ATRP catalysts

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K 10 for styrenes and acrylates are not suitable for methacrylates. 

A number of transition metal complexes have been apphed in ATRP process. It has been successful for molybdenum, chromium, rhenium, ruthenium, and iron, rhodiiun, nickel, palladium, and copper complexes. Among these, copper catalysts are superior in terms of versatihty and cost. Styrenes, (meth)acrylates, (meth)acrylamides, and acrylonitrile have been successfully polymerized using copper-mediated ATRP. The polymerization has been foimd to be tolerant to a variety of 

The tacticity of the polymer prepared by ATRP of MMA with copper complexes as catalysts has been found to be similar to that of the PMMA prepared by a conventional free-radical polymerization process. 

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ATRP catalysts may be used to generate radicals and thus alkoxyamines can be produced from alkyl halides in high yield (Scheme 9.21).174 The alkoxyaminc 102 was obtained in 92% yield 174 whereas reaction of TEMPO with PMMA under ATRP conditions is reported to provide a macromonomer (Section 9.7.2.1). 

Table 9.5 Structures of Ligands for Copper Based ATRP Catalysts...

Supported copper catalysts have also been described/29 340 The main impetus for the development of supported ATRP catalysts has been to facilitate catalyst removal and, in some cases, to allow for catalyst recycling. 

Nickel complexes (156-159) used as ATRP catalysts for polymerization of (meth)acrylates are shown in Table 9.8. 

ATRP catalysts 493—4 in situ formation 494 his-diuz.enes... 

Fig. 4.15), are active for ATRP of both styrene and methylmethacrylate (MMA) [46]. Polymerisation was well controlled with polydispersities ranging from 1.05 to 1.47. The rates of polymerisation 1 x 10 s ) showed the complexes to be more active than phosphine and amine ligated Fe complexes, and were said to rival Cu-based ATRP systems. It was quite recent that Cu(I) complexes of NHCs were tested as ATRP catalysts [47]. In this work, tetrahydropyrimidine-based carbenes were employed to yield mono-carbene and di-carbene complexes 42 and 43 (Fig. 4.15), which were tested for MMA polymerisation. The mono-carbene complex 42 gave relatively high polydispersities (1.4-1.8) and a low initiation efficiency (0.5), both indicative of poor catalyst control. The di-carbene complex 43 led to nncontrolled radical polymerisation, which was ascribed to the insolubility of the complex. 

Several nickel(II) complexes (e.g., (173)-(176)) have successfully been used to catalyze ATRP, especially when coupled with bromo-initiators, although activities are usually lower than with copper, ruthenium or iron systems.416-419 The alkylphosphine complex (175) is thermally more stable than (174) and has been used to polymerize a variety of acrylate monomers between 60 °C and 120 °C.418 Complex (176) is an unusual example of a well-defined zerovalent ATRP catalyst it displays similar activities to the Ni11 complexes, although molecular weight distributions (1.2-1.4) are higher.419 Pd(PPh3)4 has also been investigated and was reported to be less controlled than (176).420... 

Several rhodium(I) complexes have also been employed as ATRP catalysts, including Wilkinson s catalyst, (177),391 421 422 ancj complex (178).423 However, polymerizations with both compounds are not as well-controlled as the examples discussed above. In conjunction with an alkyl iodide initiator, the rhenium(V) complex (179) has been used to polymerize styrene in a living manner (Mw/Mn< 1.2).389 At 100 °C this catalyst is significantly faster than (160), and remains active even at 30 °C. A rhenium(I) catalyst has also been reported (180) which polymerizes MM A and styrene at 50 °C in 1,2-dichloroethane.424... 

A copper-based ATRP catalyst that is sufficiently stable and active can be used at very low concentrations. However, it is very important to mention that a copper(I) complex is constantly being converted to the corresponding copper(II) complex as a result of unavoidable and often diffusion-controlled radical termination reactions (k=l.0-4.0 x 109 M 1 s 1). Therefore, the deactivator (copper(II) complex) will accumulate as the reaction proceeds resulting in slowing down of the polymerization rate and limiting high monomer conversions. 

Secondly, the quaternised monomer may be replaced with a weakly basic monomer such as MEMA, which exists in its neutral, non-protonated form in alkaline media. Thus the desired zwitterionic block copolymer is prepared in its anionic/neutral form so that no isoelectric point is encountered during the copolymer synthesis. Afterwards, the solution pH can be adjusted to the isoelectric point by the addition of acid to protonate the weakly basic MEMA residues and precipitate the copolymer, which might be a useful alternative approach to column chromatography for the efficient removal of the ATRP catalyst. 

Two major challenges remain (1) the more efficient and cost-effective removal (and preferably recycling) of the ATRP catalyst and (2) the extension of aque-ous/methanolic ATRP to include other classes of monomers such as acrylates and (meth)acrylamides. 

Fig. 6 Left Strategy for consecutive chemoenzymatic and simultaneous one-pot block copolymer synthesis combining enzymatic ROP and ATRP. Right Influence of ATRP-catalyst system on the conversion of CL in the enzymatic ROP of MMA at 60 °C using ATRP-3 as initiator filled squares reaction in absence of ATRP-catalyst open circles CuBr/PMDETA (1 1 1 ratio with respect to initiator) jiWed triangles CuBr/dNbpy (1 2.1 1 ratio with respect to initiator) open inverted triangles CuBr (1 1 ratio with respect to initiator) yiHed diamonds CuBr2 (1 1 ratio to initiator). CL conversion was determined with H-NMR [26]...

There are several requirements that are generally recognized as essential to an effective ATRP catalyst [256,258,259,260]. The metal center should be able to assume at least two oxidation states, separated by one electron, like Cu(I) and Cu(II). It should also be attractive to halogens, it should possess an expandable... 

Nguyen JV, Jones CW (2004) Design, behavior, and recycling of sihca-supported CuBr-Bipyridine ATRP catalysts. Macromolecules 37 1190... 

Scheme 17. Methodology for the preparation of p(St-b-tBA-b-MA) using Cu-based ATRP catalysts [196]...

There are numerous combinations of transition metal and ligand that can be used to tailor the ATRP catalyst system to specific monomers. The ATRP systems are tolerant of many impurities and can be carried out in the presence of limited amount of oxygen and inhibitors [216,217]. This approach is so simple that it has been proposed as undergraduate experiments to prepare block copolymers [218,219]. However, the ATRP catalyst can be poisoned by acids, but the salts of methacrylic and vinylbenzoic acids have been polymerized directly in aqueous media [206]. Also, the use of protecting groups [206], followed by a deprotection step to yield the acids, has been successful in organic media [220]. While ATRP cannot be used to prepared well-defined polymers of vinyl acetate [221 ] as of yet, these goals may be realized with further catalyst development. 

All CRP chemistries should be useful in dual living polymerization techniques directed at the preparation of graft and block copolymers. However, interference may sometimes occur between, e.g., ROP catalyst and either ATRP catalyst, or end functionalities in NMP or RAFT systems. 

Moreover, Heise and Palmans investigated the possibility of conducting both polymerizations as a one-pot cascade reaction in the presence of the dual initiator, ATRP catalyst, CL and methacrylates [16]. The study revealed that certain ATRP catalysts have a strong inhibiting effect on the enzyme. A nickel catalyst, for example, completely inhibited the enzyme, while certain copper catalysts had no... 

The stracture - reactivity correlation will also help to identify new enviromnentally friendly and less expensive mediating agents. This will include a range of new ATRP catalysts, to be used at ppm amounts in benign media, new alkyl (pseudo)halide initiators, nitroxides operating at lower temperatures and apphcable to methacrylates, and also more enviromnentally friendly RAFT reagents. 

In the following text, the central atom of the ATRP catalyst will be copper, which has been most extensively studied. Both the ATRP rate (eq. 2) and control over polymerization (related to the width of the molecular weight distribution, eq. 3) are catalyst-dependent and it is cracial to correlate the stracture of the catalyst with its activity and performance. This should ultimately make possible the rational design of highly active catalysts that can be used at low concentration to mediate the controlled polymerization of various monomers in diverse reaction media. 

The activity of the ATRP catalyst is directly related to its reducing power,... 

Cyclopentadienyl derivatives of ruthenium were first complexes of this metal which were found to be able to catalyze ATRP 1, 7-10). Subsequently carborane (11-12) and alkylidene (13) rathenium complexes were employed as ATRP catalysts. 

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