Free radical reactions alkoxyamine reaction

Scheme 8.7 Chain transfer reaction of polyester containing alkoxyamine units 43 caused by free radical 45 [38],...

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... 

Polymerization can be started using an alkoxyamine as initiator such that, ideally, no reactions other than the reversible activation of dormant species and the addition of monomer to carbon-centered radicals take place. The alkoxyamine consists of a small radical species, capable of reacting with monomer, trapped by a nitroxide. Upon decomposition of the alkoxyamine in the presence of monomer, polymeric dormant species will form and grow in chain length over time. Otherwise, polymerization can be started using a conventional free-radical initiator and a nitroxide. The alkoxyamine will then be formed in situ when an initiator molecule decomposes, and, after adding a monomer unit or two, is trapped by a nitroxide. 

Another interesting use of TEMPO has been in free-radical substitution of alkyl halides. In this reaction, halides react with tributyltin hydride and TEMPO to yield A-alkoxyamine substitution products [18. This reaction is especially attractive in cases where anionic nucleophiles are sterically prevented from carrying out substitution reactions. An example of this can be seen in Barrett s synthesis of sucrose [18b], in which a stereoselective iodoetherification reaction was used to produce neopentyl alkyl iodide 13 (Scheme 5). Free radical substitution mediated by tributyltin hydride and TEMPO yielded A-alkoxyamine 14. The mechanism [19] involves TEMPO abstraction of hydrogen from tributyltin hydride [20] the stannyl radical then abstracts iodide from the substrate, and a second equivalent of TEMPO traps the resulting carbon radical. 

Subsequently, nitroxides and parent alkoxyamines were formed directly in the polymerization medium (in situ NMP) by reaction of the nitrone with the free radical initiators [270]. Two types of reactions were carried out. One was a reaction before monomer addition and the other one after the addition. In either case, a prereaction was systematically carried out at temperatures ranging from 60 to 80°C. This was followed by polymerizations at 130°C. The in situ-formed nitroxides and alkoxyamines controlled the radical polymerizations of n-butyl acrylate yielding, however, low molecular weight polymers, of < 10,000 and equal to 1.65-2.0. 

For nitroxide-mediated radical polymerizations and in the RAFT process, the same synthetic strategy as for ATRP can be used in the synthesis of AB and ABA block copolymers. The first step is coupling a functionalized alkoxyamine with a telechelic or monofunctional nonvinylic polymer to give a macroinitiator. This macroinitiator can be used in standard controlled free-radical polymerization procedures. This approach is best illustrated by the preparation of PEO-based block copolymers [81-84]. One example is the preparation of macroinitiator LMI-7 by the reaction of a monohydroxy-terminated PEO with sodium hydride followed by reaction with the chloromethyl-substituted alkoxy amine as shown in Scheme 3.16. 

In reality, NMP is not as simple as portrayed in Scheme 4.5 (reactions 1-7). Side reactions of the nitroxide or alkoxyamine (reactions 8-10) can occur, and there may be additional sources of alkyl radicals, e.g., from deliberately added conventional radical initiator (in order to speed up the polymerization) or from autoinitiation of the monomer. Furthermore, free nitroxide may be present or added at the start of the polymerization. The effects of these side reactions are discussed in Section 3.5. 

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. 

All the CRP methods have strengths that can be exploited in particular systems. TEMPO is essentially useful only for the polymerization of styrene-based monomers, whether for the preparation of statistical or block copolymers [38]. The radicals generated through the self-initiation of St help to moderate the rate of polymerization by consuming any excess TEMPO generated by termination reactions, which will not occur with other monomers. Acrylate monomers, for example, are very sensitive to the concentration of free TEMPO and therefore its build-up causes the polymerization to stop. The use of different nitroxides and alkoxyamines like DEPN [73] and TMPAH [71], which provide higher equilibrium constants and allow for faster polymerization rates, has also enabled the homo- and copolymerizations of acrylate monomers, as well as for St at lower temperatures. Block order is important, however, and chain end functionality is reduced when TMPAH functional polymers are chain extended with BA. This may... 

Recently, Grubbs and coworkers [272] have synthesized an active alkoxyamine by reaction of 2-methyl-2-nitrosopropane with 1-bromoethylbenzene, catalyzed by ligated CuBr in the presence of metallic copper. A purified alkoxyamine was used to initiate the radical polymerization of styrene and isoprene. Well-defined low polydispersity polymers formed with M /M =1.14 for polystyrene and 1.28 for polyisoprene. Subsequently, Grubbs and coworkers [273] used this alkoxyamine and successfully controlled the radical polymerization of n-butyl acylate at 125°C. Lower ratio of M /M was observed when the alkoxyamine was preheated at temperatures up to 125 for 30 mm prior to adding the monomer. This prereaction was needed for an excess of free nitroxide to be formed in situ and for polymerization to be controlled. 

With SGI as a mediator, the preformed alkoxyamine used in miniemulsion was an oil-soluble low molar mass compound with the 1 -(methoxycarbonyl)eth-1 -yl initiating radical (the so-called MONAMS A5). This type of well-defined initiator allowed good control over the initiation step, the concentration of living chains and the concentration of free nitroxide. Both molar mass and rate of polymerisation were similar to that when the reaction was carried out in bulk (Farcet et al, 2003). The target M could be larger than in the previously presented bicomponent initiating system with persulphate and metabisulphite and the PDIs were systematically lower. 

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