Various R silyl methacrylates (with R = t-butyl dimethyl (1), t-butyl diphenyl (2), and diphenyl methyl (3)) were copolymerized with methyl methacrylate (MMA) in bulk at 60 degrees C, and the reactivity ratios were determined. In all cases, the electronegativity of silicon leads to r(2) having values which were less than one. The superimposition of this effect on the steric effect produces r(1) values which are also less than one. 1 was copolymerized with methyl acrylate (MA) under the same conditions. Despite the effect of the vinyl methyl group of 1, r(1) is found to be equal to one in this case. This is confirmation of a steric effect in which both macroradical and monomer are sterically hindered. Homopolymerization reactions were carried out using methacrylates 1, 2, and 3 and triphenylsilyl methacrylate (4) in order to determine their kinetic constants, and also to examine the influence provided by the R substituents on the silicon. The following order of reactivity was revealed : 1>2>3>4. This showed that the phenyl substituent de-activated the macroradical, an observation which correlated with the relative reduction in electronic density of the monomer double bond in this series of compounds.