Synthesis, characterization and reactivity of amine-phenolate complexes towards the preparation of aliphatic polyesters

Ikpo, Nduka (2012) Synthesis, characterization and reactivity of amine-phenolate complexes towards the preparation of aliphatic polyesters. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (15MB)

Abstract

Biodegradable polymers such as poly(ε-caprolactone) (PCL), polylactide (PLA) and polycarbonate (PC) are important materials m commodity, biomedical and pharmaceutical applications. This is due to their biodegradable, biocompatible and permeable properties, and the fact that the monomers can be sourced from annually renewable feedstocks. One of the most important methods in producing these polymers is ring-opening polymerization (ROP) initiated by metal complexes. -- Many metal complexes are effective initiators for the ROP of small ring monomers; however, some undergo side reactions during polymerization that lead to low molecular weight polymers and decrease the level of control in polymer formation. Ligand design and catalyst design using main group and early transition metals have played a vital role in the development of this area. In view of this, Li, Zn, AI, Ti, and Zr complexes bearing various N-, and O-mixed donor monoanionic piperazinyl- and morpholinyl-based phenolate ligands were synthesized in this project. The complexes were fully characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography. In addition, using Brookhart's acid, cationic zinc complex was synthesized and characterized using ¹H, ¹³C, 1 ¹⁹F and ¹¹B NMR spectroscopy. -- The ROP reactions of rac-LA and ε-CL using Zn alkyl and alkoxide complexes were performed. Data showed that polymerization occured immediately using the alkoxide complexes; however, for the alkyl complexes, polymerization was possible only in the presence of alcohol. In addition, microwave irradiation (MW) was found to reduce reaction time significantly while maintaining control of the polymerization reaction. -- Furthermore, the AI and Li complexes are active for the ROP of ε-CL in the presence of benzyl alcohol (BnOH), while the Li complexes are also active in the absence of BnOH. Polymerization kinetics were monitored by observing the degree of conversion of ε-CL to PCL by ¹H NMR spectroscopy. The observed rates and activation energies are related to the steric effects and the outersphere substituents of the complexes. The polymers were analyzed by GPC, MALDI-TOF MS, DSC and TGA. Preliminary investigations into the copolymerization of epoxides with CO₂ using Zn and Al complexes were promising; however, further experiments are required to optimize the reaction method.

Actions (login required)

View Item