Bulk polymerization of lactides initiated by aluminium isopropoxide, 2a). 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-propa-nol, 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.