Dispersion polymerization of L-lactide in supercritical carbon dioxide

Triblock (A-B-A) oligomers of epsilon-caprolactone (epsilon-CL) (A) and poly(ethylene glycol) with an average molecular weight of 400 (PEG400) (B) were synthesized with three different molecular weight in the range of 2-6 kDa by changing the ratio of PEG400/epsilon-CL. These oligomers were then used in dispersion polymerization of L-lactide in supercritical carbon dioxide (scCO(2)) as stabilizers. 5% stabilizer in the polymerization recipe allowed synthesis of poly(L-lactide) (PLLA) in scCO(2) in the powder form with a weight average molecular weight of around 60 kDa with polymerization yields around 80%. Interestingly, there was almost no effect of stabilizer molecular weight on polymerization. L-lactide polymerization in scCO(2) without any stabilizer was also possible but both the PLLA molecular weight and polymerization yield were lower, and the product was as aggregates instead of powders. A stabilizer concentration of 5% in the polymerization recipe was found adequate. Further increases in the stabilizer load resulted lower molecular weights and lower yields.