Solution-state NMR studies of chain scission

As discussed below solution-state NMR methods cannot be successfully applied to materials irradiated to above their gelation dose. This situation naturally does not arise for polymers for which chain scission is the dominant reaction on irradiation. The two systems of this type which have been most extensively studied are the poly(methacrylate)s and poly(isobutylene). 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

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