Stereospecific Cyclopolymerization with Group 4 Metallocenes

Abstract

Homogeneous Ziegler-Natta catalysts are stereoselective cyclopolymerization catalysts for non-conjugated dienes. Cyclopolymerization of 1,5-hexadiene affords poly(methylene-l,3-cyclopentane) (PMCP), a polymer for which four structures of maximum order are possible. A variety of metallocene catalyst precursors have been investigated; the molecular weight and microstructure of the polymers are sensitive to the structure of the catalyst precursor as well as the reaction conditions. The selectivity for cyclization depends on reaction conditions; decreasing the olefin concentration and increasing the temperature of the reaction favor cyclization. The stereochemistry of cyclopolymers can also be controlled with appropriate choice of catalyst precursor. Diastereoselective cyclopolymerization of 1,5-hexadiene with achiral catalysts yields atactic trans-PMCP and cis-PMCP, depending on the catalyst precursor. Enantioselective cyclopolymerization with optically active catalysts yields optically active poly(methylenecyclopentane), a novel example of a polymer which is chiral by virtue of its main-chain stereochemistry.