ETHYLENE INSERTION AND BETA-HYDROGEN ELIMINATION FOR PERMETHYLSCANDOCENE ALKYL COMPLEXES - A STUDY OF THE CHAIN PROPAGATION AND TERMINATION STEPS IN ZIEGLER-NATTA POLYMERIZATION OF ETHYLENE

The rates of ethylene insertion into the Sc-C bond for Cp⋆2ScR (Cp⋆ = (η5-C5Me5),R = CH3, CH2CH3, CH2CH2CH3) have been measured at-80 °C by 13C NMR; the second order rate constants (M-1·s-1) are as follows: R = CH3, 8.1 (2) ⨯ 10-4; R = CH2CH3, 4.4 (2) ⨯ 10-4; R = CH2CH2CH3,6.1 (2) ⨯ 10-3. The slow rate for the ScCH2CH3 complex is attributed to a ground-state stabilization by a β-C-H “agostic” interaction. The distributions of molecular weights for ethylene oligomers (CH3(CH2)nCH3, = 11-47) produced from known amounts of ethylene and Cp⋆2ScCH2CH2CH3 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⋆2ScCH3 and Cp⋆2ScCH2CH3 and propagation rates equal to that for Cp⋆2ScCH2CH2CH3, the molecular weight distributions of ethylene oligomers are also accurately predicted. Cp⋆2ScCH3 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 β-hydrogen elimination for members of the series of permethylscandocene alkyl complexes Cp⋆2ScCH2CH2R (R = H, CH3, CH2CH3, C6H5, C6H4-p-CH3, C6H4-p-CF3, C6H4-p-NMe2) have been obtained by rapidly trapping Cp⋆2ScH with 2-butyne. A transition state for the β-hydrogen elimination is indicated with partial charge on the β-carbon. Hydrogen is thus transferred to the scandium center as hydride in the β-H elimination process. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.