Morpholine-2,5-diones are increasingly attractive monomers derived from amino-acids whose copolymerization with other monomers produces interesting biodegradable materials. In this study, the rapid and controlled organocatalyzed ring-opening polymerization of 3S-(isobutyl)morpholine-2,5-dione (MD) and its copolymerization with lactide (LA) was accomplished using 1,8-diazabicyclo(5,4,0)undec-7-ene and a thiourea (TU) cocatalyst. The amount of TU used for the polymerization was found to be fundamental for achieving good control. A range of polymers with molecular weights between 8.1 and 25.2 kg mol(-1) was thus produced with narrow chain distributions (D = 1.13-1.18) in short periods (5 to 10 min). Secondly, copolymers with varying compositions (MD:LA = 25:75;50:50; 75:25) were prepared (11.2 to 12.7 kg mol(-1); D = 1.09-1.26). The kinetics of these polymerizations suggest that concurrent thioimidate and cyclic imidate mechanisms are occurring and that these are governed by the quantity of TU in respect to MD.