Nb and Ta benzotriazole or benzoxazole phenoxide complexes as catalysts for the ring-opening polymerization of glycidol to synthesize hyperbranched polyglycerols

Abstract

A series of novel mononuclear (1a–7a and 1b–6b) as well as tetranuclear (8a and 9a) niobium (Nb) and tantalum (Ta) complexes of benzotriazole or benzoxazole phenoxide pro-ligands bearing different substituents at the ortho and para positions of the phenol rings were synthesized and characterized. The reaction of NbCl₅ or TaCl₅ with one equivalent of benzotriazole or benzoxazole phenoxide pro-ligands (L1H–L6H) in dry toluene or chloroform produced the corresponding chloride (1a–6a and 1b–6b) and ethoxy (7a) mononuclear Nb and Ta complexes in high yields. Furthermore, from the mononuclear Nb complexes (1a or 4a), a new structural form of tetrameric niobium complexes (8a and 9a) was synthesized through a controlled hydrolysis reaction. The molecular structures of complexes 1b, 4b, 7a, 8a and 9a were unambiguously confirmed by single crystal X-ray diffraction analyses. Furthermore, all these complexes (1a–9a and 1b–6b) were tested as catalysts for the ring-opening polymerisation (ROP) of glycidol to synthesize hyperbranched polyglycerols (HPG) by using 1,1,1-tris(hydroxymethyl)propane (TMP) as an initiator. The degree of branching (DB) observed was 0.30–0.54, which is an indication of hyperbranched structures. In particular, for the niobium complex with electron-withdrawing substituents on the benzoxazole phenoxide pro-ligand (5a), we achieved superior behavior for the ROP of glycidol in terms of activity, control of molecular weight (M_n) and molecular weight distributions (MWDs) (92% of glycidol to HPG, M_n = 10.52 kg mol⁻¹, MWDs <1.33, DB = 0.53 and T_g = −57 °C). A highly hydrophilic nature was observed for the synthesized HPG polymer by water contact angle measurement (20° to 35°).