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 (C₆H₇N) 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.