Free radical polymerization of methyl methacrylate (MMA) within the uniform channels of the mesoporous zeolite MCM-41 proceeds at 100 degrees C to give a high molecular weight polymer (PMMA). The formation of long living propagating polymer-radicals is observed by electron paramagnetic resonance (EPR). The molecular weight of PMMA within the mesopores can be controlled over a wide range by changing the monomer-to-initiator mole ratio.