The Polymerization Of PLLA In Supercritical Carbon Dioxide

Poly(lactic acid) (PLA ) is a kind of biodegrable materials with super properties. It has been used widely in medical, textile, composite materials fields. It has attracted more and more attention from researchers when the circumstance has been contaminated heavily. Now, there has been a lot of ways in which people can synthesize PLA today. PLA can be synthesized directly from lactic acid and indirectly from lactide using ring-opening polymerization. However, people were not able to avoid using lots of organic solvents in these two polymerization processes. Thus, how to get high molecular PLA under innocuous condition is becoming more and more interest.There were some researchers from America and England tried to synthesize PLA in supercritical carbon dioxide (SC-CO2). They had got small amount of PLA. The amount of polymer is too small to analyse widely on its property and structure. On the other hand, these researchers didn't get PLA with molecular weight (MW) higher than 40000 at the experiments.In this work, a 500 milliliters autoclave was used to synthesize PLLA in SC-CO2. More than ten grams PLLA can be got in each batch.The effect of reaction time, monomer concentration, reaction temperature, purity of CO₂ and amount of catalyst on the MW were studied. The experiments showed that when reaction time was 48 hours, monomer concentration was 0.5 mol per-liter, the molar ratio of monomer to catalyst was 500, and CO₂ was dried, high molecular weight PLLA could be obtained.We tried to increase the molecular weight of PLLA by adding co-solvents into SC-CO₂. Three different co-solvents including acetone, aether and hexafluoroisopropanol (HFIP) were tested. It is found that acetone was the best onewith which we could get the highest molecular weight PLLA.The influence of quantity of acetone to the molecular weight of PLLA was also investigated. It is found that the molecular weight of PLLA could reach 69880 when the molar ratio of acetone to CO₂ was about 1/180.