Polymerizations of butadiene, isoprene, and 2,3-dimethylbutadiene by Gd(OCOCCl3)3-(i-Bu)3Al-Et2AlCl and thecis polymerization mechanism for dienes

Abstract

Homopolymerizations of butadiene (BD), isoprene (IP), and 2,3-dimethylbutadiene (DMBD) were carried out by a Gd(OCOCCl 3 ) 3 -based catalyst, to investigate the effects of the energy levels of the monomers or the sterical factor of the methyl substituents on the polymerizability and the cis-selectivity of the monomers. The order of the polymerizability at 50°C was as follows: BD (4.5 kg of polymer/(mol of Gd h)) ∼ IP (4.8) > DMBD (0.6). On the other hand, the cis-selectivity of the polymers was as follows: BD (98%) > IP (94%) » DMBD (35%). These results suggest that the terminal BD and IP units are controlled by the cis configuration by the coordination between the penultimate cis-vinylene unit and the catalyst metal, whereas the penultimate DMBD unit unfavorably controls the terminal DMBD unit to the cis-1,4 configuration through the back-biting coordination with difficulty by two methyl substituents compared with the penultimate BD and IP units. The validity of the back-biting coordination was examined by MO calculation with σ-allylnickel complexes. According to the formation energy with respect to the BD-BD diad, the cis-cis form is somewhat preferable to the trans-cis form through the coordination of the penultimate BD unit by ΔE = 0.028 au (ca. 17.6 kcal/mol).