【Frontiers in HaiChuan Chemical Technology】Qingdao Institute of Energy Research achieves high-selective directional depolymerization of polycaprolactone into caprolactone monomers

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Recently, the Catalytic Polymerization and Engineering Research Center at Qingdao Institute of Energy achieved significant progress in the field of selective closed-loop depolymerization of polyester plastics. The research team achieved precise control over the selectivity of the depolymerization pathway using a confined (BisSalen)Al catalyst, enabling the directed depolymerization of polycaprolactone (PCL) into caprolactone monomers; the relevant findings were published in Angewandte Chemie International Edition. Controlling reaction selectivity is a common challenge in the field of catalysis, and it receives significant attention in areas such as small molecule transformation and macromolecule synthesis and transformation. The occurrence of side reactions stems from the competition of energy barriers along different reaction pathways; therefore, precisely activating these reaction pathways through catalyst design strategies is an important means to achieve precise control over reaction selectivity. In the field of polyester depolymerization, the chemical recycling of ring-opened polyesters back to cyclic ester monomers is an effective way to achieve material recycling. However, during depolymerization, in addition to cyclic monomers, cyclic oligomers (such as dimers and polymers) are also typically formed, a phenomenon that is particularly prominent in the depolymerization of PCL. In the solvolysis of PCL, the presence of a cyclic chain equilibrium and the high thermodynamic stability of cyclic dimers usually result in the extensive formation of such dimers; moreover, the high stability of these dimers makes it difficult for them to re-polymerize. Therefore, it is of great research significance to understand how the closed-loop depolymerization pathway of PCL can be regulated through the design of catalytic systems, so as to achieve its highly selective directional depolymerization into caprolactone monomers. In previous studies, the research team developed a design strategy for bidentate catalysts by introducing bidentates at the metal center of the catalyst to create a confined space, thereby synthesizing (BisSalen)Al catalysts with novel structures. This strategy forms a catalytic center similar to an enzyme active pocket, enabling it to exhibit good selectivity in various catalytic reactions. For example, this catalyst can selectively recognize and activate chiral monomers such as chiral lactide and phenylethyl lactide with high stereoselectivity, enabling the perfect asymmetric resolution and polymerization of racemic monomers to produce chiral polyesters with high stereoregularity ； At the same time, this catalyst can act with high regioselectivity at the ring-opening site of methyl glycolide (MeG) to synthesize fully alternating polylactic acid-glycolic acid copolymers (PLGA). Based on the above research foundation, the research team further applied this strategy for designing space-constrained catalysts to the solvolysis of PCL (Figure 1). Through the structural optimization of the catalyst, the (BisSalen)Al catalyst achieves a monomer selectivity of up to 99% and a yield of 93% in the depolymerization of PCL, eliminating the formation of cyclic dimer by-products; its performance is significantly superior to that of conventional catalysts for polyester depolymerization. To gain a deeper understanding of the high selectivity mechanism, researchers conducted detailed control experiments and DFT calculations. Studies have shown that controlled chain-end backbiting is the primary mechanism in the depolymerization process, and further revealed a significant energy barrier difference between catalyst-catalyzed carbonyl coordination and ring-closing backbiting in the formation pathways of caprolactone monomers and dimers (Figure 2). Furthermore, the recovered caprolactone monomer can be repolymerized to complete a closed-loop cycle. This study reveals the correlation between catalyst structure and depolymerization selectivity, providing theoretical guidance for the design of catalysts for directed depolymerization.