An industrial catalyst comprised of nickel carboxylate, boron fluoride, tnalkylaluminium and butadiene was developed in Japan. Kinetic study suggested that catalyst structure to be it-allylic nickel borate. In order to elucidate the mechanism for cis-polymenization, the structure of ic-crotyl nickel halides were analysed by nuclear magnetic resonance. All kinds of halides were found to possess a syn-ic-allylic structure irrespective of their different modes of polymerization, chloride, bromide

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... nd iodide yielding cis-, cis-trans-, and trans-polymer, respectively. For the cis-polymenization a mechanism involving a ir-allylic polymer terminal with an intramolecular coordination of the double bond of the penultimate unit (called 'back-biting coordination') is proposed in which a cis-double bond is formed not only from anti-x-allylic but also from the syn-2tallylic polymer terminal, because the resulting cis-double bond has little hindrance whereas the trans-one has large hindrance. Effects of additives to the catalyst were discussed. An electron donor may coordinate to the catalyst to hinder the back-biting coordination and has a tendency to give trans-polymer. On the contrary, an electron acceptor releases ligand anion to remove its steric hindrance. As a result cis-trans (1:1) polybutadiene is formed. Various phenomena such as the incorporation of a trans-unit in polymerization at a low concentration of butadiene and in the copolymerization with styrene are accounted for. 1,2-Polymerization is explained as an attack on the 'y-position of the rc-allylic terminal. The selection ofand 'y-positions is controlled by the nature of the anion ligand. The polymerization of substituted dienes is also discussed, in which a large steric hindrance is considered only for 13,y-disubstituted monomer.