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 triisopropoxide, Al(OiPr) 3 , as the initiator over a wide range of temperature and monomer‐to‐initiator molar ratio. A high increase in the bulk polymerization rate has been observed when the initiator was added with an equimolar amount of a Lewis base, such as triphenylphosphine (P(φ) 3 ) and 4‐picoline (C 6 H 7 N) added to Sn(Oct) 2 and Al(OiPr) 3 , respectively. Melt stable polylactides of high molecular weight and reasonably narrow molecular weight distribution have been accordingly prepared. The use of the Sn(Oct) 2 .P(φ) 3 equimolar combination has allowed for reaching an acceptable balance between 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 discussed by comparison with both investigated initiation systems.