This thesis reports on the synthesis, characterization, and chemistry of a new homogeneous catalytic polymerization system incorporating the metal yttrium and the hydrotris(pyrazolyl)borate (Tp) ligands. These new organoyttrium catalysts have been found to be active for the polymerization of both polar and nonpolar unsaturated species to give high molecular weight polymeric materials without the aid of an activating cocatalyst. Several catalytic precursors have been prepared and characterized based on different substituted Tp ligands having the general formula Tp$\rm\sp{R}YX\sb2$ (THF) (R = H, Me, Ph and X = halide). Reaction of these dihalide catalytic precursors with two equivalents of various alkyllithium reagents cleanly affords in high yields the yttrium (III) dialkyl complexes isolated as the tetrahydrofuranate adduct, Tp$\rm\sp{R}YR\sp\prime\sb2$(THF) (R = H, Me, Ph and R$\sp\prime$ = H, Me, Ph, $\rm CH\sb2SiMe\sb3).$ The yttrium dialkyl complexes incorporating the hydrotris(3,5-dimethylpyrazolyl)borate (Tp$\rm\sp{Me})$ ligand, $\rm Tp\sp{Me}YR\sb2$(THF) (R = Ph, $\rm CH\sb2SiMe\sb3),$ have been found to be especially active for polymerization of ethylene, alkyl methacrylates, acrylonitrile, and propylene oxide. Polyethylene produced with these catalysts is highly linear (T$\rm\sb{m}=143\sp\circ$C) and high molecular weight (M$\rm\sb{w}=2\times10\sp6)$ with moderate polydispersity. Methyl methacrylate also rapidly polymerizes with these catalysts producing poly(methyl methacrylate) with a high percent syndiotacticity (rrr = 95%) utilizing relatively short reaction times and mild conditions (30-60 min./${-}30\sp\circ$C). The copolymerization of ethylene and MMA has also been achieved producing a diblock copolymer of these two unlike species. Polymerization is consistent with the Cossee mechanism and insertion into the active Y-C bonds present in these catalysts. Polymerization and NMR data suggests the 3-methyl substituent on the Tp$\rm\sp{Me}$ ligand is responsible for the dissociation of the coordinated THF which opens a site in the crowded coordination sphere around the yttrium center, enabling insertion polymerization of various monomeric species. The synthesis and characterization of a new class of compounds, the bispoly(pyrazolyl)borate yttrium (III) halides ((Tp$\rm\sp{R})(Bp\sp{R\sp{\prime}})$Y-X), which offer a new route to desolvated monohalide lanthanide complexes, will also be discussed.
