Initiators stable radical-mediated polymerization

Metal complexes may also act as initiators in stable radical-mediated polymerization with the metal complex performing the role of the stable radical. 

The stable radical-mediated polymerization represents a promising approach to defined homopolymers and block copolymers. Recently, Hawker and Hedrick [71] demonstrated the potential use of the living radical polymerization for the preparation of fimctional polymers. Two strategies were followed. The first method involves the synthesis of functionalized monoadducts of styrene and TEMPO and their use in styrene polymerization (Scheme 23), The hydroxy functionalized initiators were prepared by hydrolyzing the ester functionality of the adduct (Scheme 24). Protected amino groups were also introduced by further modification of the latter. Hydroxy groups were treated with iV(terr-butoxycarbonyl)-4-aminophenol, as illustrated in Scheme 25. 

The first true stable radical-mediated polymerization method was the nitroxide-mediated radical polymerization (NMP) discovered by Solomon, Rizzardo and Cacioli at the Commonwealth Seientific and Industrial Research Organization (CSIRO) in 1985. The diseovery is reeounted in the article " of Solomon. This pioneering work, first published as a patent only, attracted broad attention in 1993 when Georges el al. reported the synthesis of polystyrene with low polydispersity via NMP of styrene mediated by TEMPO 2. Today, NMP is, by far, the most important and versatile SRMP method. It is an attractive, simple and straightforward teehnique, because typically only monomer and an appropriate alkoxyamine initiator R R NOR are needed to conduct the polymerization. 

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... 

In addition to styrene, the other monomers that have been particularly well studied in TEMPO-mediated radical polymerization are 4-vinylpyridine (4-VP), " 2-vinylpyridine (2-VP), and to a lesser extent 3-vinylp yridine (3-VP). The NMP led to a good control over the polymer characteristics and well-defined architectures could be easily produced, such as amphiphilic block or graft copolymers. " Not only TEMPO was used as a stable radical mediator but also SGI, which led to well-controlled polymerization systems either in solution or in emulsion initiated by a water-soluble poly (acrylic acid) (PAA) macroinitiator. 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Since TEMPO is only a regulator, not an initiator, radicals must be generated from another source the required amount of TEMPO depends on the initiator efficiency. Application of alkoxyamines (i.e., unimolecular initiators) allows for stoichiometric amounts of the initiating and mediating species to be incorporated and enables the use of multifunctional initiators, growing chains in several directions [61]. Numerous advances have been made in both the synthesis of different types of unimolecular initiators (alkoxyamines) that can be used not only for the polymerization of St-based monomers, but other monomers as well [62-69]. Most recently, the use of more reactive alkoxyamines and less reactive nitroxides has expanded the range of polymerizable monomers to acrylates, dienes, and acrylamides [70-73]. An important issue is the stability of nitroxides and other stable radicals. Apparently, slow self-destruction of the PRE helps control the polymerization [39]. Specific details about use of stable free radicals for the synthesis of copolymers can be found in later sections. 

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 2,2,6,6-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... 

Dissociation-combination living polymerizations are typified by nitroxide-mediated polymerizations, the first example of which used 2,2,6,6 tetramethyl piperidinyl-1-oxy (TEMPO) as the mediating stable free radical. The reaction steps for the polymerization of styrene, using benzoyl peroxide as the initiator, are given below. 

Careful and extensive investigations of these nitroxide-mediated polymerizations (also referred to as stable free radical polymerization) have established optimum conditions for controlled radical polymerization of a variety of vinyl monomers (Matyjaszewski, 1998,2000). Variables examined include the structure of the nitroxide and the presence of other additives to control spontaneous polymerization of monomers such as styrene. It is noteworthy that in place of alkoxyamine initiators, a mixture of a normal free radical initiator such as an azo compound or a peroxide can also be used. 

The described syntheses offer easy access to a variety of structurally different verdazyl radicals providing the opportunity to extend the initial work of Yamada and coworkers to determine if these stable radicals have the potential to solve some of the problems associated with nitroxides as mediating reagents for living-radical polymerizations. 

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 ... 

In the DC process, P-X is activated by a thermal or photochemical stimulus to produce P and the stable or persistent radical X, which is stable enough to undergo no reaction other than the combination with P (i.e., neither initiates polymerization or reacts with itself). Nitroxides such as 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) are the typical X currently utilized (nitroxide-mediated polymerization (NMP)) [9,10]. 

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