Curcumin, a natural colorant as initiator for photopolymerization of styrene: kinetics and mechanism

The photopolymerization of styrene in presence of an efficient, eco-friendly, and a cost-effective photoinitiator, curcumin, which is found in turmeric root, has been reported for the first time. The catalytic concentration (10(-6) M) of curcumin is effective to photoinitiate the polymerization of styrene. The kinetic data, inhibiting effect of benzoquinone and electron spin resonance studies, indicate that the polymerization proceeds via a free radical mechanism. The system follows non-ideal kinetics (Rp. [Cur](0.36) [Sty](1.04)) due to both primary radical termination and degradative chain transfer reactions. The broad peaks due to methine and methylene protons in H-1-NMR (nuclear magnetic resonance [NMR]) spectrum and a band of resonances at 145-146 ppm in C-13-NMR indicate atactic nature of the polystyrene formed. The maximum conversion at 30 +/- 0.2 degrees C in 17 h has been limited to 23% without gelation. The formation of radicals and mechanism of polymerization are also discussed.