Incubation period in the 2,2,4,4‐tetramethyl‐1‐piperidinyloxy‐mediated thermal autopolymerization of styrene: Kinetics and simulations

Abstract

Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (kh)]. At higher TEMPO concentrations ([N•] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition kad, also becomes relevant. From previous experimental data and simulations, initial estimates of kh ≈ 1 and kad ≈ 6 × 10−7 L mol−1 s−1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression kth[M]3 (where [M] is the monomer concentration and kth is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006

Application of Parameter Selection and Estimation Techniques in a Thermal Styrene Polymerization Model

“…Thermal initiation is assumed to occur via a three‐step process in Reaction 1 through 3. It is common to assume that the radicals A* and M* generated by Reaction 2 have the same reactivity towards monomer in Reaction 3 and 4 25–27. We have adopted this mechanism because it is recognized as the most plausible means of generating free radicals from styrene 24, 28.…”

“…Table 1 does not contain mechanistic steps related to controlled radical polymerization of styrene because the current model is focused on conventional thermal polymerization. Note, however, that recent kinetic studies involving nitroxide‐mediated styrene polymerization provide useful information about rate coefficients for thermal initiation reactions 25, 27, 39–43. For example, Kothe and Fischer devised a method to determine the rate coefficient for formation of styrene adduct (forward Reaction 1 in Table 1) by monitoring the nitroxide concentration in the initial stages of a nitroxide‐mediated polymerization 41.…”

“…Kothe and Fischer's rate constant and activation energy agree well with values determined by Moad et al44 and Kirchner et al45, 46 In Table 2, Kothe and Fischer's estimates for k 1o and ( E a / R ) 1 are selected as initial parameter values and assigned relatively small uncertainties of 15% and 10%, respectively, based on linear regression results presented by the authors. Saldivar‐Guerra et al used values from third‐order kinetic parameters3 to determined that k −1 should lie between about 1 × 10 −6 and 5 × 10 −6 s −1 at 125 °C 25. In the current model, rather than estimating k −1 , we follow the approach of Kotoulas et al26 and estimate the ratio k 1 / k −1 .…”

“…Some modelers are hesitant to adjust parameter values that have been reported in the literature, unless conflicting values have been reported in several studies. Recent work by Saldivar‐Guerra et al illustrates the traditional approach to parameter selection and estimation 25. Their model describes thermal polymerization of styrene at 125 °C, mediated by 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) and incorporates the two‐step thermal initiation scheme shown in Table 1.…”

Nitroxide‐Mediated Controlled Radical Styrene Polymerization Via a Mass‐Suspension Process

“…It is known that the length of the induction depends in a complex way on several kinetic parameters such as k it , k dim . and k h3 (thermal initiation, dimerization and rate enhancement, respectively) and we decided to use values for these parameters similar to those used in previous work by our group and others; however, in a study performed by our group there is evidence arising from experiments carried out in the absence of a radical initiator (using only monomer and TEMPO) that suggests that the value for k h3 could be much larger than the one used here (up to 1 L · (mol · s) −1 at 125 °C) instead of the relatively small value of 0.017 L · (mol · s) −1 used here, perhaps with a smaller values for the dimerization and thermal initiation constants than the ones adopted here. Not only that, the cited study also finds that hydrogen abstraction on the dimer by TEMPO can also be a significant reaction at relatively high TEMPO concentrations, but this reaction was not included in our model for the sake of simplicity.…”