Resonance poly isoprenes

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

Some polymers, such as n s-l,4-poly(isoprene) and poly(styrene), absorb UV light directly. In the case of poly(isoprene), this produces a resonance-stabilized allyl free radical through the elimination of hydrogen (2H- H ) ... 

The recombination of these free radicals causes cross-linking of the poly-(isoprene). If the free radicals are only slightly resonance-stabilized, as, for example, in poly(methyl methacrylate), then disproportionation and hence chain degradation result ... 

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