PHOTOPOLYMERIZATION OF 2 HYDROXYETHYL METHACRYLATE IN THE DEVELOPMENT OF GLASS-IONOMER CEMENTS

Abstract

The present investigation is based on a comparative study of the polymerization reactions of 2-hydroxyethyl methacrylate (HEMA) using riboflavin (RF), camphorquinone (CQ) and safranin T (ST) as photoinitiators and triethanolamine (TEOHA) as a co-initiator in aqueous and organic solvents. HEMA is involved in photoinitiated polymerization reactions occurring in glass-ionomer cements (GICs). The above photoinitiators are stable on exposure to a 15 W lamp emitting in the visible region during the irradiation period. A UV spectrophotometric method has been developed to determine the concentration of HEMA in polymerized solutions at low conversion, i.e., about 5% concentration change in the monomer. The effect of pH, solvent characteristics and HEMA / TEOHA concentration on the rate of polymerization reactions has been evaluated. The rates of the reactions have been found to increase with pH probably as a result of the deprotonation of TEOHA and the presence of a labile proton on the hydroxyl group of HEMA. The steady-state assumption of the rate of initiation being equal to the rate of termination is considered valid only at a low conversion of the monomer. Under these conditions the polymerization reactions have been found to follow pseudo first-order kinetics (within about 5 % change in HEMA concentration) and the determined apparent first- order rate constants (kobs) range from 5.44–7.63×10–4s–1 at pH 6.0–9.0 using 0.01M TEOHA. The polymerization reactions of HEMA are affected by the dielectric constant and viscosity of the medium. There is a linear relation between the rate constants and the solvent dielectric constant indicating the involvement of a polar intermediate in the reaction. The observation of a linear relation between the rate constants and the inverse of solvent viscosity indicates the effect of viscosity on the diffusional processes of reacting species and hence the decrease in rates with an increase in the viscosity of the medium. For all the photoinitiators used the polymerization reaction is dependent on the TEOHA concentration and the second- order rate constants for the interaction of HEMA (1–3M) and TEOHA 0.0025– 0.01M) range from 1.79–8.87×10–2 M–1s–1 at pH 6.0–9.0. The reactivity of the photoinitiators appears to depend on the energy of the excited triplet state, and its degree of interaction with the amine to form a radical ion pair, its dissociation and further interaction of amine radicals with HEMA to undergo polymerization. In the present work, the reactivity of the photoinitiators has been found to be in the order: RF > CQ > ST. The irradiation wavelength of the photoinitiators also increases in the same order resulting in decreased energy and, therefore, it may influence the efficacy of the photoinitiator. The polymerization of HEMA photoinitiated by the three compounds is faster in aqueous solution compared with the organic solvent under identical conditions. The results indicate that the reactivity of the photoinitiators depends upon their structural features, ionization behavior, spectral characteristics, triplet energy and solvent characteristics. It appears that the photoinitiators absorbing in the lower visible region are more efficient (RF 445 nm, CQ 468 nm) in causing polymerization of HEMA compared to those absorbing in the relatively higher visible region (ST 520 nm), as indicated by their absorption maxima. Therefore, the use of RF as a photoinitiator in glass ionomer cements would require a relatively less curing time for the setting of resin restorative materials and it appears to be the most efficient photoinitiator in the polymerization process under the conditions studied.