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(CH 3 )(OEt 2 )]BAr‘ 4 (N ∧ N = aryl-substituted α-diimine, Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArN C(H)C(H) NAr]Pd(CH 3 )(Cl) and [ArN C(Me)C(Me) NAr]Pd(CH 3 ) 2 (Ar = 2,6-( i Pr) 2 C 6 H 3 ) 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(CH 3 )(OEt 2 ) + (Ar = 2,6-( i Pr) 2 C 6 H 3, 2- t BuC 6 H 4 ) 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(CH 3 )(C 2 H 4 ) + is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.