Photoinitiated Radical Polymerisation

Most of the radical-type photoinitiators used in UV-curable adhesives consist of aromatic ketones which are known to generate free radicals upon UV-exposure, either hy homolytical cleavage of C-C bonds, or by hydrogen abstraction from a H-donor molecule [11]  

Both the benzoyl and alkyl radicals initiate the polymerisation by addition to the monomer double bond. By contrast, the ketyl radical is inactive toward vinyl double bonds, so that initiation occurs through the H-donor radical. To be efficient, a photoinitiator must effectively absorb the radiation emitted by the light source 

Chart 1 Chemical formula of radical-type photoinitiators. 

One of the problems encountered in photoinitiated radical polymerisation is the inhibitory effect of atmospheric oxygen which is known to react readily with free radicals to give inactive peroxyl radicals  

To cure thin coatings in contact with air, it is therefore necessary to work under intense illumination in order to consume rapidly the O2 dissolved in the sample and shorten the exposure time during which atmospheric oxygen diffuses into the coating. This is not a problem anymore in adhesive applications which are performed under oxygen diffusion-free conditions (laminates). After a short induction period during which the dissolved O2 is consumed by the initiator radicals, the polymerisation of the acrylate double bond proceeds as fast in the laminated sample as in an inert atmosphere and much faster than for a coating, as shown in Fig. 2 for a polyurethane-acrylate resin. 

---

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]. 

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. 

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. 

The chemical principles of functioning of acResin UV are quite unlike other UV acrylics. To elaborate, an acResin UV is already a polymeric raw material which achieves further crosslinking by exposure to UV light. This is different from other UV acrylics used as adhesive raw materials which consist of mixtures of oligomers with monomers and photoinitiators. These mixtures react after being coated onto a carrier through a free radical polymerisation of double bonds. 

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. 

If the film-former is designed to be polymerised by a free radical mechanism, free radicals can be created in the film by decomposing a photoinitiator within the film using ultra-violet radiation ... 

As a radical photoinitiator, we used 2-hydroxyisopropyl phenyl ketone (DAROCUR 1173 Ciba-Geigy), taken in an amount of 3 wt% based on polyimide (Scheme 5.13). The pattern of an exotherm obtained for the 1% solution of the polyimide (-X- = -O-), the high value of polymerisation enthalpy (352.4 J/g) and the short times of attaining the maximum peak (4.4 s) and the induction time (2.4 s) allow us to consider that this polyimide to be rather reactive from the point of view of polymerisation and formation of a crosslinked structure. 

Structure on hydrogel properties of 2-hydroxyethyl acrylate determined. " Polymers bearing tertiary amino groups have been synthesised and their fluorescence spectra found to be significantly quenched while maleic anhydride " and cyclododecanones have been found to be effective initiators of the photopolymerisation of styrene. Poly(methylphenylsilane) is also an effective photoinitiator for styrenes and acrylates via a photolytic process to give silyl radicals. Iron oxalate is also an effective photo initiator for acrylate monomers while a theoretical description of the kinetics of free radical dye-initiated polymerisation via an electron transfer process has been proposed. Using the Marcus theory it has been shown that the rate of electron transfer can affect the rate of initiation. 

Mixtures of photoinitiators have been actively studied. Michler s Ketone and benzoyl peroxide have been shown to effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 3 7, Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler s Ketone. In the interaction of benzil and thioxanthone with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation . As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene . Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine. 

In the photoinitiated polymerisation of Jl-vinylpyrrolidinone and N-vinylcaprolactam in dioxane and ethanol, the rate was higher in the latter solvent and monomerlO. This was attributed to the influence of the two additional methylene groups in the caprolactam ring which increases monomer reactivity. Other interesting effects have included the radiation dose on the photopolymerisation of diallyl oxydiethylene dicarbonate O. Here long lived radicals were produced which continue to react in the dark. The rate appears to fit a relaxation model that considers double bonds as traps with increasing lifetimes that are able to transfer to radical sites. 

The photoinduced polymerisation of methacrylic acid by sodium peroxide has been determined to have an activation energy of 17.7 kJ/molel03. From this the macroradical lifetimes were determined to be 3.9 secs. In the intermittent photoinitiation of methyl methacrylate-styrene the dependence of the rate of propagation on monomer feed composition was understood in terms of a penultimate model and not a terminal model as previously assumed 4 with some evidence for cross-termination. An eosin-periodate combination has been found to induce the photopolymerisation of acrylonitrile where the dye was found to act as both the sensitiser and reducing agent. Hydroxyl radicals are assumed to initiate polymerisation while the N-bromosuccinimide induced photopolymerisation of N,N -methylenebisacrylamide is faster in 2-propanol solution O . 

---