S. Peter Pappas

Photoinitiation of cationic and concurrent radicalcationic polymerization. Part V

Irradiation directe de sels onium. Photosensibilisation des sels onium. Systemes hybrides-polymerisation radicalaire cationique concurrente. Applications aux photoresists. References

Photoinitiation of cationic polymerization. IV. Direct and sensitized photolysis of aryl iodonium and sulfonium salts

Abstract Aryl iodonium and sulfonium salts are thermally stable photoinitiators for cationic polymerization. Laser flash photolysis of diphenyliodonium and diphenyl-4-thiophenoxyphenylsulfonium hexafluoroarsenates provides direct evidence for homolytic Ar-I and Ar-S bond cleavage to yield phenyliodinium (PhI +. ) and, primarily, diphenylsulfinium (Ph 2 S +. ) ion radicals, respectively. The radical ions were generated independently by flash-induced electron transfer from iodobenzene and diphenylsulfide to a phenanthrolinium salt. The radical ions are highly reactive with nucleophiles, including iodobenzene and cyclohexene oxide, in the case of PhI +. . Apparent secondorder rate constants were determined for the reaction of the transients with several nucleophiles. Quantum yields of acid formation from stationary photolysis of diphenyliodonium and triphenylsulfonium hexafluoroarsenates were found to be significantly higher than yields of iodobenzene and diphenylsulfide, respectively. These results may be explained by facile reaction of PhI +. (or Ph 2 S +. ) with PhI (or Ph 2 S) to yield new onium salts together with a proton. The high reactivity of PhI +. with cyclohexene oxide suggests that the transient may directly initiate cationic polymerization of epoxides. Photoinitiated cationic polymerization by photosensitization of diphenyliodonium and triphenylsulfonium salts is shown to proceed by two distinct electron transfer processes: (1) direct electron transfer from excited-state photosensitizers, and (2) indirect electron transfer from photogenerated radicals. The efficiency of the former process is attributed to instability of the reduction products (from diphenyliodonium and triphenylsulfonium salts) which dissociate in competition with undergoing energy-wastage reverse electron transfer. Amplification of photons in the production of protons (or other reactive cations) is postulated to account for the high quantum yields observed in the latter process. Potential advantages of utilizing the indirect redox process in the design of UV curable hybrid systems, which contain functionality for both radical and cationic polymerization, are noted. The sensitization results also provide evidence against the importance of triplet states of the onium salts in photoinitiator activity.

UV curing by radical, cationic and concurrent radical-cationic polymerization

Abstract UV and EB curing represent complementary technologies with respective advantages and disadvantages. This paper deals with the design and evaluation of UV curable coatings to optimize cure rate and film properties. Topics included are state-of-the-art photoinitiator systems, light intensity effects, retardation of air-inhibition, adhesion, and amplification of photons for enhanced speed of cure.

PHOTOCHEMICAL ASPECTS OF U.V. CURING

The need and desirability to reduce energy consumption and pollution is posing exciting challenges to the coatings industry. A relatively new approach which has shown promise in alleviating both of these problems is the development of “solventless” coatings and printing inks curable by ultraviolet radiation’lZa. The utihzation of light, in contrast to heat, as the energy source for curmg allows that the low molecular weight substances, necessary for proper flow characterisiics, be reactive monomers which are incorporated into the film, rather than hydrocarbon solvent which is volatilized into the atmosphere. A critical constituent in these compositions is the photoinitiator (loosely termed “photosensitizer”) which absorbs radiation and initiates the processes of polymerization and crosslinking which constitute curing, Le. the conversion of a flowable system into a solid film. The aim of this article is to provide some mechanistic insights into the process of U.V. curing with particular emphasis on the photochemical aspects. Two commercially important photoinitiator systems will be considered in detail. Both systems involve the absorption of light by aromatic ketones and the subsequent initiation of radical chain polymerization. The lirst class of ketones to be discussed, the most

Photoinitiated Cationic Polymerization

Photoinitiated cationic polymerization has not as yet achieved the commercial significance of radical polymerization in important photopolymerization processes, including UV curing and photoimaging. Several reasons for this may be advanced, including: (1) the development of photoinitiators for cationic polymerization was preceded by substantial advances in UV curing technology based on radical polymerization; (2) the early developmental work on UV curing by cationic polymerization utilized aryl diazonium salts as photoinitiators which, while highly photoactive, are thermally unstable, thereby preventing long-term storage of fully formulated compositions; and (3) the discovery of thermally stable onium salts, such as diaryliodonium and triarylsulfonium salts, as effective photoinitiators for cationic polymerization of epoxy-functional resins was made almost simultaneously by several industrial groups, which resulted in an unclear patent situation—a situation which appears, at least in part, to be resolved at present.

On the Reactions of Diphenyliodonium and Triphenylsulphonium Salts with Hydroxyl and 2-Hydroxy-2-Propyl Radicals

Hydroxyl radicals react with diphenyliodonium ions, Ph2I+, and triphenylsulphonium ions, Ph3S+, in aqueous solution at room temperature with rate constants ≥ 5 x 109 l/mol s. These reactions lead to the formation of a broad transient absorption band with a maximum at ca. 370 nm, which is attributed to OH-adducts with cyclohexadienyl structures. 2-Hydroxy-2-propyl radicals react with diphenyliodonium ions in aqueous solution at room temperature with k2 = 6 x 107 l/mol s. The transient absorption band formed in this reaction has a maximum at about 390 nm and is attributed to diphenyliodo radicals, Ph2I·. In contrast, no detectable reaction occurred with 2-hydroxy-2-propylradical and triphenylsulphonium ion.

Solvent effects on intersystem crossing in the photocyclization of methyl o-benzyloxyphenylglyoxylate

Solvent effects on the quantum yield of the phototransformation (1)→(2) may be attributed, in part, to diminished intersystem crossing efficiency with increasing solvent polarity.