Copolymerization of Isoprene and Nonconjugated α,ω-Dienes by Half-Sandwich Scandium Catalysts with and without a Coordinative Side Arm

A series of half-sandwich scandium-dialkyl complexes that bear various auxiliary ligands have been examined for the copolymerization of isoprene (IP) with nonconjugated ,-dienes such as 1,5-hexadiene (HD) and 1,6-heptadiene (HPD). Significant ligand influence on the catalytic activity and selectivity has been observed. The thf-coordinated complex [(C5Me4SiMe3)Sc(CH2SiMe3)(2)(thf)] (2) and the methoxy side arm containing the half-sandwich complex [(C5Me4C6H4OMe-o)Sc(CH2SiMe3)(2)] (3), in combination with an equivalent of [Ph3C][B(C6F5)(4)], can serve as excellent catalysts for the random cyclocopolymerizations of IP with HD and HPD. The resulting random HD-IP copolymers contain five-membered-ring methylene-1,3-cyclopentane (MCP), 3,4-polyisoprene (3,4-IP), and 1,4-polyisoprene (1,4-IP) units with controllable HD incorporation in a range of 17-82mol%. The random HPD-IP copolymers possess six-membered-ring methylene-1,3-cyclohexane (MCH), 1,4-IP, and 3,4-IP units with HPD incorporation in a range of 11-55mol%. By use of a catalyst that bears a phosphine oxide group [{C5Me4SiMe2CH2P(O)Ph-2}Sc(CH2SiMe3)(2)] (5), the alternating copolymerizations of IP with HD and HPD have been achieved for the first time in which HD and HPD are completely cyclized to the MCP and MCH units, respectively. More remarkably, in the alternating copolymerization of HPD and IP by 5, the regio- and stereospecific cis-MCH selectivity reached as high as 99%. The microstructures and compositions of these copolymers showed significant influences on their mechanical properties.