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

Citation

Cotter, W. Donald (1993) Organoscandium complexes as mechanistic probes in the Ziegler-Natta polymerization of α-olefins and dienes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11282012-135537271

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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Bercaw, John E.
Thesis Committee:
  • Unknown, Unknown
Defense Date:11 December 1992
Record Number:CaltechTHESIS:11282012-135537271
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:11282012-135537271
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7291
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:28 Nov 2012 22:32
Last Modified:26 Dec 2012 04:46

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