Vinyl polymerization. 93. Polar effects in radical polymerization of p‐substituted styrenes

Abstract

The radical polymerization of styrene and of the p-substituted styrenes p-methoxy-, p-methyl-, p-fluoro-, p-chloro-, p-bromo-, and p-cyanostyrene, was kinetically studied. The styrenes with more electron-attracting substituents show a greater overall rate of polymerization. At 30,O "C., the individual rate constants of propagation (kp) and termination (kt) were determined by a rotating sector and by usual inhibitor method using diphenylpicrylhydrazyl (DPPH) or p-benzoquinone. It was found that the following HAMMETT relationship is established in the homopolymerization of p-substituted styrenes :where cr is the HAMMETT constant, and p' is a reaction constant which was calculated as +0.6. This relation is interpreted from a participation of ionic structures in the transition state of the propagation reaction. For confirmation, MULLIKEN's electronegativity of the p-substituted styrene and the p-substituted benzyl radical was calculated by simple LCAO MO method. The relationship between the rate constants of termination, kt, and the a-values is expressed by two lines, crossing a t p-fluorostyrene which is at the maximum point. These two lines are explained by two ways; one is the diffusion controlled termination and the other is the recombination of the two polarized radical ends. ZUSAMMENFASSUNG:Die radikalische Polymerisation von Styrol und p-substituierten Styrolen, und zwar p-Methoxy-, p-Methyl-, p-Fluor-, p-Chlor-, p-Brom-und p-Cyanostyrol, wurde kinetisch untersucht. Die Bruttogeschwindigkeit der Polymerisation war bei elektronenanziehenden Substituenten groBer als bei elektronenabstoaenden. Fiir 30,O "C wurden die Absolutwerte der Geschwindigkeitskonstanten des Kettenwachstums (kp) und des Abbruchs (kt) mit Hilfe des rotierenden Sektors und mit Inhibitoren (DPPH oder p-Benzochinon) bestimmt. Es ergab sich, daB die folgende HAMMETT-Beziehung erfiillt ist : kpx/k,H = k-exp(p'.a) wobei cr die HAMMETTsChe Substituentenkonstante ist und p' eine Reaktionskonstante, die in diesem Falle zu +0,6 gefunden wurde. Diese Beziehung wird durch die Annahme er-Hart, daB beim Wachstumsschritt ionische (polare) Strukturen am Ubergangszustand beteiligt sind. Um diese Anschauung zu stutzen, wurde die MULLIKENsche Elektronega-*) Part 92 of this series, T. OTSU, A. SHIMIZU, and M. INOTO, J. Polymer Sci. 3A (1965) 615.tivitat der substituierten Styrole und der substituierten Benzylradikale nach der einfachen LCAO-MO-Methode berechnet. Die Beziehung zwischen den Geschwindigkeitskonstanten der Abbruchsreaktion (kt) und den a-Werten wird durch zwei Geraden dargestellt, die sich beim p-Fluorstyrol in einem Maximalpunkt kreuzen. Dieser Befund laat sich durch die Annahme eines difussionskontrollierten Abbruchs einerseits und eines Abbruchs durch Kombination polarisierter Radikale andrerseits deuten.

Polyaddition of Dithiol Compounds to Divinyl Compounds: The Kinetics of the Model Addition Reaction of Thiophenols to Styrenes

“…and k~ of this system equal those of substituted polystyryl radicals, 16 Tables II and III. K~/k~ are calculated based on eq 10 using the experimental data of Rx/RH and k!/k~, 16 and k~/k~ are used the experimental data, RY/RH_ In the case of styrenes, k~/ k~ decreased in the order ofMeO, Me, and then Cl. k~/~ ofthiophenols increased in the order of OH, MeO, Me, and Cl.…”

Kinetics of the Addition Reactions of Thiobenzoic Acids to Styrenes or Ethynylbenzenes as the Model of Polyaddition. Study on the Rate-Determining Step and Substituent Effect

“…But, absolute p values of TBA system are about twice those of the TP system. Imoto et al 22 claim that p changed greatly by the polarity of the attacking radical controlled by the substituent in the addition reactions of various polystyryl radicals to substituted styrenes. It was also found in the present system that the electrophilicity of the benzoylthiyl radical was larger than that of the benzenethiyl radical to St or EB.…”

Synthesis of poly(chloromethylstyrene-b-styrene) block copolymers by controlled free-radical polymerization

“…15,18,19 Kinetic studies on substituted styrenes have shown that a linear correlation can be obtained between the apparent rate coefficient, k p app ϭ Ϫ d(ln [M])/ dt, and the Hammett constant, , for various substituents, with a slope ϭ 1.5 for atom transfer radical polymerization (ATRP), 20 ϭ 1.45 for TEMPO-mediated polymerization, 16 and ϭ 0.6 for conventional free-radical polymerizations. 21 From these results ( controlled Ͼ conventional ), it was deduced that the larger k p app for electronwithdrawing substituents (EW; e.g., for CMS in comparison with styrene) would be caused not only by a larger k p but also by a larger equilibrium constant, K eq , between propagating and dormant polymer chains and/or a larger [Cu I ]/[Cu II ] ratio (in copper-mediated ATRP; i.e., a larger [P-T]/[T*] ratio in nitroxide-mediated polymerizations). Georges et al 16 estimated that the increase in K eq ϭ k d /k c should mainly be due to a decrease in k c , whereas k d would be relatively independent of the substituents.…”