The polymerization of 1–3？M 2-hydroxyethyl methacrylate (HEMA) initiated by riboflavin/triethanolamine system has been studied in the pH range 6.0–9.0. An approximate measure of the kinetics of the reaction during the initial stages (~5% HEMA conversion) has been made to avoid the effect of any variations in the volume of the medium. The concentration of HEMA in polymerized solutions has been determined by a UV spectrophotometric method at 208？nm with a precision of ±3%. The initial rate of polymerization of HEMA follows apparent first-order kinetics and the rates increase with pH. This may be due to the presence of a labile proton on the hydroxyl group of HEMA. The second-order rate constants for the interaction of triethanolamine and HEMA lie in the range of 2.36 to ？M？1？s？1 at pH 6.0–9.0 suggesting an increased activity with pH. An increase in the viscosity of HEMA solutions from 1？M to 3？M leads to a decrease in the rate of polymerization probably as a result of the decrease in the reactivity of the flavin triplet state. The effect of pH and viscosity of the medium on the rate of reaction has been evaluated. 1. Introduction Acrylic acid derivatives including 2-hydroxyethyl methacrylate monomer (HEMA) [1], urethane dimethacrylate monomer [2], proline modified acrylic acid copolymer [3], N-vinylcaprolactam-containing acrylic acid terpolymer [4], N-vinylpyrrolidone modified acrylic acid copolymer [5], and polyurethane acrylate monomer [6] have been synthesized for dental cement applications. These derivatives are intended to undergo polymerization on exposure to visible light and thus form a hardened mass (cement). Among these derivatives, HEMA is widely used in glass ionomer cements (GICs) employed as dental restorative materials [7]. Various types of GICs containing HEMA have been developed as light cure restorative materials [8]. The photoinitiated polymerization of vinyl polymers has been studied since the 1950s [9–12]. The process involves the participation of photoinitiators absorbing in the visible region. Riboflavin (RF) absorbs at 444？nm and has been used as a photoinitiator in the polymerization of HEMA along with triethanolamine (TEOHA) as a coinitiator to form a redox pair involved in the process [13–16]. RF is an efficient electron acceptor and meditates in numerous photochemical and biological electron transfer reactions [17–22]. The kinetics of polymerization reactions has been discussed by several workers [12, 23–25]. The medium characteristics, ionization behavior of reacting species, and efficiency of the photoinitiators influence the
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