A novel and powerful method of microstructure control in polyolefins is presented. By dispersing tiny amounts of sorbitol derivatives in the polymer, the subsequent crystal growth can be controlled with modest prior flow-fields; in particular high levels of preferred orientation are generated in the resultant crystals. Under shear flow, the additive forms highly extended crystals which lie parallel to the flow-field and which serve to direct the crystallisation of the polymer leading to a massive amplification of the anisotropy present in the melt.