Ring-opening (co)polymerization of six-membered substituted δ-valerolactones with alkali metal alkoxides

The ring-opening polymerization (ROP) of bio-derived six-membered (substituted) δ-valerolactones, including the 5-Me substituted δ-valerolactone (aka δ-hexalactone (HL)), 2-ethylidene-6-hepten-5-olide (EVL), 2-ethylheptane-5-olide (EHO) and the novel 2-ethylidene-6-heptan-5-olide (MH), is investigated. In comparison to the ubiquitous unsubstituted δ-valerolactone (VL), the presence of a substituent on the lactone ring appears to significantly affect the polymerizability of the monomer, whichever the catalyst/initiator system or the operating conditions. Typical Brönsted acids, organocatalysts or Lewis acidic metal complexes revealed hardly active in the ROP of HL, most likely originating from polymerization/depolymerization issues. Better efficiency was achieved from alkali metal complexes, especially using NaOMe (1–2.5 mol%) from which high-to-quantitative HL conversion was reached within 18 h at 60 °C. Oligomers (M̅n,NMR ≤ 3800 g mol−1, ÐM = 1.22–1.36) were thus synthesized from ROP, as supported by NMR spectroscopy, SEC and MALDI-ToF mass spectrometry analyses. P(HL-co-VL) random copolymers incorporating up to 44 mol% of HL into PVL were next synthesized from the simultaneous HL/VL copolymerization mediated by NaOMe (M̅n,NMR up to 9700 g mol−1, ÐM = 1.21–1.40). The ROP of the sustainable CO2/butadiene-derived EVL, EHO or MH –the original semi-hydrogenated parent lactone–, remained unsuccessful, regardless of the catalytic system.