TEMPO radical, nitroxide mediated

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

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-... 

Dynamic formation of graft polymers was synthesized by means of the radical crossover reaction of alkoxyamines by using the complementarity between nitroxide radical and styryl radical (Fig. 8.13) [40]. Copolymer 48 having alkoxyamine units on its side chain was synthesized via atom transfer radical polymerization (ATRP) of TEMPO-based alkoxyamine monomer 47 and MMA at 50°C (Scheme 8.9). The TEMPO-based alkoxyamine-terminated polystyrene 49 was prepared through the conventional nitroxide-mediated free radical polymerization (NMP) procedure [5,41], The mixture of copolymers 48 and 49 was heated in anisole... 

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... 

Scheme 1.33. Nitroxide-mediated living radical polymerization of styrene using TEMPO. After Hawker et al. (2001).

In an interesting experiment, Yagci et al. polymerized cyclohexene oxide (CHO) via a photosensitized cationic polymerization with an initiator that contained a TEMPO moiety capable of CRP [281]. Anthracene was reacted with N-ethoxy-2-methyl pyridinium hexafluorophosphate, which produced a radical cation that could then be trapped with TEMPO to create the dual initiating species capable of both cationic and nitroxide-mediated polymerizations (Scheme36). 

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. 

Solomon et al. [75] first used nitroxyl radicals and alkoxyamines in a radical polymerization, but their work was limited to production of low-molecular weight polymers. In 1993, Georges et al. [76] used a mixture of benzoyl peroxide (BPO) initiator and 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) to produce low-polydispersity and high molecular weight polystyrene. Since then many papers about SFRP (also known as nitroxide mediated polymerization or NMP), mainly focused on styrene polymerization in the presence of TEMPO, have been published. Other nitroxide mediators are being developed that are better suited to polymerization of more polar monomers such as meth(acrylates) [77]. 

An air-blown linseed oil-bearing hydroperoxide group is used as a macroinitiator in the nitroxide mediated radical polymerisation of styrene in the presence of 2,2, 6,6 -tetramethylpiperidinyl-l-oxy (TEMPO). The hydroperoxide groups formed on oil molecules serves as a macroinitiator in this polymerisation process. The amount of hydroperoxide groups and TEMPO affect the film properties. The use of an equal amount of TEMPO and free... 

Although the ability of TEMPO to mediate the polymerization of acrylates may be adversely affected by an unfavorable equilibrium constant as compared to other nitroxides, a second, and arguably more serious problem may be the persistence of TEMPO radicals in the polymerization. Unavoidable termination reactions by chain-chain coupling causes accumulation of free nitroxide in the reaction solution. Since acrylates do not exhibit an autopolymerization mechanism that can... 

Since its discovery in 1993 [27], nitroxide-mediated polymerization (NMP) has been the most extensively studied technique from the dissociation-combination dass of LRP mechanisms (Scheme 13.7). This method is also commonly termed stable free radical polymerization (SFRP). NMP reactions are distinguished by the use of stable free radical nitroxide molecules (N ) as the controlling agent [e.g. 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO), (l-diethylphosphono-2,2-dimethyl)propyl nitroxide (DEPN)]. 

Nitroxide Mediated Living Radical Polymerization. Another controlled radical polymerization developed in recent years is stable free radical mediated polymerization (SFRP), also called as nitroxide mediated radical polymerization (NMP) (78,131). This type polymerization can be reahzed through reversible deactivation of growing radicals by stable radical such as 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO). 

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]. 

Kinetic and theoretical studies The nitroxide-mediated copolymerization was far less studied than the homopolymerization although a large number of polymers produced via a radical polymerization mechanism are actually random copolymers. Early kinetic and mechanistic studies were published by Zaremski et al. for the TEMPO-mediated copolymerization of styrene with various comonomers. They discussed various regimes depending on the ability or disability of the second monomer to undergo a controlled/living NMP and determined experimentally the activation-deactivation equilibrium constants for many of those systems. 

Figure 14 Nitroxide-mediated radical polymerization, illustrating reversible activation and deactivation of polystyrene by TEMPO. The activation-deactivation equilibrium constant is defined as K= Aa/Ad.

---