Dispersion polymerization of L-lactide in supercritical carbon dioxide

Triblock (A-B-A) oligomers of ε-caprolactone (ε-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/ε-CL. These oligomers were then used in dispersion polymerization of L-lactide in supercritical carbon dioxide (scCO₂) as stabilizers. 5% stabilizer in the polymerization recipe allowed synthesis of poly(L-lactide) (PLLA) in scCO₂ 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 scCO₂ 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.