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

Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted alpha-diimine-Pd(II) complexes are presented. Syntheses of several cationi c catalyst precursors, [((NN)-N-boolean AND)Pd(CH3)(OEt2)]BAr'(4) ((NN)-N-b oolean AND = aryl-substituted pi-diimine, Ar' = 3,5-(CF3)(2)C6H3, are descr ibed. 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 su bstituents crowd axial sites in these square planar complexes. Low-temperat ure NMR studies show that the alkyl olefin complexes, ((NN)-N-boolean AND)P d(R)(olefin)(+), are the catalyst resting states and that the barriers to m igratory insertions lie in the range 17-19 kcal/mol. Following migratory in sertion, the cationic palladium alkyl complexes ((NN)-N-boolean AND)Pd(alky l)(+) formed are beta-agostic species which exhibit facile metal migration along the chain ("chain walking") via beta-hydride elimination/readdition r eactions. Model studies using palladium-n-propyl and -isopropyl systems pro vide mechanistic details of this process, which is responsible for introduc ing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN NAr)Pd(CH3)(OEt2)(+) (Ar = 2,6-(iPr)(2)C 6K3, 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 ass ociative exchange of free ethylene with bound ethylene in (N boolean AND N) Pd(CH3)(C2H4)(+) is retarded by bulky substituents. The relationship of the se exchange experiments to chain transfer is discussed.