Novel liquid-crystalline titanocene complexes with catalytic activity for polymerizations of acetylene and phenylacetylene

Abstract

We have synthesized a series of novel titanocene complexes with liquid-crystalline (LC) groups as coordination ligands, with the ultimate aim of developing LC catalytic species available for anisotropic polymerizations of acetylenes. The titanocene complexes were prepared by introducing LC groups into cyclopentadienyl ligands as substituents, (PCH506Cp)2TiCl2 (1), or coordinating them to a titanium atom as symmetric and asymmetric ligands, Cp2Ti(PCH5060)2 (2) and Cp2Ti(PCH5060)Cl (3) [where Cp = cyclopentadienyl, PCH506 = p-(trans-4-n-pentylcyclohexyl)phenoxyhexyl and PCH5060 = p-(trans-4-n-pentylcyclohexyl)phenoxyhexyloxy]. Polarizing optical microscope observations and differential scanning calorimetry measurements indicated that the titanocene complexes 2 and 3 are the first titanium complexes exhibiting LC behaviour. It is found that the titanocene complexes 1 and 2 can polymerize phenylacetylenes to give poly(phenylacetylene)s with the aid of triethylaluminium (Et3Al) as cocatalyst. The molecular weights and degrees of polymerization of the poly(phenylacetylene)s prepared were comparable or superior to those prepared with titanocene dichloride and tetra-n-butoxytitanium [Ti(O-n-Bu)4] catalysts. The LC titanocene complexes 2 and 3 were also confirmed to have catalytic activities for the polymerization of acetylene. Electron spin resonance spectra of the catalyst systems gave signals of trivalent titanium (Ti3+) with no hyperfine structure, suggesting that the catalytically active species is a mononuclear complex consisting of Ti3+ rather than a binuclear or trinuclear one.