Nitroxides, living free-radical polymerization

Living free-radical polymerization has recently attracted considerable attention since it enables the preparation of polymers with well-controlled composition and molecular architecture previously the exclusive domain of ionic polymerizations, using very robust conditions akin to those of a simple radical polymerization [77 - 86]. In one of the implementations, the grafting is achieved by employing the terminal nitroxide moieties of a monolith prepared in the presence of a stable free radical such as 2,2,5,5-tetramethyl-l-pyperidinyloxy (TEMPO). In this way, the monolith is prepared first and its dormant free-... 

Hawker CJ, Barclay GG, Orellana A et al. (1996) Initiating systems for nitroxide-mediated living free radical polymerizations synthesis and evaluation. Macromolecules 29 5245-5254... 

Lohmeijer BGG, Schubert US (2004) Expanding the supramolecular polymer LEGO system nitroxide mediated living free radical polymerization for metallo-supramolecular block copolymers with a polystyrene block. J Polym Sci Part A Polym Chem 42 4016-4027... 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Scheme 1.33 Preparation of PS by nitroxide-mediated living free-radical polymerization and its utilization for the functionalization of shortened SWCNTs.

Homopolymer PS and block copolymer poly(tert-butyl acrylate)-b-styrene, prepared by nitroxide-mediated living free-radical polymerization, were utilized for the functionalization of shortened SWCNTs through a radical coupling reaction (Scheme 1.33) [194]. 

Nitroxide mediated free radical polymerization is a living or controlled polymerization process. It can be used to initiated or terminate polymerization reactions as needed (1). The use of Phosphino, aryloxy, silyl, boryl and seleno mediating agents are described (2). 

Nitroxide-Mediated Controlled Radical Polymerization (NMCRP) was first discovered by Solomon et al., who patented their discovery in 1985 [205]. This opened up new pathways in the field of free-radical polymerization. Polymer architectures, which were the domain of the anionic polymer chemist, became accessible to the free-radical polymer chemist. However, it was not until the work of Georges et al. [206] was published in 1993, that the world of polymer chemistry became aware of the possibihties of this new class of free-radical polymerization. This was the beginning of what is today one of the leading topics in free-radical polymer chemistry Controlled or Living Free Radical Polymerization. This initiated the search for new Controlled or Living Free Radical Polymerization techniques, and soon afterwards other methods (which will be discussed later) were developed. 

Bergbreiter, D. E., Waichuk, B. Meisenheimer Rearrangement of Aiiyi N-Oxides as a Route to Initiators for Nitroxide-Mediated "Living" Free Radical Polymerizations. Macromolecules 1998, 31, 6380-6382. 

III. Nitroxide-Mediated Living Free Radical Polymerizations 3663... 

Table 1. Structure of Nitroxides Employed in Living Free Radical Polymerizations...

It is obvious from the above discussion that under the correct conditions and with the appropriate mediating nitroxide free radical, living polymerization conditions can be achieved. On the basis of this realization, numerous groups have demonstrated that the degree of structural control normally associated with more traditional living processes, such as anionic procedures, can be equally applied to nitroxide-mediated living free radical polymerizations. 

While possessing many of the key advantages of controlled/ living polymerization methods, nitroxide-mediated free-radical polymerizations do exhibit several limitations. The range of monomers that have been polymerized using nitroxide-mediated techniques include styrenics. acrylamides and (meth)acrylates but these have predominantly been reported in bulk polymerizations (i.e. without solvent) and are conducted at elevated temperature for long time periods. In addition, synthesis of the unimolecular initiator can prove troublesome (dependent upon the type required) and often requires extensive purification in order to attain sufficient purity levels to allow molecular weight control. 

The use of nitroxides as mediating radicals has been revealed to be highly successful in living free-radical polymerization and has received considerable attention for more than 10 years [203-207]. Much attention has been devoted to understanding the mechanism (Scheme 35) and kinetics for NMP [208-210]. 

There are four principal mechanisms that have been put forward to achieve living free-radical polymerization (1) Polymerization with reversible termination by coupling, the best example in this class being the alkoxyamine-initiated or nitroxide-mediated polymerization, as first described by Solomon et al. (1985) (2) polymerization with reversible termination by hgand transfer to a metal complex (usually abbreviated as ATRP),(Wang and Matyjaszewski, 1995) (3) polymerization with reversible chain transfer (also termed degenerative chain transfer)-, and (4) reversible addition/ffagmentation chain transfer (RAFT). 

Georges, M.K., Hamer, G.K., Listigovers, N.A., 1998. Block copolymer synthesis by a nitroxide-mediated living free radical polymerization process. Macromolecules 31 (25), 9087-9089. 

Veregin, R.P.N., Odell, P.G., Michalak, L.M., and Georges, M.K., 1996, Molecular Weight Distributions in Nitroxide-Mediated Living Free Radical Polymerization Kinetics of the Slow Equilibria between Growing and Dormant Chains, Macromolecides, 29 3346... 

Figure 8 Main mechanistic transformation reactions in living and/or controlled polymerization methods. ATRP, atom transfer radical polymerization RAFT, reversible addition-fragmentation chain transfer NMRP, nitroxide-mediated free radical polymerization CROP, cationic ring-opening polymerization AROP, anionic ring-opening poiymerization.

A. Nilsen, R. Braslau, Nitroxide decomposition implications toward nitroxide design for applications in living free-radical polymerization, J. Polym. Sci., Part A Polym. Chem. 2006, 44, 697—717. 

---