New catalysis for fast bulk ring-opening polymerization of lactide monomers

The ring-opening polymerization of lactides has been studied in bulk using either 2-ethylhexanoic acid tin(II) salt, Sn(Oct)(2) or aluminum triisoprop oxide, Al(OiPr)(3), as the initiator over a wide range of temperature and m onomer-to-initiator molar ratio. A high increase in the bulk polymerization rate has been observed when the initiator was added with an equimolar amou nt of a Lewis base, such as triphenylphosphine (P(phi)(3)) and 4-picoline ( C6H7N) added to Sn(Oct)(2) and Al(OiPr)(3), respectively. Melt stable polyl actides of high molecular weight and reasonably narrow molecular weight dis tribution have been accordingly prepared. The use of the Sn(Oct)(2). P(phi) (3) equimolar combination has allowed for reaching an acceptable balance be tween propagation and depolymerization rates, so that the polymerization is fast enough to be performed through a continuous single-stage process in a twin-screw extruder. A global activation mechanism is proposed and discuss ed by comparison with both investigated initiation systems.