Microwave-assisted poly(D,L-lactide) synthesis in toluene and tetrahydrofuran

Abstract

Poly(d,l-lactide) is a biocompatible and biodegradable polymer with applications in the biomedical field (drug delivery, implants) and packaging. Conventional synthesis with stannous octoate is slow (>4 h) and can climb to over 30 h. In order to reduce reaction times, we developed a microwave reactor process to ring-open polymerize d,l-lactide to form poly(d,l-lactide) in the presence of stannous octoate and an initiator, benzyl alcohol. We evaluated the suitability of toluene and tetrahydrofuran as solvents at 130, 150, and 170°C for the polymerization. Their respective dielectric loss $\left(\epsilon ,″\right)$ values are 0.1 and 0.35. Compounds with larger dielectric loss values are better at converting microwave energy to heat. The microwave's power input peaked at 420 W to reach 170°C with toluene, whereas with tetrahydrofuran the peak was 330 W; afterwards, the power input to maintain that temperature was 10 W for both solvents. A reaction in toluene at 170°C after 1 h produced poly(d,l-lactide) with a molecular weight of 31 kDa and a dispersity index of 1.5. In tetrahydrofuran, at the same temperature, the molecular weight peaked at 11 kDa after 4 h with a dispersity index of 1.2. Moreover, in the absence of microwaves the polymerization does not occur. Tetrahydrofuran is hygroscopic and water cleaves poly(d,l-lactide) chains resulting in a lower molecular weight despite the longer reaction time and larger dielectric loss compared to toluene, a water immiscible solvent.