Structure of vinyl polymers

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

Scheme 1. Molecular structures of vinyl polymer and SiH cross-linker polysiloxanes...

In the early days of polymer chemistry before the mechanism of vinyl polymerization was understood, there was considerable speculation as to structures of vinyl polymers as it was not known whether the units were regularly head to tail, head to head, tail to tail, or random in arrangement. In the late thirties and early forties, we studied the reactions of many vinyl polymers such as polyvinyl chloride, polyvinyl alcohol, polymethyl vinyl ketone and polystyrene to try to settle this point,All proved to be head to tail structxires as was later predicted to be correct from the polymerization mechanism theory. Again I eii5)hasize that, at this time, infrared spectroscopy was in its infancy and NMR was unknown so structures needed to be established by chemical means. 

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