Ring-opening bulk polymerization of L-lactide using N-2- hydroxyethylmaleimide (HEMI) as the initiator and tin(II) 2-ethylhexanoate as the catalyst produced a reactive end-functionalized poly(L-lactide) (HEMI-PLLA). Melt blends of HEMI-PLLA and conjugated soybean oil (CS) were prepared. HEMI-PLLA underwent a Diels-Alder reaction with the CS to high conversion, coupling the two immiscible components. Up to three HEMI-PLLA molecules reacted with one CS molecule to create products with varying architecture that acted as compatibilizers for the melt blend. Blends of HEMI-PLLA. and 5 wt % CS resulted in a greater than 17-fold increase in elongation to break compared to PLLA homopolymer and more than doubled the elongation to break compared to a 5 wt % CS blend with unreactive PLLA. Analysis of the blend morphology indicated that the in situ formation of the compatibilizer decreased the CS droplet diameter compared to unreactive binary blends and that an optimum droplet diameter exists for toughening PLLA with CS.