Electron spin resonance studies of reversible addition-fragmentation transfer polymerisation

The polymerisation of styrene in the presence of cumyl dithiobenzoate (RAFT agent) was studied. Electron spin resonance spectroscopy was used to determine the time-dependence of the concentrations of the intermediate radical species, as a function of temperature and relative initial amounts of reactants. The intermediate radical concentration was found to be much lower than predicted by the studies of Barner-Kowollik et al. [Barner-Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1353], but consistent with that observed by Hawthorne et al. [Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457] and Kwak et al. [Kwak, Y.; Goto, A.; Tsuji.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules 2002, 35, 3026] Several observations that are not consistent with the current RAFT model were made. Intermediate radical signals were observed long after a significant amount of initiator remained and an extra signal appeared in the ESR spectrum at longer reaction times.Conference Pape

Electron spin resonance studies of reversible addition-fragmentation transfer polymerisation

The polymerisation of styrene in the presence of cumyl dithiobenzoate (RAFT agent) was studied. Electron spin resonance spectroscopy was used to determine the time-dependence of the concentrations of the intermediate radical species, as a function of temperature and relative initial amounts of reactants. The intermediate radical concentration was found to be much lower than predicted by the studies of Barner-Kowollik et al. [Barner-Kowollik, C.; Quinn, J. F.; Morsley, D. R.; Davis, T. P. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1353], but consistent with that observed by Hawthorne et al. [Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457] and Kwak et al. [Kwak, Y.; Goto, A.; Tsuji.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules 2002, 35, 3026] Several observations that are not consistent with the current RAFT model were made. Intermediate radical signals were observed long after a significant amount of initiator remained and an extra signal appeared in the ESR spectrum at longer reaction times.Conference Pape

Kinetic and electron spin resonance analysis of RAFT polymerization of styrene [2]

The kinetic and electron spin resonance analysis of reversible addition-fragmentation chain-transfer (RAFT) polymerization of styrene was discussed. It was found that the time dependence of the intermediate radical concentration varies significantly with the type of RAFT agent. It was also found that the RAFT process is not particularly sensitive to oxygen.Lette

A 1H NMR investigation of reversible addition-fragmentation chain transfer polymerization kinetics and mechanisms. Initialization with different initiating and leaving groups

In situ 1H nuclear magnetic resonance spectroscopy was used to directly investigate the processes that occur during the early stages (the first few monomer addition steps) of azobis(isobutyronitrile)-initiated reversible addition-fragmentation chain transfer (RAFT) polymerizations of styrene in the presence of cumyl dithiobenzoate at 70 and 84°C. The change in concentration of important dithiobenzoate species and monomer as a function of time was investigated. The predominant type of growing chain under the reaction conditions carries a cumyl end group. The initialization period (the period during which the initial RAFT agent is consumed) in the presence of cumyl dithiobenzoate in homogeneous media was significantly longer than for equivalent reactions using cyanoisopropyl dithiobenzoate as RAFT agent, and the rate of monomer conversion was correspondingly slower. Very strong fragmentation selectivity of the formed intermediate radicals (to form the tertiary propagating radical) was observed during the initialization period. The rate-determining step for the initialization process was the addition (propagation) of the initiator-derived and cumyl radicals to styrene, to form the corresponding single-monomer adducts. The greater length of this period with respect to the same reaction using cyanoisopropyl dithiobenzoate as RAFT agent is suggested to be a result of slower propagation due to a smaller addition rate coefficient of the cumyl radical (which was found to be the dominant propagation process during initialization) to styrene, than for the cyanoisopropyl radical, and to a higher average termination rate for the cumyl radicals than for the cyanoisopropyl radicals. The probable (small) difference in intermediate radical concentration is considered to be a less significant contributor to the length of the period. © 2005 American Chemical Society.Articl

Beyond inhibition : a 1H NMR investigation of the early kinetics of RAFT-mediated polymerization with the same initiating and leaving groups

In situ 1H NMR (NMR) spectroscopy has been used to directly investigate the processes that occur during the early stages (typically the first few monomer addn. steps) of an AIBN-initiated reversible addn.-fragmentation chain transfer polymn. of styrene in the presence of the RAFT agent cyanoisopropyl dithiobenzoate at 70 and 84 DegC. The change in concn. 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 prodn. of RAFT-capped chains of d.p. greater than unity until all of the cyanoisopropyl dithiobenzoate was converted to its single monomer adduct. The rate-detg. step for this process was found to be the addn. (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 prodn. 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 coeff. 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 kp for initiator fragments is smaller than for long chain radicals, which is a result of this aspect of the RAFT mechanism

Evidence for termination of intermediate radical species in RAFT-mediated polymerization

The reactions of short-chain species during the initial periods of cumyl dithiobenzoate (CDB)-mediated polymns. of styrene were studied at 84 Deg. High initiator and CDB concns. were chosen such that very short chains were targeted at complete conversion, and high radical concns. were present. The main reasons why species resulting from termination of intermediate radicals were given

A 1H NMR Investigation of Reversible Addition-Fragmentation Chain Transfer Polymerization Kinetics and Mechanisms. Initialization with Different Initiating and Leaving Groups

In situ 1H NMR spectroscopy was used to directly investigate the processes that occur during the early stages (the first few monomer addn. steps) of azobis(isobutyronitrile)-initiated reversible addn.-fragmentation chain transfer (RAFT) polymns. of styrene in the presence of cumyl dithiobenzoate at 70 and 84 DegC.The change in concn. of important dithiobenzoate species and monomer as a function of time was investigated. The predominant type of growing chain under the reaction conditions carries a cumyl end group. The initialization period (the period during which the initial RAFT agent is consumed) in the presence of cumyl dithiobenzoate in homogeneous media was significantly longer than for equiv. reactions using cyanoisopropyl dithiobenzoate as RAFT agent, and the rate of monomer conversion was correspondingly slower. Very strong fragmentation selectivity of the formed intermediate radicals (to form the tertiary propagating radical) was obsd. during the initialization period. The rate-detg. step for the initialization process was the addn. (propagation) of the initiator-derived and cumyl radicals to styrene, to form the corresponding single-monomer adducts. The greater length of this period with respect to the same reaction using cyanoisopropyl dithiobenzoate as RAFT agent is suggested to be a result of slower propagation due to a smaller addn. rate coeff. of the cumyl radical (which was found to be the dominant propagation process during initialization) to styrene, than for the cyanoisopropyl radical, and to a higher av. termination rate for the cumyl radicals than for the cyanoisopropyl radicals. The probable (small) difference in intermediate radical concn. is considered to be a less significant contributor to the length of the perio

Evidence for termination of intermediate radical species in RAFT-mediated polymerization

The reactions of short-chain species during the initial periods of cumyl dithiobenzoate (CDB)-mediated polymerizations of styrene were investigated at 84°C. High initiator and CDB concentrations were chosen such that very short chains were targeted at complete conversion, and high radical concentrations were present. The high radical and RAFT agent concentrations allowed the detailed examination of the nature and concentrations of both radical and nonradical species during the initial stages of the reaction. The probability of side reactions, such as termination of intermediate radicals, was enhanced due to the high concentrations of the potential reactants that were presented.Articl

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

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 kp for initiator fragments is smaller than for long chain radicals, which is a result of this aspect of the RAFT mechanism.Articl