Photoinitiated Cationic Polymerisation

The field of photopolymerisation and radiation curing continues to expand in many areas and this is yet again reflected in the large number of reviews and related industrial articles of interest which have appeared. The mechanisms of photoinitiated polymerisation have been actively covered in this regard and include cationic types for... 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

Vinyl ethers (VE) are amongst the most reactive monomers that polymerise cationically, and they are increasingly used in photocurable resins, either neat [63] or in association with epoxides [64,65], acrylates [66], maleimides [67] or unsaturated polyesters [68]. Vinyl ether end-capped polyurethane, polyether, polysiloxane and polyesters were shown to undergo fast and extensive polymerisation when exposed to UV-radiation in the presence of an aryliodonium salt, with the formation of tridimensional polymer networks. The photoinitiated catioiuc polymerisation of a divinyl ether can be formally written as follows ... 

K. K. Diediker and P. Oldring, "Chemistry and Technology of UV EB Formulations for Coatings, Inks Paiats," Vol. 3, Photoinitiators for Free Radical and Cationic Polymerisation, ShoHum International, 1991. 

A special instance of photoinitiation was already di us in Sect. VI-B, within the context of bare cation formation. In those sterns in fact, h -energy photons were used to produce the ejection of an electron from the moiK>mer molecule. In this chapter we will briefly review other photochemical tediniques involving the framation of electronically excited intermediates, which in turn generate suitable species for the initiation of cationic polymerisation. Crivello has recently reviewed this topic and other authors have published more specialised monographs on some specific aspects of cationic photo-initiation. We will therefore reduce our coverage to the basic premises on whidi the various methods are founded and to a few comments on the more recent contributions and medianistic interpretations. 

Smets reviewed the topic of photoinitiated cationic polymerisation at the Kyoto meeting and described some novel interesting work from his own laboratory. Diazonl-um salts were used to photoinitiate the polymerisation of N-vinylcarbazole or to crosslink linear copolymers of methyl methacrylate and glycidyl methacrylate through the epoxy groups of the second comonomer. 

Crivello, J. V. Photoinitiated cationic polymerisation. In UV curing science and technology (Pappas, S. P., ed.). Stamford Technol. Mark. Corp. 1978, p. 23... 

Photoinduced cationic and anionic polymerisations continue to attract some interest. In this respect simple alkyl iodides have been shown to be effective photoinitiators of isobutyl vinyl ether in the presence of tetrabutylamraonium salts of... 

J.V. Crivello and K.K. Diethker, Photoinitiators for free radical and cationic polymerisation, in G. Bradley, (Ed.), Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints, 2nd edition, Vol. 3, SITA Technology Ltd., London, 1998. 

A linear-chained epoxy resin was formulated from phenyl glycidyl ether and nadic methyl anhydride, catalysed by benzyldimethylamine (248). An IR fibre-optic probe was used to follow the conversion of a thermosetting tetrafunctional epoxy resin in which the hardener was an aromatic diamine and a carboxylic dianhydride. A polymerisation system consisting of a cycloaliphatic diepoxide, epoxidised natural rubber (ENR), glycidyl methacrylate (GMA) and a cationic photoinitiator, triphenylsulfonium hexafluoro-antimonate, was studied (75). Multifunctional epoxy/ amine formulations (Epon 825 plus 4,4 -methylene-... 

Keywords Acrylates Crosslinking Dual cure Epoxides Glass laminates Interpenetrating polymer networks Isocyanates Photo initiated cationic polymerisation Photoinitiators Photopolymerisation Pressure sensitive adhesives Release coatings Structural adhesives Thiol/polyene UV radiation curing Vinyl ethers. 

A distinct characteristic of photoinitiated cationic polymerisation is its lack of sensitivity towards atmospheric oxygen which does not scavenge cationic species, as well as its living character due to the fact that the propagating polymer cations are not reacting among themselves. In contrast to radical-initiated polymerisation, the chain reaction will thus continue to proceed in the dark after the UV-exposure, a feature which was used to assemble nontransparent materials by UV-curable laminating adhesives [56]. 

Figure 6 Photoinitiated cationic polymerisation of an aliphatic dicycloepoxide and a dicycloepoxy silicone. Diaryliodonium salt (1 wt%) photoinitiator (/= 85 mW cm ).

Figure 8 Experimental setup for the production of nontransparent laminated assemblies by postcuring of a photoinitiated cationic polymerisation.

The following application examples from the field of photoinitiated radical and cationic polymerisation have been selected to demonstrate the multiple use of photocalorimetry. All samples were analysed in open aluminium pans, isothermally at 30 °C. Because of the fact that photopolymerisations via radicals are inhibited by oxygen the sample holder was pnrged with an inert gas. 

HAR 01] Hartwig a., Harder A., Luhring A. et al.., 9-Oxo-9H-fluoren-2-yl)-phenyl-iodonium hexafluoroantimonate(V) - a photoinitiator for the cationic polymerisation of epoxides , European Polymer Journal, vol. 37, pp. 1449-1455,2001. 

The cationic curing systems can impart requisite barrier and electrical properties unlike the free radical initiated polymerisation of acrylates. In the cationic photopolymerisation, the process involves the ring opening of oxiranes and/or oxetanes and is initiated by strong protonic acids. The use of photosensitisers like thioxanthone and anthracene derivatives and photoinitiators further enhances the polymerisation process. 

In cationic curing, high curing speeds can be achieved even though the photoinitiator requirement is less. Though cationic polymerisation process is slower than free radical polymerisation, higher belt speeds can be achieved in cationic systems as there is no problem of air inhibition. 

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