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.