Poly(lactide) (PLA) and its copolymers can be made from naturally occurring materials. They are ideal candidates as sustainable materials to replace the petroleum based polymers. However, these polymers often are brittle so toughening is needed. One of the most effective toughening methods is to reactive blend brittle polymers with materials having very different rigidity. This approach requires that the two materials chemically bond with each other at the interface to compatibilize the blends. Thus, to design and make reactive systems is a key to the success of reactive blending. In this article, we studied toughening poly(lactide-co-glycolide) (PLGA) with a rubbery material poly(trimethylene carbonate) (PTMC). We observed that PTMC spontaneously reacted with PLGA during melt blending. The reaction produced PLGA-co-PTMC copolymers that stayed at the PLGA/PTMC interface. Those copolymers not only helped to create a stable blend microstructure at a length scale of 100 rim but also promoted the bonding between the PLGA and PTMC domains. It is interesting that the reaction did not need a catalyst or initiator. We speculated that this reaction between PLGA and PTMC was a transesterification reaction. This reaction is easy to achieve and is expected to broaden the property range of the PLGA and other degradable polyesters, enabling them to replace certain types of petroleum based polymers. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2153-2158, 2010