Nitroxide mediated polymerisation

Georges and co-workers [41] reported the first controlled polymerisation using the NMP technique. The stable free radical 2,2,6,6-tetramethyl-l-piperidynyl-N-oxy (TEMPO) was initially used, with a thermal radical initiator, for the polymerisation of styrene. NMP polymerisations usnally require higher polymerisation temperatures, and it was not possible to polymerise acrylates in the presence of excess TEMPO in the early systems, due to the very low propagation rate. The radical polymerisation 

NMP makes possible the synthesis of a range of complex polymer architectures with high control over the PDI and molecular weight [46]. However, the polymerisation of methacrylate monomers, the long polymerisation time and the complexity of preparing functional initiators for further polymer modification limit the application of this technique in the routine laboratory. 

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A corresponding principle applies to controlled radical polymerisation performed in quite a number of modes such as nitroxide-mediated polymerisation (NMP), atom transfer radical polymerisation (ATRP), reversible addition fragmentation chain transfer (RAFT) or catalytic chain transfer (CCT) reactions. All of these variants of controlled radical polymerisation lead to well-defined architectures with the particular advantage that a much larger number of monomers are suitable and the reaction conditions are much less demanding than those of living ionic polymerisation reactions. 

A sample pre-treatment was evaluated to enable the production of intact cationic species of synthetic polymers holding a phosphonate group using MALDI-mass spectrometry. Thus, the polymer (496) obtained by nitroxide-mediated polymerisation was stirred for a few hours in tri-fluoroacetic acid to induce the substitution of a tert-butyl group on the nitrogen of the nitroxide end-group by a hydrogen atom (Scheme 166). 

Nitroxide-mediated living/controlled radical polymerisations (NMP)... 

Scheme 10.12 Nitroxide (T ) as the living reagent in nitroxide-mediated living/controlled radical polymerisation (NMP).

Nitroxide-mediated CRP has been investigated for use in emulsion polymerisation systems, but with mixed the results. " Atom transfer radical polymerisation (ATRP) offers greater scope than nitroxide-mediated CRP in that it is less discriminating in terms of the monomers that can be used. However, there are few reports of work on adapting ATRP to heterogeneous systems. " Given the established requirements for control of these polymerisations, both nitroxide-mediated CRP and ATRP suffer from problems brought about by... 

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... 

Functionalised PO as block and graft copolymers used as compatibilisers or to increase interactions with other materials are prepared by free radical grafting (the simplest method), metallocene-catalysed copolymerisation of olefins with functional monomers, or anionic polymerisation (silane-containing PO). They are also produced by controlled/living polymerisation techniques such as nitroxide-mediated controlled radical polymerisation, atom transfer radical polymerisation (ATRP), and reversible addition-fragmentation chain transfer (RAFT). 

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... 

Figure 5.7 Examples of alkoxyamines used as unimolecular initiator in nitroxide-mediated living radical polymerisation.

From these few reports, it can be concluded that nitroxide-mediated LRP is still a difficult process to achieve in db initio emulsion polymerisation. This is the direct consequence of the complex nucleation step and of an extensive droplet nucleation (Cunningham, 2003). Thus, the various groups decided to focus on miniemulsion polymerisation, in which such difficulties are naturally overcome. 

NMP [44-49], also known as stable free-radical-mediated polymerisation, is a type of CRP that makes use of an alkoxyamine/ nitroxide initiator to generate well-defined polymers with very low polydispersities. Historically, NMP is considered to be the pioneer for CRP, and a very easy CRP technology to implement. The advent of NMP following its successful implementation was the result of elegant work initiated by the Australian group of Rizzardo and... 

MALDI spectrum of a hydroxyl functional polymer prepared by GTP the level of the impurity is easily seen and the success of this reaction clearly observed. However, this is not a universally applicable technique. Perhaps the most topical living polymerisation at present is transition metal mediated radical polymerization. This typically gives a polymer with a tertiary halide terminal group. This group, as has nitroxide, has been found to be very labile in the mass spectrometer leading to fragmentation. 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

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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