Controlled or living radical

Ruthenium catalysts found many applications in C-C bond formation reactions (selected reviews [157-161]). Ruthenium occurs mostly in oxidation states +2 and +3, but lower as well as higher oxidation states can easily be reached. Thus ruthenium compounds are frequently used in oxidative transformations proceeding by either single or two electron transfer pathways (selected reviews [162-164]). It has long been known that ruthenium complexes can be used for the photoactivation of organic molecules (selected reviews [165, 166]). Ruthenium complexes are applied as catalysts in controlled or living radical polymerizations [167-169]. 

This means that composition of the chain and chain length is determined in seconds. Terminated chains, in principle, do not take part in further reactions (except when transfer to polymer events occur. Section 2.3). The final chemical composition distribution and molecular mass distribution is determined by the accumulation of rapidly produced dead chains (chains without an active centre). In free radical polymerisation, the active centre is a free radical. In controlled or living radical polymerisation (Section 2.5) the radical is protected against termination and continues to grow during the complete reaction time. 

Figure 1.1 Publication rate of journal papers on radical polymerization and on living, controlled or mediated radical polymerization for period 1975-2002 based on SeiFinder search (as of Mar 2005).

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. 

During the last fifteen years an explosive increase has been observed in the number of publications on CRP, including a dramatic increase in the nrrmber of patent applications and several symposia devoted partially, or entirely, to CRP." " Figure 1 illustrates resrrlts of a recent SciFinder Scholar search using the following terms controlled radicalpolymn or living radical polymn... 

CRPorLRP or controlled radical polymn or living radical polymn... 

Einally, as per Equation 18.27b, if a copolymer has a very narrow composition distribution, will be very close to (M) using a single solvent the same applies to mixed solvents if we follow the procedure mentioned earlier. This can be the case, for instance, of a radical copolymerization in a true azeotrope composition or of a block copolymer synthesized by a controlled anionic or living radical polymerization. 

Jenkins, A.D., Jones, R.G., Moad, G., 2010. Terminology for reversible-deactivation radical polymerization previously called controlled radical or living radical polymerization. Pure Appl. Chem. 82 (2), 483 91. 

Terminology for reversible-deactivation radical polymerization previously called controlled radical or living radical polymerization Pure Appl. Chem. 82 (2010) 483. 

Figure 1 Publication rate of journal papers on radical polymerization and on living, controlled, or mediated radical polymerization for the period 1975-2008 based on SciFinder search (as of March 2010). It does not distinguish forms of controlled radical polymerization. It includes most papers on ATRP, RAFT, and NMP and would also include conventional, non-RDRP, controlled radical polymerizations. It would not include papers, which do not mention the terms living , controlled , or mediated .

Several controlled and living radical polymerization techniques are available today. All are based on the reversible deactivation of growing chains. Consequently, a 2010 lUPAC recommendation proposes the term controlled reversible-deactivation radical polymerization (CRDRP) for polymerizations previously referred to as controlled radical (CRP) or living radical (LRP) polymerization. Nevertheless, due to their widespread acceptance, the terms controlled/living radical polymerization will also be used in this chapter. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

It remains a common misconception that radical-radical termination is suppressed in processes such as NMP or ATRP. Another issue, in many people s minds, is whether processes that involve an irreversible termination step, even as a minor side reaction, should be called living. Living radical polymerization appears to be an oxymoron and the heading to this section a contradiction in terms (Section 9.1.1). In any processes that involve propagating radicals, there will be a finite rate of termination commensurate with the concentration of propagating radicals and the reaction conditions. The processes that fall under the heading of living or controlled radical polymerization (e.g. NMP, ATRP, RAFT) provide no exceptions. 

Besides such dissociation into long-lived radicals in solution, numerous examples are known of radical cleavage in the gas phase into unstable or reactive radicals. Two factors, the strain of hydrocarbon molecules and the stability of the radicals, are suggested as the major controlling factors for radical fission (Riichardt and Beckhaus, 1980, 1986). 

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