The Structure of Vulcanized Rubber

Vulcanization Processes.—The vulcanization of rubber, already cited in Chapter IX as an example of the generation of a space network by cross-linking, is an extremely complex process the principal features of which are accounted for by the sequence of reactions shown on p. 455 and based largely on the investigations of Farmer and his co-workers. The first step (i) consists in removal of an allylic hydrogen from the polymer chain by a free radical supplied by an accelerator or in the absence of an accelerator the required radical may be furnished by sulfur in the chain form In the second step a sulfur molecule 

The reaction scheme presented above is supported by investigations of the products obtained in the reaction of olefins with suKur at temperatures of 140° or so, and especially by the reactions with the diisoprenoid hydrocarbons, dihydromyrcene and geraniolene, and with the hexamer, squalene.  

While free radical attack in step (i) is by no means confined to carbon atom 4, the products obtained in the reactions involving the lower polyisoprenes indicate that this process is the dominant one. Likewise in step (ii) sulfur may frequently add at carbon atom 4 rather than at atom 2. Addition in the manner shown is indicated, however, by infrared spectra, which reveal the formation of —CH=CH— groups during vulcanization. The scheme accounts also for the observed constancy of the C/H ratio during vulcanization and for the relatively low efficiency of utilization of sulfur in the formation of cross-linkages in the absence of accelerators. A preponderance of the sulfur is involved in addition without formation of cross-linkages a considerable fraction of the thus-combined sulfur may occur in five- and six-membered heterocyclic rings formed by the mechanisms indicated. 

While the action of various accelerators is not altogether clear, many of them (tetramethylthiuram disulfide, for example) may decompose with the release of radicals which accelerate step (i). Others (e.g., the diphenylguanidine type) appear to promote the normal reactions of sulfur, perhaps by catalyzing the conversion of cyclic sulfur (Ss) to a more soluble and reactive form.2 27 efficiency of sulfur utiliza- 

In addition to the two major processes, cross-linking and chain modification (or cyclization), chain scission doubtless occurs also to varying degrees during conventional vulcanizations. Processes of this nature are not difficult to envisage in the presence of free radicals. The radical intermediate (II) may, for example, undergo /3-fission as follows  

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Further deviations from eqn (4.13) are to be expected where segments are highly extended and Gaussian statistics no longer apply, because of neglect of the experimentally observed C2 term and by all the other imperfections of the Gaussian theory of high elasticity. In spite of this eqn (4.13) (or rather its simplifications (4.18) and (4.19)) has been of importance in the elucidation of the structure of vulcanized rubbers (see Chapter 8). 

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