Polyolefin-containing block copolymers were catalytically synthesized using a postpolymerization modification strategy. A traditional olefin polymerization followed by a tandem hydroformylation/hydrogenation of the olefinic terminated polymer was implemented to yield a hydroxyl terminated polyolefin. The hydroxyl terminated polyolefin was then used as a macroinitiator for the ring-opening polymerization of cyclic esters to yield the corresponding diblock copolymer. This methodology was applied to an array of different polyolefins and two cyclic monomers (epsilon-caprolactone and lactide) showcasing the versatility of the protocol. Most noticeably, isotactic polypropylene was quantitatively incorporated into block copolymers. Additionally, the hydro-phobicity of polyethylene films was successfully decreased by the addition of small amounts of a block copolymer.