Poly branching during formation

The NMR spectroscopy of poly(vinyl chloride), which was reduced with tributyltin hydride, showed that the original polymer contained a number of short four-carbon branches [300]. This, however, may not be typical of all poly(vinyl chloride) polymers formed by free-radical polymerization. It conflicts with other evidence from NMR spectroscopy that chloromethyl groups are the principal short chain branches in poly(vinyl chloride) [301, 302]. The pendant chloromethyl groups were found to occur with a frequency of 2-3/1,000 carbons. The formation of these branches, as seen by Bovey and coworkers, depends upon head to head additions of monomers during the polymer formation. Such additions are followed by 1,2 chlorine shifts with subsequent propagations [301, 302]. Evidence from still other studies also shows that some head to head placement occurs in the growth reaction [303]. It was suggested that this may be not only... 

As indicated previously, points of branching are also thought to feature in the degradation of poly(vinyl chloride). It was seen in Section 4.2.4. that branching which occurs during polymerization of vinyl chloride is likely to result in the formation of a tertiary chlorine atom. The high reactivity of tertiary chlorine in heterolytic reactions suggests that a branch point is a favourable site for unimolecular elimination. 

The (o,a), (o"-trilithiumpolystyrene (96) was also functionalized with carbon dioxide to form the corresponding (o,a), a)"-tricarboxypolystyrene (98) as shown in Scheme 32. Although significant amounts of dimer (11-12%) were observed when carbonation was effected in the presence of THF [241] or after end-capping the styryllithium chain ends with 1,1-diphenylethylene [141], procedures that were previously shown to be effective for quantitative carboxylation of poly(styryl)lithium, less than 2% dimer formation and formation of a tricarboxylated, three-arm, star-branched polystyrene with a functionality of 2.9s were obtained upon carboxylation of a freeze-dried [141] sample of 96. Previous studies indicate that dimer and trimer formation during carboxylation are enhanced by chain-end association [141, 241]. 

The smaller and simpler substances which act as starting materials for the synthesis of polymers are known as Monomers. Bifunctional monomers (those having only two reactive sites) polymerise to give linear or branched molecules. Presence of poly functional monomers during the process of polymerisation may cause cross-linking of chains and thus result in the formation of a three-dimensional network. 

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