Chirality constraints in crystal-crystal transformations: Isotactic poly(1-butene) versus syndiotactic polypropylene

Solid-state crystal-crystal transformations in polymers are limited by conf ormational and steric constraints when helical hands are involved in the in itial, final, or both stages of the transformation. The constraints are ill ustrated by (a) the phase II to phase I transformation in isotactic poly(l- butene), which preserves the chirality of the helices, and (b) the generati on of unusual, chiral crystal phases of syndiotactic polypropylene when sta rting from an extended-chain crystal phase, whereas the stable structure is fully antichiral. In the latter case, the isochirality of the resulting he lical phase indicates that the transformation is a cooperative process.