The influence of the comonomer in the synthesis of poly(propylene-co-1-pentene) copolymers, with rac-(dimethylsilylbis(1-indenyl))ZrCl2/Methylaluminoxane at low temperature, has been studied. Changes in the catalyst activity and molecular weight have been analyzed as a function of copolymer composition, and associated with both content and nature of chain defects. The thorough characterization of chain-end double bonds by means of the H-1 NMR technique highlights the particular chain termination pathway, which underlies the so-called comonomer effect. A specific termination mechanism is proposed based on the preferential regio-irregular interaction of the active site with 1-pentene molecules, instead of the one related to the beta-H atom of the last regular inserted unit, either propene or 1-pentene. (C) 2016 Wiley Periodicals, Inc.