Alkoxyamine formation

In the second approach, the alkoxyamine is fonned in situ typically from the nitroxide and radicals generated using a conventional initiator (Scheme 9.5). The initiator used in the early work of Georges et was BPO (Scheme 9.18). The yield of alkoxyamine based on BPO is not quantitative and various side reactions are known to accompany alkoxyamine formation (Section 3.5.2.4). When the... 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

The monomer 359 has been formed in situ by decomposing the initiator (A1BN) in the presence of the corresponding nilroxide in a solution of S or 2-ethoxyethyl acrylate.744 The kinetics dictate that alkoxyamine formation, by coupling of the nitroxide with cyanoisopropyl radicals, will take place before eopolymerization. 

Scheme 10.6 Alkoxyamine formation by trapping of the cumyl radical by TEMPO.

The very small number of growing polymer chains, when compared to the monomer concentration results in a very low overall concentration of free control agent and leads to inefficient capping of chain ends. One solution to this problem is the addition of a free or unbound control agent to the polymerization medium. This can take the form of a low molecular weight alkoxyamine, ATRP initiator, RAFT agent or, alternatively, free deactivator such as nitroxide or Cu(II). This species is often called a sacrificial agent. This solution also leads to the formation of free polymer that must ultimately be removed from the brush. 

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... 

Figure 8.13 Schematic representation for the dynamic formation of graft polymer through radical crossover reactions of alkoxyamine units [32],...

Scheme 8.10 Dynamic formation of graft polymer 50 prepared form copolymer 48 and TEMPO-based alkoxyamine-terminated polystyrene 49 [40].

Scheme 8.11 Thermodynamic formation of crosslinked polymer 54 via radical crossover reaction of alkoxyamines in copolymers 52 and 53 [42],...

Higaki, Y Otsuka, H. Takahara, A. Dynamic formation of grafted polymers via radical crossover reaction of alkoxyamines. Macromolecules 2004, 37, 1696-1701. 

Scheme 10.11 shows a PRE-mediated 5-exo-trig radical cyclisation in which the controlled thermal formation of active radicals from the dormant alkoxyamine 2 is facilitated by steric compression of the alkoxyamine C—O bond by the bulky N-alkyl and O-alkyl groups [8]. Intramolecular H-bonding between a —CH2—OH and the nitroxyl oxygen of the incipient nitroxide in a six-membered cyclic transition structure further facilitated the dissociation of 2. After cyclisation, the resultant primary cyclopentylmethyl radical was trapped by the free nitroxide to form the new dormant isomerised alkoxyamine 3, which is more stable than 2 since the O-alkyl is now primary. The same reaction using TEMPO as the nitroxide component did not work presumably because the C—O bond in the alkoxyamine precursor is much stronger. 

The formation of oximes and hydrazones can be used to modify proteins due to some attractive properties, that is, some stability in aqueous media and compatibility with many functionalities of proteins [134], Their synthesis can be accomplished through the nucleophilic addition-elimination (condensation) of hydrazines and alkoxyamines with aldehydes and ketones, respectively. Typically, it has been used to modify the N-terminus [135-137] of peptides and proteins. For example, oxime formation has been used to modify BSA and diphtheria toxin with Shigella sonnei O-specific oligosaccharides [138],... 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

With the recent development of living radical polymerization, the problem of gel formation during radical polymerization possibly can be controlled. This is because termination by radical chain coupling is virtually eliminated. Thus Hawker reported the preparation of soluble hyperbranched polystyrene using alkoxyamine IV as a living radical polymerization initiator [12]. 

The oxidation of amines, acylhydrazines, and alkoxy amines described in this section involves the formation of nitrenes or other intermediates, depending on the nature of the nitrogen substituent and the oxidant, although lead tetraacetate is commonly employed. For example, a nitrenium ion or an amino lead derivative was proposed as the intermediate in the oxidation of alkoxyamines with lead tetraacetate 2. However, evidence has been provided that the jV-acetoxy species is the intermediate in the aziridination of alkenes with V-aminophthal-imide and /V-aminoquinazolinone, where a mechanism analogous to the Bartlett mechanism for the peracid epoxidation of alkenes should be operating3,4. 

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