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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.imageValidity of the rigorous nitroxide quasi‐equilibrium assumption as a function of the nitroxide equilibrium constant.

Nanostructured titania reservoirs were synthesized by the sol-gel method. An antiepileptic drug phenytoin was encapsulated for the targeted drug delivery. NMR studies confirmed that the drug does not undergo substantial modifications during the sol-gel process. The water/alkoxide ratio r w was varied from 2 to 24 to evaluate the role of hydrolysis degree on the drug-matrix interactions. The interactions were found to be of hydrogen-type between amine and carbonyl groups of phenytoin and hydroxyl groups of titania. Tridental complex was found to be the most favorable out of three complexes proposed. To form this complex for each phenytoin molecule, two hydroxyl groups of titania are needed. It was found that when the water/alkoxide ratio r w ) 16 the hydroxylation degree is the highest, which allows us to bind the largest amount of the drug.