The effect of phenyl substituents on the activity of some zirconocene photoinitiators

In this paper we report the synthesis of several unbridged zirconocenes, 1a.c, 2 and 5, each bearing a phenyl substituent in the 2-position of the cyclopentadienyl ring. We also report their photochemical behaviour and we compare the results with those obtained with the corresponding unsubstituted metallocenes 3a.c and 4. Study of their electronic spectra and EPR/spin trapping experiments showed photogeneration of ligand- and zirconium-centred radicals, thus making these complexes suitable as photoinitiators for radical polymerisation processes. The substitution of the hydrogen atom in the 2-position with a phenyl group had positive effects both on the absorption coefficients, improving the light absorption efficiency, and on the reaction yields of tert-butyl acrylate polymerisation. These results can be explained in terms of the combined effects of greater persistence of the radical in solution and/or increased efficiency of the radical in initiating the polymerisation process.