Thioxanthone based water-soluble photoinitiators for acrylamide photopolymerization Demet Karaca Balta, Gokhan Temel, Meral Aydin, Nergis Arsu * Department of Chemistry, Yildiz Technical University, Davutpasa, Istanbul 34210, Turkey article info Article history: Received 30 October 2009 Received in revised form 29 March 2010 Accepted 31 March 2010 Available online 4 April 2010 Keywords: Free radical polymerization Thioxanthone Type II system Water-soluble photoinitiator abstract The two water-soluble thioxanthone-based 2-(carboxymethoxy) thioxanthone, 1A and 2-thioxanthone-thioacetic acid sodium salts, 2A were synthesized and characterized by spectroscopic methods. The quantum yield for fluorescence emission (u f ) in water was found to be 0.50 for 1A and 0.05 for 2A. The long phosphorescence lifetime, that is 201 and 114 ms in a matrix at 77 K for 1A and 2A, respectively, indicates a p–p* nature of the lowest triplet state. The triplet lifetimes were determined by laser flash photolysis as 12 and 12.6 ls for 1A and 2A. Polymerization of acrylamide (AAm) was initiated by both initiators in water. Decarboxylation of initiators was achieved during 20 min of irradiation. It was found that 2A initiated polymerization of AAm in water more efficiently than 1A. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The range of industrial applications of water-soluble polymerization systems has recently increased and has been mainly stimulated by the screen printing sector in the production of stencils, using aqueous polyvinyl alco- hol/acetates and nylon [1]. Water-soluble chromium com- pounds, thiocyanates and peroxo salts were used as sensitizers for the photocrosslinking of these systems. But short shelf life and environmental concern about using materials such as chromium compounds were major draw- backs [2]. The applications of water-soluble photoinitiators for UV-cured coatings started in the early 1980s and are a relatively new concept. They have received increasing interest due to their widespread applications in the graft- ing of monomers onto natural polymers and UV-curing of water-based ink systems [1]. Due to the expecting poten- tial interest of working in aqueous or hydrophilic media, water soluble and hydrophilic photoinitiators have been developed. The introduction of water solubilizing groups onto a conventional photoinitiator is a relatively simple task. Water solubilizing groups that can be used include non-ionic ethers and polyethers, hydroxyethers, ionic substituents such as quaternary ammonium salts, sulfo- nates, carboxylic acids and thiosulfates [1,3,4]. Several water-soluble photoinitiators such as benzophenone and thioxanthone derivatives [3–9] are known, but the studies about waterborne curing systems are still in progress and in some cases their solubility in water, particularly when high concentrations of initiators are required, is limited. These initiators have some drawbacks such as low solubil- ity in water, low efficiency and oxygen inhibition. We have recently reported the use of acetic acid deriv- atives of thioxanthone structures, shown in Fig. 1, as photoinitiators for free radical polymerization [10,11]. They exhibit a one-component nature and the light absorbing and hydrogen donating sites are contained in the photoinitiator molecules. We report here, the synthesis of novel water-soluble thioxanthone photoinitiators which were developed by introducing Na + into the skeleton of the corresponding oil soluble compounds (see Fig. 1). 2. Experimental part 2.1. Materials 2-Thioxanthone-thioacetic acid (TXSCH 2 COOH) and 2- (carboxymethoxy) thioxanthone (TXOCH 2 COOH) were 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.03.022 * Corresponding author. Tel.: +90 212 3834186; fax: +90 212 3834134. E-mail address: narsu@yildiz.edu.tr (N. Arsu). European Polymer Journal 46 (2010) 1374–1379 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj