Available online at www.sciencedirect.com Journal of Photochemistry and Photobiology A: Chemistry 196 (2008) 33–37 Chemical incorporation of thioxanthone into -cyclodextrin and its use in aqueous photopolymerization of methyl methacrylate Demet Karaca Balta a , Emine Bagdatli a , Nergis Arsu a,∗ , Nuket Ocal a , Yusuf Yagci b,∗ a Yildiz Technical University, Department of Chemistry, Davutpasa Campus, Istanbul 34210, Turkey b Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey Received 6 August 2007; received in revised form 8 November 2007; accepted 9 November 2007 Available online 19 November 2007 Abstract Photoinitiated free radical polymerization of methyl methacrylate (MMA) in aqueous solution via hydrogen abstraction mechanism was described. For this purpose, thioxanthone (TX) chromophoric group was chemically incorporated into -cyclodextrin (-CD) by a simple esterification process. The resulting thioxanthone photoinitiator (TX–-CD) exhibited similar spectral characteristics and photoactivity to that of the parent TX molecule. Host guest complexes of MMA with TX–-CD in water, in the presence of N-methyldiethanol amine (NMDEA) as a hydrogen donor, facilitated photoinitiated free radical polymerization in aqueous medium. The postulated mechanism is based on the intermolecular reaction of photoexcited triplet TX moiety of TX–-CD with NMDEA. The resulting amino alkyl radicals initiate the polymerization. © 2007 Elsevier B.V. All rights reserved. Keywords: Photopolymerization; Thioxanthone; Cyclodextrin; Host guest complexes; Aqueous polymerization 1. Introduction Cyclodextrins (CDs) are cyclic oligosaccharides built from six, seven, eight or nine optically active glucopyranose units (-CD, -CD, -CD or -CD, respectively) with a hydropho- bic cavity and hydrophilic exterior [1]. Because of their special molecular structure, these molecules have capability to enclose small hydrophobic molecules into their cavity and consequently to form host/guest compounds in aqueous solution and in emul- sion [2–5]. Photoinitiated free radical polymerization is a well-accepted technology, which finds industrial application in coatings on var- ious materials, adhesives, printing inks and photoresists [6–10]. Environmental issues involving conventional organic solvents are one of the major concerns in such applications. Photopoly- merization in aqueous solution is a highly effective approach to use water instead of the organic solvents. Pioneering work by Ritter demonstrated that hydrophobic vinyl monomers became water soluble due to the inclusion/ complexation of CD and can ∗ Corresponding authors. Tel.: +90 2124491824; fax: +90 2124491888. E-mail addresses: narsu@yildiz.edu.tr (N. Arsu), yusuf@itu.edu.tr (Y. Yagci). easily be polymerized in aqueous solution in the presence of a water soluble thermal free radical initiator [11]. During the polymerization, the CD gradually slipped off from the growing chain and remained in aqueous phase. The concomitantly precip- itated polymer was obtained in high yields. Photoinitiators play a vital role in photopolymerization as they generate initiating species upon photolysis. The same research group also showed that the complexation of a Type I (-cleavage) photoinitia- tor, namely 2-hydroxy-1-phenylpropan-1-one, with methylated -CD (Me–-CD) results in the formation of water soluble host/guest complex[12]. Compare to the bare photoinitiator, this complex exhibited much higher initiation efficiency in the poly- merization of water soluble monomer, N-isopropylacrylamide [12]. In another study, Li et al. demonstrated [13] that the host/guest complexation of Me–-CD with more hydropho- bic photoinitiator, 2,2-dimethoxy-2-phenyl acetophenone, gave stable water soluble compound with high photoactivity and effi- ciency of polymerization. Type II photoinitiators are a second class of photoinitiators and based on compounds whose triplet excited states are reacted with hydrogen donors thereby producing an initiating radical [14–18] (Scheme 1). Because the initiation is based on bimolecu- lar reaction, they are generally slower than Type I photoinitiators, which are based on unimolecular formation of radicals. On the 1010-6030/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2007.11.009