Zirconium and titanium-based photoinitiators for acrylate radical polymerisation

Photoinitiated polymerisation is widely employed in many commercial processes, e.g.: photoimaging, UV curing or holography. The most commonly used photoinitiators for the polymerisation of acrylic esters are organic aromatic ketones. However, organometallic complexes possess a number of features that make them potential candidates for their use as photoinitiators: i.the excited states resulting in the absorption of UV/visible radiation may undergo fast dissociative-type processes with the formation of coordinatively unsaturated and/or highly reactive radical species; ii.the energy of the electronic transition is often positioned in the visible region of the spectrum and can be easily tuned by a rational variation of the structure of the co-ordinated ligands. Despite their importance in organometallic chemistry, up to now very few studies have been performed on the photoinitiation activity in the olefin polymerisation of cyclopentadienyl derivatives of Group IV metals and on the processes that occur immediately after the photoexcitation. Here we report the synthesis and photochemical characterisation of a series of bicyclic metallocenes titanium and zirconium, whose substituents differ in both electronic and steric properties. The analysis of the electronic spectra shows that the energy modulation of the lowest electronic transition, which appears LMCT in character, was driven both by the substituent on the indenyl ligand and by the metal centre. Furthermore, the excited state resulting from irradiation of the complexes at the wavelength of LMCT transition undergoes indenyl ligand/metal bond dissociation with formation of a radical pair, as evidenced by EPR spectroscopy coupled with the spin trapping technique. Therefore, the ability of the ligand- and metal-centred radicals to polymerise tert-butyl acrylate and styrene monomers was tested and the photoinitiator activity of the series of metallocenes synthesised was compared.