A closer look at ε-caprolactone polymerization catalyzed by alkyl aluminum complexes: the effect of induction period on overall catalytic activity.

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time. We propose that an induction period for Al complexes for conversion to real catalytic species, Al alkoxide, should be considered because the total conversion time consists of both an induction period and polymer propagation time. Herein, the polymerization rate of a series of Al complexes bearing ketimine ligands was investigated. The catalytic results indicated complexes with more steric hindrance with an electron-withdrawing group on the ligands, or the fact that less chelating ligands demonstrated greater propagation activity. The opposite trend for these structural effects was observed on the measurement of induction periods. These features on ligands of aluminum complexes are responsible for facilitating the conversion process to Al alkoxides. The overall catalytic performance should consider both the induction period and the propagation time.