Date of Award

7-13-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Harmon B. Abrahamson

Abstract

Metal-catalyzed ring-opening polymerization of cyclic esters is an important strategy to prepare high molecular weight polyester materials, which are biodegradable and are produced from renewable resources. Examples of such polymers include polylactide and polycaprolactone. These environmentally friendly polymers have found great utility as drug delivery agents and as packaging materials owing to their biodegradability compared to petroleum-based plastics. Lanthanide phenolate compounds have been investigated as catalysts for the ring-opening polymerization of lactide and ϵ-caprolactone and the results are presented in this dissertation. Exploring the chemistry of lanthanide phenolate compounds has become one of the most valuable and exciting areas of lanthanide chemistry because of their useful application in catalysis. The effects of monoanionic, multidentate, phenolate ligands on the ring-opening polymerization of cyclic esters have been investigated and comparative studies have been carried out with a new dianionic, multidentate, phenolate ligand. Studies of the kinetics of ring-opening polymerization and polymer end group analysis have been undertaken to understand polymerization mechanism. The lanthanide phenolate compounds have been synthesized via two strategies: (1) acid-base transamination reactions using one equivalent of Ln[N(SiMe 3)2]3 (Ln = La, Nd, Sm, Yb) and equivalent amounts of the phenols to yield heteroleptic, trivalent lanthanide phenolate compounds; and (2) exploring the one electron reduction property of divalent lanthanide phenolate compounds formed by reacting Ln[N(SiMe3)2] 2(THF)2 (Ln = Yb, Sm) and corresponding phenols. These divalent lanthanides are oxidized by mild oxidizing agents to yield trivalent lanthanide phenolate compounds. The synthesized divalent and trivalent lanthanide phenolate compounds have been characterized using nuclear magnetic resonance and infrared spectroscopy, elemental analysis and in some cases, X-ray crystallography. All compounds were assessed as catalyst precursors towards the ring-opening polymerization of both L-lactide and ϵ-caprolactone. These polymerization studies revealed that catalysts containing larger lanthanide metals were more active than those with smaller metals. Furthermore, replacement of the [N(SiMe3)2] initiating group with [BPh4 ] suppressed catalytic activity. Detailed studies of the kinetics of ring-opening polymerization of L-lactide by the neutral lanthanum compounds, and polymer end-group analysis by electrospray ionization (ESI) mass spectrometry revealed that the monoanionic, bidentate phenolate ligand [(L)-)] is an effective initiating group and accounted for a second order kinetics in lactide consumption while the dianionic, tridentate phenolate ligand [(LI)2- remains bound to the lanthanide metal center during polymerization reactions.

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