Organoscandium complexes as mechanistic probes in the Ziegler-Natta polymerization of α-olefins and dienes

Author: Cotter, W. Donald

Year: 1993

Degree: Dissertation (Ph.D.)

Advisor: Bercaw, John E.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/9wx4-9x03

Abstract

Measurement of the molecular-weight distributions of alkanes prepared by treatment of permethylscandocene alkyl complexes at -78°C reveals that under the reaction conditions the reaction may be described as "living."

The stoichiometric insertion of 2-butyne across the scandium-carbon σ-bond in Cp*_2ScCH_3 proceeds without a visible kinetic deuterium isotope effect, either for substitution at the alkyne methyl groups or at the scandium-methyl group. The first result reveals that steric isotope effects are not important in this reaction, which models the propagation step in Ziegler-Natta polymerization. The second result suggests that an α-agostic interaction is likely not present in the transition state for 2-butyne insertion.

The polymerization of conjugated dienes with organoscandium complexes of the type {[(η^5- C_5Me_4)SiMe_2NCMe_3]ScR}_2 (R = n-propyl, n-butyl) leads to polymers with very narrow molecular-weight distributions and microstructures rich in 1,2-repeat units and an unusual cyclic repeat unit derived from two sequential 1,2-insertions. The 1,4-repeat unit characteristic of rubber account for about 40 mass-% of the products. Allyl complexes are readily prepared from treatment of alkylscandium complexes with conjugated dienes, or from treatment of the scandium hydride complex {[(η^5-C_5Me_4)SiMe_2NCMe_3]ScH(PMe_3)}_2 with allenes. This hydride complex, however, reacts with butadiene to form an ethylene-bridged complex and ethane.

Both the hydride complex {[(η^5-C_5Me_4)SiMe_2NCMe_3]ScR}_2 and the alkyl complex {[(η^5- C_5Me_4)SiMe_2NCMe_3]Sc(CH_2CH_2CH_3)]_2 catalyze the cyclopolymerization of 1,5-hexadiene. Although polymers with narrow molecular weight distributions (1.1-1.2) can be obtained, the polydispersity index is very sensitive to the presence of trimethylphosphine and, to a lesser extent, to the nature of the initiating group. Chain-transfer processes could not be studied directly due to deleterious reactivity with scandium-hydride trapping reagents. Cyclopolymerization is not accompanied by a measurable a deuterium isotope effect on the cyclization step, which suggests that trans-fused [5,4]-bicyclic transition states are allowed for this step.

Variable-temperature NMR spectroscopy was used to examine the dynamic solution behavior of several ansa-(cyclopentadienyl)amidoscandium alkyl complexes. On the basis of these studies, it is proposed that the primary equilibrium when trimethylphosphine is present is between a phosphine-bound scandium alkyl and a free, 12-electron monomeric scandium alkyl.

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