Preparation of PLA blends by polycondensation of D,L-lactic acid using supported 12-tungstophosphoric acid as a heterogeneous catalyst

Poly(lactic acid) (PLA) is a significant polymer that is based on renewable biomass resources. The production of PLA by polycondensation using heterogeneous catalysis is a focus for sustainable and economical processes. A series of samples comprising 12-tungstophosphoric acid (H3PW) supported on activated carbon, silica, and alumina induced the catalytic polymerization of D, L-lactic acid to form blends of PLA. The catalysts were characterized by multiple techniques to confirm the integrity of the Keggin anion as well as the acidity, which is the key property for relating structure to activity. The best reaction conditions were established for H3PW/C and tested for the other supported catalysts. The obtained polymer was a blend that was characterized as an enantiomeric excess (ee) of as much as 95% L-PLA (PLLA) with a mass average molar mass (M-w) of approximately 14,900 daltons. The role of H3PW in these polymerizations was demonstrated, i.e., without the Keggin acid, only oligomeric units (M-w < 10,000 daltons) could be obtained. Additionally, inverse relationships between the M-w of PLA and the enthalpy (-Delta H) of the strongest sites of the catalysts were distinguished, i.e., PLA(Mw-H3PW/C) > PLA(Mw-H3PW/Al2O3) > PLA(Mw-H3PW/SiO2), whereas the acidity (-Delta H) order was as follows: H3PW/SiO2 > H3PW/Al2O3 > H3PW/C. These findings could be attributed to the correct tuning of strength and the accessibility of the sites to produce longer polymeric chains.