An ESR Approach to the Estimation of the Rate Constants of the Addition and Fragmentation Processes Involved in the RAFT Polymerization of Styrene

Abstract

Dedicated to the memory of Emeritus Professor Dr. Hanns FischerThe reversible-addition-fragmentation chain transfer (RAFT) controlled radical polymerization of such vinylic monomers as styrene (= ethenylbenzene) has gained increasing popularity in current years. While there is a general agreement on the mechanism of RAFT polymerization, there is an ongoing debate about the values of the rate constants of its key steps, i.e., the addition of the propagating radicals to the mediator and the fragmentation of the resulting spin adducts. By carrying out an ESR spectroscopic investigation of the AIBN-initiated polymerization of styrene (AIBN = 2,2'-azobis[2-methylpropanenitrile]), mediated by benzyl (diethoxyphosphoryl)dithioformate (5) as RAFT agent, we were able to detect and characterize four different radical species involved in the process. By reproducing their concentration-time profiles through a kinetic model, the addition and fragmentation rate constants at 908 of the propagating radicals to and from the mediator were estimated to be ca.10 , respectively. The validity of the kinetic model was supported by hybrid meta DFT calculations with the BB1K functional that predicted addition-and fragmentation-rate-constant values in good agreement with those estimated from the ESR experiments.

RAFT Polymerization Kinetics: Combination of Apparently Conflicting Models

“…Despite the good agreement with conversion/polydispersity data, each model is in conflict with aspects of experimental results as well as agreeing with other aspects of the experiments. The intermediate termination model predicts radical concentrations in the correct order of magnitude as those observed experimentally (typically 10 −7 −10 −5 ). ,,– However, the intermediate termination model gives equilibrium constants many orders of magnitude lower than those obtained from quantum calculations – and experiments. , In addition to the conflicting equilibrium constant predictions, the intermediate termination model predicts a significant number of 3-armed star polymers as products of the polymerization . Although the star polymers have been detected in MALDI-TOF experiments specifically designed to detect them – and experiments using short polymer chains, these 3-armed polymers have not being consistently observed in SEC experiments, despite the intermediate termination model predicting large quantities of these 3-armed star polymers .…”

Full Molecular Weight Distribution in RAFT Polymerization. New Mechanistic Insight by Direct Integration of the Equations

“…One school of thought proposes that the induction/retardation phenomena are due to the slow fragmentation (SF) of the intermediate or adduct radicals 2,5 9–11. Another line of thought assumes that these phenomena are due to the cross‐termination reaction of the adduct or intermediate radical (IRT, intermediate radical termination) with living radicals 12–17. These two theories propose values for the kinetic coefficient of the fragmentation reaction which differ by several orders of magnitude; the SF theory postulates the value of this coefficient to be about 10 −2 s −1 , while the IRT theory estimates this value to be about 10 4 s −1 .…”

Enhanced Control of Phosphinoylcarbodithioate-Mediated RAFT Polymerization: Key Role of Substituents at the Phosphorus Center

“…The concept of organophosphorus RAFT agent was suggested by Moad et al at CSIRO back in 1998 . Soon after, Laus and Alberti et al have published a series of articles dedicated to the synthesis and application of diethoxyphosphoryl- and diethoxyphosphoro­thioylcarbodithioates as chain transfer agents for RAFT polymerization. − Around the same time, an efficient synthesis of these compounds under mild conditions was reported by Fox et al Because of their strongly electron-withdrawing dialkoxyphosphoryl Z-group, these compounds demonstrate moderate efficiency in RAFT polymerization with poor control over number-average molar masses M n and high dispersities Đ . In another context, these P-RAFT agents and derived polymers have found an application as dienophiles for the hetero-Diels–Alder (HDA) cycloaddition and formation of diblock copolymers by “click” coupling between P-RAFT and diene-terminated polymers. − A few years later, our team reported the synthesis of tungsten, chromium, and molybdenum pentacarbonyl complexes with cyanomethyl and 1-phenylethyl diphenylphosphino­carbodithioates and their application in RAFT polymerization. , In parallel, Chen et al have reported a few more examples of tungsten complexes with benzyl and allyl R-groups .…”