The synthesis of a linear polymer having a tertiary amino group in a part of the main chain has been attempted by several methods. However, it was difficult for the linear polyamine to form because of the complicated reaction of the amino group. In the polymerization of weakly basic cyclic amines, e.g., 1-cyanoethylazetidine or 1-cyanoethylaziridine, high molecular weight polymers, poly-1-cyanoethyltrimethyleneimine (poly-CET) and poly-1-cyanoethyl-ethyleneimine (poly-CEE), were obtained. An attempt was made to convert poly-CET and poly-CEE into poly(l-benzylalkyleneimine)s (poly-BET and poly-BEE). Linear poly-BET was prepared by the alkylation of poly-CET (MW about 6000) with benzylbromide, followed by the elimination of the cyanoethyl group of polymer by heating. The poly-BET obtained was a white, greasy substance with a molecular weight of about 5000; its structure was determined by NMR spectroscopy. The cyanoethyl groups were replaced completely by benzyl groups. However, poly-BEE with a complete exchange to benzyl group was not obtained from poly-CEE due to the remaining cyanoethyl groups. Residual cyanoethyl groups replaced about 40% of the amino groups in poly-CEE.