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.,
The influence of solvent on the rates and mechanisms of chemical reactions is of great importance and has been discussed by many workers [
Chemical structures of HEMA and photoinitiators.
Riboflavin (RF), camphorquinone (CQ), and safranin T (ST) were obtained from Sigma. Triethanolamine (TEOHA, Sigma) and 2-hydroxyethyl methacrylate (HEMA, Aldrich) were distilled under reduced pressure before use. Water was purified using a Millipore Milli-Q system.
Polymerization of HEMA (monomer/solvent ratio 1.21 : 10, 1 M) was carried out in the presence of photoinitiators, RF, CQ, and ST (absorbance of each photoinitiator at the
All spectral measurements on fresh and polymerized solutions of HEMA were carried out on a Shimadzu UV-1601 recording spectrophotometer using quartz cells of 10 mm path length.
Fluorescence measurements of RF in various HEMA solutions were carried out at room temperature (~25°C) using a SpectraMax 5 fluorimeter (Molecular Devices, USA) in the end point mode using
The measurement of the intensity of General Electric 15 W fluorescent lamp was carried out by potassium ferrioxalate actinometry [
The assay of HEMA in fresh and polymerized solutions was carried out by mixing a small amount of the solution with 0.05 M phosphate buffer, pH 7.0, and measurement of absorbance at 208 nm (molar absorptivity 8000 M−1 cm−1). At this dilution the photoinitiator has negligible absorption at the analytical wavelength. The concentration of the samples was calculated using the following least squares regression equation:
The rate of the reactions between dipolar molecules is dependent on the dielectric constant,
The polymerization of HEMA was carried out in aqueous and organic solvents (acetonitrile, methanol, and ethanol) containing RF as photoinitiator and 0.01 M TEOHA as a coinitiator. The reactions in 1-propanol and 1-butanol could not be carried out due to the insolubility of RF in these solvents. The analytical data were subjected to kinetic treatment and the reactions were found to follow pseudo first-order kinetics in the initial stages (~5% HEMA loss) using a low intensity radiation source. The steady-state assumption of the rate of initiation being equal to the rate of termination in polymerization reactions is considered valid only at a low conversion of monomer [
Apparent first-order rate constants (
Solvent | Dielectric constanta (25°C) | Viscositya (mPa·s)−1 |
|
||
---|---|---|---|---|---|
RF | CQ | ST | |||
Water | 78.5 | 1.000 | 5.05 | 4.04 | 2.98 |
Acetonitrile | 37.5 | 2.898 | 3.71 | 3.02 | 2.10 |
Methanol | 32.6 | 1.838 | 3.53 | 2.90 | 1.93 |
Ethanol | 24.3 | 0.931 | 3.12 | 2.62 | 1.71 |
1-Propanol | 20.1 | 0.514 | — | 2.45 | 1.56 |
1-Butanol | 17.1 | 0.393 | — | 2.39 | 1.45 |
Plots of
The extent of solvent interaction with the dipolar intermediate would determine the degree to which it leads to the formation of radicals. In these reactions the primary photochemical process is considered as being the electron transfer between reactants. In such a case the transition state is more polar than the reactant and the rate of reaction would increase with the dielectric constant of the medium as observed in the case of the photolysis of formylmethylflavin [
It has been reported that the rates of polymerization of HEMA are decreased with a decrease in medium polarity, that is, from water to acetonitrile, as a result of singlet state quenching in organic solvents [
Fluorescence intensity of 1 × 10−5 M RF in water and organic solvents.
Solvent | Relative fluorescence intensity at 520 nm |
---|---|
Water | 100.0 |
Ethanol | 87.1 |
Methanol | 86.7 |
Acetonitrile | 84.1 |
The results of the effect of solvent dielectric constant on the rate of polymerization of HEMA in the presence of CQ as photoinitiator may be considered on the basis of the data discussed above in the case of RF as a photoinitiator. The values of
The apparent first-order rate constants for the polymerization of HEMA in ST/TEOHA system in aqueous and organic solvent are reported in Table
The slopes of the plots of
Another important factor that may influence the rate of a chemical reaction is the viscosity of the medium. This appears to control the solute diffusion and hence the rate of a reaction. A pervious study has shown that the 3Fl quenching by a substrate is proportional to the inverse of solvent viscosity as expected for a diffusional process [
Polymerization reactions of HEMA in water at 1, 2, and 3 M concentrations in the presence of different photoinitiators have shown that the rates are decreased with an increase in the viscosity of the medium [
Plots of
The effect of viscosity on the rate of polymerization of HEMA using CQ as a photoinitiator shows a similar behavior as observed in the case of RF. A plot of
The results obtained with ST as a photoinitiator in the polymerization of HEMA are similar to those of RF and CQ. A plot of
In order to provide further explanation of the reactivity of the three photoinitiators (RF, CQ, and ST) used in this study, a consideration of the spectral and structural characteristics of these compounds is necessary.
RF exhibits an absorption maximum at 445 nm and undergoes
It needs to be emphasized that, under the assay conditions of HEMA, on the dilution of photolysed solutions, the maximum TEOHA concentration used (0.01 M) would be too low to undergo complexation with HEMA in a molar ratio. The UV spectra of HEMA at such a dilution did not show any change in UV absorption in the presence of TEOHA. This suggests that there is no possibility of interaction between these compounds to affect the rate constants.
The mechanisms of polymerization of HEMA using RF [
A general scheme for the polymerization of HEMA in the presence of photoinitiators.
The photoinitiator (PI) on the absorption of light is promoted to the excited singlet state (1PI*) followed by intersystem crossing (isc) to the excited triplet state (3PI*). 3PI* is quenched by the amine (Am) by electron transfer to form a semireduced 3PI•− and a semioxidized 3Am•+ radical pair [3(PI•−+ Am•+)]. This is followed by proton transfer from the Am•+ radical to the PI•− radical to produce neutral PI and Am radicals. The free radicals thus formed in the reaction would add to the double bonds of HEMA monomer and initiate the polymerization process. The rate and extent of polymerization would depend on the solvent polarity and viscosity.
The polymerization of HEMA in the presence of RF, CQ, and ST as photoinitiators and TEOHA as a coinitiator shows that RF is more efficient than CQ and ST. The rate of polymerization is a linear function of the solvent dielectric constant indicating the involvement of polar intermediates in the photoinitiated reaction. The effect of decrease in fluorescence intensity of RF on the rate of the reaction is due to a decrease in solvent polarity causing the quenching of the excited singlet state. The rate of the reaction is inversely proportional to the solvent viscosity as a result of the diffusion controlled process. The study highlights the role of solvent characteristics in the efficiency of the polymerization of HEMA in the presence of the photoinitiators used.
There is no conflict of interests.