The thermal polymerization of styrene: A proton NMR study

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Abstract
The proton nuclear spin–lattice relaxation has been measured as a function of reaction time during the thermal polymerization of styrene. Initially the proton relaxation in the monomer is exponential and the measured T1 is 21 s, a value typical of oxygen free organic liquids. Within a few hours of the start of the experiment, the relaxation is observed to be the sum of two exponential components. The component with the longer relaxation time is due to the monomer. The second component which has a much shorter relaxation time is assigned to the polymer formed during the reaction. For a polymerization carried out at 86°C the proportion of monomer is observed to decrease initially at rate of 0.93%/h. T1 for the monomer decreases linearly until about 35% of the monomer remains when the polymerization rate becomes less. The results are independent of the NMR frequency and an analysis of the results indicates that the translational diffusion of the monomer decreases substantially during the reaction. T1 for the polymer depends on the NMR frequency. Initially it decreases with reaction time, goes through a minimum, then increases. The value of the T1 at the minimum suggests that the molecular motions involving the hindered rotation about the carbon backbone of the long polymer chain are being observed.