Ring-Opening Polymerization of Tri-Substituted Six-Membered Lactone Derived from CO2/Butadiene

The selective ring-opening homo-polymerization of CO2/butadiene-derived lactone monomers has emerged as an appealing approach for synthesizing chemically recyclable polyesters from CO2. Previous research has only concentrated on di-substituted six-membered lactones. In this study, a newly designed tri-substituted six-membered CO2/butadiene-derived lactone monomer, 3,3,6-triethyltetrahydro-2H-pyran-2-one (Et-HL), was polymerized successfully through selective ring-opening polymerization (ROP) using NaOMe, tBu-P4/BnOH, or tBu-P4. The tBu-P4/BnOH affords linear-poly(Et-HL) with typical living polymerization behaviors, while a maximum number-average molecular weight (Mn) of 1050 kg mol-1 and a dispersity (Đ) of 1.52 was achieved for cyclic-poly(Et-HL) using only tBu-P4. Catalytic methods were developed for monomer recycling of both linear- and cyclic-poly(Et-HL). Direct observation of key intermediates by Nuclear Magnetic Resonance (NMR) reveals the mechanistic differences between Et-HL and HL. A tail-to-head strain-releasing mechanism was proposed to rationalize the selective formation of cyclic polymers for both HL and Et-HL using only tBu-P4. In the case of linear polymers, HL and Et-HL share a similar mechanism involving initiator anion attacking the monomer ester bond. This work represents the first example for the ROP of six-membered lactones bearing more than 2 substituents, simultaneously offering fundamental understanding for the Thorpe-Ingold effect on the ROP of CO2/butadiene-derived six-membered lactones for the first time.