Ligand monomer choice

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

Both methods require that the polymerization of the first monomer not be carried to completion, usually 90% conversion is the maximum conversion, because the extent of normal bimolecular termination increases as the monomer concentration decreases. This would result in loss of polymer chains with halogen end groups and a corresponding loss of the ability to propagate when the second monomer is added. The final product would he a block copolymer contaminated with homopolymer A. Similarly, the isolated macroinitiator method requires isolation of RA X prior to complete conversion so that there is a minimum loss of functional groups for initiation. Loss of functionality is also minimized by adjusting the choice and amount of the components of the reaction system (activator, deactivator, ligand, solvent) and other reaction conditions (concentration, temperature) to minimize normal termination. 

Concerning the use of ATRP with MIPs, the major limitation for this technique in the context of MIP synthesis is the small choice of monomers with suitable functional groups. Typical monomers used for molecular imprinting such as methacrylic acid (MAA) are incompatible, as they inhibit the metal-ligand complex involved in ATRP. With other monomers like methacrylamide [59] and vinylpyridine [60] it is difficult to achieve high monomer conversion. Template molecules also often carry functional groups that may inhibit the catalyst. All this seems to make ATRP not the best choice for molecular imprinting. Nevertheless,... 

The second type includes a functionalized monomer copolymerized with styrene and DVB. For example, j)-bromostyrene may be included in the reaction mixture to provide the desired concentration of functional groups in the support. These groups may eventually be converted into phosphines by reaction with lithium diphenylphosphide. Such resins with low phosphine concentrations are the supports of choice for attachment of monophosphine-substituted metal clusters because the ligands are sparsely and almost randomly distributed in the polymers (5). 

Armes et al. have intensively studied the aqueous ATRP of various monomers. Using CuBr/bpy as a catalyst, methacrylic acid polymerization has been shown to be possible in aqueous media at pH values between 6 and 9 [200]. The polymerization occurs very slowly (80% conversion after 21 h at 90 °C, [monomer] [initiator] [catalyst] = 28 1 1) yielding polymers with low molecular weight (M = 2.9xl0 g mol ) and a polydispersity of Mw/Mn = 1.3. This is probably due to a loss of catalytic species occurring from the competitive coordination of carboxylic acids to the copper centers, as mentioned for the case of acrylic acid. The choice of pH is important at pH <6, protonation of the bipyridyl ligand occurs, resulting in loss of control. The choice of initiator is equally important the polymerization is only controlled when the methoxy-capped macroinitiator 46 is used. 

HEMA) [212]). Monomer 42 (MFC) spontaneously polymerizes at room temperature in aqueous solution and the choice of the slowest catalyst is therefore justified. Because poly-38 [212] is not soluble in water, the polymerization is run in a 50 50 MeOH water mixture. Consequently, rates are lower than in water (95% conversion requires 3-4 h reaction time at room temperature). A comparison of polymerization rates in aqueous and non-aqueous media reveals strong solvent effects. Polar solvents have been found to increase the polymerization rate, possibly because of the combined effect of an increase of rate constant [213] and a competitive coordination of the solvent and the ligand in the copper species [214]. 

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