Degenerative transfer controlled radical

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

If an overall conclusion could be made, it might be considered that the counterradicals vary considerably (Scheme 3). They can either be stable (e.g., nitroxyls, arylazooxyls), semi persistent (e.g., from thiourams) and also metallic (e.g., acetoacetato metals). In addition, if these radicals either terminate or transfer, non-living (or inactive) species will be produced. But, in order to preserve the living character, the radicals must propagate and in specific cases (e.g., iodine transfer polymerization or degenerative transfer) active species will be obtained. The more that one of these latter steps is favored, the more living is the tendency of the radical polymerization, with a very high kinetic control of this reaction. 

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

This technique for controlling radical polymerizations is based on one of the oldest technique, that of chain transfer, and has often been used in telomeriza-tion [83]. Similar to the concept of degenerative transfer with alkyl iodides [50, 51, 84], reversible addition fragmentation chain transfer with dithio esters (RAFT) [52-55, 85] is successful because the rate constant of chain transfer is faster than the rate constant of propagation. Analogous to both nitroxide-medi-... 

Figure 6.26 Reaction scheme of controlled free-radical polymerization, based on degenerative chain transfer, of butyl acrylate (R = C4H9COO-) in the presence of secondary alkyl iodide (R = CH3CH(Ph)-, X = I) as the degenerative transfer agent. The latter alone does not initiate polymerization. (After Matyjaszewski et al., 1995.)...

The fifty chapters submitted for publication in the ACS Symposium series could not fit into one volume and therefore we decided to split them into two volumes. In order to balance the size of each volume we did not divide the chapters into volumes related to mechanisms and materials but rather to those related to atom transfer radical polymerization (ATRP) and to other controlled/living radical polymerization methods reversible-addition fragmentation transfer (RAFT) and other degenerative transfer techniques, as well as stable free radical pol5mierizations (SFRP) including nitroxide mediated polymerization (NMP) and organometallic mediated radical polymerization (OMRP). 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

Matyjaszewski et al. systematically investigated the effect of electron donors (ED), such as pyridine and triethylamine, on the CRP of VAc with Co(acac)2. They proposed that the polymerization mechanism of VAc with Co(acac)2 in the absence of electron donor was a degenerative transfer process as shown in scheme 3(a). The polymerization in the presence of electron donor was a stable free radical polymerization controlled by the reversible homolytic cleavage of cobalt(III) dormant species as shown in scheme 3 (b). ... 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Controlled radical polymerization techniques are suitable for synthesizing polymers with a high level of architectural control. Notably, they not only allow a copolymerization with functional monomers (as shown previously for free-radical polymerization), but also a simple functionalization of the chain end by the initiator. Miniemulsion systems were found suitable for conducting controlled radical polymerizations [58-61], including atom transfer radical polymerization (ATRP), RAFT, degenerative iodine transfer [58], and nitroxide-mediated polymerization (NMP). Recently, the details of ATRP in miniemulsion were described in several reviews [62, 63], while the kinetics of RAFT polymerization in miniemulsion was discussed by Tobita [64]. Consequently, no detailed descriptions of the process wiU be provided at this point. 

The absence of control of the incorporation of monomers into the polymeric chain implies that many macroscopic properties carmot be influenced to a large extent. Therefore, much effort has been directed toward the development of controlled radical polymerization (CRP) methods for the preparation of various copolymers (for a review, see Reference 31). CRPs offer the possibility of producing polymers with relatively well-defined properties, while at the same time maintaining the simplicity of radical processes.These methods are based on the idea of establishing equilibrium between the active and dormant species in solution phase. In particular, the methods include three major techniques called stable free-radical polymerization, degenerative chain transfer technique, and atom transfer radical polymerization, pioneered by Ando et and Matyjaszewski et Although such syntheses pose significant technical problems, these difficulties have all been successively overcome in the past few years. Nevertheless, the procedure of preparation of the resultant copolymers with controlled monomer sequence distribution remains somewhat complicated. 

K. Matyjaszewski, S. Gaynor, J. -S. Wang, Controlled radical polymerizations the use of alkyl iodides in degenerative transfer. Macromolecules 1995, 28, 2093-2095. 

Atom Transfer Radical Polymerization. ATRP is one of the most successful controlled/living radical polymerization (CRP) systems, in addition to NMP and degenerative transfer processes, such as RAFT (5,233,234). The key feature of all of them is the dynamic equilibration between the active radicals and varions tsqjes of dormant species (see Living Radical Polymerization). 

Control by degenerative transfer (DT) involves perhaps the smallest change from a eonventional free radical process of all the controlled/living polymerization proeesses developed to date. A recent review of various methods of telomer synthesis [180] diseusses the different types of transfer agents and monomers and the contribution of the teehniques of telomerization to CRP (includes discussion of iodine transfer polymerization, RAFT, and macromolecular design through interchange of xanthates (MADIX)) [181,182]. 

DT relies on a thermodynamically neutral (degenerative) transfer reaction. The key for control is a minimal energy barrier for that reaction. Conventional free radical initiators are used, i.e., peroxides and diazenes, at temperatures typical for radical polymerization and the polymerization is carried out in the presence of a compound with a labile group or atom which can be either reversibly abstracted or added-fragmented by the growing radical. The simplest examples are reactions in the presence of alkyl iodides [33,183-184] Scheme 11 ... 

Destarac, M., et al. (2001). Synthesis method for polymers by controlled radical polymerization with xanthates. In PCT bit. Appl. WO 0142312, Rhodia Chimie, Fr., 46 pp. Gaynor, S. G., Wang, J.-S., and Matyjaszewski, K. (1995). Controlled radical polymerization by degenerative transfer elfect of the structure of the transfer agent. Macromolecules, 28(24) 8051-8056. 

For example, controlled free radical polymerization of styrene based on a degenerative transfer process with iodine exchange was carried out in oil-in-... 

Polymerization, recently reviewed" ) only pertains to the use of cobalt and is not spedfic for the reversible deactivation mechanism vide infra). The term OMRP covets aU metals. The use of this aaonym was initially limited to the reversible deactivation mechanism outlined in Figure 1. However, it has recently been shown that organometallic compoimds may also act as transfer agents for the controlled radical polymerization that follows the degenerative transfer prindple, as outlined later in Section 3.11.4. In this chapter, both these two controlled polymerization methods, which may in certain cases interplay, will be outlined. When addressing each specific mechanism, an additional qualifier will be added to the acronym, OMRP-RT for reversible termination and OMRP-DT for degenerative transfer, whereas the OMRP term will be used in a more general situation. 

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