Highly efficient recyclable hydrated-clay supported catalytic system for atom transfer radical polymerization w

However, the use of Cu halide based homogeneous catalyst systems in high concentrations severely limits the viability of ATRP for commercial applications. The removal of catalyst from the polymers prepared using ATRP generally requires uneconomical techniques such as passing the final polymer solution through adsorbent, repeated solvent washing, extraction and selective precipitation. Recently, new initiating systems such as activator generated or regenerated electron transfer ATRP (AGET or ARGET ATRP, respectively) have been developed which require the use of a reduced concentration of catalyst. Still, the concentration of catalyst is not sufficiently low enough to directly use the polymers for commercial applications. The strategies which have been employed with limited success for the removal of catalyst include the use of solid supported catalyst systems based on silica, ion-exchange resins, cross-linked beads, reversible supported and hybrid catalysts, and selective soluble biphasic and polymer anchored catalyst systems. The major problem associated with covalent and physisorption strategies for catalyst attachment is that they limit the mobility of the catalyst thereby making its accessibility to the fast diffusing radicals in the solution extremely difficult. Thus, almost all the catalyst immobilization strategies produce polymers with broad MWD due to uncontrolled chain growth and termination except the ones that leach catalyst into the solution. An ideal solid supported catalyst system would be the one that has characteristics of both heterogeneous catalysis for easy recovery/reuse and homogeneous catalysis for effective control of the polymerization. We have recently reported the use of hydrated clay as a means to efficiently remove the catalyst from ATRP solution. In this study we report the development of a highly efficient and novel supported catalyst system using hydrated sodium montmorillonite (Naclay) as a catalyst support for AGET ATRP of benzyl methacrylate (BnMA) and methyl methacrylate (MMA) in anisole in the presence of sodium ascorbate (NaAsc) as a reducing agent (Scheme 1). The affinity of water for the clay as well as for the catalyst complex is exploited to contain the catalyst at the surface. The hydrated Na-clay supported catalyst was recycled 21 times in an efficacious manner for the polymerization of BnMA with no or negligible loss in catalytic activity of the recovered supported catalyst. The supported catalyst was prepared by mixing equimolar amounts of CuBr2 and N,N,N 0,N00,N00-pentamethyldiethylenetriamine (L) with Na-clay (Cu(II)/clay = 10 wt%) in methanol at 30 1C and was stirred for 30 min. Thereafter, the solvent was removed and the resulting blue colored CuBr2–L loaded Na-clay was dried under vacuum for another 30 min at 30 1C. The supported catalyst was analyzed using X-ray diffraction (XRD) which showed expanded intercalary layers of 2.13 Å, confirming the presence of intercalated CuBr2–L complex (Fig. 2a, see ESIw). The AGET ATRP of BnMA was carried out under a N2 atmosphere using Na-clay catalyst in anisole at 30 1C. An aqueous solution of NaAsc was added dropwise in order to generate the active catalyst species, CuBr–L for the polymerization. The heterogeneous solution was stirred for an hour and the blue color of the Na-clay catalyst turned to green indicating the generation of Cu via reduction of Cu. Then, the required amount of initiator, ethyl bromoisobutyrate (I) was added and the polymerization was allowed to go for few hours (Table 1). Interestingly, the supernatant remained colorless throughout the polymerization suggesting there was no leaching of the catalyst complex into the organic phase. The reaction was terminated and stirred under air for Scheme 1 Hydrated Na-clay supported catalytic system for reversible and recyclable atom transfer radical polymerization in anisole.