Polymerisation Radical traps

ARs are useful counters of active alkyl radicals [37] and inhibitors of radical polymerisation [38]. Aliphatic and aromatic ARs have approximately identical inhibition. These radicals are similar to quinines, and are considerably stronger radical inhibitors than nitroso compounds. The comparison of reactivity of spin traps and ARs shows that ARs are 2-5 orders of magnitude more effective radical acceptors than nitrones and nitroso compounds. Therefore, new effective acceptors of radicals are generated already at the early stages of short-living radical trapping. 

TEMPO is widely used as a radical trap, as a structural probe for biological systems in conjunction with EPR spectroscopy, as a reagent in organic synthesis, and as a mediator in controlled free radical polymerisation. As well as alcohol oxidation, TEMPO also finds use in the oxidation of other functional groups, including amines, phosphines, phenols, anilines, sulfides and organometallic compounds [144]. 

Abstract The radical trapping technique employing the stable aminoxyl (nitroxide) 1 as a radical scavenger, has been used to study (a) the initiation stage in the mechanism of formation of alternating copolymers (b) the reaction of diphenylphosphinoyl and dimethyl phosphoryl radicals with monomers (c) the unusual polymerisation characteristics of maleates and fumarates. 

The radical trapping technique can be used to study the initiation step in polymerisation where the initiating species is a phosphorus-centred radical. Addition of phosphorus-centred radicals to alkenes was found to be competitive with direct trapping by the aminoxyl. 

In some systems the polymer may precipitate in the course of the reaction and this again greatly affects the kinetics of polymerisation, e. g., in a radical polymerisation the precipitation may lead to the formation of "trapped radicals. Moreover, separation into two phases affects the concentration of monomer around the growing centres and this may... 

In trapping experiments, nitroxides will only trap carbon-centred radicals, and not oxygen-centred ones. This is particularly important since oxygen-centred radicals are often used as initiators (Section 10.2). The nitroxide should also not undergo other reactions, such as addition to double bonds or H-abstraction this increases the probability that it will trap selectively carbon-centred radicals which act as chain carriers in many synthetically useful organic reactions, as propagating species in polymerisations and as reactive intermediates in biological pathways. 

The most important methodology for the aliphatic C-C bond formation via radical reactions is the addition of the radical to an alkene double bond, both inter -and intramolecularly (with the 5-exo-ring cyclisation mode preferred in the latter case). This reaction leads to adduct radicals that must be converted to non-radical products before polymerisations can take place. For this reason, polymerisation is avoided either by intermolecular trapping of adduct radicals or by intramolecular, homolytic bond cleavage. Hydrogen atom donors X-H, heteroatom donors X-Z or electron donors M"+ are used as trapping agents (Scheme 7.1). 

In the photoinitiated polymerisation of Jl-vinylpyrrolidinone and N-vinylcaprolactam in dioxane and ethanol, the rate was higher in the latter solvent and monomerlO. This was attributed to the influence of the two additional methylene groups in the caprolactam ring which increases monomer reactivity. Other interesting effects have included the radiation dose on the photopolymerisation of diallyl oxydiethylene dicarbonate O. Here long lived radicals were produced which continue to react in the dark. The rate appears to fit a relaxation model that considers double bonds as traps with increasing lifetimes that are able to transfer to radical sites. 

Trapping Reactions with 2-t-Butylacrylic Acid Methylester XIV. This trapping reaction, which mimics the initiation step of the polymerisation process, has been used to obtain information on the reactivity of the primary radicals formed upon irradiation [ 1 ] [26]. Photolysis of BOMB in the presence of a threefold excess of XIV affords the benzoyl derivative XV in 87 % yield (Figure 9). Again VII was etlso isolated (64 %), whereas no stable addition product of -aminoalkyl radical could be identified. This result suggests that the benzoyl radical is mainly responsible for the polymerisation of vinylic monomers and is in agreement with previous studies on benzil ketals and benzoin ethers [ ]. But, as -aminoalkyl radicals have also been shown to initiate acrylate polymerization [ ], further investigations will be devoted to the elucidation of the role of this primary photoproduct in the overall polymerisation process. 

Practically, the polymerisation can be carried out in two different ways (Figure 5.3). The first method is to simply add the deactivator into the polymerisation medium containing monomer and radical initiator and then adapt the experimental conditions so that the trapped radical can be reactivated. The second method consists of using a pre-made dormant species of small size (where R-T is considered as a unimolecular initiator) to start the polymerisation without the need of a radical initiator. In both cases, initiation must be fast, so that all growing chains are created within a short time span. It should, however, be noted that the second method leads to a better control over molar mass as the initiator efficiency is usually close to 1, which might not always be the case when a classical radical initiator is used. 

Nitroxides represent a very important class of radical deactivators (Bertin et al, 1998 Hawker et al, 2001 Hawker, 2002). They are stable radicals able to terminate with carbon-centred radicals at near diffusion controlled rates. The trapping reaction forms an alkoxyamine (Figure 5.5) that is very stable at low temperatures, and therefore corresponds to an irreversible termination step. However, at elevated temperature, the C—O bond may undergo homolytic cleavage, producing back the propagating radical and nitroxide. This equilibrium between propagating radical and inactive alkoxyamine is the key step in nitroxide-mediated LRP activation is thus purely a thermal process. The polymerisation... 

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