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

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 (ε″) 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.