Advances in cationic photopolymerization based on alkoxyaluminate photoacid generators

Abstract

Cationic photopolymerization represents an energy efficient method for the UV- and visible light curing of a great variety of monomer classes highly abundant in coating industries. The production of materials from heterocyclic and vinyl monomers finds wide application, especially regarding epoxy monomers, which show outstanding chemical, mechanical and electric properties in thin films. Compared to the omnipresent free radical photopolymerization, this curing method shows great advantages by its insensitivity towards oxygen, rapid reactions and its unique post-curing behaviour in the absence of light. These features arise from the acid initiated polymerization reaction following the decomposition of photoacid generators (PAGs), which liberate protons and carbocations upon the irradiation with light (Figure 1). Within our research group we faced the challenges of developing a new PAG with benefits in acid strength, initiation ability, hydrolysis stability, facile preparation and absence of heavy metals. As common state-of-the-art PAGs as BF4-, PF6-, AsF6-, SbF6- and B(ArF)4- could not combine these demands, we introduced a novel class of PAG based on the most weakly coordinating anion (WCA), namely tetrakis(perfluoro-t-butyloxy)aluminate.1-3 By the combination with diaryliodonium and triarylsulphonium cations, which are known for its highly efficient photodecomposition, the resulting cationic photoinitiators showed beneficial curing behaviour for a wide range of applications. 4-6 The reactivity was investigated in thin film UV-curing of different cationically polymerizable monomers in Photo-DSC studies. In classical cationic UV-curing, but also in Radical-Induced Cationic Frontal Polymerization (RICFP) the alkoxyaluminate-based PAGs show great advantages and, thus, expand the possibilities of cationic photopolymerization towards efficient curing of materials from cationically polymerizable monomers.