Synthesis stable free radical polymerization

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]. 

Other teams worked on the functionalization of the aminoxyl group situated at the co position. For instance, the method of Ding et al. [342] is original for the synthesis of a novel series of poly(sodium styrenesulfonate) (PSSNa) macromonomers (compound 3 in Scheme 74) based on stable free radical polymerization in the presence of TEMPO. 

CRP provides a versatile route for the preparation of (co) polymers with controlled molecular weight, narrow molecular weight distribution (i.e., Mw/Mn, or PDI < 1.5), designed architectures, and useful end-functionalities. Various methods for CRP have been developed however, the most successful techniques include ATRP, stable free radical polymerization, " and reversible addition fragmentation chain transfer (RAFT) polymerization. " " CRP techniques have been explored for the synthesis of gels " " and cross-linked nanoparticles of well-controlled polymers in the presence of cross-linkers. 

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).[ ]... 

No.ll, 22nd May 2001, p.3594-9 BLOCK COPOLYMER SYNTHESIS BY A MINIEMULSION STABLE FREE RADICAL POLYMERIZATION PROCESS... 

Keoshkerian, B., Mac Leod, P.J. and Georges, M.K. (2001) Block copolymer synthesis by a miniemulsion stable free-radical polymerization process. Macromolecules, 34, 3594—3599. 

YOU 00] Yousi Z., JiAN L., Rongchuan Z. et al, Synthesis of block copolymer from dissimilar vinyl monomer by stable free radical polymerization . Macromolecules, vol. 33, pp. 4745-4749, 2000. 

Pasquale, A. J., Long, T. E., Synthesis of star-shaped polystyrenes via nitrogen-mediated stable free-radical polymerization /. Polym. Sci. A, Polym Chem. (2000) 39, pp. 216-223... 

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. 

A parallel study has reported the synthesis of crosslinked polymer microspheres in supercritical carbon dioxide [54]. Heterogeneous free-radical polymerization of divinyl benzene and ethyl benzene were carried out at 65 C and 310 bar using AIBN initiator to form the crosslinked polymer. It is shown that in the absence of surfactants as stabilizers, polymerization of the mixture containing 80 % divinyl benzene + 20 % ethyl benzene leads to poly(divinylbenzene) microspheres of about 2.4 micron diameter [Figure 14]. In the presence of a carbon dioxide-soluble diblock copolymer as a stabilizer, polymerization of the mixture with the same monomer ratio proceeds as an emulsion and lead to smaller crosslinked particles (ca. 0.3 micron). Thermal analysis shows that the crosslinked polymer that is formed from these polymerizations is stable up to 400 C. 

Yildirim, T. G., et al. (1999). Synthesis of block copol5mers by transformation of photosensitized cationic polymerization to stable free radical pol5merization. Polymer,... 

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