Propagation group 4 metallocene catalysts

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

Collins, S. Ward, D. G Suddaby, K. H. Group-transfer polymerization using metallocene catalysts Propagation mechanisms and control of polymer stereochemistry. Macrotnolecules 1994, 27, 7222-7224. 

Olefin polymerization with metallocene catalysts involves initiation, chain propagation, formation of a dead chain with a saturated chain end through the chain transfer agent, p-hydride elimination to form a dead chain with a vinyl terminal double bond, insertion of a macromer with a vinyl end group, and catalyst deactivation. Assuming that all the reactions associated with each step are first order, the reaction processes can be expressed as... 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

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