Racemate configuration

Another important application of NMR to polymer systems is the elucidation of the stereochemical configurations of Polymer chains. Poly (methyl methacrylate) was first studied by Bovey in 1960. It is now possible to analyse for the statistical frequency of occurrence of all possible combinations of up to four successive pairs of units (dyads) capable of occurring with either the same (meso) or opposite (racemic) configurations. 

J. Prud homme I think here the same situation arises as with hydrogenated 1,4-polydimethylbutadiene. It would appear that when two methylene units separate two chiral carbons, each methylene unit shows little sensitivity to the meso and racemic configurations of the two adjacent chiral carbons. Carman et al. have reported spectra measured on alkanes which show that when two tertiary chiral carbons are separated by two methylene units, the difference in the chemical shifts of the methylene units in the meso and racemic configurations is close to 0.3 ppm. It was not possible to observe this kind of resolution in the spectra of the present polymers. Only a broadening effect occurred. 

When measuring vinyl polymer tactlclty, one prefers the longest complete n-ad distribution available as well as the translated simplest comonomer distribution, possibly m versus r. An alternative exists to the m versus r distribution In the form of number average or mean sequence lengths. If any vinyl homopolymer Is viewed conceptually as a copolymer of meso and racemic dyads, mean sequence lengths can be determined for continuous runs of both meso and racemic configurations (32), that Is,... 

Other Monomers. Ivin and co-workers have examined the metathesis of a number of cyclic olefins using n.m.r., and have observed fine structure due to cisjtrans isomerism of double bonds and meso/racemic configurations of neighbouring rings in the chain. Harris and Robins have continued their work on Si n.m.r. of poly(siloxanes) with a comprehensive study of end-group eifects. 

These meso and racemic configurations are fixed during the formation of the polymer and cannot be transformed (e.g. m into r, and thereby altering the tacticity) by heat or solution. In many polymers the tacticity does not greatly alter the properties of the polymer, e.g. PMM A or PVC, but where it affects the ability of a polymer to crystallize, e.g. PP, then the difference is very significant. Crystalline syndic tactic or isotactic PP are stiff polymers (softening point 150 °C) whereas atactic PP is a viscous gum. 

In the case of chiral homopolymers, the meso and racemic diads are treated statistically as if they were different comonomers. Some care has to be taken when calculating statistical models for chiral homopolymers because two contiguous units are required to identify meso or racemic configurations. There are examples in the literature where the order of a particular statistical model has been confused by the simple treatment of a chiral homopolymer as an m, r copolymer (6). 

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