Stable Free-Radical Polymerization SFRP Process

3 Stable Free-Radical Polymerization (SFRP) Process 

Monomers that cannot be polymerized by other conventional living-radical polymerization methods can be polymerized by the SFRP process. Examples include chloromethylstyrene and water-soluble monomers such as /(-toluenesulfonate, sodium salt. While most of the earlier work in the SFRP process was performed under bulk conditions, some solution polymerizations, as well as dispersion polymerizations [20] have been performed. Recent focus has shifted to the more commercially compatible emulsion and mini-emulsion processes with good success [21]. 

While styrene and its derivatives polymerized very well under SFRP conditions, acrylate polymerizations proved to be a problem. Although styrene/acrylate ran- 

Further expanding the list of monomers that can be polymerized by the SFRP process, the polymerization of 1,3-dienes has been reported in the synthesis of block copolymers [25] and, more recently, homopolymers [26], 

Reported limitations of the SFRP process include the high temperatures required to enable the polymerization to proceed at reasonable rates and the limited range of monomers that can be used. As outlined above, this latter concern has been addressed and should no longer be considered an issue. 

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Over the past few years there has been a tremendous interest in living radical polymerizations. One type of living radical polymerization is stable free radical polymerization, SFRP, where a stable free radical such as TEMPO (2,2,6,6-tetramethylpiperidinoxyl) is used to reversibly cap the growing polymer chain (L2). SFRP has the advantage over conventional radical polymerization in that the polymers prepared are living and can be used for further polymerization to make blocks or other complex architectures. The polymers prepared by the SFRP process have a narrower molecular weight distribution compared to polymers prepared by conventional radical polymerization in the case of block copolymers this may be a desirable attribute. This article focuses on the use of the SFRP process to prepare random copolymers. 

Anionic and later cationic pol3Tnerization gave most of examples of living pol3rmerization systems until recently, when more sophisticated methods of manipulation with free-radical polymerization processes become available. These methods are based on the use of the compounds which reversibly react with propagating radical and convert it to the so-called dormant species . When the equilibrium between the active and dormant species is regulated by special catalysts based on a transition metal, this process is called atom transfer radical polymerization (ATRP). If this equilibrium is provided by stable radicals such as nitroxides, the process is called stable free-radical polymerization (SFRP). In the case when dormant species are formed via a chain transfer rather than reversible termination reactions, this process is referred to as reversible addition fragmentation chain transfer (RAFT) polymerization. All these techniques allow to produce macromolecules of desired architecture and molecular masses. 

A bewildering array of names are used to describe the various controlled/living radial polymerization techniques currently in use. These include stable free radical polymerization (SFRP) [35-38], nitroxide mediated polymerization (NMP) [39], atom transfer radical polymerization (ATRP) [40-42 ] and degenerate transfer processes (DT) which include radical addition-fragmentation transfer (RAFT) [43, 44] and catalyst chain transfer (CCT). These techniques have been used to polymerize many monomers, including styrene (both linear and star polymers) acrylates, dienes, acrylamides, methacrylates, and ethylene oxide. Research activity in this field is currently expanding at a very high rate, as is indicated by the many papers published and patents issued. 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

NMRP (also called NMP, nitroxide-mediated polymerization) or SFRP (stable free radical polymerization) was first disclosed by Solomon et al. from CSIRO (Commonwealth Scientific Industrial Research Organization) in 1984. Their patent [49] describing a unimolecular process in which an... 

One and two electron oxidative addition processes that involve electron transfer between alkyl radicals and transition metal species have been exploited in organic synthesis for many years. These reactions can ultimately result in the formation of stable metal-alkyl complexes. The formation of such organometallic species during ATRP would have several implications on the role of the catalyst. The relative bond dissociation energies of the the Mt-R, Mt-X, and R-X bonds would ultimately dictate whether polymerization would be inhibited by the formation of a Mt-R bond, whether initiation efficiency might just be reduced, or whether the entire polymerization could be mediated through the reversible formation of such a Mt-R bond (as in stable free radical polymerization, or SFRP).[ ]... 

In the 1990s the groups of Rizzardo and Georges reported a stable free radical polymerization process (SFRP) allowing the preparation of polystyrene with a narrow polydispersity. In the presence of stable free radicals, such as the mainly used 2,2,6,6-tetramethylpiperidine-A-oxyl (TEMPO), macromolecules based on styrene and styrene derivatives with well defined structures were synthesized [263,264],... 

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