Racemic placements

When adjacent monomers in a backbone share the same stereoconfiguration, the placement is known as a meso diad. When adjacent monomers have opposing stereoconfigurations, the placement is known as a racemic diad. Thus, a purely isotactic polymer comprises all meso placements, and a syndiotactic polymer consists of all racemic placements. 

Stereodefects are the result of one or more racemic diads interrupting a sequence of meso diads. Figure 5.7 illustrates the two principal types of stereodefect. In Fig. 5.7 a), a single racemic placement results in the subsequent methyl groups being placed on the opposite side of the chain from those of the preceding sequence. In Fig 5.7 b), a pair of racemic placements interrupts the meso sequence. In this case, both the meso sequences have their methyl groups on the same side of the chains. 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

Isoselective polymerization of one enantiomer or the other of a pair of enantiomers results in an optically active polymer [Ciardelli, 1987 Delfini et al., 1985 Pino et al., 1963]. For example, polymerization of (5)-3-methyl-l-pentene yields the all-.S polymer. The optical activity of the polymer would be maximum for the 100% isotactic polymer. Each racemic placement of the S-monomer decreases the observed optical activity in the polymer. 

Although the treatment described above has been limited to polymerizations proceeding by meso and racemic placements, the various models can also be adopted to describe the stereochemistry of 1,4-propagation in 1,3-dienes. 

Figure 2.19 Sketches of some stereochemically variable chains, of polypropylene (two meso placements), poly(methyethylphosphazene) (two racemic placements), and poly(methylphenyl-siloxane) (meso placement followed by racemic placement).

At present, no evidence exists for long runs of either meso or racemic placements in the Wurtz dechlorination reaction used in the synthesis of polysilylenes. Deviation from a random stereochemical polymerization has been suggested by West et al. (3) in the case of poly(phenylmethylsilylene) (PPMS). An analysis of Si NMR triads indicates that addition of monomer units with the same stereochemistry is preferred, which results in an increase in the intensity of syndiotactic and isotactic triad structures compared with a random stereochemical polymerization. 

Scheme 1 introduces the parameters that are used to describe the stereoselectivity of the monomer enchainment process. For chain-end control, the parameters Pm and refer to the probability of meso and racemic placements, respectively (the Bovey formalism is a convenient way to describe polymer tacticity, with a small m for meso, and a small r for racemic relationships between adjacent stereogenic centers). A Pm equal to unity indicates isotac-ticity, while a P equal to unity signifies syndiotac-ticity. For site-control mechanisms, the parameter a represents the degree of enantiotopic selectivity of the enchainment. When a is either 1 or 0 an isotactic polymer forms, while an a parameter of 0.5 produces an atactic polymer. Polymer architectures relevant to this review are shown in Figure 1. 

If Pr is the probability of a racemic placement for a growing vinyl polymer chain, deduce an expression for the ratios of the intensities of the C NMR peaks for the rr, mr and mm a-carbon triads and evaluate the ratios for Pj = 0.6. Assume that Bernoullian statistics apply, i.e. that the probability of a racemic placement is independent of the nature of the previous placements. (This is not necessarily true for a real system.)... 

If the Me2C(CN)- group derived from [j ,)5- C]AIBN is attached to a monomeric unit without a chiral centre, as for poly(vinylidene chloride) or the 1,4-unit in poly butadiene, there is only one set of signals from the methyl carbons. The same result is found for poly(ethyl acrylate) [11] even though the monomeric unit possesses a chiral site in this case, the two sets must be almost coincident presumably because the terminal meso and racemic placements of the monomeric unit produce very similar shielding of the methyl groups in the initiator fragments. 

Spin-spin interactions between nuclei in copolymer chains also cause signal broadening and interfere with resolution. Such interactions make it difficult to obtain information about copolymer structure from studies of methine or methylene resonance in copolymers. Methylene protons in racemic placements in vinyl copolymers should be detected as triplets whereas those in meso placements should yield twelve peak patterns. The high multiplicity of such signals causes them to be broad and this can hinder resolution efforts if the chemical shifts of the uncoupled methylene protons in the various sequences do not differ appreciably. 

The first portion of this article will concern vinyl polymers which contain predominantly meso (isotactic) or racemic (syndiotactic) placements. Attention will then be given to the study of homopolymers containing appreciable aimounts of both meso and racemic placements, since the study of such materials is necessary if trends toward stereospecifity or stereoselection... 

When the relative amounts of various stereosequences can be measured by nmr, it is possible to discern what statistical relationships exist among the various structural features and thus learn something about the stereo-regulating processes occurring during polymer formation. In such considerations, stationary probabilities (e.g., P(m), P(mr), etc.) may be used to represent the relative amounts of dyads, triads and n-add placements and conditional probabilities (e.g., P(m/m), P(r/mm), etc.) may be used to represent the probabilities that m or r placements follow particular sequences in the polymer. The expression P(m/r), for example, refers to the probability that a meso placement follows a racemic placement in a polymer chain. 

The differences between the activation enthalpies and entropies for meso and racemic placement can be obtained from a plot of n(Pm/Pr) against reciprocal temperature. Thus,... 

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