Anionic polymerisation photoinitiated by ruthenium polypyridyl complexes

Abstract

A new group of photoinitiators of ethylcyanoacrylate monomers based on ruthenium polypyridyl complexes has been investigated. The photoinitiators, of general formula [Ru(DMB)2(L)2]2+, where L = 4-picoline, 2-picoline, undergo a photosubstitution reaction, when irradiated with visible light (436 nm). The labile ligand L is released in solution and initiates polymerisation, with a propagation step following an anionic mechanism. A detailed photolysis analysis carried out on the first ligand loss reaction for [Ru(DMB)2(4-picoline)2]2+ supported the mechanism proposed. The polymerisation was studied by calorimetry, which enabled us to calculate propagation rate constants. Propagation rate constants determined for the photoinduced polymerisation were consistently lower than in the case of "conventional" 4-picoline initiated polymerisation. The photoinitiation of ethylcyanoacrylate monomers by [Ru(DMB)2(2- picoline)2]2+ was also investigated. Once again the rate of polymerisation was found to be much lower than expected. A comparison of the polymers obtained by photopolymerisation initiated either by [Ru(DMB)2(4-picoline)2]2+ or [Ru(DMB)2(2-picoline)2]2+ showed evidence of an influence of the initiating ligand on the final polymer. A detailed NMR spectroscopy study of the photopolymerisation of ethylcyanoacrylate by [Ru(DMB)2(4-picoline)2]2+ was carried out. A comparative polymer end-group analysis between photoinitiated polymerisation and "conventional" polymerisation showed the same pattern on the final polymer: the existence of an Nalkyl-4-methyl-picolinium salt. This spectroscopic evidence came therefore to support the general mechanism of anionic photopolymerisation initiated by a pyridine derivative released in solution by photolysis.