Difference in propagation rate constants between photo‐and thermal‐initiated polymerization of styrene – effects of light irradiation

Propagation rate constants ( k p ) for photo‐ and thermal‐initiated radical polymerizations of styrene were determined by electron spin resonance (ESR) spectroscopy. The k p value for the former at 70°C ((420 ± 30) M −1 · S −1 ) was more than twice as large as that for the latter ((190 ± 10) M −1 · s −1 ) although the former value was almost the same as that obtained with the pulsed‐laser polymerization method. To evaluate the origin of the difference in the k p values between photo‐ and thermal‐initiated polymerization the absorption spectrum of a model of the propagating radical was measured and both the influence of the wavelength of the irradiation light on k p values and the activation energy of the photo‐initiated radical polymerization of styrene were determined. k p decreases for wave‐lengths shorter than 370 nm and is in agreement with the k p value for thermal‐initiated polymerization. No wavelength dependence of k p was obtained for dienes whose propagating radical does not absorb above 300 nm. The difference in k p values for photo‐ and thermal‐initiated polymerizations is considered to be due to the photoactivation (photoinduced excitation) of the propagating radical of styrene.