From zirconium to titanium: the effect of the metal in t-butylacrylate photoinitiated polymerisation

We report here the synthesis, photochemistry and photoinitiator activity of some titanocenes (1-4) and compare the results with those obtained for the corresponding zirconocenes (5-8). Analysis of the electronic spectra showed that the energy modulation of the lowest electronic transition, which appears to be LMCT in character, is driven both by the substituent on the cyclopentadienyl moiety and by the metal centre. Furthermore, the excited state resulting from irradiation of the complexes at the wavelength of the LMCT transition undergoes ligand-metal bond dissociation with formation of a radical pair, as evidenced by EPR spectroscopy coupled with spin trapping techniques. All the complexes were very active, compared with known organometallic photoinitiators, for the free-radical photopolymerisation of t-butylacrylate. The titanium complexes 1-4, which can be used with visible light, were more active than the zirconium derivatives 5-8. The better yields in photopolymerisation can be interpreted on the basis of the combined effect of two factors: (i) the photoreactivity of the complexes in solution and (ii) the high persistence of the Ti(III) radical species in solution, which guarantees high concentrations of initiators in the polymerisation process.