Cationic photopolymerizations

The second section focuses on emerging classes of photopolymerizations that are being developed as alternatives to acrylates. Three types of polymerization systems are included cationic photopolymerizations, initiator-free charge-transfer polymerizations, and a thiol-ene reaction system. The last section covers four interesting emerging applications of photopolymerization technology. 

Table 2 Influence of the temperature on the cationic photopolymerization of the divinyl ether of triethyleneglycol (DVE-3) (1 = 5 mW cm-2). Film laminated between 2 KBr crystals ...

Cationic photopolymerizations of epoxides and vinyl ethers offer tremendous potential in the area of high-speed, solvent-free curing of films and coatings. The polymers formed exhibit excellent clarity, adhesion, abrasion, and chemical... 

In contrast, the polymer obtained by the cationic photopolymerization of divinyl ether, DVE-3, showed significant weight losses at temperatures in excess of 200°C. In dynamic TGA tests, the onset of polymer decomposition was found to be 370°C for this material. 

Quantitative aspects of photopolymerization have been described in Sec. 3-4c. There are some differences between radical and cationic photopolymerizations. The dependence of Rp on light intensify is half-order for radical polymerization, but first-order for cationic polymerization. Radical photopolymerizations stop immediately on cessation of irradiation. Most cationic photopolymerizations, once initiated, continue in the absence of light because most of the reaction systems chosen are living polymerizations (Sec. 5-2g). 

Lewis acids such as BF3 and SbCl5, almost always in conjuction with water or some other protogen, initiate polymerization of cyclic ethers. The initiator and coinitiator form an initiator-coinitiator complex [e.g., BF3 H20, H+(SbCl6) ], which acts as a proton donor (Sec. 5-2a-2). Cationic photopolymerizations are achieved when similar proton donors are formed by the photolysis of diaryliodonium and triarylsulfonium salts (Sec. 5-2a-4). 

Kinetics of Cationic Photopolymerization Chemical Systems in UV Processing... 

As outlined in Scheme 2, for cationic photopolymerization a photoinduced formation of species X+ or Lewis acids is required [162]. Such species are formed both by PET between neutral donors and acceptors (see Scheme 3), between neutral donors and positive charged acceptors (see Schemes 9 and 11), respectively, and by an indirect PET between nucleophilic radicals and onium salts or halogen compounds (see Eq. (16) and Scheme 12). Therefore, combinations of compounds, whose light-induced reactions are based on the pathways given above, are usable as photoinitiators for cationic polymerizations, too. Prerequisites for the use of cations... 

So far, only a few examples of cationic photopolymerizations using PET corresponding to Scheme 3 have been described [10,13,165]. In the ternary system cyclohexene oxide, 9.10-dicyano anthracene and polynuclear aromatics, the polymerization of the former is initiated by the radical cations of the aromatic hydrocarbons formed via the PET with the dicyano compound. 

Presumably, a PET is involved also in the cationic photopolymerization of epoxides with metal arene complexes [181, 182]. 

Onium salts such as triarylsulfonium and diaryliodonium can initiate both free radical and cationic photopolymerization [57]. Direct photolysis, effective only for wavelengths below about 260 nm, produces phenyl radicals and cation radicals as illustrated in Eqs. (23,24). Because these systems absorb... 

It can be concluded that cationic photopolymerization of glycidyl ethers with onium salt initiators should be considered as a dual-step process 1) liberation of the active initiator species by irradiation and 2) heat treatment to complete the polymerization reaction... 

Various bifunctional resins are based on acrylic epoxide monomers. Such systems can photopolymerize by the radical and/or cationic mechanism. With iron arene photoinitiators in the presence of an oxidant, radical as well as cationic photopolymerization of these monomers is possible . Onium -type photoinitiators form radical species upon photolysis, as shown in Figs. 3 and 4. The local radical concentration is, however, too low to permit the polymerization of such systems... 

The absorption spectra of the iodonium borates depend on the solvent [53, 54]. In acetonitrile, the absorption spectra are equal to the additive spectra obtained from the components (up to 300 nm) [55, 56]. In less polar solvents onium borates exhibit a weak, extended absorption tail in the 320-450 nm region that is attributed to an intra-ion-pair ground-state charge transfer transition from the borate anion to the iodonium cation. Photopolymerization using the iodonium borates can be effectively initiated only by UV irradiation and by violet light of the visible region. 

TP initiated cationic photopolymerization is an additional research target in TP photosciences [225]. The crosslinking of higher functional epoxies benefits from the lower shrinkage in comparison with acrylates [269]. In particular, those TP active systems with the capability to transfer an electron according to route A in... 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

Cheng, X. Guo, L.J. Fu, P.-F. Room temperature and low pressure nanoimprinting based on cationic photopolymerization of novel epoxysilicone monomers. Adv. Mater. 2005, 17, 1419-1424. 

Hult, A. Ito, H. MacDonald, S.A. Willson, C.G. Process for Preparing Negative Relief Imaging with Cationic Photopolymerization U.S. Patent 4,551,418, November 5, 1985. 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Cho and Hong (2005) used photodifferential scanning calorimetry to investigate the photocuring kinetics of UV-initiated cationic photopolymerization of 1,4-cyclohexane dimethanol divinyl ether (CHVE) monomer with and without a photosensitizer, 2,4-diethylthioxanthone (DETX) in the presence of a diaryliodonium-salt photoinitiator. Two kinetic parameters, the rate constant (k) and the order of the initiation reaction (m), were determined for the CHVE system using an auto-catalytic kinetics model as shown in the following equation ... 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

As far as the polymerization reactions are concerned in UV curing and imaging areas, they are mostly based on a radical process. Cationic photopolymerization is noticeably less used. Anionic photopolymerization is rather inexistent. Photolatent base generation technology is expected to be developed in the future. 

Sulfonic (-SO3H) and Sulfinic (-SO2H) Acid-Based Nonsalt Photoinitiators Photosensitive tosylate esters of nitrobenzyl [65] and benzoin [73], sulfonyl ketones [67,68], and diphenyl disulfones [68] can be employed in cationic photopolymerization, especially in photolithography and chemical amplification processes. The advantages of these photoinitiators are the ease of synthesis, the absorption wavelengths suitable for deep-UV curing and the high yield of acid formation [65]. 

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