Solvent-Free Synthesis of Poly(lactic acid) by Reactive Extrusion: Role of α-Tocopherol and Catalyst Loading

Abstract Poly(lactic acid) (PLA) was synthesized by ring-opening polymerization (ROP) of l-lactide through reactive extrusion using Sn(Oct)2 as the catalyst, 1-dodecanol as the initiator, and α-tocopherol as a natural antioxidant. This work combined reaction-time monitoring with structure–property analysis to define the optimum processing window and formulation. In the first stage, samples collected during reactive extrusion were analyzed by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR), showing the progressive consumption of l-lactide and the formation of PLA. The main monomer-related signals disappeared after approximately 12 min, while the most favorable thermal response was obtained after 18 min, indicating an optimum balance between conversion and thermal exposure. In the second stage, the effects of α-tocopherol and catalyst:monomer ratio were evaluated by mechanical, thermal, colorimetric, gel permeation chromatography (GPC), and FTIR analyses. For the 1:1000 formulation, α-tocopherol increased the molecular weight to 30,926 g mol–1 and the tensile strength to 34.1 MPa while also enhancing crystallinity and preserving a lighter, less yellow appearance. Among the antioxidant-containing samples, the 1:1000 catalyst:monomer ratio provided the best overall balance between conversion, molecular weight, mechanical performance, and thermal behavior. FTIR confirmed successful PLA formation in the optimized formulations, whereas the 1:5000 sample retained lactide-related bands, consistent with incomplete conversion and inferior properties. These results demonstrate that reactive extrusion is a viable solvent-free route for PLA synthesis when residence time, natural antioxidant stabilization, and catalyst loading are jointly optimized.