Controlled ring-opening polymerization : Polymers with designed macromolecular architechture

by Stridsberg, Kajsa

This thesis summarizes the development of new catalyst systems for ring-opening polymerization and their use in the production of macromolecules with advanced architecture. The influences of reaction conditions, i.e. reaction temperature, solvent and reaction time, on the polymerization kinetics have been evaluated for the ring-opening polymerization (ROP) of 1,5-dioxepan-2-one (DXO) initiated by a cyclic tin-alkoxide. The purpose has been to achieve a controlled ring-opening polymerization of lactones and lactides, resulting in polymers with desirable properties. The mechanism and kinetics of controlled ring-opening polymerization of L-lactide (L-LA) have been investigated, leading to hydroxy telechelic polymers to be used in macromer reactions of various natures. The ROP mechanism of DXO with stannous octoate as catalyst has been investigated theoretically with hybrid density functional methods. A new mechanism has been proposed which provides an explanation of the experimental observations. The ROP mechanism has been shown to involve the formation of a tin-alkoxide complex, which subsequently coordinates a monomer. It has been demonstrated that the ring-opening of the monomer proceeds via a concerted four-center transition state. Elastomeric tri-block copolymers have been obtained from L-LA and DXO with a difunctional tin-initiator. A two-step process has been developed to achieve a well-defined triblock copolymer, poly(L-lactide-b-1,5-dioxepan-2-one-b-L-lactide) (poly(L-LA-b-DXO-b-L- LA)), in good yield. Thermal analysis of the poly(L-LA-b-DXO-b-L-LA) has been used to examine the morphology of the block copolymer. The crystallinity and melting temperature have been shown to increase with increasing amount of L-LA in the copolymer, but the glass transition temperature was only slightly influenced by the polymer composition. Poly(L-LA-b-DXO-b-L-LA) has been subjected to hydrolytic degradation in phosphate buffer solution. The influence of molecular weight and chemical composition on the hydrolysability has been investigated. The molecular weight change, weight loss and composition changes have been characterized in order to determine the degradation pathway. The degradation of poly(L-LA-b-DXO-b-L-LA) was characterized by a significant decrease in molecular weight immediately after immersion in the buffer solution and a progressive increase the amount of L-LA in the remaining copolymer with increasing degradation time. The primary degradation products formed during hydrolysis have been detected as lactic acid and 3-(2hydroxyethyl)-propanoic acid. Results indicate that the composition had no effect on the rate of degradation. The major factor determining the degradation rate was the original molecular weight.