Polybutyrolactone tri- and diblock copolymers with welldefined structures and narrow molar distributions were prepared by trifluoromethanesulfonic acid-organocatalyzed ring-opening polymerization of β-butyrolactone initiated with dihydroxylated poly(hydrogenated butadiene) and hydroxylated polystyrene. Study of the phase separation behavior of these block copolymers in the bulk and in thin film shows their ability to segregate even for low molecular weights, giving rise to spherical, cylindrical, and lamellar morphologies with periodicities in the range 10−20 nm. The Flory−Huggins interaction parameters estimated from the order-to-disorder transition temperatures are in the same range or higher than those of other block copolymers associating biodegradable and polyolefin blocks.