Carbon(3) symmetric single-site metal alkoxides for the ring-opening polymerization of lactides

This thesis is composed of two parts.; The first part deals with the microstructural analysis of polylactides using multidimensional nuclear magnetic resonance spectroscopy. The 1H, 13C{lcub}1H{rcub} and HETCOR NMR spectra of poly(meso-lactide), poly(racemic-lactide) and poly(atactic-lactide) are reported and the spectroscopic data are discussed in terms of four alternative explanations based on tetrad and triad/pentad sensitivities.; The second part explores the use of single site catalysis towards the control of the microstructure of the polymer. Using the trispyrazolylborate ligand framework, we can control the nature of the metal, that is, to keep it a single site. Novel single site achiral and chiral C-3 symmetric magnesium and zinc trispyrazolylborate alkoxides for the ring opening polymerization of lactides and related monomers have been synthesized. These catalyst precursors polymerize lactides and related cyclic esters in CH2Cl2 solution at 22°C. These tristbutyl pyrazolyl hydroborate metal alkoxides show a marked preference for meso over racemic lactide. The menthone derived catalyst [tris{lcub}7(R)isopropyl-4(R)methyl-4,5,6,7-tetrahydro-2H-indazolyl{rcub}borate] magnesium phenoxide shows greater selectively; it selectively polymerizes meso lactide in the presence of racemic lactide. In contrast, the zinc analogue shows lower diastereoselectivity. The properties of these catalysts together with their reactions with various substrates are described.; Finally, in an ongoing effort to improve enantioselectivity, the synthesis and characterization of novel bidentate chiral C2/C1 (cyclooctane-1,5-diyl)bis(2-indazolyl)boratemetal complexes of potassium and thallium are reported. The potential utility of these new negatively charged ligands is noted with respect to stereoselective metal catalyzed reactions.