Cyclopolymerization of Si-Containing α,ω-Diolefins by a Pyridylamidohafnium Catalyst with High Cyclization Selectivity and Stereoselectivity

Cyclopolymerization of unsymmetric and symmetric Si-containing α,ω-diolefins by dimethylpyridylamidohafnium/organoboron catalytic system with high cyclization selectivity and stereoselectivity was first reported here. Compared with the constrained geometry catalyst [Me₂Si­(η⁵-Me₄C₅)­(N^tBu)]­TiCl₂ and the typical metallocene catalyst <i>rac</i>-Et­(Ind)₂ZrCl₂, the Hf complex has proved to be one more promising catalyst for the cyclopolymerization of unsymmetric 3,3-dimethyl-3-sila-1,5-hexadiene (DMSHD). The poly­(DMSHD) with high molecular weight (up to 953 kg/mol), high cyclization selectivity (100%), high <i>cis</i> selectivity (>90%), and high isotactic selectivity (>90%) could be easily obtained by the Hf catalyst under mild conditions. Moreover, the poly­(DMSHD) also possessed a high glass transition temperature (up to 99.8 °C) and high melting temperature (<i>T</i>_m >200 °C). The insertion reaction of the vinyl group into Hf–C_Ar bond was further investigated, and a possible cyclization mechanism of DMSHD was proposed according to density functional theory calculation. Meanwhile, the Hf catalyst also showed notable catalytic activity for the cyclopolymerization of the symmetric Si-containing α,ω-diolefins including 4,4-dimethyl-4-sila-1,6-heptadiene, 4-methyl-4-phenyl-4-sila-1,6-heptadiene and 4,4-diphenyl-4-sila-1,6-heptadiene. Although the cyclization selectivity is somewhat low, the copolymerization by Hf catalyst still shows excellent <i>cis</i>/<i>trans</i> selectivity and isotactic selectivity