A sample of isotactic poly-1-pentene has been synthesized by using a metallocene catalyst, and two specimens have been prepared displaying the most stable Form I of this polymer, reported to adopt a 31 helical conformation. The solid-state (CNMR)-C-13 study of this helical form shows an exceptional splitting of the resonances, indicative of an important magnetic inequivalence in the unit cell. The maximum separation (amounting to 4.7 ppm) is obtained for carbon 2B: the methylene next to the terminal methyl in the lateral branch. This exceptional splitting is interpreted as originating from either intermolecular (packing) or intramolecular (conformational) differences (or both), leading to well non-equivalent sites for all the carbons. The comparative analysis of the relaxation times in the rotating frame between isotactic poly-1-pentene and isotactic polypropylene indicates that the two polymers display a rather similar relaxation behavior for both the crystalline and amorphous components. (C) 2016 Elsevier Ltd. All rights reserved.