Novel photo acid generator for cationic polymerization

Background
Cationic photopolymerization is currently used in a variety of applications like coatings (e.g. silicon release, decoration and protection coatings), lithographic techniques, resists, printed circuit boards. The initiating species in cationic photopolymerization is generated due to (UV)-light induced cleavage of so called photo acid generators (PAGs). The state of the art PAGs are usually iodonium or sulfonium salts of fluorometallate anions (e.g. BF4-; PF6-; AsF6-; SbF6-) or [BAr4F]--anions. These anions suffer from different drawbacks like toxicity, hydrolysis or tough synthesis and therefore high price.
Cationic photopolymerization offers not only wide range of polymerizeable monomers (e.g. epoxides, oxetanes, vinyl ether) but also a clear advantage compared to the classical free radical photo¬polymerization, which is the independence of the process from oxygen. Therefore a broad (expanding) field of applications opens for a novel PAG.

Technology
We investigated a new aluminum based PAG which is easy to synthesize (sulphonium as well as iodonium salt) at high yields. The educts are inexpensive and commercially available. Furthermore it shows a remarkable stability against hydrolysis. This high stability and the abandonment of any heavy metals in our novel PAG prompts a low toxicity. The high stability against hydrolysis is a consequence of the almost perfect steric shielding and electronic stabilization of the used fluorinated alkoxy aluminate. A further benefit of this shielding / stabilization is a good solubility of the PAG in all, until now examined, monomers.
The reactivity was also tested in different monomers and found to be at least on a par with other state of the art acid generators. In several cases the reactivity of our novel PAG is clearly superior to the state of the art systems. Especially the high reactivity towards the usually rather inactive glycidylethers (e.g. Bisphenol A diglycidyl ether BADGE) is remarkable. Also a remarkable high polymerization rate in the first few seconds of the polymerization could be observed. This is of importance for coating applications since this value determines how long it takes to achieve a tack free surface and limits therefore often the whole speed of production. Despite the high reactivity of our novel PAG no reduced pot life was observed.