Emulsion polymerization of vinyl monomers by transition metal compounds

A number of transition metal ions and complexes of the type which effect the stereospecific polymerization of butadiene in aqueous emulsion catalyze the polymerization of other vinyl monomers as well. In contrast to experience with butadiene, the structure of all products was typical of free‐radical propagation reactions. The catalysis was somewhat selective, a given transition metal compound being active for some monomers but not for others. A mechanism involving coordinate complex formation between catalyst and monomer, generation of a free‐radical initiator by redox reaction within the complex, and noncoordinated free‐radical propagation is postulated. Copolymerization studies support such a mechanism; copolymer compositions were always typical of free‐radical catalysis, even when the catalyst was active in homopolymerization of only one member of the monomer pair. Butadiene polymerizations using many of the same catalysts clearly proceed by different, little understood mechanisms. Attempted copolymerizations with butadiene gave only polybutadiene; the comonomers had no effect on polymer microstructure but generally reduced polymerization rate.