Poly [(epsilon-caprolactam)-co-(epsilon-laurolactam)] was prepared at two different experimental arrangements - pseudoadiabatically and isothermally. Polymerization activity of two initiators c.-caprolactam magnesium bromide (CLMgBr) or sodium salt of F-caprolactam (CLNa) in combination with N-benzoyl-epsilon-caprolactam (BzCL) was compared. The copolymerizations were carried out in the whole concentration scale of both monomers and in the temperature range from 120 to 240 degrees C. Prepared materials were evaluated by means of polymer yield, DSC, DMA and WAXS. The results have shown fundamental differences between both initiators. Copolymers prepared by initiation with CLNa have random character, one melting endotherm and display one crystalline form opposite copolymers prepared by initiation with CLMgBr having heterogeneous character proved especially by two melting endotherms (similar to 140 and similar to 210 degrees C) and two types of crystalline form (alpha and gamma). (c) 2005 Elsevier Ltd. All rights reserved.