Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ?-Caprolactone and Substituted ?-Caprolactones and Their Copolymerizations

Two series of titanium complexes, includingsalicylbenzoxazole titanium complexes (1-4) and salicylbenzo-thiazole titanium complexes (5-8), were successfully synthesizedand characterized by NMR spectroscopy, elemental analysis, andX-ray diffraction crystallography (for2and5). The1H NMRspectra of complexes7and8revealfluxional behavior in solutionat room temperature, and the activation parameters weredetermined by lineshape analysis of variable-temperature (VT)NMR spectra in toluene-d8: for7,Delta H???= 73.0 +/- 1.8 kJ mol-1,Delta S???= 22.1 +/- 5.5 J mol-1K-1; for8,Delta H???= 73.7 +/- 1.2 kJ mol-1,Delta S???=20.3 +/- 3.8 J mol-1K-1. The positive values of Delta S???suggested thatthe isomerization occurred via a dissociative mechanism. Allcomplexes were active initiators for the ring-opening polymer-ization of epsilon-caprolactone (epsilon-CL) and three substituted epsilon-CLs:gamma-methyl-epsilon-caprolactone (gamma MeCL),gamma-ethyl-epsilon-caprolactone (gamma EtCL),and gamma-phenyl-epsilon-caprolactone (gamma PhCL). Of all complexes, complex5was found to be the most active initiator in this study. Thecopolymerizations between epsilon-CL and three substituted epsilon-CLs produced completely random copolymers. The polymerization wasproposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titaniumcomplex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiledthat the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titaniumcomplexes were the key factors in enhancing the catalytic rate