Monomer-Recyclable Polyesters From Ring-Opening Polymeri-zation of a CO2/Butadiene-derived Tri-Substituted Six-Membered Lactone

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 lac-tone 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. Similar thermal properties were found between poly(Et-HL) and polyHL derived from Et-HL’s di-substituted precursor, 3,6-diethyltetrahydro-2H-pyran-2-one (HL). The ceiling temperature (Tc) was calculated to be 265 K (-8℃) at [Et-HL]0 = 1.0 mol L-1 in THF for poly(Et-HL), which is only slightly lower than HL (Tc = -6℃, [HL]0 = 1.0 mol L-1). 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. tBu-P4 efficiently promotes the deprotonation of the α-hydrogen adjacent to the carbonyl in HL, presumably yielding an enolate as the nucleophile responsible for the initial ring opening in the for-mation of cyclic-polyHL. However, Et-HL's additional substituent prohibits this process, instead favoring direct attack on the ester bond of the monomer by tBu-P4 in the initial ring opening to afford cyclic-poly(Et-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. The topological shift from cyclic to linear for poly(Et-HL) is more sensitive to initiator: tBu-P4 ratio (1:1 vs polyHL's 10:1), reflecting the impact of the additional substituent in Et-HL. This work represents the first example for the ROP of six-membered lactones bearing more than 2 substituents, simultaneously offering fundamental understand-ing for the Thorpe-Ingold effect on the ROP of CO2/butadiene-derived six-membered lactones for the first time.