The comonomer sequence distributions of a series of ethylene-dimethylaminoethylmethacrylate copolymers (EDAM) were measured by quantitative high-resolution C-13 n.m.r. spectroscopy. It was found that both the crystallinity and the lamellar thickness of EDAM samples increase linearly with the increase of the numerical average sequence length of ethylene run. It was further demonstrated that only ethylene longer than a critical length can participate in crystallization. Such a critical length was found to be independent of the molecular weight and composition of EDAM samples under the same crystallization condition.