Determination of Rate Constants in the Carbocationic Polymerization of Styrene: Effect of Temperature, Solvent Polarity, and Lewis Acid

The electrophilicity parameter (E = 9.6) of the 1-phenylethyl cation, 1+, has been determined and combined with the nucleophilicity parameter (N = 0.78, s = 0.95) of styrene (St) to predict diffusion-limited propagation in the cationic polymerization of St by the linear free energy relationship log k = s(N + E). This prediction has been experimentally verified using two different diffusion clock methods, which provided a value of kp± ≈ 2 × 109 L mol-1 s-1, 6 orders of magnitude higher than previously accepted, for the absolute rate constant of propagation of the TiCl4-induced polymerization of St in methylcyclohexane/methyl chloride 60/40 (v/v) at −80 °C. The kp± value remained unchanged in the temperature range −50 to −80 °C, indicating that propagation does not have an enthalpic barrier; however, it increased moderately with increasing solvent polarity. The nature of the Lewis acid has little effect on kp± as similar values have been obtained with TiCl4 or SnCl4. The apparent rate constant of ionization, , the rate constant of deactivation, k-i, and the apparent equilibrium constant of ionization, , have also been determined as a function of temperature. The increases slightly and k-i increases moderately with increasing temperature; therefore, and the overall polymerization rate decrease moderately with increasing temperature

Influence of Chain Length on the Electrophilic Reactivity of Carbocations

The electrophilic reactivities of the bis(p-methylphenyl)carbenium ion 1b+ and its macromolecular analogue 1a+ have been compared in slow reactions with allylsilanes and in fast reactions with silyl enol ethers. Treatment of 1b-Cl with TiCl4 or GaCl3 and of 1a-Cl with GaCl3 in CH2Cl2 at −70 °C gave solutions of the carbocations 1b+ and 1a+, the concentration of which was measured photometrically. Addition of allylsilanes led to the exponential decay of the benzhydryl cation concentrations, from which the second-order rate constants have been derived. The rate constants determined for the reactions of 1a+ or 1b+ with allyltriphenylsilane and with allylchlorodimethylsilane, respectively, agree within experimental error. Laser flash photolyses of 1a-Cl and 1b-Cl in acetonitrile/dichloromethane mixtures yielded the corresponding benzhydryl cations, which showed pseudo-first-order decay in the presence of variable concentrations of 1-trimethylsiloxycyclohexene or 1-trimethylsiloxycyclopentene. The second-order rate constants ranged from 1 × 108 to 5 × 108 L mol-1 s-1 but differed for 1a+ and 1b+ by less than 30%. It is concluded, therefore, that macromolecular carbocations and their low molecular weight analogues have the same reactivity

Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles^†^,^‡

Twenty-three diarylcarbenium ions and 38 π-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and π-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 °C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938−957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table ). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with π-nucleophiles (Figure ) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-π-complexes, and hydride donors (Table ) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table ). The reactivity parameters in Figure are, therefore, recommended for the characterization of any new electrophiles and nucleophiles in the reactivity range covered. The linear correlation between the electrophilicity parameters E of benzhydryl cations and the corresponding substituent constants σ+ provides Hammett σ+ constants for 10 substituents from −1.19 to −2.11, i.e., in a range with only very few previous entries

Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles^†^,^‡

Twenty-three diarylcarbenium ions and 38 π-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and π-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 °C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938−957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table ). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with π-nucleophiles (Figure ) are also suitable for characterizing the nucleophilic reactivities of alkynes, metal-π-complexes, and hydride donors (Table ) and for characterizing the electrophilic reactivities of heterosubstituted and metal-coordinated carbenium ions (Table ). The reactivity parameters in Figure are, therefore, recommended for the characterization of any new electrophiles and nucleophiles in the reactivity range covered. The linear correlation between the electrophilicity parameters E of benzhydryl cations and the corresponding substituent constants σ+ provides Hammett σ+ constants for 10 substituents from −1.19 to −2.11, i.e., in a range with only very few previous entries