Mechanistic Studies of Pd(II)−α-Diimine-Catalyzed Olefin Polymerizations1

Abstract

Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted α-diimine−Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [(N∧N)Pd(CH3)(OEt2)]BAr‘4 (N∧N = aryl-substituted α-diimine, Ar‘ = 3,5-(CF3)2C6H3), are described. X-ray structural analyses of [ArNC(H)C(H)NAr]Pd(CH3)(Cl) and [ArNC(Me)C(Me)NAr]Pd(CH3)2 (Ar = 2,6-(iPr)2C6H3) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, (N∧N)Pd(R)(olefin)+, are the catalyst resting states and that the barriers to migratory insertions lie in the range 17−19 kcal/mol. Following migratory insertion, the cationic palladium alkyl complexes (N∧N)Pd(alkyl)+ formed are β-agostic species which exhibit facile metal migration along the chain (“chain walking”) via β-hydride elimination/readdition reactions. Model studies using palladium−n-propyl and −isopropyl systems provide mechanistic details of this process, which is responsible for introducing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN∧NAr)Pd(CH3)(OEt2)+ (Ar = 2,6-(iPr)2C6H3, 2-tBuC6H4) occurs by C−H activation of β-C−H bonds of the ortho isopropyl and tert-butyl substituents and loss of methane. The rate of associative exchange of free ethylene with bound ethylene in (N∧N)Pd(CH3)(C2H4)+ is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.