A process is disclosed whereby a polymeric resin (containing ester and/or carbonate bonds) is readily converted into a resin having a modified molecular structure. Accordingly, a cyclic carbonate (a monomer or oligomer) is transesterified with the polymer resin in the melt, preferably upon extrusion, optionally in the presence of a catalyst. Significantly, the cyclic carbonate which may include any of a variety of functional groups may, by the inventive process, be inserted into the structure of the polymer, effecting a modification to the structure and the properties of the resin. Among the disclosed beneficial modifications thus imparted to the resin are improved thermal stability, altered rheology and optical properties.