Limitations in the synthesis of high molecular weight polymers via nitroxide-mediated controlled radical polymerization: Modeling studies

The nitroxide-mediated controlled radical polymerization (NM-CRP) of styrene was modeled at the mechanistic level using the method of moments. The mechanistic models developed described the kinetics and the molecular weight development of the living free-radical polymerization process. A base model was constructed which included initiator decomposition, propagation, end-chain coupling, and termination by recombination and disproportionation. Using an Evans-Polanyi description of the activation energy (E = E-0 + alphaDeltaH(R)), the base model was fit to a set of experimental data for the living free radical polymerization of styrene at 87 degreesC(1) to obtain the heat of reaction for decoupling (DeltaH(R) for the di-tert-butyl nitroxide coupling agent) and to fit the intrinsic barrier (E-0) for propagation/depropagation. The remaining rate parameters were primarily obtained from the literature, while some were taken from previous modeling work in our laboratory.(2,3) The fit of the base model to the experimental data was then compared to the fit obtained when chain transfer to monomer and both chain transfer to monomer and styrene thermal initiation were included in the mechanism. It was found that including styrene thermal initiation was critical to being able to obtain good agreement between the model and the experimental data. The fitted parameters obtained after including styrene thermal initiation were E-0 for propagation/ depropagation = 10.78 +/- 0.08 kcal/mol and DeltaH(R) for the decoupling reaction = 22.70 +/- 0.40 kcal/mol. Using these fitted parameters, the model was used to predict the evolution of M-n and M-w of the polymer product at different times and temperatures and with a macroinitiator. The importance of reactions such as chain transfer to polymer and the reaction between a nitroxide radical and a polymeric radical to form a hydroxy amine was also investigated, and it was found that these reactions were negligible.