Despite a great deal of research on nitroxide mediated radical polymerization (NMRP), its kinetic mechanism is not fully known yet. The focus of this work was to contribute further to the understanding of the kinetics of NMRP processes through refinements of a comprehensive mathematical model developed by our group for the bimolecular system. This work considered important secondary reactions that can occur during the NMRP process. It also analyzed important kinetic aspects via a sensitivity study on some key parameters of the system. It was observed that an irreversible reaction between the TEMPO controller and the chemical initiator is very important and must be considered in the NMRP mechanism, in order to be able to describe properly both conversion and average molecular weight data. 相似文献
NMRP is a controlled polymerization technique with the ability to produce polymers with a highly controlled microstructure. The properties of the thus obtained polymers make it desirable to scale this technique to an industrial level, but there are still some challenges to be faced, e.g., to develop emulsion NMRP at low temperatures (lower than about 100 °C) with inexpensive, commercially available nitroxides such as TEMPO. Here, the emulsion NMRP of styrene using TEMPO at temperatures lower than 100 °C is described. An optimal control of molecular weights and polydispersities and a fast polymerization rate are obtained.
Simulations of polymerization rate, molecular weight development and evolution of the concentrations of species participating in the reaction mechanism over a range of operating conditions, and a parameter sensitivity analysis showing the effects of temperature, activation/deactivation equilibrium constant and initial concentrations of controller and initiator (if present) on these variables are presented for the nitroxide‐mediated radical polymerization of styrene. The simulations were performed with a computer program based on a detailed reaction mechanism. The simulated profiles of conversion, number average molecular weight ( ), and polydispersity agree well with experimental data. Previously unknown activation energies for reactions involved in the mechanism are estimated. The temperature dependence of the kinetic rate constants obtained in this study will be useful for future modeling and optimization studies.
Summary: The range of validity of two popular versions of the nitroxide quasi‐equilibrium (NQE) approximation used in the theory of kinetics of alkoxyamine mediated styrene polymerization, are systematically tested by simulation comparing the approximate and exact solutions of the equations describing the system. The validity of the different versions of the NQE approximation is analyzed in terms of the relative magnitude of (dN/dt)/(dP/dt). The approximation with a rigorous NQE, kc[P][N] = kd[P N], where P, N and P N are living, nitroxide radicals and dormant species respectively, with kinetic constants kc and kd, is found valid only for small values of the equilibrium constant K (10−11–10−12 mol · L−1) and its validity is found to depend strongly of the value of K. On the other hand, the relaxed NQE approximation of Fischer and Fukuda, kc[P][N] = kd[P N]0 was found to be remarkably good up to values of K around 10−8 mol · L−1. This upper bound is numerically found to be 2–3 orders of magnitude smaller than the theoretical one given by Fischer. The relaxed NQE is a better one due to the fact that it never completely neglects dN/dt. It is found that the difference between these approximations lies essentially in the number of significant figures taken for the approximation; still this subtle difference results in dramatic changes in the predicted course of the reaction. Some results confirm previous findings, but a deeper understanding of the physico‐chemical phenomena and their mathematical representation and another viewpoint of the theory is offered. Additionally, experiments and simulations indicate that polymerization rate data alone are not reliable to estimate the value of K, as recently suggested.
Validity of the rigorous nitroxide quasi‐equilibrium assumption as a function of the nitroxide equilibrium constant. 相似文献
Summary: This study examined the kinetics of nitroxide-mediated radical polymerization of styrene with unimolecular (alkoxyamine) initiators. Control of polymerization rate and polymer molecular weight in unimolecular nitroxide-mediated radical polymerization was studied by looking at the effects of the three main factors: initiator concentration, temperature, and initiator molecular weight on polymerization rate, molecular weight and polydispersity. In addition, the behavior of the unimolecular initiating systems was compared to that of the corresponding bimolecular system. The effective TEMPO concentration and degree of self-initiation of styrene were proved to be significant in dictating magnitudes of molecular weight averages and widths of molecular weight distribution. 相似文献
A mechanistic model is developed for high‐temperature (138 °C) styrene semibatch thermally and conventionally initiated FRP, as well as NMP with a two‐component initiating system (tert‐butyl peroxyacetate, 4‐hydroxy‐TEMPO). The model, using kinetic coefficients from literature, provides a good representation of the FRP experimental results. Implementation of a gel effect correlation to represent the change in the diffusion‐controlled termination rate coefficient with conversion improves the fit to the thermally initiated system, but is not required to represent the production of low molecular weight material ( Dalton) by conventionally initiated FRP or NMP. The low initiator efficiency found in NMP is well explained by a reaction network involving combination of free nitroxide with methyl radicals formed from initiator decomposition.
