Group 4 complexes of salicylbenzoxazole ligands as effective catalysts for the ring-opening polymerization of lactides, epoxides and copolymerization of ε-caprolactone with L-lactide

• High yield synthetic route to new Group 4 compounds containing salicylbenzoxazole ligands.
• Efficient catalysts for the synthesis of PLA.
• The synthesis of isoenriched PLAs.
• The kinetics and mechanistic studies with the ROP of lactides is reported.
• Active catalysts toward the copolymerization of lactide and caprolactone.

A series of salicylbenzoxazole pro-ligand 2-(5-X-benzoxazol-2-yl)-6-R1-4-R2-phenol, L1H-L4H were subjected to reaction with titanium, zirconium and hafnium alkoxides. The substituents (R1, R2, X) present in the pro-ligand back bone are (H, H, H) for L1H; (H, H, Cl) for L2H; (Br, Br, H) for L3H; and (Cl, Cl, H) for L4H. The mononuclear Ti complexes, 1a-4a of [M(L)2(OiPr)2] formulation were prepared by combining the pro-ligands L1H-L4H with Ti(OiPr)4 in dry toluene. However, with the addition of H2O (0.5 M equiv.), the controlled hydrolysis of Ti complex 4a in a mixed solvent of THF/toluene produced oxo-bridged dinuclear Ti complex 5a having [M(μ-O)(L)2(OiPr)]2 formulation in good yield (≥75%) using L4H as the pro-ligand. The composition of the mononuclear Zr and Hf complexes of L1H-L4H are found to be quite similar to that of the Ti complexes. Additionally, mononuclear Zr complexes 1b-4b and Hf complexes 1c-4c were prepared. The oxo-bridged dinuclear Zr and Hf complexes 5b and 5c were isolated from the controlled hydrolysis reactions of mononuclear Zr and Hf complexes 4b and 4c. All these complexes were well characterized and crystal structure of six of the fifteen complexes were determined. These complexes were used as initiators for the ring-opening polymerization (ROP) of rac-LA, copolymerization of L-LA with ε-CL and homopolymerization of rac-epoxides. In particular, the dinuclear Zr complex (5b) was found to exhibit extremely high activity in these polymerization studies. Iso-enriched polymers were observed upon using the Zr and Hf complexes of the pro-ligands L3H or L4H as the initiator. DFT studies have been carried out on the ROP of lactides initiated by the Ti, Zr and Hf complexes 1a, 1b and 1c. The results indicated that the activation barrier to reach the transition state using Zr complexes is lower and hence facile as compared to the Ti and Hf complexes.

Group IV complexes of a series of salicylbenzoxazoles (L1H-L4H) are prepared. Alkoxide bearing mono nuclear complexes are formed with these pro-ligands (L1H-L4H). However, control hydrolysis of L4H supported mono nuclear complexes resulted in alkoxide bearing oxo-bridged dinuclear complexes. These complexes are employed for homopolymerization of rac-LA, rac-epoxides and co-polymerization of L-LA with ε-CL. The complexes of L4H exhibited superior reactivity for the polymerization reactions.Figure optionsDownload high-quality image (256 K)Download as PowerPoint slide