Reverse ATRP

So-called reverse ATRP has been described where a conventional radical initiator (e.. AIBN) and a transition metal complex in its higher oxidation state are iised. One of the first systems explored was CuBr2/133/AIBN/MMA. It is important that the initiator is completely consumed early in the polymerization. Tlie use of peroxide initiators in reverse ATRP can be problematical depending on the catalyst used and the reaction temperature. The system CuBr2/133/BPO/MMA at 60°C was found to provide no control.- In ATRP at lower temperatures (40 °C), the system CuCl/133/BPO/MMA was successful though dispersities obtained were relatively broad, Radicals are produced from die redox reaction between the catalyst in its reduced fonn and BPO. 

The molecular weight in reverse ATRP will depend on the concentration of the initiator (T) and the initiator efficiency if) and ideally is given by eq. 11. Side reactions between the catalyst and the initiator and the radicals fonned from the initiator may lead to efficiencies being lower than those observed in conventional radical polymerization. 

Experiments have been described where a combination of direct and reverse 

ATRP is used. In this case eq. 12 should apply, V [ML  

In combination ATRP, the catalyst is again present in its more stable oxidized fonn. A slow decomposing conventional initiator (e.g. AIBN) is used together with a normal ATRP initiator. Initiator concentrations and rate of radical generation are chosen such that most chains arc initiated by the ATRP initiator so 

An alternative approach to initiating an ATRP is to use azobisisobutjTonitrile (AIBN), a radical source used commonly in conventional radical polymerizations. By generating free radicals in the presence of a transition metal hahde/hgand complex in its higher oxidation state, the required dynamic equilihrium can be gaierated Initiation 

Although initiation using azo compounds has been successfnl with a number of systems, the use of peroxides has been less sneeessful. 

Successful miniemulsion polymerisations were also achieved with the same monomer and the same Cu(II)/dialkylbipyridine complex, in the additional presence of hexadecane high shear of the initial system was provided by ultrasonication. A monomer-soluble initiator (AIBN) and a water-soluble one (V-50) were employed (Matyjaszewski et at, 2000b). The latter, however, led to better controlled molar masses, with higher initiator efficiency. Stable latexes were achieved with particle diameter of 300 nm, but the amount of required surfactant was very large (i.e. 13.5 wt% based on monomer). In a more 

The latexes described above (Li Matyjaszewski, 2003b) were subjected to chain extension by feeding a new load of -butyl methacrylate and surfactant after 98.3% monomer conversion from the first stage. Such results that demonstrated livingness of the first block can be considered as the first step towards the synthesis of block copolymers via ATRP in an aqueous dispersed system. 

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Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

ATRP as discussed to this point is normal ATRP which uses RX and a transition metal in its lower oxidation state to establish the equilibrium between dormant and propagating species. Reverse ATRP involves generating the same ATRP system by using a combination of a... 

Copper-based ATRP in emulsion has been achieved successfully under reverse ATRP conditions, where one starts with a conventional free radical initiator and CuBr2/dNbpy as a catalyst [226]. Typical commercial water-soluble free radical initiators such as potassium peroxodisulfate (in a phosphate buffer, pH = 7), 2,2 -azo-... 

ATRP can be approached from both sides of the equilibrium, that is, beginning from an alkyl halide and a low oxidation state metal, or from a radical and the higher oxidation state metal this latter approach is termed reverse ATRP (rATRP) [81,189,190]. Qiu et al. used this technique to prepare block copolymers, also of MMA and St [ 191 ]. They used a hexasubstituted ethane thermal in-iferter, diethyl 2,3-dicyano-2,3-di (p-tolyl)succinate, which decomposes reversibly to form two radicals when heated. The new radical is either deactivated by the CuCl2/bpy complex or adds MMA monomer, followed by deactivation, both of which will produce the dormant species in the ATRP equilibrium. The rATRP... 

Note that the addition of a higher oxidation state transition metal and metal zero which acts to reduce the stable oxidation state transition metal complex was covered in an improvement to a reverse ATRP (26) in US Patent 6,541,580, (40) which is based on a provisional apphcation filed 4/97. This application also discussed use of metal zero as the only source of the catalyst. 

ATRP polymerization in miniemulsion has recently attracted more attention and met with greater success. Some difficulties with conventional initiation were attributed to catalyst oxidation during the homogenization/sonication step particularly when more active, less oxidatively stable, catalysts are used. This problem was solved using reverse ATRP or combinations of reverse and normal A I RP " that meant the catalyst could be added in its oxidized form (Section... 

The use of reverse ATRP enlarges the scope of substrate polymers for controlled polymerizations even further because the required radical initiators can in many cases be provided more easily than ATRP initiators on surfaces of inert polymers. For example, poly(vinylidene fluoride) microfiltration membranes were irradiated with UV light and then exposed to air to create hydro(peroxide) species [11]. These were then used to initiate the reverse ATRP of methyl methacrylate in the presence of copper(I) chloride, 2,2 -bipyridine, and benzoylperoxide (Figure 3.5). Similar reaction schemes are applicable to (hydro)perox-ide patterns created directly on polymer surfaces using the lithographic methods discussed in Chapter 2. 

Figure S.5 Reverse ATRP of methyl methacrylate on PVDF microfiltration membranes activated hy UV irradiation [11],...

Living CROP to ATRP or reverse ATRP to form AB- and ABA type-block copolymers, were also performed [77, 78]. One or two bromopropionyl end groups were introduced onto PTHF by using functional initiator and termination approaches in the CROP ofTHF (Scheme 11.19). 

In a recent report, Chang et al. [201] proposed a simple strategy for the one-step synthesis of PSt-b-PCL by using a combination of conventional free radical or reverse ATRP and AROP. These strategies involved the use of a symmetric bifunctional initiator (2,2-azobis[2-methyl-N-(2-hydroxyethyl) propionamide]) that was able to combine two dissimilar polymerization systems simultaneously. 

The grafting-from technique involves the immobilization of initiators onto the substrate followed by in situ surface-initiated polymerization to generate a tethered polymeric phase. This approach has generally become the most attractive way to prepare thick, covalently tethered polymer brushes with a high grafting density. A variety of synthesis methods such as radical chain transfer reaction,reverse ATRP, living anionic surface-initiated polymerization, ATRP, " dispersion polymerization, and... 

MA, used as first monomer, exceeded 70%, a significant percentage of dead PMA chains contaminated the final diblock copolymer. Conversely, when MA was added to the solution of PBA in [BMIMJPFe, clean diblock was formed, essentially free of homopolymer, and BA can be polymerized to complete conversion. Ma and coworkers (42,49) also described successful synthesis of block copolymers, where St was polymerized by chlorine-end-capped PMMA as macroinitiator through reverse ATRP in [BMIMlPFe, [BMIMJBF4, and [Ci2MIM]BF4. 

Problem 11.9 While in a normal ATRR the initiating radicals are generated from an alkyl halide in the presence of a transition metal in its lower oxidation state (e.g., CuBr(dNbpy)2), ATRP can also be initiated by using a thermal free-radical initiator (e.g., AIBN) along with the transition metal compound in its higher oxidation state (e.g., CuBr2(dNbpy)2). Write a general scheme for this latter approach which is named reverse ATRP . 

The reverse ATRP using AIBN as the initiator has been performed successfully for copper-based heterogeneous (Xia and Matyjaszewski, 1999) and homogeneous (Xia and Matyjaszewski, 1997) systems in solution and in emulsion as well as for iron complexes (Matyjaszewski and Xia, 2001). A general outline of reverse ATRP is shown in Scheme PI 1.9.1. As shown in this Scheme, the starting materials in reverse ATRP are a thermal free radical initiator (I-I), transition metal halide in the oxidized state (XMt ), and monomer (M), while the propagation step resembles a normal ATRP. It may be noted, however, that the reverse ATRP initiated by peroxides sometimes behaves quite differently than that initiated by azo compounds like AIBN. The differences between the benzoyl peroxide (BPO) and AIBN systems possibly arise due to an electron transfer and the formation of a copper benzoate species in the BPO system (Xia and Matyjaszewski, 1999). 

Scheme Pll.9.1 General scheme of reverse ATRP using AIBN (represented by I-I) thermal initiator (Problem 11.9).

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