Platinum Metals Rev., 1962, 6, (4), 135
Rhodium as a Polymerisation Catalyst
Further Studies on Polybutadiene Preparation
The use of aqueous solutions of rhodium salts as stereo-specific catalysts in the polymerisation of 1, 4 butadiene has recently been reported by the research laboratories of the United States Rubber Company. This development may make possible the large-scale production of cis -polybutadiene by the commonly-used technique of emulsion-polymerisation.
Further aspects of this work have now been reported from the laboratories of the Shell Development Company, Emeryville, U.S.A., by A. J. Canale, W. A. Hewett, T. M. Shryne and E. A. Youngman (Chem. & Ind., 1962, (24), 1054). These workers demonstrated that, in addition to trans-I, 4 polymerisation of butadiene possible with Rh3+ salts, a much broader steric control over such polymerisations is possible with other ions and complex compounds of the platinum group metals. Under the polymerisation conditions studied, for example, almost complete trans-I, 4 additions were obtained using rhodium trichloride, cyclo-octadiene rhodium chloride and iridium trichloride as catalysts. Palladium, in the form of palladium chloride or ammonium chlorpalladate, gave high proportions of I, 2 polymerisations but the polymers had low molecular weights. One of the highest proportions of cis- I, 4 additions obtained resulted from the use of ruthenium trichloride with triphenyl phosphine or tri-n -butyl phosphine. Cobalt catalysed polymerisations, however, yielded the highest proportion of cis- I, 4 polymers, and also gave the highest molecular weights.
The fact that certain compounds that normally inhibit the “classical” free radical polymerisations had no effect on the reaction studied indicates that a completely different mechanism is operative in these instances. While no specific mechanisms have been suggested to date, it is clear that the ligands, the nature of the transition metal ions and their valency states exert powerful influences on the steric course of the polymerisation, its rate and the molecular weight of the products. It is postulated that the transition metal hydride may play a part in the initiation of some polymerisations.