Mechanistic Studies of Pd(II)-Catalyzed Copolymerization of Ethylene and Vinylalkoxysilanes: Evidence for a β-Silyl Elimination Chain Transfer Mechanism

Copolymerizations of ethylene with vinyltrialkoxysilanes are reported using both a “traditional” cationic Pd(II) aryldiimine catalyst, t-1 (aryl = 2,6-diisopropylphenyl), and a “sandwich-type” aryldiimine catalyst, s-2 (aryl = 8-tolylnaphthyl). Incorporation levels of vinyltrialkoxysilanes between 0.25 and 2.0 mol % were achieved with remarkably little rate retardation relative to ethylene homopolymerizations. In the case of the traditional catalyst system, molecular weights decrease as the level of comonomer increases and only one trialkoxysilyl group is incorporated per chain. Molecular weight distributions of ca. 2 are observed. For the sandwich catalyst, higher molecular weights are observed with many more trialkoxysilyl groups incorporated per chain. Polymers with molecular weight distributions of ca. 1.2–1.4 are obtained. Detailed NMR mechanistic studies have revealed the formation of intermediate π-complexes of the type (diimine)Pd(alkyl)(vinyltrialkoxysilane)+. 1,2-Migratory insertions of these co...