Ring-opening polymerization of ε-caprolactone initiated with titanium n-propoxide or titanium phenoxide

Ring-opening polymerization of epsilon-caprolactone initiated by titanium n-propoxide Ti(O-n-Pr)(4) was compared to the one initiated by titanium phenoxide or Ti(OPh)(4). Polymerization was confirmed to proceed via a coordination-insertion mechanism for both initiators after end groups analysis by H-1 and C-13 NMR (nuclear magnetic resonance) spectroscopy. Bulk polymerization at 100 degrees C and with a M-0/I-0 (monomer/initiator ratio) equal to 300 was studied and compared with both initiators. The polymerization with Ti(OPh)4 exhibits a slower kinetic of reaction and the polymer synthesized an higher number-average molecular weight and higher polydispersity than with Ti(OPr)(4). These results are consistent with evolution of the average number of active aryloxide groups per initiator ((N) over bar (aag)) during polymerization for this initiator. Indeed, (N) over bar (aag) determined by H-1 NMR increases gradually during the po merization to 2 for around 70% conversion and then decreases at the end of the polymerization and even over the time required for total conversion. This decrease of (N) over bar (aag) at the end of the conversion may be explained by important transesterification reactions with phenoxyl end groups of the polymer, which lead to the formation of larger poly(epsilon-caprolactone) chains. That is consistent with the increase in the number-average molecular weight of the polymer beyond the end of the conversion. The rheological study also confirms this result.