Stable Free-Radical Polymerization SFRP

Various stable radicals such as nitroxide, triazolinyl, trityl, and dithiocarbamate have been used as the mediating or persistent radical (deactivator) for SFRP. Nitroxides are generally more efficient than the others. Cyclic nitroxide radicals such as 2,2,6,6-tetramethyl-l-piper-idinoxyl (TEMPO) have been extensively studied. SFRP with nitroxides is called nitroxide-mediated polymerization (NMP). Polymerization is carried out by two methods that parallel those used in ATRP [Bertin et al., 1998 Georges, 1993 Flawker, 1997 Flawker et al., 2001], One method involves the thermal decomposition of an alkoxyamine such as 

6-tetramethyl-l-(l-phenylethoxy)piperidine into a reactive radical and a stable radical (Eq. 3-236). The other method involves a mixture of a conventional radical initiator such as 

AIBN or benzoyl peroxide and the nitroxide radical. Nitroxide radicals are sufficiently stable (due to steric hindrance) that they can be stored at ambient temperatures without change and some are available for purchase from chemical vendors. 

The reactive radical initiates polymerization while the stable radical mediates the reaction by reacting with propagating radicals to lower their concentration. The overall process (Eqs. 3-237-3-239) is analogous to ATRP. The nitroxide radical, although unreactive with 

Some improvements occurred by changing to nitroxides with a hydrogen on at least one of the a-carbons of the piperidine ring, in contrast to TEMPO, which has no hydrogens on a-carbons. However, the major breakthrough came by using sterically hindered alicyclic nitroxides with a hydrogen on one of the a-carbons. f-Butyl 2-methyl-1-phenylpropyl nitroxide (LVIII) and f-butyl l-diethylphosphono-2,2-dimethylpropyl nitroxide (LIX) are examples of 

NMP has not been extended to methacrylate monomers, in contrast to ATRP, which is successful with methacrylates. Many attempts to polymerize methacrylates by NMP have been unsuccessful, resulting in low conversions and/or broad PDI [Hawker et al., 2001]. This is generally ascribed to degradation of propagating radicals via jl-hydrogen abstraction 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can he synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styiene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms  

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Stable free radical polymerization (SFRP), 20 442, 443 Stable node(s)... 

The first workable capping agents for controlled radical polymerization were discovered by Rizzardo et al. [77, 78] who used nitroxides. The nitroxide reacts reversibly with radical chain ends but itself does not initiate the monomer. They called their new system Stable Free Radical Polymerization (SFRP). Scheme 32a depicts an example of SFRP using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy). SFRP was developed independently by Georges at Xerox for the synthesis of styrene block polymer as dispersing agents [79]. 

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 (SFRP), atom transfer radical polymerization (ATRP), and the degenerative chain transfer technique (DCTT) [17]. Although such syntheses pose significant technical problems, these difficulties have all been successively overcome in the last few years. Nevertheless, the procedure of preparation of the resulting copolymers remains somewhat complicated. 

In a stable free-radical polymerization (SFRP), the initiated polymer chains are reversibly capped by a stable radical, for example, the 2,2,6,6-tetra-methylpyridin-l-oxyl radical (TEMPO). Stable PS dispersions via miniemulsion polymerization were prepared by MacLeod et al. with an optimized ratio... 

Controlled free-radical polymerization (CFRP) has been used successfully to produce block, graft, and other controlled architecture copolymers within the last decade for a variety of free radically polymerizable monomers. The main techniques include reversible addition fragmentation and transfer (RAFT) polymerization, stable free-radical polymerization (SFRP) mediated by nitroxide/alkoxyamine based radicals, atom transfer radical polymerization (ATRP), diphenyl ethylene (DPE) mediated polymerization, and novel precipitation/emulsion polymerization based methods like free-radical retrograde precipitation polymerization (FRRPP). ... 

Stable Free-Radical Polymerization (SFRP) Process... 

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. 

Since its discovery in 1993 [27], nitroxide-mediated polymerization (NMP) has been the most extensively studied technique from the dissociation-combination dass of LRP mechanisms (Scheme 13.7). This method is also commonly termed stable free radical polymerization (SFRP). NMP reactions are distinguished by the use of stable free radical nitroxide molecules (N ) as the controlling agent [e.g. 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO), (l-diethylphosphono-2,2-dimethyl)propyl nitroxide (DEPN)]. 

Thus, in stable free radical polymerization (SFRP), also called nitroxide-mediated polymerization or NMP (which was discovered while using TEMPO as a radical scavenger in investigating the rate of initiation during free radical polymerization), it is believed that reversible combination of a polymer radical, P, with a stable niUoxyl radical, N, takes place forming an adduct, P-N, that exists as a dormant species ... 

Describe the principal criteria that can be used to identify a living polymerization/controlled polymerization (LP/CP). What are the various ways in which living polymerization can be accomplished What are the advantages of living/controlled radical polymerization (LRP/CRP) compared to other methods of LP/CP Describe how the main requirements of LP/CP are ful lied in the following LRP/CRP methods (a) nitroxide mediated polymerization (NMP) or stable free radical polymerization (SFRP), (b) atom transfer radical polymerization (ATRP), and (c) polymerization by reversible addition fragmentation chain transfer (RAFT) ... 

Three basic concepts concerning stable free-radical polymerization (SFRP) also called nitroxide-mediated radical polymerization (NMP), atom-transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT) have been developed during the last decade. 

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