Aldehyde-telechelic polyisobutylenes free of other end groups such as alcohols, olefins, etc., are made by the hydroformylation of polyisobutylene having one or more olefinic end groups in the presence of a rhodium catalyst in combination with an organic phosphorus ligand. Very high conversions of olefin to aldehyde are obtained. Depending upon the number of aldehyde end groups per macromolecule, the telechelic polyisobutylene polymers can be utilized as precursors to form various types of block copolymers, e.g., AB, ABA, or radial block copolymers, with other polymers such as polyurethanes, polyesters, polyamines, and the like.