Polyesters are of increasing importance as they can function as substitutes for other bulk -and commodity polymers, due to their high potential with respect to chemical recycling and biodegradability, but methodologies to achieve their recycling, degradation and valorization, as well as the optimization of their properties, still require significant improvement. Important milestones have been the introduction of covalent adaptive networks (CANs) and so-called vitrimers, which rely on a catalytic transesterification reaction at elevated temperature, as well as the discovery that aliphatic longchain polyester, which can be produced from renewable feedstocks and recycled by transesterification, exhibit properties similar to high-density polyethylene (HDPE). In this context, the modification of polymers exhibits a great potential and can produce unusual properties, such as enormous electro- and barocaloric effects, but methods for the post synthetic modification of polyesters, yielding functionalized polymers with tuned properties, are often limited to blending, curing, grafting or derivatization with reagents, whereas the modification of the main chain or back bone often involves specifically designed polymers. In particular for statistic co-polyesters, formed by random incorporation of a second minor monomer in polyesters, it has been demonstrated that biodegradation can be facilitated, while important properties of the original polyester are maintained. In the current manuscript, we report a novel type of transesterification reaction for polyesters that relies on a hydrogen transfer ester metathesis (HTEM) sequence (Fig. ), which represents the first example of a catalytic hydrogen borrowing reaction with polymers as substrates. The combination of the latter with a concurrent catalytic transesterification allows for a dual catalytic system, which gives rise to the efficient main chain modification and conversion of a homo-polyester into block -or randomly sequenced co-polyester in the presence of a suitable cooperative catalyst-system and in the absence of any reagent. It is demonstrated that with formation of constitutional isomers, the properties of the utilized polyester can be modified in an atom-economic reaction. Figure . Previously reported dehydrogenative polymer synthesis and hydrogenative depolymerization (A) and the novel introduced hydrogen borrowing reaction in this work (B). Concept of the hydrogen transfer ester metathesis of polyesters using the example of polycaprolactone (C) as well as potential pre-catalysts (D).