Polymerization free radical-mediated

SFRMP stable free radical mediated polymerization... 

This chapter contains a survey of free-radical-mediated multicomponent reactions (MCRs), which permit the coupling of three or more components. Even though they are not technically classified as MCRs, remarkable intramolecular radical cascade processes have been developed. Some examples, such as those shown in Scheme 6.3, use an isonitrile or acrylonitrile as the intermolecular component for each reaction [6]. These examples demonstrate the tremendous power of the combination of inter- and intramolecular radical cascade processes in organic synthesis. Readers are advised to be aware of remarkable intramolecular aspects of modem radical chemistry through excellent review articles published elsewhere [1, 7]. It should also be noted that there has also been remarkable progress in the area of living radical polymerizations, but this will not be covered here. 

This is an example of a step in the free-radical-mediated polymerization of ethylene, forming polyethylene. As in the previous example, note that a single-barbed arrow is used for each electron that is moving. Arrow pushing is illustrated below ... 

Most polymerizations in this section can be categorized as stable (free) radical-mediated polymerizations (sometimes abbreviated as SFRMP). In the following discussion systems have been classed according to the type of stable radical involved, which usually correlates with the type of bond homolyzed in the activation process. Those described include systems where the stable radical is a sulfur-ccntcrcd radical (Section 9.3.2), a scicnium-ccntcrcd radical (Section 9.3.3), a carbon-centered radical (Sections 9.3.4 and 9.3.5), an oxygen-centered radical (Sections 9.3.6, 9.3.7), or a nitrogen-centered radical (Section 9.3.8). Wc also consider polymerization mediated by cobalt complexes (Section 9.3.9) and certain monomers (Section 9.3.5). 

Most of the common synthetic polymers are formed by the mechanism of free radical mediated addition polymerization where the monomer units are activated by free radical formation and... 

Nitroxide Mediated Living Radical Polymerization. Another controlled radical polymerization developed in recent years is stable free radical mediated polymerization (SFRP), also called as nitroxide mediated radical polymerization (NMP) (78,131). This type polymerization can be reahzed through reversible deactivation of growing radicals by stable radical such as 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO). 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Polymerization employing Co complexes as catalysts or else polymers incorporating functionality that includes Co ions represent aspects of polymerization reactions of interest here. Cobalt-mediated free-radical polymerization of acrylic monomers has been reviewed.55 Co11 porphyrins act as traps for dialkylcyanomethyl radicals.1098 Alkyl complexes of Co(TMesP)... 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

He also prepared a poly(styrene-g-styrene) polymer by this technique [114], The lack of crosslinking in these systems is indeed proof of the control achieved with this technique. An eight-arm star polystyrene has also been prepared starting from a calixarene derivative under ATRP conditions [115]. On the other hand, Sawamoto and his coworkers used multifunctional chloroacetate initiator sites and mediation with Ru2+ complexes for the living free-radical polymerization of star poly(methylmethacrylate) [116,117]. More recent work by Hedrick et al. [84] has demonstrated major progress in the use of dendritic initiators [98] in combination with ATRP and other methodologies to produce a variety of structure controlled, starlike poly(methylmethacrylate). 

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