Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted R-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 R-diimine, Ar′ ) 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArNd C(H)C(H)dNAr]Pd(CH 3 )(Cl) and [ArNdC(Me)C(Me)dNAr]Pd(CH 3 ) 2 (Ar ) 2,6-(iPr) 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 palladiumn-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-(iPr) 2 C 6 H 3 , 2-tBuC 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.

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A series of stable dialkyl complexes of Pd, (alpha-diimine)PdR2 (alpha-diimine = aryl-substituted diimine, R = n-Pr, n-Bu, i-Bu), have been prepared via Grignard alkylation of the corresponding (alpha-diimine)PdCl2 complexes. Protonation of these dialkyl species at low temperature results in loss of alkane and formation of cationic Pd beta-agostic alkyl complexes, which have been observed as intermediates in the polymerization of ethylene and propylene by these Pd catalysts. Studies of the structure and dynamic behavior of these alkyl complexes are presented, along with the results of trapping reactions of these species with ligands such as NCMe, CO, and C2H4. Trapping with ethylene results in formation of cationic alkyl ethylene complexes which model the catalyst resting state in these systems. These complexes have been used to obtain mechanistic details and kinetic parameters of several processes, including isomerization of the alkyl ethylene complexes, associative and dissociative exchange with free ethylene, and migratory insertion rates of both primary and secondary alkyl ethylene species. These studies indicate that the overall branching observed in polyethylenes produced by these Pd catalysts is governed both by the kinetics of migratory insertion and by the equilibria involving the alkyl ethylene complexes.

Variable-temperature 1H and 13C NMR spectroscopy has been used to study the dynamics of the β-agostic isopropyl complex (ArNC(R)−C(R)NAr)Pd(CH(CH2-μ-H)(CH3))+BAr4‘- (Ar = 2,6-C6H3(i-Pr)2). 1H and 13C line shape analysis suggests two independent processes occur:  interchange of the agostic and nonagostic methyl groups and exchange of hydrogens within the agostic methyl group, which is best regarded as in-place methyl group rotation. Barriers for both of these processes are similar, with ΔG ⧧ ca. 9 kcal/mol. The methyl groups undergo interchange without inversion at the Pd(II) center.

Late transition metal catalysts bearing R-diimine ligands allow ethylene and R-olefin homoand copolymerizations to polyolefins with unprecedented structures. The polypropylenes made with these new late metal catalysts have very complex microstructures that include combinations of features not seen in any known polypropylenes. These unusual structures include long branches, branches on branches including isobutyl branches, and moderate highly variable levels of 1,3-enchainment leading to runs of methylenes in the backbone of many different well-defined lengths. These features vary with the nature of the catalyst used for polymerization and with the polymerization conditions. Many of the polypropylenes are made primarily by 1,2-insertions to give syndiotactic placements via chain end control. A mechanistic description of catalyst behavior has been developed to explain these observed microstructures.