| In Chapter One, we described the synthesis and characterization of [PO]PdMe(L) (L = pyridine or other bases), and the homopolymerization of ethylene using [PO]PdMe(py) catalysts. Linear polyethylene that contains low levels of Me branches, one C=C unit per chain (mostly 1- or 2-olefins), and Mn in the range 6,000 to 19,000 was obtained. An ethylene polymerization mechanism was proposed based on the high linearity of the polymer, which involves insertion and chain transfer of [PO]Pd(R)(ethylene) species (II), and ethylene trapping and much slower chain-walking of the [PO]Pd(CH2CH 2R) species (III) formed by insertion of II.;Chapter Two described the studies towards the understanding of copolymerization mechanisms of ethylene and vinyl ethers. A soluble base-free dimer {[PO-3,5- tBu]PdMe}2 (3) reacts with excess ethylene or tert-butyl vinyl ether (tBuVE) to form [PO-3,5- tBu]PdMe(monomer) (monomer = ethylene, 2d or tBuVE, 2e) adducts, in which the monomers bind through pi-bonds to Pd. The binding affinities of the two monomers were compared. The activation barriers for monomer insertion of 2d,e were studied. 2d inserts ethylene to form [PO-3,5- tBu]Pd(CH2CH2)nR alkyl species, which inserts ethylene more rapidly than 2d. 2e undergoes 1,2 tBuVE insertion to form the O-chelated [PO-3,5- tBu]PdCH2CH(O tBu)Me species (4), which undergoes very slow ethylene insertion due to the strong chelation. The O-chelated [PO-3,5-tBu]PdCH 2CH(O tBu)Me is therefore one form of catalyst inhibition by tBuVE. 4 also undergoes beta-OR elimination followed by allylic C-H activation to form [PO-3,5- tBu]Pd(allyl) species (5), which is inactive for ethylene polymerization. The formation of [PO-3,5-tBu]Pd(allyl) species is therefore considered as one catalyst deactivation pathway. Finally, the predicted incorporation ratio of tBuVE based on the thermodynamic and kinetic studies was compared to batch copolymerization results.;In Chapter Three, we described the synthesis of the phosphine-bis-arenesulfonate ligand PPh(2-SO3Li-4-Me-Ph)2 (Li2[OPO]) and a series of palladium complexes. (Li-OPO)PdMe(L) species (L = pyridine or 4-(5-nonyl)pyridine) self assemble into tetranuclear complexes {(Li-OPO)PdMe(L)} 4, which are in equilibrium with their monomeric correspondents in solution. The ethylene polymerization was tested using the {(Li-OPO)PdMe(L)}4 catalysts. Under conditions where the tetrameric structure remains substantially intact, the PE contains a substantial high molecular weight fraction, while under conditions where fragmentation is more extensive, the PE contains a large low molecular weight fraction. These results suggest that the tetrameric assembly gives rise to the high molecular weight polymer.;In Chapter Four, the copolymerization of ethylene and vinyl fluoride (VF) with both (PO)PdMe(py) and {(Li-OPO)PdMe(py)}n (n = 1 or 4) were described. Both catalytic systems generate linear copolymers of ethylene/VF, at very low yields. Control experiments in the presence of excess galvinoxyl (a radical inhibitor) ruled out a radical polymerization mechanism, and an insertion mechanism was proposed for both systems. The copolymers from (PO)PdMe(py) contain a dilute VF incorporation (< 0.5 mol%), mostly in-chain -CH 2CHF-CH2- units and very minor to no -CH2CHFCH 3 end units, providing limited information for copolymerization mechanisms. On the contrary, {(Li-OPO)PdMe(py)}4, whether it is mostly a tetramer or a monomeric fragment, catalyze the formation of copolymers with 2.5 - 3.6 mol% of VF incorporation. The VF placements in these copolymers include in-chain -CH2CHFCH2- units (major), chain-end -CH2CHFCH 3 (minor) and -CH2CH=CHF (minor) units. (Abstract shortened by UMI.)... |
