Direct condensation of d,l- and l-lactic acids: effect of AlCl3 catalyst on microstructural arrangements of homopolymers and copolymers

The effect of AlCl3 as a catalyst in direct condensation of homopolymerization and copolymerization of (rac and l)-lactic acid was studied. The morphological effect of dimer formation via racemization during polymerization with respect to time and catalyst concentration was investigated at 180°C under vacuum conditions (176 mmHg). The sequence distribution of triads and hexads was analyzed using 13C NMR spectroscopy. A combination of Avrami and Ozawa equations for mathematical modelling of the non-isothermal crystallization kinetics of the synthesized poly(d,l-lactic acid) produced consistent results with the experimental data under these conditions. It is shown that AlCl3 promotes racemization which leads to formation of ill-defined polymers, as demonstrated by determinations of molecular weight distribution (Mn < 5279 g/mol, PDI = 1.4-3), optical purity (91.67% - 99.36%) and melting temperature (143-155°C). The activation energy for crystal growth during the cooling of molten samples of poly(d,l-lactic acid), as determined by Kissinger method, is found to be 45 kcal per mol. In this respect, the microstructural effect of copolymers on their thermal behaviour is discussed.