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