Hydrated sodium montmorillonite (Na-clay) has been used as a catalyst support for the heterogeneous atom transfer radical polymerization of benzyl methacrylate in the presence of various concentrations of water, reducing agent, and CuBr(2) in anisole at ambient temperature. The polymerization was promoted via reduction of Cu(II) to Cu(I) through the addition of sodium ascorbate (NaAsc) as a reducing agent in aqueous solution. The polymerizaton proceeded in a controlled manner and produced poly(benzyl methacylate) with moderately narrow molecular weight distribution (MWD) when performed under optimum conditions of hydration (10 wt % <= H(2)O/Na-clay <= 21 wt %) and reducing agent (0.15 <= [ NaAsc]/[ I] <= 0.23). The polymerization was uncontrolled if hydration and NaAsc exceed above their optimum range of concentrations. Apparent rate of the polymerization (k(app)) increased in the presence of decane-anisole (1/3, v/v) mixture solvent. Selective adsorption of decane at the interfaces of the hydrated clay was attributed for the rate enhancement due to increased polymer and hydrophobic interface interaction. The polymerization progressed in a controlled manner as confirmed by the first-order time-conversion plot, linear increase in molecular weights, and moderately narrow MWDs over conversion. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5049-5056, 2011