Biomacromolecules, Vol.20, No.5, 1965-1974, 2019
Bulk Organocatalytic Synthetic Access to Statistical Copolyesters from L-Lactide and epsilon-Caprolactone Using Benzoic Acid
The development of synthetic strategies to produce statistical copolymers based on L-lactide (L-LA) and epsilon-caprolactone (CL), denoted as P(LA-stat-CL), remains highly challenging in polymer chemistry. This is due to the differing reactivity of the two monomers during their ring opening copolymerization (ROcP). Yet, P(LA-stat-CL) materials are highly sought after as they combine the properties of both polylactide (PLA) and poly(epsilon-caprolactone) (PCL). Here, benzoic acid (BA), a naturally occurring, cheap, readily recyclable, and thermally stable weak acid, is shown to trigger the organocatalyzed ring-opening copolymerization (OROcP) of L-LA and CL under solvent-free conditions at 155 degrees C, in presence of various alcohols as initiators, with good control over molar masses and dispersities (1.11 < D < 1.35) of the resulting copolyesters. Various compositions can be achieved, and the formation of statistical compounds is shown through characterization by H-1, C-13, and diffusion ordered spectroscopy NMR spectroscopies and by differential scanning calorimetry, as well as through the determination of reactivity ratios (r(LA) = 0.86, r(CL) = 0.86), using the visualization of the sum of squared residuals space method. Furthermore, this BA-OROcP process can be exploited to access metal-free PLA-b-P(LA-stat-CL)-b-PLA triblock copolymers, using a diol as an initiator. Finally, residual traces of BA remaining in P(LA-stat-CL) copolymers (<0.125 mol %) do not show any cytotoxicity toward hepatocyt-like HepaRG cells, demonstrating the safety of this organic catalyst.