Controlled Free-Radical Polymerization ofn-Butyl Acrylate by Reversible Addition−Fragmentation Chain Transfer in the Presence oftert-Butyl Dithiobenzoate. A Kinetic Study

Abstract

The RAFT polymerization of n-butyl acrylate was studied in the presence of various concentrations of tert-butyl dithiobenzoate (t-BDB) as a chain transfer agent, and various concentrations of AIBN as an initiator at 60 and 90 °C. The polymerizations exhibited the expected features of controlled systems. In particular, the linear increase in number-average molar mass (M n) with monomer conversion, from very low conversion values, indicated a large apparent chain transfer constant to t-BDB. The kinetic study showed two typical characteristics already observed in RAFT polymerizations of various acrylates: the existence of an induction period and a strong rate retardation, both enhanced by the increase in t-BDB initial concentration. ESR spectroscopy was applied at 90 °C and revealed the existence of an unexpectedly large concentration of intermediate radicals. The system was examined and discussed considering either the possibility of cross-termination between a propagating macroradical and an intermediate one, with fast fragmentation of the intermediate radicals or the absence of cross-termination but slow fragmentation.

Living Radical Polymerization by the RAFT Process

“…For example, there is significant retardation in the polymerization of acrylate esters in the presence of dithiobenzoate esters. [9,44,52,64,84,[136][137][138] For polymerization of MA with benzyl 20 or cyanoisopropyl dithiobenzoate 8 as RAFT agent at 60 • C, we found retardation from the onset of polymerization. The retardation appeared not to be directly related to consumption of the initial RAFT agent (which was rapid), with the dithioester being completely consumed at the first time/conversion point.…”

“…The intermediates have been observed directly by EPR in RAFT polymerizations of styrene and acrylate esters and in model reactions with dithiobenzoate and dithiophosphonate RAFT agents. [112,[131][132][133][134][135][136] The intermediates 3 or 5 are not detectable during MMA polymerizations or in polymerizations with aliphatic dithioester, trithiocarbonate, xanthate, or dithiocarbamate RAFT agents. This is attributed to the greater rate of fragmentation and the correspondingly shorter lifetime of the adduct in these polymerizations.…”

Scope for Accessing the Chain Length Dependence of the Termination Rate Coefficient for Disparate Length Radicals in Acrylate Free Radical Polymerization

“…For one, acrylates undergo side reactions such as inter-and intramolecular chain transfer (i.e., chain transfer to polymer and backbiting, respectively), and thus, whether or not these side reactions will impact the method's ability to elucidate the termination kinetic coefficient from only the polymerization rate data needs to be examined. [2,[45][46][47][48][49] Additionally, the RAFT-CLD-T method relies on accurate online determination of the polymerization rate and -given their rapid polymerization -acrylates seem as an attractive option for experimental validation of this procedure. Thus, the impact of fast propagation, backbiting and mid-chain radical reactions on the method's ability to obtain accurately the chain length dependence of k t for both similar and disparate size radicals is investigated with the goal of aiding the experimentalist in choosing the optimum polymerization system for validating the recently introduced k s;l t methodology.…”

Use of Spin Trap Technique for Kinetic Investigation of Elementary Steps of RAFT-Polymerization

“…The most convincing argument for the currently accepted mechanism of RAFT polymerization is the direct monitoring of the radical intermediates formation by means of ESR (Chernikova et al, 2004, Hawthorne et al, 1999, Golubev et al, 2005, Kwak et al, 2002. The concentrations of these intermediates and their changes during the process provide valuable data for the kinetic modeling of RAFT polymerization.…”

“…To resolve these issues, we have been using ESR spectroscopy to measure rate coefficients. An advantage of ESR is that this procedure allows direct observation of the formation of radical intermediates and confirmation of their structure (Chernikova et al, 2004, Hawthorne et al, 1999, Golubev et al, 2005. This is possible because radical intermediates are less reactive than other radicals involved in polymerization and their steady-state concentrations are sufficient for their direct detection with modern radiospectrometers.…”