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Synthesis and Characterization of Benzotriazole or Benzoxazole Phenoxide Ligands Supported Nb and Ta Metal Complexes and their Application for the Synthesis of Hyperbranched Polyglycerols by Ring-Opening Polymerisation of Glycidol

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 position of the phenol rings were synthesized and characterized. Reaction of the 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. Further from the mononuclear Nb complexes (1a or 4a), a new structural form of tetrameric niobium complexes (8a and 9a) was synthesized through control 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 hyper branched polyglycerols (HPG) by using 1,1,1-Tris(hydroxymethyl)propane (TMP) as initiator. The degree of branching (DB) observed was 0.30–0.54, which is the indication of hyperbranched structures. In particular, with niobium complex with electron withdrawing substituents on the benzoxazole phenoxide pro-ligand (5a), we achieved superior behavior for ROP of glycidol in terms of activity, control of molecular weight (Mn) and molecular weight distributions (MWDs) (92 % of glycidol to HPG, Mn = 10.52 Kg/mol, MWDs < 1.33, DB = 0.53 and Tg = -57 °C). Highly hydrophilic nature was observed for the synthesized HPG polymer by water contact angle measurement (20o to 35 o).

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Publication details

The article was received on 02 Aug 2017, accepted on 07 Nov 2017 and first published on 08 Nov 2017


Article type: Paper
DOI: 10.1039/C7DT02839C
Citation: Dalton Trans., 2017, Accepted Manuscript
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    Synthesis and Characterization of Benzotriazole or Benzoxazole Phenoxide Ligands Supported Nb and Ta Metal Complexes and their Application for the Synthesis of Hyperbranched Polyglycerols by Ring-Opening Polymerisation of Glycidol

    S. pappuru, D. Chakraborty and R. V, Dalton Trans., 2017, Accepted Manuscript , DOI: 10.1039/C7DT02839C

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