Assessing the importance of diffusion-controlled effects on polymerization rate and molecular weight development in nitroxide-mediated radical polymerization of styrene

A previously derived kinetic model for the nitroxide-mediated radical polymerization (NMRP) of styrene has been modified by considering diffusion-controlled (DC) effects on the bimolecular radical termination, monomer propagation, dormant polymer activation, and polymer radical deactivation reactions. Free-volume theory was used to incorporate the DC-effects into the model. It was found that DC-termination enhances the living behavior of the system, whereas DC-propagation, DC-activation and DC-deactivation worsen it. Although the inclusion of overall DC-effects into the kinetic model improved the performance of the model by slightly reducing the deviations obtained from experimental data of polymerization rate and molecular weight in the bimolecular NMRP of styrene with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and dibenzoyl peroxide (BPO), it does not seem to justify adding the extra four free-volume parameters. In the case of the semi-batch addition of azo-bis-iso-butyronitrile (AIBN) (several single shots at definite time intervals) in the NMRP of styrene, recently reported in the literature, it was found that DC effects are more significant, but it was observed that there was a strong dependence of polymerization rate on the frequency of addition of the shots of initiator (a maximum on polymerization rate being observed at a given frequency of addition of the shots), which could not be adequately explained in terms of DC-effects.