Catalytic ring-opening polymerization of cyclic esters to biodegradable polyesters using N,N'- and N,O-ligand supported Cu(II), Mg(II) and Zn(II) complexes.
Over the past decades, there has been a tremendous increase in market demand for polyesters and their co-polymers. Of interest, polycaprolactone (PCL) and polylactides (PLA) which are biodegradable have found widespread applications in the packaging and biomedical fields. Polyesters are produced via ring-opening polymerization (ROP) process using metal-based metal-catalyst/initiators, with industrial production relying on tin(II) compounds. Despite the intense research efforts devoted to this area, there are still considerable limitations. For example, in case of chiral lactides monomers very few catalytic systems are capable of stereoselective synthesis. In addition, there is also lack of control of the polymerization process to curb side reactions which results in low molecular weight polymers with broad molecular distributions. Furthermore, the toxicological effects associated with tin compounds pose a danger if polymers are applied in the biomedical field since it is difficult to completely remove remnant catalyst from the polymer matrices. Thus, this thesis investigated the synthesis of less toxic metal complexes such as zinc, copper and magnesium supported by strategically designed ligands and their application in ROP. Four different class of ligands were explored as supports namely formamidine, N-hydroxy formamidine, Schiff base phenoxide and chiral amino pyridyl ligands and thirty complexes were synthesized and reported in this thesis. The steric and electronic properties of the ligands were fine-tuned to influence the catalytic activity and the polymer properties. The effect of the nature of the metal—oxygen bond which is prerequisite for ring-opening polymerization was investigated. Complexes with acetate and alkoxide reactive ligands were synthesised where the oxygen was not part of the ligand system. N-hydroxy formamidine and Schiff base phenoxide ligands contain the oxygen heteroatom as part of the ligand backbone. All the complexes polymerized caprolactone and lactides with appreciable activity, however for hydroxy formamidine ligands the polymerization complexes were more active only in the presence of co-initiator. The effect of auxiliary ligands such as acetates, alkoxides was also investigated. The polymerization data showed that catalytic activity depended on the metal identity, steric crowding and auxiliary ligands. Generally, zinc acetate complexes were more active achieving complete monomer conversion within 68 h compared to 120 h for the copper analogues.Magnesium-amino phenolate complexes showed greater activity, attaining 99% monomer conversion in less than 32 h as compared to 55 h for the zinc analogues. The zinc pyridyl alky and alkoxide complexes showed excellent activity, achieving 100% monomer conversion within 1 min at room temperature. Bulk substituents and electron withdrawing substituent resulted in reduced catalytic activity. All catalytic systems produced low molecular weight polymers ranging from 1200 to 10 500 g mol-1 with relatively broad molecular weight distributions and PDIs that lie between 1.2 and 2 pointing to semi-living polymerization. Chiral ligand supported catalysts showed good stereoselectivity in polymerization of rac-lactide (rac-LA) with Pr values ca 0.70. The role of the solvent was studied, and it was observed that coordinating solvent such as THF retards the polymerization as they compete with the monomer for catalytic active sites. Detailed abstracts are given in each of chapters 3 to 6.
Doctoral Degree. University of KwaZulu-Natal, Durban.