Two- and Three-Component Visible Light Photoinitiating Systems for Radical Polymerization Based on Onium Salts: An Overview of Mechanistic and Laser Flash Photolysis Studies

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 260728, 9 pages doi:10.1155/2012/260728 Review Article Two- and Three-Component Visible Light Photoinitiating Systems for Radical Polymerization Based on Onium Salts: An Overview of Mechanistic and Laser Flash Photolysis Studies Marı́a L. Gómez, Carlos M. Previtali, and Hernán A. Montejano Departamento de Quı́mica, Universidad Nacional de Rı́o Cuarto, Campus Universitario, 5800 Rı́o Cuarto, Argentina Correspondence should be addressed to Hernán A. Montejano, Received 20 February 2012; Revised 7 May 2012; Accepted 22 June 2012 Academic Editor: Xavier Allonas Copyright © 2012 Marı́a L. Gómez et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A review of our work on two- and-three component photoinitiator systems is presented. The emphasis is in on visible light polymerization in aqueous media. The systems discussed comprise a synthetic dye as sensitizer and an onium salt as coinitiator, or a dye-amine-onium salt with the amine as coinitiator and the onium salt as an enhancer of the polymerization efficiency. The effect of the composition of the system on the photopolymerization kinetics was analyzed. To this end, the photophysics and photochemistry of the dye under polymerization conditions was explored by means of stationary and time-resolved spectroscopic methods. Different dyes and onium salts were investigated. The action mechanism of the different photoinitiators systems is discussed. 1. Introduction The development of new materials by means of photopolymerization requires investigations aimed at discovering more efficient photoinitiator systems. Most of traditional photoinitiating systems employed UV radiation to generate active species. The use of sensitizers for visible light and the development of photopolymerizable system in aqueous media, make them environmental friendly and useful for applications in green chemistry or in the development of material for biological uses. Therefore, the development of photoinitiators based on a synthetic dye and a coinitiator was the subject of a large amount of work. Among the different coinitiators explored, a particular class corresponds to onium salts. The development of applications involving onium salts (OS) in photoinitiation of polymerization [1] has been a topic of interest since the late 60s. Studies have been reported on ammonium salts [2], phosphonium [3], sulfonium [4], and iodonium salts [5] with aryl and/or alkyl groups on its structure. For similar applications, arsonium [6], pyridinium [7] and aryl tropylium [8] salts, have also been proposed in photoinitiation systems; these salts are capable of undergoing photochemical decomposition, producing active species suitable to initiate polymerization. When these salts may act as electron acceptors, photosensitization was proposed to take place by reduction and posterior fragmentation of the cation [9, 10]. Onium salts could act as a source of free radicals, radical cations, or Brönsted acids when exposed to light [11] and may be employed as efficient photoinitiators of radical, cationic, or mixed polymerization, [12–14]. The most widely used OS are diaryliodonium salts with chloride or perchlorate counterion. They have been employed extensively for cationic [15– 18] and vinyl [19, 20] polymerization. The main practical limitation of the use of these salts is related to its low molar absorption coefficient above 300 nm. To overcome this drawback, several photosensitizers are used to extend the spectral range suitable for electronic excitation. Aromatic ketones and polycyclic aromatic hydrocarbons (PAHs) are mainly used as photosensitizers for UV C radiation (300– 400 nm) [21–23]. Numerous papers have dealt with the mechanism of active species generation when PAHs are employed as 2 International Journal of Photoenergy Table 1: Names, formulae, and abbreviations of sensitizers, electron donor, and onium salts. Compound Sensitizers Formulae Abbreviation N Na+ −O Resazurin O O N CH2 CHOH Rz OHCH2 CH2 OHCH2 CH2 Safranine O Electron donor Saf OHCH2 CH2 N CH2 CHOH OHCH2 CH2 Triethanolamine Onium salts TEOA I+ Cl− Diphenyliodonium chloride DPI S+ CF3 SO3 − TPS Triphenylsulfonium triflate P+ Cl− TPP Tetraphenylphosphonium chloride As+ Cl− TPAs Tetraphenylarsonium chloride hydrate sensitizers in binary photoinitiating systems [24, 25]. Electron transfer and/or energy transfer from the excited PAH to the iodonium cation were suggested as the first reaction step in the initiation mechanism. In some cases, it is not clear whether the singlet or triplet excited state of the sensitizer is responsible for the interaction with the onium salt [24, 26, 27]. In a previous paper [28], we have investigated the interaction of diphenyliodonium cation (DPI+ ) with the excited singlet and triplet states of eight polycyclic aromatic hydrocarbons in acetonitrile solution. These results confirmed the electron transfer nature of the quenching process: ∗ PAH + DPI+ −→ PAH+• + DPI• (1) Singlet and triplet quenching rate constants were measured and they correlate well with the reaction driving force as expected for an electron transfer process. Moreover, the experimental data could be fitted to a Rehm-Weller mechanism with normal parameters. This result is a further confirmation of the electron transfer that occurs in the deactivation of electronically excited states of the PAHs by DPI+ . The electron transfer reaction is followed by a very fast breakdown of DPI• radical [29, 30] to form the phenyl radical (Ph• ), which is a reactive radical suitable for the initiation of vinyl polymerization. In the present paper, we will focus on the research carried out by our group on two- and three-component photoinitiating systems for free radical polymerization containing onium salts. We will describe two-component photoinitiating systems employing sensitizers (SENs) adequate for visible light [31] and an onium salt, and three-component systems based on a sensitizer, an electron donor (ED), and an onium salt [32–34]. International Journal of Photoenergy The most common photoinitiator systems in the visible region (400–780 nm) for radical polymerization involve a dye and an amine as an electron donor [35–38]. Active radicals are formed in a bimolecular electron transfer quenching of the triplet state of the dye by the amine. We have previously reported on the photoreduction of several dyes (flavins, safranine, resazurin, and resorufin) by amines and the use of these systems as suitable photoinitiators of vinyl polymerization using visible irradiation [36, 39–42]. Two-component systems based on an onium salts in conjunction with synthetic dyes are also an alternative for photoinitiation [7, 31, 43]. These systems ha (...truncated)