Wavelength tunability in photoinitiated cationic polymerization
Onium salts, widely used cationic photoinitiators, have absorption in the short wavelength of the UV spectrum. However, their spectral sensitivity can be tuned up to the visible region by using several methods. Photosensitization of onium salts, their charge transfer complex formation and their free radical promotion are some techniques that are used to provide wavelength flexibility. This chapter summarizes these methods. Upon irradiation photosensitizers such as anthracene, perylene and phenothiazine form exciplexes with onium salts generating reactive species via electron transfer. Furthermore, ground state charge transfer formation Correspondence/Reprint request: Dr. Y. Yagci, Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey. E-mail: yusuf@itu.edu.tr
Cationic photopolymerization kinetics of neat coating formulations involving poly(propylene glycol) diglycidyl ether and glycerol diglycidyl ether
Kinetic analysis of formulations based on poly(propylene glycol) diglycidyl ether and glycerol diglycidyl ether was carried out using photodifferential scanning calorimeter. The effects of formulation monomer ratios at different temperatures were studied as photocalorimetric optimization studies of formulation ratios involving diglycidyl ethers are less reported. The nature of cure was found to depend more on the in situ physical parameters rather than on the mechanism of the reaction. The variations in the observed kinetic parameters can be related to interplay of mobility of reactive species, distance of counter ion and the diffusional restrictions. The final conversion was found to increase with increase in poly(propylene glycol) diglycidyl ether content, while the thermal stability increased with an increase in glycerol diglycidyl ether content. Autocatalytic kinetic model was also evaluated, and the system underwent early gelation. The studies show that the intermediate formulations are promising compositions for surface-coating applications.
Lichtinitiierte Polymer‐ und Polymerisationsreaktionen. 27. Einfluß von Aryloniumverbindungen auf die durch aromatische Carbonylverbindungen photoinitiierte radikalische Polymerisation
Die Geschwindigkeit der Photopolymerisation von Methylmethacrylat, initiiert durch photoreduzierbare aromatische Ketone oder α-spaltbare Carbonylverbindungen (Benzoinderivate, Benzilketale) wird durch Zusatz von Aryloniumsalzen (Coinitiatoren, ArnX+:ArN, Ar2I+, Ar3S+) erhoht. Diese Wirkung beruht auf der Umwandlung von inhibierenden, nucleophilen Initiatorradikalen (Ketyl- bzw. substituierte Benzylradikale) in reaktive Arylradikale durch Reaktion mit den Coinitiatoren. Die effektivitatssteigernde Beeinflussung der Aryloniumverbindungen entspricht ihrem Elektronenacceptorvermogen; daher gilt: ArN > Ar2I+ > Ar3S+. Bei photoreduzierbaren aromatischen Ketonen wirkt als weitere Reaktion die Elektronenubertragung aus Triplettzustanden auf Aryloniumsalze, die bei hoheren Arn X+ mit der Triplettloschung durch MMA konkurriert. 
 
 
 
The polymerization rate of methyl methacrylate initiated by photoreducable aromatic ketones or carbonyl compounds reacting by α-scission (benzoin derivatives, benzil ketales) can be increased by addition of arene onium compounds (coinitiators, ArnX+:ArN, Ar2I+, Ar3S+). This effect is caused by conversion of the inhibiting nucleophilic initiator radicals (ketyl or substituted benzyl radicals) into reactive arene radicals. The effectivity of arene onium compounds corresponds to their electron acceptability, that is ArN > Ar2I+ > Ar3S+. In the case of photoreducable aromatic ketones the electron transfer from the excited triplet state to arene onium salts acts as a further reaction which competes with triplett quenching by MMA.