Aluminum coordination complexes in polymerization catalysis

Plommer, Hart (2019) Aluminum coordination complexes in polymerization catalysis. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Polymers based on renewable feedstocks are desirable alternatives to fossil fuel-based conventional plastics. Aliphatic polycarbonates and some polyesters including poly(ε-caprolactone) (PCL) are biodegradable polymers commonly produced by ring-opening copolymerization (ROCOP) and ring-opening polymerization (ROP). Controlled, active, inexpensive, and non-toxic catalysts are needed for these ROP reactions. In this thesis, aluminum amino-phenolate complexes bearing different pendent donors were synthesized and fully characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. The complexes were investigated as catalysts for ROP of cyclohexene oxide (CHO), ROCOP of CHO/CO₂, and ROP of ε-caprolactone (ε-CL). The produced polymers were analyzed by gel permeation chromatography (GPC), matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and differential scanning calorimetry (DSC). Complexes containing different outer-sphere heteroatoms (NMe in N-piperazinyl or O in N-morpholinyl) showed markedly different reactivities in ROP of CHO, with the N-morpholinyl catalyst being more active. In ROCOP of CHO/CO₂, pendent donors within the aluminum complexes had considerable influence on the selectivity towards copolymer. In addition, removal of the pendent donor generated catalyst systems that were more active. Generally, the copolymers produced were of strictly alternating character when bis(triphenylphosphoranylidene)iminium chloride (PPNCl) was used as a co-catalyst. Cationic aluminum complexes were prepared which contained a weakly coordinating anion (WCA) and the cationic aluminum centre was stabilized by a bidentate N-morpholinyl moiety bound through both the oxygen and nitrogen atoms. When the aluminum complexes containing a WCA stable to alcoholysis were combined with protic co-initiators (EtOH, glycerol carbonate) they formed living catalyst systems for the ROP of ε-CL, producing poly(ε-caprolactone) (PCL) with narrow dispersity (Đ = 1.00–1.05). Kinetic studies allowed the determination of activation parameters, which were consistent with a coordination-insertion mechanism.

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