Summary: Reversible addition‐fragmentation chain transfer (RAFT) polymerization is a recent and very versatile controlled radical polymerization technique that has enabled the synthesis of a wide range of macromolecules with well‐defined structures, compositions, and functionalities. The RAFT process is based on a reversible addition‐fragmentation reaction mediated by thiocarbonylthio compounds used as chain transfer agents (CTAs). A great variety of CTAs have been designed and synthesized so far with different kinds of substituents. In this review, all of the CTAs encountered in the literature from 1998 to date are reported and classified according to several criteria : i) the structure of their substituents, ii) the various monomers that they have been polymerized with, and iii) the type of polymerization that has been performed (solution, dispersed media, surface initiated, and copolymerization). Moreover, the influence of various parameters is discussed, especially the CTA structure relative to the monomer and the experimental conditions (temperature, pressure, initiation, CTA/initiator ratio, concentration), in order to optimise the kinetics and the efficiency of the molecular‐weight‐distribution control.

Poly(vinyl ester) stars have been synthesized via different macromolecular design via interchange of xanthate (MADIX)/reversible addition-fragmentation chain transfer (RAFT) polymerization methodologies. Two approaches were investigated. The first method involved attaching the xanthate functionality to the core via a nonfragmenting covalent bond (Z-group approach). The second approach involved attaching the xanthate functionality to the core via a fragmenting covalent bond (R-group approach). The R-group approach yielded well-defined poly(vinyl acetate), poly(vinyl pivalate), and poly-(vinyl neodecanoate) stars with narrow polydispersities (PDI e 1.4). In contrast, the molecular weight distributions of poly(vinyl acetate) stars prepared using the Z-approach tended to broaden at moderate to high conversions. We attribute this broadening to steric congestion around the xanthate functionality, restricting the access of monomer to the CdS bonds. The R-group approach was also found to be superior for preparing precursor stars suitable for hydrolysis to poly(vinyl alcohol). Hydrolysis of stars generated by the Z-group approach resulted in destruction of the architecture, as the process also cleaved the xanthate linkage at the nexus of the arms and core. Preliminary experiments on using the R-group approach to mediate the star-polymerization of vinyl-functional glycomonomers demonstrated the possibility of generating complex glycopolymer architectures. However, some significant problems were observed, and this synthetic approach requires further optimization.

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Homopolymers of N-acryloylmorpholine (NAM), a water-soluble bisubstituted acrylamide derivative, have been synthesized by reversible addition-fragmentation chain transfer polymerization (RAFT). Several dithioesters were used as chain transfer agents:  carboxymethyl dithiobenzoate (CMDB), tert-butyl dithiobenzoate (tBDB), menthonyl dithiobenzoate (MDB), and a bifunctional dithiobenzoate, 1,3-bis(2-(thiobenzoylthio)prop-2-yl)benzene (TPB). Whereas CMDB is a commercial reagent, tBDB and MDB were synthesized by a novel biphasic process based on a thioacylation reaction and leading to very high yields. The performances of the four dithiobenzoates were compared in term of kinetics and molecular weight distribution control. Very good control of NAM polymerization was obtained with tBDB and MDB, with a linear increase of M n vs conversion over the whole conversion range and with polydispersity indices (PDI) below 1.1, as determined by aqueous size exclusion chromatography with on-line light scattering detection. In addition, a degradation phenomenon of the dithioester functions was evidenced during the course of the polymerization, correlated with a M n vs conversion curve leveling off and even sometimes decreasing above 80% conversion. Such observations were assumed to be the consequence of the formation of side products in the polymerization media, subsequently acting as nondegradative irreversible transfer agents.

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A matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI−TOF MS) study of N-acryloylmorpholine (NAM) homopolymers, as obtained by reversible addition−fragmentation chain transfer (RAFT) polymerization technique, is reported. Polymerization of NAM was performed in dioxane using azobis(isobutyronitrile) (AIBN) as initiator and tert-butyl dithiobenzoate as RAFT chain transfer agent. Polymer samples of low molecular weights (<10 000 g mol-1) and low polydispersity indices (PDI < 1.1) were obtained, which are essential requirements for such MALDI−TOF MS analysis. First, analysis of poly(NAM) samples in linear mode led to values very close to both size exclusion chromatography/light scattering (SEC/LS) and 1H NMR values as well as to the theoretical ones. Then, an accurate examination of chain end groups was performed using the reflectron mode. Two main chain populations were identified:  (i) dormant chains (i.e., initiated by a tert-butyl and terminated by a dithiobenzoate group) together with sulfine and thioester-ended chains probably resulting from oxidation of dithiobenzoate chain ends during storage; (ii) proton-terminated chains mainly produced by fragmentation of the former chains in the spectrometer. In addition, some chains which could correspond to termination reactions onto the intermediate radicals involved in the RAFT equilibrium were suspected. Finally, comparison of polymer samples before and after aminolysis indicated that dithioester-ended chains constituted the majority of chains initially present in these samples. This study confirms that it is indeed possible to use MALDI−TOF MS to investigate the structure of polymer chains synthesized by the RAFT technique.

Summary: Potential sources of inhibition have been investigated via in situ Fourier transform – near infra‐red (FT‐NIR) and off‐line 1H NMR spectroscopy in the RAFT/MADIX bulk polymerization of vinyl acetate (VA) in the presence of an O‐isopropyl xanthate, i.e. methyl (isopropoxycarbonothioyl)sulfanyl acetate. The very high reactivity of the vinyl acetate propagating radical makes it vulnerable to oxygen, by‐products generated during the xanthate synthesis, and stabilizers present as impurities. These impurities induce strong and variable inhibition periods in the polymerization. In addition, the MADIX process, using xanthates as reversible chain transfer agents, has been confirmed to be an efficient method for living VA polymerization. Congruent data are obtained when the monomer consumption with time is followed via both 1H NMR and in situ FT‐NIR spectroscopy. The xanthate‐mediated polymerization of VA exhibits no retardation effects and excellent control of the molecular weight distribution can be achieved, leading to poly(VA) of molecular weights exceeding 50 000 g · mol−1 with relatively low polydispersities.image

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