For the solid-solid transformation from form Ⅱ to form Ⅰ of isotactic polybutene-1 (iPB),the temperature dependence of form Ⅰ nucleation and growth was deemed to control the transformation process.However,the relationship between form Ⅰ formation and form Ⅱ disappearance in the transformation process is not clear.In this work,the spontaneous crystal transformation from form Ⅱ to Ⅰ of iPB with 81 mol%mmmm sequence concentration is studied firstly by tracking the two processes,the decay of form Ⅱ and the yielding of form Ⅰ in a wide range of temperature spanning from 0 ℃ to 50 ℃ and in a long transformation time ranging from 5 min to 65 days with in situ FTIR and WAXD.Unlike the literature reports,the decay rate of form Ⅱ is firstly found to be lower than the yielding rate of form Ⅰ at all studied temperatures,especially at low transition temperature.This is attributed to the amorphous chains which locate near crystal lamella participating into the nucleation of form Ⅱ.The regular chain folding and growth of iPB form Ⅰ from amorphous chains containing short isotactic sequences also lead to an increase in crystallinity of form Ⅰ compared with that of initial form Ⅱ crystallized at 60 ℃.An increase in the annealing temperature results in decrease in crystallinity and increase in lamellae thickness of iPB form Ⅰ.