A study of the crystallization from the melt and of the polymorphic behavior of isotactic poly(1-butene) (1PB) prepared with metallocene catalysts is presented Samples of isotactic polybutene Of different stereoregularity, containing different concentrations of stereodefects (rr triads defects) and regiodefects (4,1 units) have been prepared with different C-2- and Cl-symmetric metallocene catalysts The concentration of del'ects and the molecular mass are controlled through a rational choice of the catalyst Structure and conditions of polymerization. Highly isotactic samples with concentration of 1r stereodefects lower than 2 mol %. crystallize from the melt in the metastable form II This is it common polymorphic behavior observed in 1PB samples prepared with Ziegler-Natta catalysts and extensively reported in the literature. More stereodefective 1PB samples, containing concentration of stereodefects higher than 2 mol %. crystallize form the melt as Mixtures of form II and form I. the fraction of crystals of form I increases with increasing concentration of defects and decreasing cooling rate from the melt, and samples with concentration of rr defects of 3-4 mol % crystallize from the melt in the pure form I at low cooling rates This is the first experimental observation of the crystallization of the stable trigonal form I of 1PB front the melt at atmospheric pressure. This result may be or interest because the crystallization of the metastable form II from the melt and the spontaneous transformation at room temperature of form II into the stable form I has been considered for long time can unavoidable problem that stands in the way of the industrial development of the Ziegler-Natta iPB