Ethylene insertion and β-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene

Abstract

The rates of ethylene insertion into the Sc-C bond for Cp*_2ScR (Cp* = (η^5-C_5Me_5), R = CH_3, CH_2CH_3, CH_2CH_2CH_3) have been measured at -80 °C by ^(13)C NMR; the second order rate constants (M^(-1) S^(-1)) are as follows: R = CH_3, 8.1 (2) X 10^(-4); R = CH_2CH_3,4.4 (2) X 10^(-4); R = CH_2CH_2CH_3, 6.1 (2) X 10^(-3). The slow rate for the ScCH_2CH_3 complex is attributed to a ground-state stabilization by a β-C-H "agostic" interaction. The distributions of molecular weights for ethylene oligomers (CH_3(CH_2)_nCH_3, n = 11-47) produced from known amounts of ethylene and Cp*_2ScCH_2CH_2CH_3 at -80 °C satisfactorily fit a Poisson distribution, indicative of a "living" Ziegler-Natta polymerization system. From the measured, slower initiation rates of insertion for Cp*ScCH_3 and Cp*_2ScCH_2CH_3 and propagation rates equal to that for Cp_2ScCH_2CH_2CH_3, the molecular weight distributions of ethylene oligomers are also accurately predicted. Cp*ScCH_3 undergoes a single insertion with 2-butyne with a moderate enthalpy of activation and a large, negative entropy of activation. The second-order rate constants for the insertion of 3-phenyl-2-propyne, 2-pentyne, and 4-methyl-2-pentyne have been measured. The rates for &hydrogen elimination for members of the series of permethylscandocene alkyl complexes Cp*_2ScCH_2CH_2R (R = H, CH_3, CH_2CH_3, C_6H_5, C_6H_4-p-CH_3, C_6H_4-p-CF_3, C_6H_4-p-NMe_2) have been obtained by rapidly trapping Cp*ScH with 2-butyne. A transition state for the β-hydrogen elimination is indicated with partial charge on the &carbon. Hydrogen is thus transferred to the scandium center as hydride in the β-H elimination process.

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