Structure of Poly vinyl chloride

There has also been a trend in recent years to the use of free-radical initiators that decompose more rapidly than the traditional initiators such as caproyl and lauryl peroxides. Currently used initiators include peroxydicarbonates, t-butylperpiva-late, azobis-(2,4-dimethylvaleronitrile) and acetyl cyclohexylsulphonyl peroxide. 

X-ray studies indicate that the vinyl chloride polymer as normally prepared in commercial processes is substantially amorphous although some small amount of crystallinity (about 5% as measured by X-ray diffraction methods) is present. It has been reported by Fuller d in 1940 and Natta and Carradini in 1956 that examination of the crystalline zones indicates a repeat distance of 5.1 A which is consistent with a syndiotactic (i.e. alternating) structure. Later studies using NMR techniques indicate that conventional PVC is about 55% syndiotactic and the rest largely atactic in structure. 

PVC has a rather limited thermal stability. This is rather surprising since it is known that low molecular weight materials containing similar structures cire far more stable. It would thus appear that this instability is due to imperfections or weak points in the structure at which degradation can commence. The 

By noting the amount of chlorine that could be removed in this way they were able to determine whether the polymer was formed by head-to-tail linkage 

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Much work has been carried out in order to elucidate the molecular structure of poly(vinyl chloride). In 1939, Marvel, Sample and Roy dechlorinated PVC with zinc dust to give linked cyclic structures (Figure 12.9). 

Show the structure of poly(vinyl chloride), a polymer made from H C CHCl, by drawing several repeating units. 

The head-to-tail structure of poly (vinyl chloride) permits the continuous regeneration of an allylic chloride moiety as hydrogen chloride elimination proceeds along a chain. Thus, once initiated, loss of hydrogen chloride may proceed along a polymer chain without abatement. 

Problem 30.1 Give the shorthand structures of poly(vinyl chloride) and nylon 6,6 in Section 30.1. 

In the structure of poly(vinyl chloride) (PVC), alternate carbon atoms are linked to a chlorine atom, so that the polymer contains 56 per cent m/m of chlorine. The plasticizer-free rigid PVC burns away in a flame in open air but ceases to burn as soon as it is removed from the direct flame. Its oxygen index is 43 to 48 per cent. 

Figure 2.4 Isotactic and syndiotactic structures of poly(vinyl chloride). Allyn and Bacon Molecular Model Set for Organic Chemistry.

A cornerstone of the chemistry of polymers is a publication on the structure of poly(vinyl chloride) by Marvel and coworkers [42]. This paper, which is still cited, established the head-to-tail structure of PVC. The approach used involved the dechlorination of uncross-linked poly(vinyl chloride) in dioxane solution with zinc. From statistical considerations, it had been predicted that random attack by zinc on the polymer chains should produce different degrees of dehalogenation depending on whether the polymer were of a head-to-tail or a head-to-head, tail-to-tail structure. Since the experiment resulted in the removal of 84-87of the available chlorine, as predicted, and the resultant product consisted of polymeric cyclopropane units with occasional, isolated... 

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