Beyond Inhibition:  A ¹H NMR Investigation of the Early Kinetics of RAFT-Mediated Polymerization with the Same Initiating and Leaving Groups

Abstract

In situ 1H nuclear magnetic resonance (NMR) spectroscopy has been used to directly investigate the processes that occur during the early stages (typically the first few monomer addition steps) of an AIBN-initiated reversible addition−fragmentation chain transfer polymerization of styrene in the presence of the RAFT agent cyanoisopropyl dithiobenzoate at 70 and 84 °C. The change in concentration of important dithiobenzoate species as a function of time has been investigated. It was found that the reaction was extremely selective during the period of consumption of the initial RAFT agent (defined as the initialization period), with almost no production of RAFT-capped chains of degree of polymerization greater than unity until all of the cyanoisopropyl dithiobenzoate was converted to its single monomer adduct. The rate-determining step for this process was found to be the addition (propagation) of the cyanoisopropyl radicals to styrene. During the period where the initial RAFT agent was consumed, fragmentation of formed intermediate radicals strongly favored the production of the tertiary cyanoisopropyl radicals, which were the only significant propagating species during that period. This led to a greater rate of propagation during that period, since the propagation rate coefficient for the cyanoisopropyl radical is greater than that of polystyryl radicals. It was found that inhibition effects can occur in the presence of RAFT agents in homogeneous media when the k p for initiator fragments is smaller than for long chain radicals, which is a result of this aspect of the RAFT mechanism.

RAFT‐Mediated Polymerization—A Story of Incompatible Data?

“…However, these large values for the equilibrium constant are backed up by high level ab initio quantum chemical calculations, which for many systems yield very reliable results. Furthermore, these ab initio rate data are able to explain reasonably well the initialization experiments by Klumperman and coworkers [13] as outlined earlier in this paper, albeit not in detail. [12] This in turn would suggest that most of the important aspects of the reaction dynamics are covered, yielding credibility to the high values for K. However, using the lower values for K and reasonable values for radicalradical termination rate coefficients, the experimental initialization data are well described, [17] including some important qualitative details.…”

“…The fragmentation rate coefficients, k frag , for these two reactions were found to be 0. 13 , respectively, were found. Two potential causes for these negative barriers are suggested; they are (i) truly barrierless reactions preceded by weak complex formation, or (ii) an artifact of the procedure.…”

“…[13][14][15] The majority of evidence for this process to occur was gathered from in situ 1 [14] Under those specific experimental conditions, an almost linear decrease of the RAFT agent concentration as a function of time was observed. The RAFT agent was fully consumed after approximately 45 min, after which polymerization commenced.…”

Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization of Vinyl Monomers Initiated by Poly(methyl methacrylate) Peroxide

“…An initialization period [31][32][33], especially at non-or-low concentration of PMMAP, was observed in Fig. 1.…”

“…1. The prolonged initialization period in the presence of non-or-low concentration of PMMAP was due to the low initiation rate which induced the low rate of the consumption of RAFT agent [31][32][33]. Unlike benzoperoxide, the peroxide bonds in PMMAP can not decompose completely [27,28].…”