Mechanistic study of Sn(Oct)₂-catalyzed ε-caprolactone polymerization using Sn(Oct)₂/BF₃ dual catalyst

A novel method was used to investigate the mechanism of Sn(Oct)2-catalyzed e-caprolactone polymerization by using Sn(Oct)2/BF3 dual catalyst. The bulk polymerization was conducted at 110 and 130°C with different Sn(Oct)2/BF3 ratios. The polymerization kinetics was followed using gel permeation chromatography, and the molecular structures of the low-molecular weight polymers were examined using 1H-nuclear magnetic resonance (NMR). A polymerization induction period was observed in polymerizations containing the Sn(Oct)2 catalyst, but it was not observed in the system containing only BF3. After the induction period, BF3 and Sn(Oct)2 initiated the polymerization separately. For Sn(Oct)2 catalyst with no purposely added alcohol, the actual initiation species is a tin hydroxide species formed in situ by the reaction of Sn(Oct)2 and adventitious water. For BF3 catalyst, the active species is the protonic acid formed by the reaction of BF3 with the adventitious water. When mixed, the Sn(Oct)2 reacts with the adventitious water faster than the BF3, preventing the BF3 catalyzing any polymerizations during the induction period.