In conclusion, we reported a catalytic reaction that allows to transfer a linear polyester such as polycaprolactone (PCL) into a novel co-polyester, consisting of an additional repeating unit without the need for any reagent. For different polyesters as starting materials, i.e. pure PCL and PHA, the reaction was found to yield polyesters of almost identical HA/CL ratios (45/55) after long reaction times, indicating the presence of a thermodynamic minimum ratio between the two polyesters. Our mechanistic investigations show that two catalytic reactions are operative under the employed conditions, a hydrogen transfer ester metathesis, resulting in truly chemically modified ester groups via hydrogen transfer reaction, and a (common) basecatalyzed transesterification. We further provide evidence that the hydrogen transfer ester metathesis (HTEM) proceeds via a reversible aldehyde formation. Notably, the crystallinity of the modified materials is only slightly reduced which suggests that mechanically attractive materials are formed. Of course, the reported polyester modification and the correspondingly reduced crystallinity are expected to have a significant impact on the physical and mechanical properties of the polyesters. Therefore, we are currently focusing our efforts on the application of the reported post-synthetic modification to tune properties and facilitate chemical recycling.