Rate Enhancement and Retardation Strategies in Living Free Radical Polymerizations Mediated by Nitroxides and Other Persistent Species:  A Theoretical Assessment

Abstract

Three strategies to improve the performance of living and controlled polymerizations mediated by persistent radicals are analyzed kinetically to explore their advantages and limits. First, rate enhancements by an additional conventional initiation or by the monomer self-initiation are found only compatible with high degrees of polymer livingness and control if the additional initiation rate is much smaller than the activation rate of the dormant polymer. Second, rate-enhancing reductions of the persistent radical concentration by reactions with additives or by a natural decay lead to living and controlled polymers even if these processes are surprisingly fast. The enhanced monomer conversion rates generally diminish the detrimental effects of a direct precursor decay or of the radical disproportionation to unreactive products. Third, a retarding initial excess of the persistent species can force unfavorably fast systems to livingness and control. The limits of these procedures are specified. Analytic equations are given which facilitate the extraction of kinetic data from conversions, degrees of polymerization and polydispersities. They agree with experimental findings.

Nitroxide decomposition: Implications toward nitroxide design for applications in living free‐radical polymerization

“…Alternatively, several groups have enhanced polymerization rates by the continuous addition of radical initiators to consume excess nitroScheme 8 Scheme 7 xide. [153][154][155][156][157] The organic acid salt 2-fluoro-1-methylpyridinium p-toluenesulfonate has been used to trap the mediating nitroxide as its oxammonium salt 47 and hydroxylamine anion salt 48 in much the same way as a Brønsted acid. 158,159 The addition of acetic anhydride has also been shown to increase TEMPO-mediated radical polymerization rates.…”

On the Nitroxide Quasi‐Equilibrium in the Alkoxyamine‐Mediated Radical Polymerization of Styrene

“…In the regime of low K the behavior of the system can be represented simply by Equation (15) and (16) instead of (5) and (6).…”

“…[15] However, instead of using the analytical solution of (20) derived for the case of constant R i (which has been already done by Fukuda et al [25,26] ), we decided to use the more realistic variable initiation rate given by R i ¼ k th [M] 3 and solved Equation (20) numerically (notice that the analytical implicit solution given by Equation (10-12) also requires numerical calculations in order to recover the actual time trajectories of the species). The results of the comparison are shown in Figure 5 and 6 respectively for conversion as well as for N and P concentrations and are similar to those presented by Fukuda et al for constant R i .…”

“…Furthermore, Souaille and Fischer, [13][14][15] based on an elegant phase-space analysis of the differential equations that describe the evolution of the nitroxide and living radicals, developed a general theory that sets limits for the existence of the relaxed quasi-equilibrium regime for the nitroxide in alkoxyamine-mediated polymerizations. They found that a necessary and sufficient condition for this was:…”

“…Souaille and Fischer [15] extended their theory to the case where there is an additional source of radicals with generation rate R i , such as the thermal autoinitiation of styrene, while Fischer [16] analyzes several practical cases showing that all of them fulfill the conditions for the validity of nitroxide quasi-equilibrium.…”