A study of the thermal polymerization of styrene by depolarized Rayleigh light scattering spectroscopy

The technique of depolarized Rayleigh light scattering spectroscopy has been applied to follow the kinetics of the thermal polymerization of styrene. By measuring the spectrum of the depolarized scattered light we are able to separate the intensity of light scattered by the polymer being formed from the intensity of light scattered by the monomer. The spectrum consists of a wide Lorentzian due to the monomer and a central spike with instrumental width due to the polymer. This separation occurs because the polymer relaxation time which is determined by the longest Rouse–Zimm mode or by overall rotation is four orders of magnitude longer than the relaxation time of the monomer which is governed by overall rotation. Using the intensity data, the initial rate of the polymerization reaction is measured and is found to agree with the rate determined in previous experiments using classical techniques. After about one-third of the reaction is complete, the rate increases by 50%. This increase in rate is interpreted as resulting from the large increase in viscosity preventing the termination of the reaction by coupling of polymer radicals. Also the rotational relaxation time of the monomer is determined for the first 80% of the reaction and is found to increase linearly with time. However the value of the monomer rotational relaxation time at 80% of reaction is only 1.5 times the inital value. This indicates the monomer rotational motion is not determined by the large macroscopic viscosity, but by a local viscosity in the region of the monomer.