Olefin Polymerization with Single Component Organoscandium and Organoyttrium Catalysts

Abstract

Single component catalysts for polymerization of olefins based on organoscandium and organoyttrium compounds of the types (η ⁵-C₅Me₅)₂Sc-R, {(η ⁵-C₅Me₄)₂SiMe₂} Sc-R, meso-{(η ⁵-C₅H₃-3-CMe₃)₂SiMe₂} Sc-R, {(η ⁵-C₅Me₄) SiMe₂(η ¹-NCMe₃)} Sc-R, and the C ₂-symmetric complexes rac-{(η ⁵-C₅ H₂-2-SiMe₃-4-CMe₃)₂ SiMe₂}Y-R (R = H, alkyl) have been prepared. All of these compounds rapidly polymerize ethylene. {(η ⁵-C₅Me4)₂SiMe₂}Sc-R and meso-{(η ⁵-C₅H₃-3-CMe₃)₂SiMe₂} Sc-R catalyze the head-to-tail dimerization of α olefins. (η ⁵-C₅Me4)SiMe₂(η ¹-NCMe₃)}Sc-R produces low molecular weight, essentially atactic polymers from α olefins. [rac-{(η ⁵-C₅H₂-2-SiMe₃-4-CMe₃) ₂SiMe₂}Y]₂(μ-H)₂ polymerizes propylene, 1-butene, 1-pentene and 1-hexene to highly isotactic, moderate molecular weight polymers. ¹³C NMR spectra of the polypropylene so obtained indicates greater than 99% mmmm pentads, with greater than 99.9% 1,2-insertions. The possibility that an α agostic C-H interaction is assisting olefin insertion for these catalysts has been probed using Grubbs’ test of “deuterium isotopic perturbation of stereochemistry.” Our findings implicate an a agostic structure for the transition state for chain propagation. Ground state structures for the ethyl derivatives may also have β agostic interactions. The α agostic transition state has one less rotational degree of freedom, reinforcing the enantiofacial preference of the chiral bis(cyclopentadienyl)metal site, and thus likely contributes to the high enantioselectivity of the C₂-symmetricrac-{(η ⁵ -C₅H₂-2-SiMe₃-4-CMe₃)₂ SiMe₂}Y-R catalyst system.