Photoinitiators for the free radical polymerisation of t-butyl acrylate: unbridged bicyclic zirconium complexes

Although metallocene complexes of Ti, Zr, and Hf[1] are well known and important catalysts and organometallic reagents, to now little attention has been devoted to their photochemical activity and very little is known about the correlation between their electronic properties[2] and their catalytic activity. Group IV metallocenes are well known catalytic precursors in homogeneous polymerisation of ?-olefins. Recently, some classes of organometallic compounds have proved to be effective photoinitiators for the free radical polymerisation of acrylic esters[3]. Among them, metallocene derivatives have aroused interest because it is possible to tune the efficiency of the photopolymerisation processes by modification of the substituents array. This paper reports the photochemical behaviour of several zirconocenes, whose substituents differ in both electron-donor and steric properties[4]. The study of the electronic spectra and EPR spin-trapping experiments showed the photogeneration of ligand- and zirconium-centred radicals at wavelengths higher than 320 nm. This feature makes these complexes suitable as photoinitiators for radical polymerisation processes. tert-Butyl acrylate polymerisation has been studied in detail and the results are particularly encouraging.