ATRP catalysts radical polymerization

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

Block copolymers were synthesized by a combination of fipase-catalyzed polymerization and atom transfer radical polymerization (ATRE). " " At first, the polymerization of 10-hydroxydecanoic acid was carried out by using lipase CA as catalyst. The terminal hydroxy group was modified by the reaction with a-bromopropionyl bromide, followed by ATRP of styrene using CuCE2,2 -bipyridine as catalyst system to give the polyester-polystyrene block copolymer. Trichloromethyl-terminated poly(e-CL), which was synthesized by lipase CA-catalyzed polymerization with 2,2,2-trichloroethanol initiator, was used as initiator for ATRP of styrene. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

Acrylate monomers may also be polymerized by atom transfer radical polymerization (ATRP). The reader is referred to Section 9.1.3.3 for an overview of catalyst systems. 

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. 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

PE-g-PS graft copolymer was produced by a coupling reaction, too. a-Carboxyl PS prepared by an ATRP technique was reacted with PE-g-glycidyl methacrylate (GMA) to produce PE-g-PS [121]. A PP-fc-PMMA block copolymer was synthesized using a magnesium bromide terminated PP as an initiator for the radical polymerization of MMA, which was prepared from the vinylidene terminated PP obtained with the Et(Ind)2ZrCl2/MAO catalyst system [122],... 

Louie and Grubbs prepared an iron-based catalyst for atom transfer radical polymerization (ATRP) [49]. By heating a solution of Iz Prim and FeX2 (X = Br, Cl), crystals of Fe(Iz Prim)2X2 were obtained. These complexes mediated the homogeneous ATRP of styrene and methyl methacrylate with... 

Three key conditions must be met to design a uniformly reactive, recoverable, and recyclable polymerization catalyst (1) the synthetic protocol used to make the immobilized catalyst must lead to only one type of active site on the surface, (2) the support material must be able to allow sufficient transport of reactants to and polymer from the active site, and (3) at the end of the reaction, the active site must not be irreversibly changed or decomposed [23]. Research in our lab has thus far sought to investigate these points using the atom transfer radical polymerization (ATRP) of methyl methacrylate as a model reaction. 

Hong SC, Matyjaszewski K (2002) Fundamentals of supported catalysts for Atom Transfer Radical Polymerization (ATRP) and apphcation of an immobUizEd/soluble hybrid catalyst system to ATRP. Macromolecules 35 7592... 

To promote a polymerization, the newly formed carbon-halogen bond must be capable of being reactivated and the new radical must be able to add another alkene. This was accomplished for the radical polymerizations of St and methyl acrylate (MA), which were initiated by 1-phenylethyl bromide and catalyzed by a Cu(I)/2,2 -bipyridine (bpy) complex [42,79-81]. The process was called Atom Transfer Radical Polymerization (ATRP) to reflect its origins in ATRA. A successful ATRP relies on fast initiation, where all the initiator is consumed quickly, and fast deactivation of the active species by the higher oxidation state metal. The resulting polymers are well defined and have predictable molecular weights and low polydispersities. Other reports used different initiator or catalyst systems, but obtained similar results [43,82]. Numerous examples of using ATRP to prepare well-defined polymers can now be found [44-47,49]. Scheme 4 illustrates the concepts of ATRA and ATRP. To simplify schemes 3,4 and 5, termination was omitted. 

Howdle reported that a one-pot, simultaneous synthesis of block copolymers by enzymatic ROP and ATRP employing initiator 3, CL and MMA is possible in supercritical C02 (scC02) [17]. scC02 is a unique solvent because it combines gaslike and liquid-like properties most importantly in this case it plasticizes and liquefies polymers very effectively, allowing enhanced mass transport which contributes to more efficient polymerization, particularly important for a supported solid phase enzyme catalyst (see also Chapter 13) [28]. The authors also show that the CL acts as a scC02 co-solvent which was crucial to allow the radical polymerization to remain homogeneous and controlled [18]. The unique ability... 

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