AbstractThe mechanism of two‐dimensional polymerization of octadecyl acrylate in lamellar crystal was investigated in two‐step postpolymerizations by temperature increase and stepwiseγ‐ray irradiation in lieu of the usual one‐step reaction. Two‐step postpolymerizations by these procedures are interpreted satisfactorily by the cone model, which assumes that the polymerization probability of the monomer molecules in a single layer is distributed conically around the initiation point. It was found that the propagating radicals were living, even in the saturated stage, and the effect of the polymer chains already formed on the propagating and terminating reactions was evident. Furthermore, molecular weight distributions of the resultant poly(octadecyl acrylate) measured by gel permeation chromatography (GPC) were broad. The values ofMw/Mnfor the two‐step post polymerizations were 4.71–7.03, whereas those for one‐step reaction