Macromolecular Cross-Metathesis (MCM) reactions on commercial Polybutadiene (PBD) and Polyisoprene (PIP) homopolymers, in the presence of first generation Grubbs and Hoveyda-Grubbs catalysts, are reported. C-13-NMR resonances of butadiene(B)/isoprene(I) heterosequences show block-copolymer formation, with blocks longer than a hundred monomer units, even for long reaction times. DSC scans of MCM products show that, with MCM time, PBD crystallinity progressively disappears while the well separated T-g of the two homopolymers merge to an intermediate single T-g. This indicates that PBD and PIP blocks are completely miscible when their lengths are reduced to a few hundred units. C-13 NMR spectra also show that, for the considered MCM process, double bond isomerizations are definitely more frequent than heterosequence formations. (C) 2017 Elsevier Ltd. All rights reserved.