Aryloxy and benzyloxy compounds of hafnium: Synthesis, structural characterization and studies on solvent-free ring-opening polymerization of ε-caprolactone and δ-valerolactone

A large variety of aryloxy compounds and benzyloxy derivatives of Hf(IV) were synthesized from Hf(O-tBu)4 using the alcoholysis route. These compounds were completely characterized using various spectroscopic and analytical methods along with single crystal X-ray diffraction studies. Multinuclear NMR studies prove high degree of fluxional behavior of these compounds in solution. They adopt a dimeric structure in the solid-state as seen from X-ray diffraction studies on a few of them. These compounds are powerful catalysts for the ring-opening polymerization of ε-caprolactone (CL) and δ-valerolactone (VL) resulting in high number average molecular weight (Mn) polymers and controlled molecular weight distributions (MWDs). Significant control in the polymerization was achieved with these compounds as initiators in comparison to their titanium and zirconium analogues. Kinetic studies reveal that the rates of such polymerizations are in general slower than the corresponding titanium and zirconium derivatives. 1H NMR and MALDI-TOF studies of low molecular weight oligomers suggest that these polymerizations proceed by the activated monomer mechanism.