Improving lactic acid melt polycondensation: The role of co‐catalyst

Abstract A study on lactic acid polycondensation under melt conditions was carried out and a preliminary assessment revealed tin powder as a very good catalyst for poly(lactic acid) (PLA) synthesis by melt polycondensation while confirming previous information on SnCl 2 good performance. However, these catalysts also promoted side reactions leading to racemization and yellowing of the final polymer. The use of p ‐toluenesulphonic acid ( p ‐TSA) or triphenylphosphine (PPh 3 ) as co‐catalysts proved to be very effective hindering colour formation and allowing synthesizing PLA samples with enhanced properties. The addition of these compounds to neat tin powder increased the PLA optical purity, whereas their addition to SnCl 2 speeded up the polymerization. A significant increase in molecular weight, from 32,500 to 52,000 g mol −1 , was recorded, with the new catalytic system SnCl 2 /PPh 3 showing catalytic activity comparable with the one reported in the literature for SnCl 2 / p‐ TSA. Several characterization techniques were used for assessing polymer samples: the molecular weights were determined by SEC, thermal behavior measured by DSC, and racemization extent calculated from specific rotation measurements. UV/vis spectroscopy was confirmed as a powerful technique for evaluating yellowing of final polymers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013