Photochemical [2 + 2] Cycloaddition Enables the Synthesis of Highly Thermally Stable and Acid/Base-Resistant Polyesters from a Nonpolymerizable α,β-Conjugated Valerolactone

We report a simple strategy to transform a nonpolymerizable six-membered α,β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO), into polymerizable bicyclic lactones via photochemical [2 + 2] cycloaddition. Two bicyclic lactones, M1 and M2, were obtained by the photochemical [2 + 2] cycloaddition of tetramethylethylene and DPO. Ring-opening polymerization (ROP) of M1 and M2 catalyzed by diphenyl phosphate (DPP), La[N(SiMe3)2]3, and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris (dimethylamino) phosphoranylide-namino]-2λ5, 4λ5-catenadi(phosphazene) (tBu-P4) were conducted. M1 is highly polymerizable, either DPP or La[N(SiMe3)2]3 could catalyze its living ROP under mild conditions, affording the well-defined PM1 with a predictable molar mass and low dispersity. M2 could only be polymerized with tBu-P4 as the catalyst, also generating the same polymer PM1. PM1 has high thermal stability, with a Td,5% being up to 376 °C. Ring-opening copolymerization (ROcP) of M1 and δ-valerolactone (δ-VL) catalyzed by La[N(SiMe3)2]3 afforded a series of random copolymers with enhanced thermal stabilities. Both PM1 and the copolymer containing 10 mol % M1 exhibited excellent resistance to acidic and basic hydrolysis. Our results demonstrate that direct photochemical [2 + 2] cycloaddition of α,β-conjugated valerolactone is not only a strategy to tune its polymerizability, but also allows for the synthesis of highly thermally stable aliphatic polyesters, inaccessible by other methods.

A Versatile Step-Growth Polymerization Route to Functional Polyesters from an Activated Diester Monomer

This work describes a versatile and efficient condensation polymerization route to aliphatic polyesters by organo-catalyzed (4-dimethylaminopyridine) transesterification reactions between an activated pentafluorophenyl-diester of adipic acid and structurally different diols. By introducing "monofunctional impurity" or "stoichiometric imbalance," this methodology can afford well-defined end-functionalized polyesters with predictable molecular weights and narrow dispersity under mild conditions without any necessity for the removal of the byproducts to accelerate the polymerization reaction, which remains a major challenge in conventional polyester synthesis with non-activated diesters. Wide substrate scope with structurally different monomers and the synthesis of block copolymers by chain extension following either ring-opening polymerization or controlled radical polymerization have been successfully demonstrated. Some of the polyesters synthesized by this newly introduced approach show high thermal stability, crystallinity, and enzymatic degradation in aqueous environments.

Tunable and recyclable polyesters from CO2 and butadiene

Carbon dioxide is inexpensive and abundant, and its prevalence as waste makes it attractive as a sustainable chemical feedstock. Although there are examples of copolymerizations of CO₂ with high-energy monomers, the direct copolymerization of CO₂ with olefins has not been reported. Here an alternative route to functionalizable, recyclable polyesters derived from CO₂, butadiene and hydrogen via an intermediary lactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one, is described. Catalytic ring-opening polymerization of the lactone by 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields polyesters with molar masses up to 13.6 kg mol^-1 and pendent vinyl side chains that can undergo post-polymerization functionalization. The polymer has a low ceiling temperature of 138 °C, allowing for facile chemical recycling, and is inherently biodegradable under aerobic aqueous conditions (OECD-301B protocol). These results show that a well-defined polyester can be derived from CO₂, olefins and hydrogen, expanding access to new polymer feedstocks that were once considered unfeasible.