Competitive Chain Transfer by β-Hydrogen and β-Methyl Elimination for a Model Ziegler-Natta Olefin Polymerization System [Me_2Si(η^5-C_5Me_4)_2]Sc{CH_2CH(CH_3)_2}(PMe_3)

Abstract

The reaction of OpSc(H) (PMe_3) (Op = {(η^5-C_5Me_4)_2SiMe_2}) with isobutene produces OpSc(CH_3)(PMe_3) along with isobutene, 2-methylpentane, isobutene, 2-methyl-l-pentene, propane, and n-pentane. These products arise from a series of reactions involving olefin insertion, β-CH_3 and (faster) β-H elimination which proceed until only the 2-methyl-1-alkenes (C_4H_8, C_6H_(12), etc.) and the predominant organoscandium product OpSc(CH_3)(PMe_3) remain. A transient observed in the reaction sequence has been unambiguously characterized as OpSc(CH_2CH_2CH_3) (PMe_3). Slower u bond metathesis involving the methyl C-H bonds of PMe_3 and the Sc-C bonds of the scandium alkyls accounts for the observation of saturated alkanes (2-methylalkanes (C_4H_(10), C_6H_(14), etc.), normal alkanes (C_3H_8, C_5H_(12), etc.), and a minor organoscandium product OpScCH_2-PMe_2 in the product mixture. b-Ethyl migration is not observed for the closely related 2-ethylbutyl derivative, OpSC{CH_2CH(C_2H_5)CH_2CH_3}(PMe_3), obtained from reaction of 2-ethyl-1-butene with OpSc(H)(PMe_3).

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