The effect of random insertion of low amount of 1-butene of less than about 11 mol% into the isotactic polypropylene chain on structure formation at non-isothermal crystallization at different rate of cooling was investigated by X-ray scattering, density measurements, and atomic force and polarizing optical microscopy. Emphasis is put on the evaluation of the condition of crystallization for replacement of lamellar crystals by mesomorphic nodules on increasing the cooling rate/supercooling. In the polypropylene homopolymer, mesophase formation occurs on cooling at rates larger about 150-200 K s(-1), while in case of poly(propylene-ran-l-butene) mesophase formation is observed on cooling at a lower rate of about 100 K s(-1). It is suggested that the lowering of the critical rate of cooling for mesophase formation in poly(propylene-ran-1-butene) is due to a reduction of the maximum rate of formation of monoclinic/orthorhombic crystals at low supercooling, compared to the homopolymer. The data of the present study allowed the establishment of a non-equilibrium phase diagram which shows ranges of existence of phases as a function of the cooling rate on solidification the quiescent liquid and the concentration on 1-butene co-units. (C) 2009 Elsevier Ltd. All rights reserved.