Zirconocenes as photoinitiators for free-radical polymerization of acrylates

In this paper we report the synthesis of several unbridged zirconocenes, 1a-c, 2 and 5, bearing a phenyl substituent in the position 2 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. The study of the electronic spectra and EPR/spin trapping experiments showed the photogeneration of ligand- and zirconium-centred radicals thus making these complexes suitable as photoinitiators for radical polymerisation processes. The introduction of a phenyl group had a positive effect both on the absorption coefficients, improving the light absorption efficiency, and on the reaction yields of t-butylacrylate polymerisation. These results can be explained as the combined effect of an higher persistence of the radical in solution and/or an increased efficiency of the radical in initiating the polymerisation process.