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Abstract

This thesis describes new Pd(II)-alkyl catalysts for ethylene polymerization and strategies for enhancing their catalytic performance. These catalysts feature chelating, electronically-asymmetric ligands that contain strong phosphine and weak sulfonate or phosphonate donor groups. The effects of the phosphonate donor group and the remote binding of Lewis acids to the phosphonate on catalyst performance was explored. Synthetic studies on the self-assembly of Pd(II)-alkyl complexes containing phosphine-bis-arenesulfonate ligands to tetrameric cage structures that can polymerize ethylene to high molecular weight polyethylene is also described. Chapter One introduces Pd(II)-alkyl complexes that contain ancillary phosphine-arenesulfonate (PO) ligands. (PO)PdRL catalysts are unique in their ability to copolymerize ethylene and a wide variety of polar vinyl monomers to highly linear functionalized polyethylene. The ability to produce new types of functionalized polymers with enhanced properties motivates the development of new PO-type Pd catalysts and new strategies for enhancing the catalytic performance of these catalysts. Chapter Two describes Pd(II)-alkyl complexes containing phosphine-monoalkyl phosphonate ligands ([1-PAr2-2-PO2(OR)-Ph]– [PPO]–). The complexes (κ2-1-PAr2-2-PO2(OR)-Ph)PdMeL ((PPO)PdMeL) were prepared and characterized in the solid state and in solution. The reaction of the Pd complexes with one or two equiv of B(C6F5)3 generates the borane adduct (κ2-1-PAr2-2-P(O)(O-B(C6F5)3)(OR)-Ph)PdMeL or the base-free, borane adduct {(κ2-1-PAr2-2-P(O)(O-B(C6F5)3)(OR)-Ph)PdMe}2, respectively. The borane adducts were characterized by X-ray crystallography and in solution. The reactivity of the (PPO)PdMeL complexes with other Lewis acids is also described. Chapter Three describes the ethylene polymerization performance of the complexes prepared in Chapter Two. Remote binding of B(C6F5)3 to (PPO)PdMeL (L =pyridine or lutidine) or {(PPO)PdMe}2 ethylene polymerization catalysts that contain phosphine-monoalkyl phosphonate ligands significantly increases the catalyst activity and the molecular weight of the polyethylene (PE) product. X-ray structural data, trends in ligand lability, and comparative studies of BF3 activation suggest that these allosteric effects are primarily electronic in origin. The B(C6F5)3 binding enhances Rgrowth by increasing the degree of positive charge on the Pd center. This effect does not result in the large increase in Rtransfer and concomitant reduction in PE molecular weight seen in previous studies of analogous (PO)PdRL catalysts that contain phosphine-arenesulfonate ligands, because of the operation of an unusual dissociative chain transfer process, which is inhibited by the increased charge at Pd. Chapter Four describes synthetic studies that were undertaken to further probe the factors that influence the self-assembly of four (Li-OPO)PdRL units containing phosphine-bis-arenesulfonate [Li-OPO]– ligands into tetranuclear cage structures. The factors that favor the self-assembly of the (Li-OPO)PdRL units around the periphery of a central Li4S4O12•Li2Cl2 expanded core over a Li4S4O12 double-four-ring (D4R) core were explored. New [Li-OPO]– ligands were synthesized with either meta- or para-substituents, and the generation of tetrameric Pd assemblies based on Li4S4O12•Li2(OAc)2 and Li4S4O12•Li2Br2 cores was explored to determine if cage core expansion can be achieved using other M2X2 units besides Li2Cl2.

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