Alkoxyamines for NMP

Nitroxide-mediated polymerization (NMP) [3] and atom transfer radical polymerization (ATRP) [4, 5] are the two main methods of CRP based on a reversible termination reaction. This corresponds to an equilibrium between the active macromolecular radical and a dormant covalent counterpart, which is either an alkoxyamine for NMP or an alkyl halide for ATRP (Fig. 1). Activation of the alkoxyamine is a thermal process and requires elevated temperatures, whereas in... 

Various methods have been used to form low molecular weight alkoxyamine initiators for NMP. Most involve forming an appropriate carbon-centered radical in the presence of a nitroxide. Initiators that generate carbon-ccntcrcd radicals may be thermally decomposed in the presence of a nitroxide. For example, alkoxyamine 100 is formed by decomposition of AIBN in the presence of TEMPO (Scheme 9.19). 1,1 Carbon-centered radicals may also be generated photochemically. 70... 

The thermal decomposition of the phenylelhyl alkoxyamine with TEMPO and the fraction of living ends in TEMPO-mediated S polymerization has been studied by Priddy and coworkers.143 179 They concluded that to achieve >90% living ends conversions and/or nitroxide concentrations should be chosen to give V/ less than 10000.143 However, disproportionation or elimination is most important during polymerizations of methacrylates and accounts for NMP being less successful with... 

Functional alkoxyamines used as initiators for NMP include 283-287. The functional alkoxyamines can be formed in situ by use of a functional azo compound or peroxide. NMP has been shown to be compatible with hydroxy, epoxy, amide and tertiary amine groups in the initiator. Carboxylic acid groups can cause problems but may be tolerated in some circumstances.106... 

Scheme 9.48 (For NMP X—I is an alkoxyamine and X is a nitroxide, A and U are specific monomers, M is any monomer, is a copolymer chain note that PnB, PnAB, Pr,BB and PnA, P BA, PnAA are not distinguished)...

NMP is based on the concept of a dynamic equilibration between dormant alkoxyamines and propagating radicals as shown in eqn [55].The choice of the persistent radical is cmcial for controlled polymerization. While styrene can be easily moderated by 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), other monomers required the development of nitroxides that contain hydrogen atoms at the a-C. There are two different initiation methods for NMP. Conventional radical initiators (i.e., AIBN, BPO) in conjunction with a persistent radical were initially used to prepare polymers by NMP, but these systems were limited in the choice of monomer. Functionality could be incorporated via a functionalized initiator or a functionalized persistent radical. For example, Baumert and Mulhaupt prepared carboxylic acid-terminated polystyrene, poly(styrene-co-acrylonitrile), and polystyrene-b-poly (styrene-co-acrylonitrile) by the use of the functionalized initiator 4,4 -azobis(4-cyanopentanecarboxylic acid). The polymerization was controlled by the addition of 2,2,6,6-tetramethyl-l-piperidyloxyl radical, and polymers with... 

Alkoxyamines such as S-TEMPO and BS-TEMPO in Figure 4 work as an initiating dormant species in nitroxide-mediated polymerization (NMP). They are synthesized and purified independently from the NMP mn or are prepared in situ in the NMP run. In the latter, for example, a mixture of the conventional initiator benzoyl peroxide (BPO), monomer, and TEMPO in a suitable ratio is heated to generate BS-TEMPO and its analogues with two or more monomer units, which will work as initiating alkoxyamines. For kinetic studies, the use of a purified alkoxyamine is preferable to avoid unnecessary complexities. 

Beyond these achievements, this strategy will allow a novel chemistry based on photosensitive alkoxyamines to be developed, which could extend the range of monomers available for NMP and be implemented for controlled multilayered micropatteming applications. 

All these results highlighted the cmcial role of the alkoxya-mine structure for NMP. The best alkoxyamine should then present a high dissociation rate constant value and a rate constant of first monomer addition at least equal to the propagation rate constant. This makes the determination of the dissociation and recombination rate coefSdents of model alkoxyamines of high importance for further improvement of this process. 

Contrary to ATRP and RAFT techniques, the removal or the transformation of NMP (co)polymers o-end-groups has not been extensively studied. Two distinct solutions can be considered (1) a radical pathway where functionalization occuued after decomposition of the (macro)alkoxyamine, and (2) a nonradical route in which the (macro)alkoxyamine is reacted directly. An illustration of end-group transformations achieved so far for NMP is given in Figure 18. 

Later it was shown that the alkoxyamine end group was connected to a single styrene terminal unit (Scheme 4.17) and that the MMA penultimate unit caused a significantly lower cleavage temperature. Consequently, the copolymerization of MMA with a small amount of styrene could be performed at 78 °C, an unprecedented low temperature for NMP. 

A wide range of nitroxidcs and derived alkoxyamincs has now been explored for application in NMP. Experimental work and theoretical studies have been carried out to establish structure-property correlations and provide further understanding of the kinetics and mechanism. Important parameters are the value of the activation-deactivation equilibrium constant K and the values of kaa and (Scheme 9.17), the combination disproportionation ratio for the reaction of the nilroxide with Ihe propagating radical (Section 9.3.6.3) and the intrinsic stability of the nitroxide and the alkoxyamine under the polymerization conditions (Section 9.3.6.4). The values of K, k3Cl and ktieact are influenced by several factors.11-1 "7-"9 ... 

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

The same heterobifunctional initiator, 2-phenyl-2-[(2,2,6,6-tetramethy-piperidino)oxy] ethyl 2-bromo-2-methyl propanoate, was employed for the synthesis of PMMA-fo-PfBuA-fo-PS triblock terpolymers via the combination of ATRP and NMP [136]. Styrene was initially polymerized through the alkoxyamine function in bulk at 125 °C, leading to PS chains with bromine end groups. Subsequent addition of fBuA in the presence of CuBr/PMDETA provided the PS-fr-PfBuA diblock. Further addition of CuCl, to achieve halogen exchange and MMA yielded the desired triblock copolymer with... 

The tendency of nitrones to react with radicals has been widely used in new synthetic routes to well-defined polymers with low polydispersity. The recent progress in controlled radical polymerization (CRP), mainly nitroxide-mediated polymerization (NMP) (695), is based on the direct transformation of nitrones to nitroxides and alkoxyamines in the polymerization medium (696, 697). In polymer chemistry, NMP has become popular as a method for preparing living polymers (698) under mild, chemoselective conditions with good control over both, the polydispersity and molecular weight. 

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... 

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