SYNTHESIS OF NOVEL POLYMER MATERIALS VIA ORGANOCATALYTIC RING-OPENING POLYMERIZATION (ROP) OF CYCLIC LACTONES

Abstract

The Ring-opening polymerization (ROP) of cyclic lactones has come a long way from 2001 with the aid of organocatalysts. The field of organocatalysts has recent developments to afford precisely tailored biodegradable polyesters via ROP of cyclic esters. However, the organocatalytic ROP of cyclic lactones is still on a laboratory-scale and shows industrial implementation limitations. A series of conformationally flexible bis(thio)urea was developed to understand the structure-function relationship of the multi H-bonding (thio)ureas in the ring-opening polymerization of lactones. The rates of the ROPs showed a strong dependence upon the length and identity of the tether. The bis(thio)urea with five methylene-unit long tether exhibits the fastest ROP and remains active at low catalyst loadings under solvent-free conditions. The organocatalytic ROP of (thiono)macrolactones was conducted for the first time. The driving force of the ROPs of (thiono)macrolactones was studied and displayed entropically driven ROPs with minimal or negligible contribution from enthalpy for the ROP yet, retain the characteristics of living polymerization even at elevated temperatures. The polymers of thionomacrolactones showed altered material properties compared to its polylactones. An oxidative crosslinking process was carried out for the poly(thionolactone)s to synthesize a porous, flexible crosslinked polymer that could facilitate the extraction of Au3+ from an aqueous solution, demonstrating the synthesized crosslinked polymer's potential to use as a water filter for a host of inorganic materials. With the discovery of highly efficient and effective multi H-bonding bis-(thio)ureas for the ROPs, a novel series of bis-(thio)urea catalyst was developed as an enantio-selective chiral catalyst for the ROP of rac-Lactide. This catalyst shows high stereoselectivity and faster reaction rates at mild conditions for the polymerization of rac-LA, forming stereoblock PLA with precise control in molecular weight and enhanced thermal properties. The ROP of a series of substituted lactones was carried out using a cooperative catalyst system - magnesium salts in the presence of a hyperactive imidate catalyst. This study shows the usage of this mixture of catalysts in performing controlled ROP of substituted 7 membered cyclic lactones which do not polymerize even in the presence of an active imidate H-bonding catalyst systems. A series of block-copolymers were synthesized by switching the catalyst and the temperature.