The Synthesis of Biodegradable Polycarbonates and Polyesters from Renewable Resources Using Highly Active (Porphyrin)Cr(III) Chloride Catalysts

A series of porphyrin metal(III) compounds of general formula LMX (where L = tetraphenylporphyrin, tetrakispentafluorophenylporphyrin and octaethylporphyrin, M = Al, Cr, Co, and X = Cl and OEt) has been synthesized and studied as catalysts for the ring opening polymerization of propylene oxide (PO) as well as copolymerization of PO with CO2 to produce polypropylene oxide (PPO), polypropylene carbonate (PPC) and propylene carbonate (PC), respectively. The electronic nature of the porphyrin ligand and the addition of Lewis basic cocatalysts e.g. DMAP and PPNCl showed a dramatic influence on the catalytic TOF, product selectivity and polymeric microstructure. The origin of these effects was attributed to the electronic factors associated with the individual reaction steps and equilibriums. Introduction Recently the copolymerization of aliphatic epoxides and CO2 by metal coordinate catalysis has received significant attention as an environmentally benign synthetic route to the With sincere thanks to my Advisor Prof. Malcolm H. Chisholm for his guidance and support. production of polycarbonates (Scheme 1). This copolymerization process is often associated with a competing reaction, which produces the cyclic carbonate. Scheme 1. Copolymerization of epoxide and CO2 by metal catalyst of general formula LM-OR, where L is the ancillary ligand and M is the Lewis acidic metal center. This atom efficient process utilizes CO2 as an inexpensive, sustainable, nontoxic C1 feedstock and is an attractive alternative to the conventional synthesis, which utilizes petrochemical feedstocks and toxic phosgene derivatives. Following the pioneering work of Inoue, there has been an explosion of interest in this copolymerization reaction, however, the aliphatic polycarbonates are still mainly used as sacrificial binders in ceramic industry. More widespread application of these materials requires low cost production and improved physical and mechanical properties, which in turn need higher catalytic turnover and control in the polymerization process. Hence matters pertaining to the catalytic process become crucial, see Scheme 2. The ring opening of the epoxide, namely propylene oxide by the metal catalyst produces the metal alkoxide intermediate which subsequently reacts with CO2 to produce the metal carbonate species. The relative reactivity of these two competing intermediates determine the fate of the reaction and polypropylene carbonate is formed as the major product when k2 > k3 > k1. Additionally, nucleophilic attack of the metal alkoxide group on adjacent carbonate functionality forms the cyclic propylene carbonate. Thus a better understanding of the individual reaction steps and equilibria involved in the polymerization process will be useful to improve catalytic activity and selectivity.