Revealing Model Dependencies in “Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”

In a recent article (W. Meiser, M. Buback, Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study, Macromol. Rapid Commun. 2011 , 18, 1490‐1494), it is claimed that evidence is found that unequivocally proves that quantum mechanical calculations assessing the equilibrium constant and fragmentation rate coefficients in dithiobenzoate‐mediated reversible addition fragmentation transfer (RAFT) systems are beset with a considerable uncertainty. In the present work, we show that these claims made by Meiser and Buback are beset with a model dependency, as a critical key parameter in their data analysis – the addition rate coefficient of the radicals attacking the C=S double bond in the dithiobenzoate – induces a model insensitivity into the data analysis. Contrary to the claims made by Meiser and Buback, their experimental results can be brought into agreement with the quantum chemical calculations if a lower addition rate coefficient of cyanoisopropyl radicals (CIP) to the CIP dithiobenzoate (CPDB) is assumed. To resolve the model dependency, the addition rate coefficient of CIP radicals to CPDB needs to be determined as a matter of priority.     

RAFT kinetics revisited: Revival of the RAFT debate

Recently, two electron spin resonance (ESR)‐based methods for the determination of addition and fragmentation rate coefficients in dithiobenzoate‐mediated reversible addition fragmentation transfer (RAFT) polymerization were introduced, one being based on a spin‐trapping method and the other on single‐laser pulse initiation in conjunction with ESR detection at microsecond time resolution. For the RAFT‐intermediate radical fragmentation rate, coefficient data differing by six orders of magnitude were obtained, which cannot be explained by the usual model dependencies, that is the so‐called cross‐termination versus stable intermediate model. Even under consideration of fast cross‐termination in both cases, the large difference persists. Both the experimental designs are thus critically reviewed to identify potential error sources and to explain the vast difference in the individual results. Both techniques appear to be robust and only small interferences could be identified. Finally, recommendations for the refinement of the individual techniques are given to achieve a consistent kinetic picture of the underpinning reaction equilibria. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

“Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study”—Response to a Comment

We have presented an EPR‐based approach for deducing the RAFT equilibrium constant, K_eq, of a dithiobenzoate‐mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011 , 32, 1490]. Our value is by four orders of magnitude below K_eq from ab initio calculations for the identical monomer‐free system. Junkers et al. [Macromol. Rapid Commun. 2011 , 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so‐obtained intermediate radical as well as high cross‐termination rate. By identification of all side products, our EPR‐based method is shown to be model independent and to provide reliable K_eq values, which demonstrate the validity of the intermediate radical termination model.     

Computational Studies of RAFT Polymerization–Mechanistic Insights and Practical Applications

Summary: Computational chemistry is a valuable complement to experiments in the study of polymerization processes. This article reviews the contribution of computational chemistry to understanding the kinetics and mechanism of reversible addition fragmentation chain transfer (RAFT) polymerization. Current computational techniques are appraised, showing that barriers and enthalpies can now be calculated with kcal accuracy. The utility of computational data is then demonstrated by showing how the calculated barriers and enthalpies enable appropriate kinetic models to be chosen for RAFT. Further insights are provided by a systematic analysis of structure‐reactivity trends. The development of the first computer‐designed RAFT agent illustrates the practical utility of these investigations.

     

Synthesis and characterization of telechelic polymethacrylates via RAFT polymerization

The reversible addition–fragmentation chain transfer (RAFT) polymerization technique has been employed to synthesize linear α,ω ‐telechelic polymers with either hydroxyl or carboxyl end groups. Methyl methacrylate, butyl methacrylate, and butyl acrylate were polymerized with RAFT polymerization. The polymerizations exhibited the usual characteristics of living processes. Telechelic polymethacrylates were obtained from a hydroxyl monofunctional RAFT polymer with a two‐step chain‐end modification procedure of the dithioester end group. The procedure consisted of an aminolysis followed by a Michael addition on the resulting thiol. The different steps of the procedure were followed by detailed analysis. It was found that this route was always accompanied by side reactions, resulting in disulfides and hydrogen‐terminated polymer chains as side products next to the hydroxyl‐terminated telechelic polymers. Telechelic poly(butyl acrylates) with carboxyl end groups were produced in a single step procedure with difunctional trithiocarbonates as RAFT agents. The high yield in terms of end group functionality was confirmed by a new critical‐liquid‐chromatography method, in which the polymers were separated based on acid‐functionality and by mass spectrometry analysis. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 959–973, 2005     

Determination of Addition and Fragmentation Rate Coefficients in RAFT Polymerization via Time‐Resolved ESR Spectroscopy after Laser Pulse Initiation

Summary: A method for simultaneous determination of both the addition and fragmentation rate coefficients of the RAFT equilibrium reactions is presented, which is based on laser single pulse initiation in conjunction with microsecond time‐resolved ESR spectroscopy. The build‐up and subsequent decay in concentration of the intermediate radical are measured and k_ad and k_β values are deduced from fitting the concentration versus time profiles to simple kinetic models.

Normalized ESR signal intensity vs. time after firing an initiating laser pulse in BMPT‐mediated butyl acrylate polymerization.     

Synthesis of well‐defined copolymer of acrylonitrile and maleic anhydride via RAFT polymerization

Well‐defined copolymer of acrylonitrile (AN) and maleic anhydride (MAn) has been successfully synthesized via reversible addition‐fragmentation chain transfer polymerization. The polymerization kinetics and “living”/controlled features were thoroughly studied and confirmed. The thermal properties and spinnability of the prepared copolymers were investigated via differential scanning calorimetry, thermogravimetric analyzer, and electrospinning subsequently. When PAN‐co‐PMAn was used as precursors, nonwoven with “crosslinked” structures was obtained during electrospinning. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 5263–5269     

Facile synthesis of star‐shaped copolymers via combination of RAFT and ring opening polymerization

Reversible addition fragmentation chain transfer (RAFT) polymerization and bifunctional sparteine/thiourea organocatalyst‐mediated ring opening polymerization (ROP) were combined to produce poly(L ‐lactide) star polymers and poly(L ‐lactide‐co‐styrene) miktoarm star copolymers architecture following a facile experimental procedure, and without the need for specialist equipment. RAFT was used to copolymerize ethyl acrylate (EA) and hydroxyethyl acrylate (HEA) into poly(EA‐co‐HEA) co‐oligomers of degree of polymerization 10 with 2, 3, and 4 units of HEA, which were in turn used as multifunctional initiators for the ROP of L ‐lactide, using a bifunctional thiourea organocatalytic system. Furthermore, taking advantage of the living nature of RAFT polymerization, the multifunctional initiators were chain extended with styrene (poly((EA‐co‐HEA)‐b‐styrene) copolymers), and used as initiators for the ROP of L ‐lactide, to yield miktoarm star copolymers. The ROP reactions were allowed to proceed to high conversions (>95%) with good control over molecular weights (ca. 28,000‐230,000 g/mol) and polymer structures being observed, although the molecular weight distributions are generally broader (1.3–1.9) than those normally observed for ROP reactions. The orthogonality of both polymerization techniques, coupled with the ubiquity of HEA, which is used as a monomer for RAFT polymerization and as an initiator for ROP, offer a versatile approach to star‐shaped copolymers. Furthermore, this approach offers a practical approach to the synthesis of polylactide star polymers without a glove box or stringent reaction conditions. The phase separation properties of the miktoarm star copolymers were demonstrated via thermal analyses. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6396–6408, 2009     

Controlled/living polymerization of methacrylamide in aqueous media via the RAFT process

The polymerization of methacrylamide (MAM) was performed in aqueous media via reversible addition fragmentation chain transfer (RAFT) polymerization with the dithiobenzoate chain‐transfer agent (CTA) 4‐cyanopentanoic acid dithiobenzoate (CTP) and 4,4′‐azobis(4‐cyanopentanoic acid) (V‐501) as initiator. The polymerization in unbuffered water at 70 °C with a CTP/V‐501 ratio of 1.5 was controlled for the first 3 h, after which the molecular weight distribution broadened and a substantial deviation of the experimental from the theoretical molecular weight occurred, presumably because of a loss of CTA functionality at longer polymerization times. Conducting the polymerization in an acidic buffer afforded a well‐defined homopolymer (M_n = 23,800 g/mol, M_w/M_n = 1.08). To demonstrate the controlled/living nature of the system, a block copolymer of MAM and acrylamide was successfully prepared (M_n = 33,800 g/mol, M_w/M_n = 1.25) from a polymethacrylamide macro‐CTA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3141–3152, 2005     

Synthesis of chiral amphiphilic diblock copolymers via consecutive RAFT polymerizations and their aggregation behavior in aqueous solution

Two chiral amphiphilic diblock copolymers with different relative lengths of the hydrophobic and hydrophilic blocks, poly(6‐O‐p‐vinylbenzyl‐1,2:3,4‐Di‐O‐isopropylidene‐D ‐galactopyranose)‐b‐poly(N‐isopropylacrylamide) or poly(VBCPG)‐b‐poly(NIPAAM) and poly(20‐(hydroxymethyl)‐pregna‐1,4‐dien‐3‐one methacrylate)‐b‐poly(N‐isopropylacrylamide) or poly(MAC‐HPD)‐b‐poly(NIPAAM) were synthesized via consecutive reversible addition‐fragmentation chain‐transfer polymerizations of VBCPG or MAC‐HPD and NIPAAM. The chemical structures of these diblock copolymers were characterized by ¹H nuclear magnetic resonance spectroscopy. These amphiphilic diblock copolymers could self‐assemble into micelles in aqueous solution, and the morphologies of micelles were investigated by transmission electron microscopy. By comparison with the lower critical solution temperatures (LCST) of poly(NIPAAM) homopolymer in deionized water (32 °C), a higher LCST of the chiral amphiphilic diblock copolymer (poly(VBCPG)‐b‐poly(NIPAAM)) was observed and the LCST increased with the relative length of the poly(VBCPG) block in the copolymer from 35 to 47 °C, respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7690–7701, 2008     

Short chain amphiphilic diblock co‐oligomers via RAFT polymerization

We demonstrate the ability of the reversible addition‐fragmentation chain transfer (RAFT) process to produce well‐defined block co‐oligomers for which each block has a narrow molecular weight distribution and degrees of polymerization ranging from 2 to 33. We exploit RAFT versatility to control the structure of the co‐oligomers and produce amphiphilic block co‐oligomers of styrene, acrylic acid and ethylene glycol. A detailed study shows that the amphiphilic diblock co‐oligomers self‐assemble in solution and form micelles or particles, depending on the hydrophobicity of the diblock. These oligomers present an excellent alternative to traditional amphiphilic molecules, by combining the properties of polymers with those of single molecule surfactants. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of cationic N‐[3‐(dimethylamino)propyl]methacrylamide brushes on silicon wafer via surface‐initiated RAFT polymerization

Surface‐initiated reversible addition‐fragmentation chain transfer (SI‐RAFT) polymerization of N‐[3‐(dimethylamino)propyl]methacrylamide (DMAPMA) on the silicon wafer was conducted in attempt to create controllable cationic polymer films. The RAFT agent‐immobilized substrate was prepared by the silanization of hydroxyl groups on silicon wafer with 3‐aminopropylthriethoxysilane (APTS) and by the amide reaction of amine groups of APTS with ester groups of 4‐cyano‐4‐((thiobenzoyl) sulfanyl) pentanoic succinimide ester (CPSE); followed by the RAFT polymerization of DMAPMA using a “free” RAFT agent, that is, 4‐cyanopentanoic acid dithiobenzoate (CPAD) and an initiator, that is, 4,4′‐azobis‐4‐cyanopentanoic acid (CPA). The formation of homogeneous tethered poly(N‐[3‐(dimethylamino)propyl]methacrylamide) [poly(DMAPMA)] brushes, whose thickness can be tuned by reaction time varying, is evidenced by using the combination of grazing angle attenuated total reflectance‐Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, and water contact‐angle measurements. The calculation of grafting parameters from the number‐average molecular weight, M _n (g/mol) and ellipsometric thickness, h (nm) values indicated the synthesis of densely grafted poly(DMAPMA) films and allowed us to predict a polymerization time for forming a “brush‐like” conformation for the chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Living star polymer formation (RAFT) studied via electrospray ionization mass spectrometry

A mass spectrometry analysis has been performed on complex architecture polymeric material produced during reversible addition fragmentation chain transfer (RAFT) polymerizations yielding star polymers. Para‐acetoxystyrene (AcOSty) has been polymerized at 60 °C, using azobisisobutyronitrile (AIBN) as the thermally decomposing initiator, in the presence of the R‐group approach tetrafunctional RAFT agent (1,2,4,5‐tetrakis‐(2‐phenyl‐thioacetyl‐sulfanylmethyl)‐benzene). In addition to ideal star material, a variety of products unique to this mode of polymerization have been identified. These include star–star couples, stars terminated with initiator fragments, star–star couples terminated with initiator fragments and linear polymers, supporting the notion that these species are responsible for the structured molecular‐weight distributions measured for these systems when analyzed via gel permeation chromatography. The analysis begins with a study of AcOSty polymerizing (i) in the absence of any mediating agent and (ii) in the presence of a monofunctional RAFT agent, revealing the mode of termination of propagating poly(AcOSty) radicals as combination and that some ionization biases exist among variants of poly (AcOSty). The interpretation of the mass spectrometry data has been aided by a novel kinetic model of star polymerizations, allowing the rationalization of experimental observations with theoretical expectations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1873–1892, 2008     

High molecular weight and low dispersity polyacrylonitrile by low temperature RAFT polymerization

High molecular weight polyacrylonitrile (PAN) with low dispersity has been successfully synthesized utilizing reversible addition‐fragmentation chain transfer (RAFT) polymerization. A comprehensive study was performed to understand the influence of reaction temperature, RAFT agent structure, and [M]₀:[CTA]₀[I]₀ on the polymerization kinetics, molecular weight, and dispersity. Enhanced control is attributed to reduction of side reactions by conducting the polymerization at lower temperature, and optimizing the radical exchange between active and dormant states via appropriate selection of RAFT agent and initiator. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 553–562     

Chain Transfer in Degenerative RAFT Polymerization Revisited: A Comparative Study of Literature Methods

Under the validity of the degenerative transfer mechanism, the activation/deactivation process in reversible addition‐fragmentation chain transfer (RAFT) polymerization can be formally quantified by transfer coefficients, depending on the chemical structure of the participating radicals and dormant species. In the present work, the different literature methods to experimentally determine these RAFT transfer coefficients are reviewed and theoretically re‐evaluated. The accuracy of each method is mapped for a broad range of reaction conditions and RAFT transfer reactivities. General guidelines on when which method should be applied are formulated.

     

Macromolecular design via reversible addition–fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerization

Among the living radical polymerization techniques, reversible addition–fragmentation chain transfer (RAFT) and macromolecular design via the interchange of xanthates (MADIX) polymerizations appear to be the most versatile processes in terms of the reaction conditions, the variety of monomers for which polymerization can be controlled, tolerance to functionalities, and the range of polymeric architectures that can be produced. This review highlights the progress made in RAFT/MADIX polymerization since the first report in 1998. It addresses, in turn, the mechanism and kinetics of the process, examines the various components of the system, including the synthesis paths of the thiocarbonyl‐thio compounds used as chain‐transfer agents, and the conditions of polymerization, and gives an account of the wide range of monomers that have been successfully polymerized to date, as well as the various polymeric architectures that have been produced. In the last section, this review describes the future challenges that the process will face and shows its opening to a wider scientific community as a synthetic tool for the production of functional macromolecules and materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43:5347–5393, 2005     

Quantum Chemical Mapping of Initialization Processes in RAFT Polymerization

Summary: We present the first ab initio simulation of a reversible addition fragmentation chain transfer (RAFT) polymerization. Using ab initio molecular orbital theory, we calculate the equilibrium constants for the first eight addition–fragmentation steps in the cyanoisopropyl dithiobenzoate‐mediated polymerization of styrene. We then simulate the concentration profiles for the RAFT agent, and its unimeric and dimeric adducts, assuming standard experimental parameters for styrene homopolymerization and the addition of the styryl radical to the RAFT agent. The simulated data show excellent agreement with published experimental data, highlighting the accuracy of quantum chemistry. In contrast, the currently used chain‐length independent models fail to describe even the qualitative trends in the data, regardless of whether the fragmentation reaction is assumed to be fast or slow. The calculated chain‐length dependent equilibrium constants are large, in agreement with the earlier proposed slow fragmentation model.

Ab initio kinetic modelling of concentration profiles during the RAFT initialization period.     

Graft block copolymers of propargyl methacrylate and vinyl acetate via a combination of RAFT/MADIX and click chemistry: Reaction analysis

Propargyl methacrylate with its acetylene function protected with a silyl group is polymerized via the reversible addition fragmentation chain transfer (RAFT) process, using cyanoisopropyl dithiobenzoate (CPDB) as RAFT agent, and subsequently deprotected to afford a polymer backbone where each repeated unit is decorated with an acetylene functionality (1000 < M_n < 13,600 g mol⁻¹, 1.07 < PDI < 1.29). In parallel, an azide functionalized xanthate (ethoxythiocarbonylsulfanyl‐acetic acid 3‐azido‐propyl ester) was employed to prepare narrow polydisperse poly(vinyl acetate) (M_n = 850 g mol⁻¹, PDI = 1.20). The two polymers are conjugated by Huisgen 1,3‐dipolar cycloaddition to afford narrow polydisperse comb polymer (1.12 < PDI < 1.18, 3400 < M_n < 12,500 g mol⁻¹, based on linear polystyrene calibration, 4500 < M < 15,600 g mol⁻¹). The study places special emphasis on following the copper catalyzed 1,3‐dipolar cycloaddition via Fourier Transform Infrared Spectroscopy (FTIR) as well as via on‐line UV–Vis photospectrometry on several model compounds, i.e. the nonmonomer inserted azido‐xanthate RAFT/MADIX agent as well as a 2‐propargyl‐2‐bromopropionate and 3‐azidopropyl‐2‐bromopropionate model compounds. A suitable absorption band in the VIS at 666 nm (tentatively assigned to a charge transfer complex between copper(I) and the forming triazole moieties) is identified as a promising sensor for following the click reaction kinetics, thus allowing for the rapid assessment of reaction completion in an on‐line fashion. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 155–173, 2008     

Synthesis and characterization of fluorescence end‐labeled polystyrene via reversible addition‐fragmentation chain transfer (RAFT) polymerization

Fluorescence end‐labeled polystyrene (PS) with heteroaromatic carbazole or indole group were prepared conveniently via reversible addition‐fragmentation chain transfer (RAFT) polymerization using dithiocarbamates, ethyl 2‐(9H‐carbazole‐9‐carbonothioylthio)propanoate (ECCP) and benzyl 2‐phenyl‐1H‐indole‐1‐carbodithioate (BPIC) as RAFT agents. The end functionality of obtained PS with different molecular weights was high. The steady‐state and the time‐resolved fluorescence techniques had been used to study the fluorescence behaviors of obtained end‐labeled PS. The fluorescence of dithiocarbamates resulting PS in solid powder cannot be monitored; however, they exhibited structured absorptions and emissions in solvent DMF and the fluorescence lifetimes of PS had no obvious change with molecular weights increasing. These observations suggested that the polymer chains were possibly stretched adequately in DMF, that is, the fluorescence end group was exposed into solvent molecules and little quenching of excited state occurred upon incorporation into polymer chain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6198–6205, 2008