Bulk polymerization of lactides initiated by aluminium isopropoxide, 2: Beneficial effect of Lewis bases and transfer agents

A high increase in the bulk polymerization rate of L,L-lactide and racemic D,L-lactide is observed when aluminium isopropoxide is added with an equimolar amount of a Lewis base, such as triphenylphosphine (P(φ)₃) and better 4-picoline (C₆H₇N). Polylactide of high molecular weight and narrow molecular weight distribution is accordingly prepared in a controlled manner when the Al(OiPr)₃ · C₆H₇N combination is used as an initiator over a large temperature range from 110 to 180°C. A secondary alcohol, such as 2-propanol, has proved to be an efficient transfer agent, which allows the amount of Al per kg of polylactide to be advantageously reduced. All the alcohol molecules participate in the ring-opening polymerization even at an [iPrOH]₀/[Al] ratio as high as 10³.