Preparation of Poly(lactic acid) blends by heterogenous catalysis using supported 12-tungstophosphoric acid

Preparation of poly(lactic acid) (PLA) by catalytic reactions is a prominent research area for application of this material at the industrial scale. Considered a green polymer, the main challenge in its synthesis includes producing an economical material with high level of enantioselective isomer and low level of contaminants. Supported H3PW12O40 (H3PW) are important catalysts for several processes, especially those that takes place non-polar solvents, which eliminate any leaching concerns of these materials. This work addresses the polymerization of a racemic mixture of D,L-lactic acid, which resulted blends of PLLA and PDLA via heterogeneous catalysis using supported 12-tungstophosphoric acid on activated carbon (C), silica (SiO2) and alumina (Al2O3). These catalysts were characterized by multiple techniques to confirm the integrity of the Keggin anion, as well as acidity as the key property to relate structure-activity. Evidence of the structure was proven by 31P MAS NMR spectroscopy (Figure 1), which is correlated to the highest possible acidity of these materials (Table 1). Optimal conditions (20 wt. % H3PW/Support calcined at 400 ºC; 0.1 wt. % of the catalyst relative to D,L-lactic acid; temperature of 180 ºC and 15 h reaction time) were developed to prepare blends of PLA with up to 95% of enantiomeric excess for poly(L-lactic acid). The average molar mass (Mw) of the formed PLA was up to approximately 17,700 Daltons, and this molar mass was well correlated to the acidity of the catalysts, as revealed by the different blank tests.