Living radical polymerization active species

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

As with ruthenium, iron belongs to the group 8 series of elements and can similarly take various oxidation states (—2 to +4), among which Fe(II), Fe-(I), and Fe(0) species have been reported to be active in Kharasch addition reactions.33 For metal-catalyzed living radical polymerizations, several Fe(II) and Fe-(I) complexes have thus far been employed and proved more active than the Ru(II) counterparts in most cases (Figure 2). The iron-based systems are attractive due to the low price and the nontoxic nature of iron. 

Metal-catalyzed living or controlled radical polymerizations can generally be achieved with initiating systems consisting of an organic halide as an initiator and a metal complex as a catalyst or an activator as described above. However, these polymerizations are slow in most cases due to low concentration of the radical species, as required by the general principle, the dormant-active species equilibria, for living radical polymerization (see the Introduction). 

Living radical polymerization (LRP) has attracted growing attention as a powerful synthetic tool for well-defined polymers 1,2). The basic concept of LRP is the reversible activation of the dormant species Polymer-X to the propagating radical Polymer (Scheme la) 1-3). A number of activation-deactivation cycles are requisite for good control of chain length distribution. 

CRP (also referred to as living radical polymerization ) is a family of promising techniques for the synthesis of macromolecules with well-defined molecular weight, low polydispersities (often close to unity) and various architectures under mild conditions at 20-120°C, with minimal requirements for purification of monomers and solvents. A common feature of the variants is the existence of an equilibrium between active free radicals and dormant species. The exchange between active radicals and dormant species allows slow but simultaneous growth of all chains while keeping the concentration of radicals low enough to minimize termination. The ideal CRP is achieved if all chains are initiated... 

Atom Transfer Radical Polymerization. ATRP is one of the most successful controlled/living radical polymerization (CRP) systems, in addition to NMP and degenerative transfer processes, such as RAFT (5,233,234). The key feature of all of them is the dynamic equilibration between the active radicals and varions tsqjes of dormant species (see Living Radical Polymerization). 

Second, even though the metal center can take two consecutive valence states, some metal centers in a lower valent state may prefer to form a carbon-metal bond upon meeting with the growing radical species rather than activate the more abundant carbon-halogen bond in the dormant termini (Scheme 2-(4)). In this case, if the carbon-metal bonds may be homolytically cleaved with a fast and reversible equilibrium, the polymerization would result in another controlled/living radical polymerization, the so-called organometallic-mediated... 

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