Efficacy of photoinitiators such as riboflavin (RF), camphorquinone (CQ), and safranin T (ST) and triethanolamine as a coinitiator has been compared in carrying out the polymerization of 2-hydroxyethyl methacrylate (HEMA) in aqueous and organic solvents. HEMA solutions were polymerized in the presence of RF, CQ, and ST using a low intensity visible radiation source. HEMA was assayed by a UV spectrophotometric method during the initial stages of the reactions (i.e., ~5% change). A comparison of the efficacy of photoinitiators in causing HEMA polymerization showed that RF is more efficient than CQ and ST. The rate of polymerization is directly related to solvent dielectric constant and inversely related to the solvent viscosity. RF is the most efficient photoinitiator in the polymerization of HEMA and the highest rate of reaction occurs in aqueous solutions. A general scheme for the polymerization of HEMA in the presence of photoinitiators is presented. 1. Introduction The influence of solvent on the rates and mechanisms of chemical reactions is of great importance and has been discussed by many workers [1–5]. 2-Hydroxyethyl methacrylate (HEMA) is a component of resin-modified glass-ionomer cements used as restorative materials in dentistry. It undergoes polymerization in the presence of a photoinitiator during the setting process on bonding to the teeth [6]. The efficacy of photoinitiators in the polymerization of HEMA may be affected by medium characteristics including the polarity, viscosity, and the extent of radical formation involved in the reaction. Several studies have been carried out on the effect of solvent on the polymerization of HEMA using dilatometry [7, 8], gas chromatography [9], Raman spectroscopy [10], ATR-FTIR spectroscopy [11], and differential scanning calorimetry (DSC) [12]. The primary photochemical processes in polymerization may be dependent on the solvent and, therefore, the dielectric constant of the medium could affect the initial quantum yield of the process [8]. Most of the work on the polymerization of HEMA in aqueous solution has been carried out using water-soluble photoinitiators and information is lacking on their behavior in organic solvents. It would be worthwhile to evaluate the efficiency of these photoinitiators in the polymerization of HEMA in organic solvents. The present work is based on a study of the effect of solvent dielectric constant and viscosity on the rate of polymerization of HEMA in aqueous and organic solvents using a UV spectrophotometric method. Riboflavin ( ？nm) [13], camphorquinone ( ？nm) [14],
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