Role of Back-Biting in the Stereoselectivity of Ni(II)-Catalyzed Butadiene Polymerization

We have reported new and remarkable information on the relevance of back-biting to determining the stereoselectivity in the polymerization of butadiene. In fact, the experimental results, with respect to the first insertion of the butadiene unit in the metal– methyl r bond, show that it inserts preferentially in a 1,4-trans configuration, whereas during the chain propagation, more than 90% of the monomer is in a 1,4-cis arrangement. It is reasonable to assume that the butadiene coordinates in the same manner (probably η4-cis) both in the first insertion and during the chain growth. The insertion of the first g4-cis coordinated butadiene unit in the metal–methyl bond leads to the formation of an anti-allyl species that, in the absence of back-biting, can easily isomerize to the more thermodynamically stable syn-allyl. The insertion of the next monomer unit produces a 1,4-trans unit. During the propagation, 1,4-cis units are produced because, on account of back-biting coordination of the double bond of the penultimate inserted unit, allyl isomerization is unfavored.