Ring-opening polymerization of a 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 CO₂-derived 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. ^tBu-P₄/BnOH affords linear-poly(Et-HL) with typical living polymerization behaviors, while a maximum number-average molecular weight (M_n) of 1050 kg mol⁻¹ and a dispersity (Đ) of 1.52 were 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 DEtP. A tail-to-head strain-releasing mechanism was proposed to rationalize the selective formation of cyclic polymers for both DEtP and Et-HL using only ^tBu-P₄. In the case of linear polymers, DEtP and Et-HL share a similar mechanism involving the initiator anion attacking the monomer ester bond. This work represents the first example of the ROP of six-membered lactones bearing more than 2 substituents, simultaneously offering a fundamental understanding of the Thorpe–Ingold effect on the ROP of CO₂/butadiene-derived six-membered lactones for the first time.