Mechanistic studies of Pd(II)-α-diimine-catalyzed olefin polymerizations

Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted alpha-diimine-Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [((NN)-N-boolean AND)Pd(CH3)(OEt2)]BAr'(4) ((NN)-N-boolean AND = aryl-substituted pi-diimine, Ar' = 3,5-(CF3)(2)C6H3, are described. X-ray structural analyses of [ArN= C(H)C(H)=NAr]Pd(CH3)(Cl) and [ArN=C(Me)C(Me)=NAr]Pd(CH3)(2) (Ar = 2,6-(iPr)(2)C6H3) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, ((NN)-N-boolean AND)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 ((NN)-N-boolean AND)Pd(alkyl)(+) formed are beta-agostic species which exhibit facile metal migration along the chain ("chain walking") via beta-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)(2)C6K3, 2-tBuC(6)H(4)) occurs by C-H activation of beta-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 boolean AND N)Pd(CH3)(C2H4)(+) is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.