Kinetics and mechanisms in carbocationic polymerization: The quest for true rate constants

Abstract

This article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (kp) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, kp = 1.35 × 104 L mol−1 s−1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded kp = 1.01 × 107 L mol−1 s−1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl4 from various laboratories yielded of ΔH ∼−25 and −34.5 kJ/mol for high and low TMPCl/TiCl4 ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ∼ −40 kJ/mol, whereas H2O/TiCl4 in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = −12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005

Cationic polymerization in rotating packed bed reactor: Experimental and modeling

“…The kinetic scheme adopted for the synthesis of IIR by cationic polymerization involves initiation, propagation, chain transfer to the monomer, and termination as shown in Scheme 15, 22, 24, 51, 52…”

“…The rate constants of propagation ( k p ) in the cationic polymerization vary considerably, e.g., k p 's from 10 3 to 10 9 L mol −1 s −1 have been reported for isobutylene polymerization 13, 14. It is proposed that the discrepancy in k p reported for various cationic polymerizations is due to different mechanistic interpretations and assumptions 15. Two general methods, diffusion clock (DC) and ionic species concentration methods, are typically used to determine the propagation rate constants in cationic polymerization of vinyl monomers.…”

Kinetic study on the living/controlled cationic polymerization of p‐methoxystyrene coinitiated by tris(pentafluorophenyl)borane

“…PREDICI simulation was earlier successfully applied for the estimation of various rate constant during the exhaustive mechanistic study of the living cationic polymerization of isobutylene. 50,51 To extract the reliable rate constants from experimental data, a separate kinetic experiment was performed in NMR tube under the experimental conditions closed to those reported in Figure 1 (see Fig. 8 caption for exact experimental conditions), where the conversion of initiator and monomer were determined every 2-3 min.…”

Generalization of a parameter set selection procedure based on orthogonal projections and the D‐optimality criterion

“…The orthogonalization method has been one of the most popular approaches for parameter set selection 23–31. This technique sequentially applies orthogonalization projections of sensitivity vectors to select one parameter at a time.…”