Synthesis, characterization and reactivity of amine-phenolate complexes towards the ROP of rac-lactide and the cycloaddition reactions of carbon dioxide and epoxides

Abstract

Polylactide (PLA) is an important polymer due to its mechanical properties, biodegradability, biocompatibility, and renewable nature. Two main pathways to produce polylactide have been described in the literature: the polycondensation of lactic acid and the ring-opening polymerization (ROP) of lactide (LA). The latter is the most efficient route to produce high molecular weight polymers of low dispersity in the presence of a catalyst and/or an initiator. Furthermore, the transformation of CO₂ into cyclic carbonates is economical and uses a waste feedstock (i.e., CO₂ emissions). In this thesis, tetradentate amino-bis(phenolate) ligands were used to prepare catalysts for rac-lactide polymerization and the cycloaddition reactions of epoxides and CO₂. In these ligands, the substituents on the phenolate groups can be varied. Tetrametallic lithium and sodium complexes were synthesized and characterized; both of these metals are appealing for research in this area because of their low toxicity and cost. The complexes were fully characterized by elemental analysis, nuclear magnetic resonance spectroscopy, X-ray crystallography, and mass spectrometry. The ROP reactions of rac-lactide using tetrametallic lithium and sodium complexes were studied in the melt and solution phase in the presence and absence of benzyl alcohol as co-initiator. All complexes were capable of ring-opening rac-lactide to produce polylactide with and without benzyl alcohol. Data also showed that the complex containing the earth-abundant metal, sodium, gave excellent results. The isolated polymer was characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and gel permeation chromatography (GPC). In addition, iron(III), cobalt(II), and cobalt(III) amino-bis(phenolate) complexes were synthesized and characterized. Their activity in the cycloaddition reaction of propylene oxide and various epoxides with CO₂ to yield cyclic carbonates was investigated. The effect of factors such as reaction conditions and the electronic and steric properties of the substituents on the phenolate rings was studied. The activation energy for the formation of cyclic propylene carbonate using iron(III) complexes was also determined to be close to previously reported values in the literature.