Polymerization of 2-hydroxyethyl methacrylate initiated by excitation of [Co(NH3)5N3]2+ at 365 and 546 nm

The polymerization of 2-hydroxyethyl methacrylate (HEMA) was studied by using azidopentaammine cobalt(III) as a photoinitiator in water/acetonitrile solution (7/3 by volume) and in pure aqueous solution. In the former case, a homogenous polymerization was observed whereas precipitation of poly(HEMA) occurred in pure aqueous solution. The studies were performed by using monochromatic radiation at λ=365 and 546 nm. In both cases the cobalt azide complex, [Co(NH3)5N3]2+, was found to be an efficient photoinitiator of polymerization. In water/acetonitrile solution the polymerization rate, Rp, by excitation of the complex at 365 or 546 nm was found to be proportional to [HEMA] and to the square root of the absorbed light intensity, Ia. A polymerization kinetic scheme implicating radical–radical combination in the termination step is proposed. In water/acetonitrile solution, the ratio kp/(kt,a)0.5 was evaluated to be 0.35. When the polymerization was performed in pure aqueous solution, the plot of Rp as a function of monomer concentration shows an autoacceleration phenomenon. However at low conversion percentage and low concentration of the monomer, and in spite of the precipitation of the polymer, the polymerization scheme appears to be analogous to that observed in water/acetonitrile. At high concentration of the monomer and for prolonged irradiation at 365 or 546nm, a gel was formed.