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 degrees 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 wavelengths 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.