen as a chain error repair process, where the aberrant formation of the first transdouble bond is corrected by the formation of a second before resumption of cisdouble bond formation. An analogous phenomenon has been observed in thelargely isotactic polymerisation of certain alpha-olefins [ 2 ] , where 1 3C N M Rspectroscopy has shown that the small proportions of syndiotactic (r) junctionsthat occur are found in pairs, as evidenced by the relatively intense rmmr andmmrr pentad signals. Here the catalyst site, which normally selects the sameprochiral face of the monomer in each cycle, occasionally reacts at the other face,leading to an aberrant r junction. Choice of the original prochiral face in the next



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catalytic cycle results in the continuous formation of isotactic polymer: thismechanism is marked by the presence of pairs of sy ndiotactic (r) junc tions.Alternatively the catalyst, having ch osen a different prochiral face, continues todo so. The result is the formation of a polymer containing isotactic b locks joined

by single syndiotactic (r) junction s, i.e. the initial error is propagated, and is visiblein the 1 3 C N M R spectrum as the occurrence of mrmm and mmrm pentads. Hereagain an analogous situation exists in som e R O M P's of norbornen e and deriva-tives, and is seen in the polymerisation of norbornene using the benzoquinone-modified OsCl3 catalyst, Figure 1.1 l(b). In this case, and in contrast to the ReC l5polymer of Figure 1.1 l(a), the var ious tt lines are three to four times as intense asthe tc lines, with a conco m itant increase in the intensity of what must be the tttcom ponent over the ttc and ctt lines in the olefinic trans resonance. This indicatesthat the small percentage of trans double bonds occur in tn blocks (n > 2). Thesame phenom enon is observed in polym ers formed from 1-methylnorbornene[63] , Figure 1.12. At the cis junction in these high cis polymers, monomeraddition occurs in a head-tail manner (see below) but the small proportion oftransjunctions show s no bias. However, it may be clearly seen that in the po lymerformed using the W C l6/M e 4Sn catalyst, Figure 1.12(a), trans double bonds tendto occur in pairs, as evidenced by the low intensity ttt/ctc signals, whereas in thepolymers formed from the OsCl 3 catalyst, Figure 1.12(b), there is a tendency toform blocks.

If there is propagation through metallacarbenes of octahedral symmetry witha vacant alternating ligand position such as described a bove, these species m ay bechiral, with the formation of tactic polymer. Furthermore, cis double bondformation will be associated w ith syndiotactic junc tions and trans double bondswith isotactic junctions, as in Scheme 12. If, however, the catalyst site is achiral, or

Scheme 12chiral but undergoing racem isation faster that prop agation , then atactic polym erwill result, and r or m dyads may be associated with either cis or trans doublebond [67]. Initially (see below) with poly(norbornene) no fine structure wasobserved, which could be attributed to th is tacticity effect, bu t it was realised thatpolymerisation using one enantiomeric form of a chiral norbornene derivative(Scheme 13) would translate the tacticity effect into a bias toward head-head(HH) and tail-tail (TT) addition for syndiotactic polymers, and head-tail (HT)addition for isotactic polymers [65].



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f a )



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Figure 1.11 13C NMR spectra of high cis poly(norbornene): (a) 90% cis prepared using Rea modified OsCl3 catalyst

(b )



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( a ) ( b )Figure 1.12 Olefinic region of the 1 3 C NMR spect rum of poly( l -methylnorboraene)formed with (a) the WCi 6 /M e 4 Sn catalyst an d (b) the Os C l 3 catalyst: (a) Rep roduc ed bypermission of Huthig & Wepf Verlag from [63]

Scheme 13

ROMP



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This analysis was made possible because the chemical shifts of the variousolefinic carbon double bonds in these unsymmetrically substituted norbornenederivatives are very sensitive to w hether they are in an H H , TT or H T /T H unit, ascan be seen for the case of poly(l-m ethylnorborn ene) [ 6 3 ] in Figure 1.12. It wastherefore possible to examine a range of metathesis catalysts for their ability toproduce tactic polymers. In fact, a range of tacticities was ob served, w ith extremesin behaviour being represented by the ReCl5 catalyst, which produced an all-d ssyndiotactic polymer [65] and the W(mesityl) (CO)3 catalyst, which produceda high trans isotactic polymer [ 7 1] .

1.6.3 R E G I O S E L E C T I V I T Y I N R O M PThe above method of tacticity determination depends upon there being noregioselectivity, i.e. no bias towards HT or HH/TT addition in the polymerisa-tion of the racemate, and in fact this is the case with 5-substituted norbornenederivatives.

Placement of a methyl substituent on the double bond results in completeregioselectivity [7 2 ] , but much m ore interesting is the case w here an alkyl groupis in the bridgehead position, as in poly(l-alkylnorbornene) [63, 73]. Thesemonomers exhibit a strong catalyst- and substitutent-dependent selectivity,which again may be observed in the 1 3C NMR spectra of the polymer, Fig-ure 1.13. For example, high trans polymer may be prepared using either Ru C l3 orOsCl3 as catalyst, but whereas the R uC l3 catalyst is non-regioselective the O sC l3catalyst exhibits a strong bias towards the HT addition of monomer (Fig-ure 1.12(a)). This effect may be explained in terms of different polarities of therespective [ M t ] - = C + C ^ pi-bonds as they engage the m onom er dou ble bond,Scheme 8, in a [2 + 2 ] cycload dition reaction which is the initial step of theROM P reaction [74,75]. As expected, steric effects are also important, and the morebulky ethyl substituent induces a HT bias in the polymer formed using the RuCl3catalyst [73] and enhances the HT bias in the O sC l3 case [70][73], Figure 1.13(b).

In this context a particularly interesting and unique exam ple o f the alternatingcopolymerisation of enantiomers was demonstrated in the polymerisation of 1-methylnorbornene with the ReC l5 catalyst [63 ,75] . Th e ana lysis relied on the factthat the hydrogenated forms of these polymers (but see more recent work, p. 52),unlike their unsaturated precursors, exhibited fine structure due to the presenceof ring dyad units of different tacticities.This catalyst gave a poly(l-methylnorbornene) which on 1 3 C NMR analysis ,Figure 1.14, was show n to be all-cis and all H T, in contrast to the O sC l3 catalyst,which produced an all-trans and all HT polymer, Figure 1.13. Both polymerswere hydrogenated, and it was found that whereas in the OsCl 3 case one lineexhibited doublet fine structure, which must be due to the m/r effects, the ReCl 5

polymer gave only the down-field line, indicating that the polym er w as tactic. TheCopyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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Figure 1.13 Olefinic region from the 13C NMR spectrum of all-trans polymer formed fromcatalyst systems, (a) Reproduced by permission of the Society of Chemical Industry, London, froby permission of the Society of Chemical Industry, London, from [73]Copyright 1996 John Wiley & Sons Retrieved from: www


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Figure 1.14 Olefinic region of the 13C NMR spectrum of poly(l-methylnorbornene)formed using a ReCl5 catalyst; the polymer is all HT all-cis and syndiotactic (compareFigure 1.12 (a) andd (b)). Reproduced by permission of the Society of Chemical Industry,London, from Br. Polym. J., 1984,16, 2

fact that it was synd iotactic was shown by using the OsC l3 catalyst to polymeriseoptically resolved m onom er, wh ich must result in an isotactic all-trans polymer.The 1 3C N M R spectrum of the hydrogenated product gave only the up-field lineof the original m/r pair, thereby proving the syndiotactic nature of the poly(l-methylnorbornene) prepared using the R eC l5 catalyst. Such a polymer can onlyform at a catalyst site w hich alternates in chirality in each catalytic cycle, and thusis required to cho ose alternate enantiomeric forms of the m onomer in successivecatalytic cycles. An alternating cop olym er of enantiom ers was thereby formed. Itwas therefore highly significant that, in this context, we were unable to formring-opened polymer from op tically resolved m onom ers with the ReC l5 catalyst.Here again we may draw parallels with Ziegler-Natta polymerisations,Scheme 14. In the syndiotactic polymerisation of propylene [76] the catalyst is

ppm



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Scheme 14selecting a different prochiral face of a monomer (which exists in only onemolecular form). In the case of the 1-methylnorbornene monomer, Scheme 15,reaction is restricted to one face (exo) of the molecule [61], but two chiral formsare available. In each case the polym er is H - T biased, and the catalyst sitealternates in chirality in each catalytic cycle.

Scheme 15

Scheme 16Copyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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1 3C NMR studies of the ROMP of certain 7-substituted norbornadienederivatives provided a remarkable example of a substituent-dependent re-gioselectivity, Scheme 16. 7-methylnorbornadiene [62] and 7-f-butoxynorbor-nadiene [77] were polymerised using a range of catalysts; whereas the 7-Mederivative behaved in the expected manner with almost exclusive attack at theanti face of the molecule (13C N M R spectra of the polym ers are discussed below),catalyst attack occurred with almost equal facility at both syn and anti faces in the7-r-butoxy derivative.In this reaction it is envisaged that the lone pair of electrons on the 7-oxy sub-stituent interacts with the electrons of the syn double bond, and the normal [2 + 2 ]cycloaddition, which occurs on anti attack, b ecom es a facile pseu do [3 + 2 ]cycloaddition, overcoming the apparent steric crowding at the syn face [62].

1.6.4 D I R E C T O B S E R V A T I O N O F T A C T I C I T Y1 3C N M R spectra of polymers formed when there is unsymmetric substitution inthe norbornene monom er, as show n above, have been very useful in dem onstrat-ing the regioselectivity of various cata lyst systems. In additio n, these substituentsare responsible for a decrease in the conformational mobility of the polymerchain, and consequently fine structure w hich m ay be due to tacticity is resolved incertain cases. The situation is complicated, however, by the possibility that suchsplittings may be due to longer range H T effects when H T, H H and TT sequencesare present in the polymer chain. Positioning the substituent at C 7 retains thechain stiffening effect without splittings due to a regio effect; the observed finestructure may then be attributed to tacticity effects, espec ially in high cis or hightrans polymers where remote c and t effects do not interfere.

These 7-substituted derivatives are also important because m uch of the abov emechanistic interpretation depends upon the assumption that attack on thenorbornene molecule occurs at the exo face. The result of ring-opening poly-merisation of mixtures of syn- and anft'-7-methylnorbornene [6 1 ] show s that thisassumption is valid. Thus, only poly(nr/-7-methylnorbornene) was obtainedfrom the polymerisation of syn/anti mixtures, although a small proportion of synisomer was incorporated in som e cases. W ith particularly active catalysts the synisomer could be homopolymerised. More recently, and in relation to the re-gioselectivity studies discussed ab ove, 7-m ethylnorbornadiene was prepared andpolymerised [62].The importance of these polymers (for NMR analysis) lies in the excellentresolution of the 1 3 C N M R spectra which may be achieved and the fact that ringtacticity may be observed directly in addition to cis/trans ratios and distribution.For exam ple, the spectrum of the high trans polym er of anft'-7-methylnorbornene,Figure 1.15(b), which is atactic, showing sensitivity to m/r dyads, may becompared with its tactic high cis analogue, Figure 1.15(a). The syndiotacticnature of this latter polymer is inferred from the known behaviour of the ReCl 5catalyst discussed earlier. Other catalyst system s produce a variety of microstruc-



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(a)



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Figure 1.15 * 3C NMR spectra of poly(anft'-7-methylnorbornene): (a) syndpolymerprepared using the ReCl5 catalyst; (b) atactic all-trans polymer prRuCl5 catalyst. Reproduced by kind permission of Elsevier Science Publi

( b )



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Figure 1.16 1 3 C NMR spectra of poly(anfi-7-methylnorbornene): (a) an intermediatecis-tactic polym er prep ared using the W (mesit) (CO )3/EtAlCI 2 cata lyst system, and (b) anatactic polymer of similar cis content prepared using the WCl 6 /B u 4 Sn catalyst system.Rep roduce d by kind permission of Elsevier Science Pub lishers from [6 1]

(a)

ppm

(b )

ppm



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ture types, and it is interesting that one can subtly change the behaviour of, forexample, a W-based catalyst by changing the oxidation state and ligation. TheW(mesityl) (CO)3 complex and the W VI hexachloride catalyst both producepolym ers o f intermediate and similar cis double bond content, Figure 1.16(a)and(b) respectively, but in the former case cis double b onds are associated solely withr dyads and trans with m, whereas in the latter case cis or trans double bonds maybe associated with m or r dyads [61 ].

In keeping with the general principle that polymerisation of monomers thathave a pair of dou ble bonds capable of chelation at the catalyst site leads to theformation of high cis polymer [66], polymers formed from 7-methylnorbor-nadiene were generally high cis. Resolution of the various microstructuralfeatures is also observed in the* 3C spectra of these polymers, but paradoxically itis in the C 7 , tt line, Figure 1.17 (which shows no fine structure in the 7-methylnorbornene case), which is clearly resolved here; exactly the oppositesituation holds for the C 7 , cc line. One can therefore estimate cis content,blockiness and tacticity of the various double bond dyads from this resonancealone , which m ay be checked for consistency by reference to the fine structure ofother resonances in the spectrum.

Figure 1.17 The C 7 resonance in the 13C NMR spectrum of poly(7-methylnorbor-nadiene) prepared using the W Cl6/M e4Sn catalyst system. Reproduced by permission ofHuthig & Wepf Verlag from [62]



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(a)



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Figure 1.18 125 MHz 13C NMR spectrum of (a) poly(l-methylnorbornene), all cis, all HTcomplex, (b) the same polymer prepared using the ReCl5 catalystCopyright 1996 John Wiley & Sons Retrieved from: www


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An important consequence of the foregoing discussion is that it is im possible topredict which resonance will be split by any of the possible microstructuralfeatures, and one must therefore be careful not to assume that a polymer is, forexample, tactic simply because no fine structure is resolved. Also, spectra ofpolymers taken on modern high field instrum ents (125 M H z for 13C) may showup fine structure not resolved on lower field instruments. A most appositeexample of this was observed recently in the *3 C N M R spectroscopy of polymersformed from 1-methylnorbornene using a tungsten alkylidene complex [78].A spectrum was taken initially at 62.5 M Hz and fine structure was not observed.The polymer was high cis, all HT, assumed to be syndiotactic and thoughtinitially to be another example of the alternating copolymerisation of enan-tiomers described in detail above. However, on obtaining the spectrum at highfield (125 M Hz, Figure 1.18(a), each line exhibited considerable fine structure,showing that the polymer was in fact on ly partially syndiotactic [ 79 ]. In contrast,it was gratifying to observe that the alternating copolym er of enantiom ers formedfrom this monomer using the ReCl2 catalyst, Figure 1.14, when re-examined at125 M Hz), Figure 1.18(b), had a spectrum alm ost devo id of fine structure, therebydemonstrating its tactic nature and allowing the assignment of some of the linesin the more complex spectrum of the atactic polymer.

1.7 REFERENCES[1 ] F.A. Bovey, high resolution carbon-1 3 studies of polym er structure s, in K J . Ivin(Ed.), Structural Studies of Macromolecules by Spectroscopic Methods, John Wiley& Sons, Lon don, 1976.[2 ] F.A. Bovey, Chain Structure and Conformation of M acromolecules, Academic Press,

London, 1982.[3] J.L. Koenig, Chemical Microstructure of Polymer Chains, John Wiley & Sons,Chichester, 1980.[4] J.L. Koe nig, Spectroscopy of Polymers, AC S Professional Reference Book, Am ericanChem ical Society, W ashing ton, 1992.[5] A.E. Tonelli, NM R Spectroscopy and Polymer Microstructure, VCH Publishers,Berlin, 1989.[6 ] A.H. Faw cett , Synthetic ma cromo lecules, p. 333 in G. W eb b (Ed.), Special PeriodicalReport on Nuclear Magnetic Resonance, The Royal Society of Chemistry, Cam-bridge, 1993.[7 ] F.A. Bovey and J. Tiers, J. Polym. Sci., 1960, 44 ,17 3.[8 ] A.E. Tonelli, Ch apte r 2 , p. 55, in this book .[9 ] F. Heatley and A. Zam belli, Macromolecules, 1 9 6 9 , 2 , 6 1 8 .[10 ] A. Zam belli and A. Segre, J. Polym. ScL, B, 1968, 6 ,473 .[11 ] N . Ishihara, T. Seimiya, M. K ura m oto and M . Uoi , Macromolecules, 1986,19 ,2464.[12 ] M . M inagawa, H. Yam ada, K. Yam aguchi an d F . Yoshi , Macromolecules, 1992,25 ,503.[13] A.M. Aerdts, J .W. de Haan and A .L. Ge rm an, Macromolecules, 1993, 26, 1965.[14] A.H. Fawc et t and W . Dd am da, MakromoL Chem., 1982 ,183, 2799.Copyright 1996 John Wiley & Sons Retrieved from: www.knovel.com


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[15 ] J.K. Becconsall, P.A. Curnuck and M .C Mclvor, Appl. Spectrosc, 1971,4, 307.[16 ] D.T. Pegg, D.M . Doddrell and M.R. Bendall, J. Chem. Phys., 1982,77, 2745.[17] R.E. Emst, G. Bodenhausen and A. Waokaun, Principles of Nuclear M agneticResonance, Clarendon Press, Oxford, 1987.[1 8 ] H. Friebolin, Basic One- and Tw o-Dimensional NM R Spectroscopy, VCH , New Yorkand Weinheim, 1991.[19 ] F.C. Schilling, F. A. Bovey, M. D . Bruch and S. A. Kozlowski, Macromolecules, 1985,18,1418.[20 ] P.A. Mirau and F.A. Bovey, Macromolecules, 1986,19, 210.[21 ] J J . K otyk, P.A. Berger and E.E. Remsen, Macromolecules, 1990, 23, 5167.[22 ] G.R. Quinting and R. Cai, Macromolecules, 1994,27,6301.[23 ] A.G. Ferrige and J.C. Lindon, J. Magn. Reson,,1978, 31, 337.[24] A.H. Fawcett, S. Fee and L.C. Waring, Polymer, 1983,4,1571.[25 ] J.A. Frankland, H.G.M. Edwards, A.F. Johnston, LR. Lewis and S. Poshyachinda,Specirochim . Ada, Part A, 1991,47A, 1511.[26] D.M. Grant and E.G. Paul, J. Am. Chem. Soc,, 1964,86, 2984.[27] J.C. Randal, J. Polym. ScL, Polym. Phys. Ed., 1973,11, 275.[28] J.C. Randal (Ed.), NMR and Macromolecules, ACS Symp.Ser., No. 247,1984, 256.[29] J.C. Randal, C J. Ruff and M. Keltermans, Reel Trav. Chim. Pays-Bos, 1991,110,543.[30] A.H. Fawcett, KJ. Ivin and C. Stewart, Org. Magn. R eson., 1978, 11, 360, andreferences cited therein.[31] A. Kaji, Y. Akitomo and M. M urano , J. Polym. Set, Part A: Polym. Chem., 1991,29,1987.

[32] Q. Zhu, F. Horii and R. Kitamaru, J. Polym. ScL, Part A : Polym. Chem .,1990, 28,2741.[33] A.H. Fawcett, M. Hania, K.-W. Lo and A. Patty, J. Polym. ScL, Part A: Polym.Chem.,1994,32, 815.[34] A.H. Fawcett and M. Hania, unpublished results.[35] F . Heatley, Y.Z. Luo , J.F. Ding, R.H. Mobbs and C. Booth, Macromolecules, 1989,21, 2713.[36] F. Heatley, G.E. Yu, M.D. Draper and C. Booth, Eur. Polym. J., 1991, 27,471.[37] F . Ciardelli, O. Pierone and A. Fissi, Chapter 14, p. 347 of this book.[38 ] A.H. Fawcett, J. Guthrie, M.S. Otte rbum and D.Y.S. Szeto, J. Polym. ScL, Polym.Lett., 1988,26,459.[39] A.H. Fawcett, D.Y.S. Szeto and D. Pepper, in preparation.[40] A. Zambelli, P. Locatelli, G. Bajo and F.A. Bovey, Macromolecules, 1975,8,687.[41] R. Mani and C M . Burns, Macromolecules, 1991, 24, 5476.[42] A.H. Fawcett, F. Heatley, K J . Ivin, C D . Stewart and P . Watt, Macromolecules,1977,10 , 765.[43] R.H. Cole, P.W. Winsor, A.H. Fawcett and S. Fee, Macromolecules, 1987, 20 ,157 .[44] N . Spasky and P . Sigwalt, Bull. Soc. Chim. Fr., 1967,4617.[45] A.H. Fawcett and R.K. Malcolm, Polym . Int., 1994,35,41.[46] S.A. Cham bers, A.H. Fawcett, J.F. Malone and S. Fee, Macromolecules, 1990, 23,2757.[47] J.R. Ebdon, The characterization of diene polymers by high resolution protonmagnetic resonance, in K. J. Ivin (Ed.), Structural Studies of Macromolecules bySpectroscopic Methods, John Wiley & Sons, London, 1976, p. 241.[48] C-W. Chau, A.H. Fawcett, J.N. Mulemwa and C-E. Tan, Polymer, 1992, 193,257.[49] A.H. Fawcett, J.N. Mulemwa and C-E. Tan, Polym. Commun., 1984,25, 300.



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1. NMR Character is at Ion of Macro Molecules in Solution

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