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The influence of the comonomer in the synthesis ofpoly(propylene-co21-pentene) copolymers, with rac-(dimethyl-silylbis(1-indenyl))ZrCl2/Methylaluminoxane at low temperature, has been studied. Changes in the catalyst activity andmolecular weight have been analyzed as a function of copolymer composition, and associated with both content and natureof chain defects. The thorough characterization of chain-end double bonds by means of the 1H NMR technique highlights the particular chain termination pathway, which underlies the so-called comonomer effect. A specific termination mechanismis proposed based on the preferential regio-irregular interaction of the active site with 1-pentene molecules, instead of theone related to the b-H atom of the last regular inserted unit, either propene or 1-pentene.