Propylene-butene-1 copolymer (PBC) prepared with a titanium catalyst system was fractionated by 5-80 degrees C. PBC was shown to have a wide composition distribution ranging between 12-47 mol % of butene-1. Most of the fractions showed nearly the same crystallinity regardless of the butene-l content. However, the solubility of the crystalline parts, which turned out to be the driving force for the fractionation, varied over the butene-1 content. Evidence that PBC shows isomorphism was obtained by the analysis of fractionated polymers, using C-13 nuclear magnetic resonance spectrometry, X-ray diffraction, differential scanning calorimetry, and gel permeation chromatography.