Platinum Metals Rev., 1962, 6, (2), 46
Rhodium as a Polymerisation Catalyst
Preparation of Polybutadiene
In the production of synthetic rubbers from polymers and co-polymers of a series of dienes, notably butadiene, the properties and utility of the resulting polymer depend to a very great extent on the conditions of polymerisation and on the nature of the catalyst employed. Diene polymers and co-polymers are mixtures of 1,2 and 1,4 additions - each in the cis- and trans- form. Different methods of polymerisation cause certain structures to predominate and yield polymers of widely differing properties.
The stereo-specific activity of trivalent rhodium when used as a catalyst in the emulsion-polymerisation of 1,4 butadiene has recently been reported by R. E. Rinehart, H. P. Smith, H. S. Witt and H. Romeyn, of the United States Rubber Company (J.A.C.S. 1961, 83, (23), 4864-4865). The chloride, nitrate and some other salts of trivalent rhodium were employed in dilute solution in water or ethanol and yielded polymers containing more than 98 per cent of the trans- structure. This form of the polymer is hard, brittle and crystalline, compared with the usual rubbery material which consists largely of the cis- structure. The novelty of this finding lies in the fact that stereo-specific polymerisations of dienes had not previously been possible by the commonly-used emulsion polymerisation technique, as catalysts which could be employed are affected by water.
Applications of this technique for the production of cis -polybutadiene offers very interesting commercial possibilities. This type of rubber is said to have considerable advantages over natural and styrene-butadiene rubbers in the manufacture of heavy-duty tyres, in that it exhibits higher recovery rates from load strains and better resistance to undesirable temperature rises.