Polymer Communication Photochemically masked benzophenone: Photoinitiated free radical polymerization by using benzodioxinone Mehmet Atilla Tasdelen, Volkan Kumbaraci, Naciye Talinli * , Yusuf Yagci * Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey Received 10 August 2006; accepted 5 September 2006 Available online 25 September 2006 Abstract A new photoinitiating system for free radical polymerization of methyl methacrylate (MMA) is reported. This system consists of benzodi- oxinone and hydrogen donors such as triethylamine (TEA), N,N-dimethyl ethanol amine (DMEA) and tetrahydrofuran (THF). A feasible mech- anism involves photoinduced formation of benzophenone from benzodioxinone and subsequent hydrogen abstraction of photoexcited benzophenone from hydrogen donors to yield radicals capable of initiating polymerization of MMA. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Photolysis; Polymerization; Benzodioxinone 1. Introduction Photoinitiated polymerizations have received considerable attention because of their rapid curing even at ambient temper- atures, environmental issues, and low energy consumption. These advantages lead to important industrial advantages including coatings on various materials, adhesives, printing inks and photoresists [1e5]. Among the most studied photoinitiator systems are those in which initiating free radicals are formed by a bimolecular process consisting of a triplet excited state of an aromatic carbonyl compound and a hydrogen donor compound as a co-initiator. The photopoly- merization of vinyl monomers is usually initiated by the radi- cals produced from the hydrogen donor. The ketyl radicals are usually not reactive toward vinyl monomers due to the steric hindrance and the delocalization of unpaired electron. The overall mechanism of the photoinitiation is represented in Scheme 1. Typical photoinitiators of this type include benzophenone [6e8] and derivatives, thioxanthones [9e11], benzyl [8], and quinones [8] while alcohols, ethers, amines and thiols are used as hydrogen donors. Among these type of photoinitiators, benzophenone is most widely used because of the high quan- tum efficiency of the hydrogen abstraction particularly with amines. Because the initiation is based on a bimolecular reac- tion, benzophenones initiate generally slower than cleavage type photoinitiators. These systems are, therefore, more sensi- tive to the quenching of excited triplet states, which are the reactive precursors of light-induced initiation. Indeed, quench- ing [12] by monomers with low triplet energy (e.g., styrene and or N-vinyl carbazole) or by oxygen or self quenching [13] by ground state benzophenone is often observed and may lead to relatively low curing rates. Ar 2 C hv Ar 2 CO 3 * * 3 + R-H Ar 2 C OH + R R + Monomer Polymer (1) (2) (3) O Ar 2 C O Scheme 1. Photoinitiated free radical polymerization by aromatic carbonyl compounds. * Corresponding authors. Tel.: þ90 212 285 32 41; fax: þ90 212 285 63 86. E-mail addresses: talinlin@itu.edu.tr (N. Talinli), yusuf@itu.edu.tr (Y. Yagci). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.09.020 Polymer 47 (2006) 7611e7614 www.elsevier.com/locate/polymer