This study aims to assess the relative influence of microstructure, molar mass and morphology on the local a relaxation of isotactic polypropylene (iPP). For that purpose, three families of samples have been prepared. The Ziegler-Natta type provides a wide molar mass range where both the isotactic average length (n(1)) and the nature of the isotacticity interruptions change according to chain size. The metallocene type allows to analyse also the samples in which molar mass and n(1) are related but the nature of the defects stays the same. Finally, three propylene-like EP copolymers (containing only isolated ethylene units) have been synthesised via metallocene catalysis in order to study samples which combine both relatively high molar masses and short n(1). Among the factors considered, microstructure and n(1) in particular have been found to drive mainly the quality of the a relaxation. Concerning the intensity, it has been observed that some microstructural features related to n(1) must be fulfilled for this dynamics to take place. Provided this requirements are met, the intensity of the a relaxation depends on the final crystalline distribution resulting from a given molar mass, microstructure and processing conditions. (c) 2006 Elsevier Ltd. All rights reserved.