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-2<i>H</i>-pyran-2-one (<b>DPO</b>), into polymerizable bicyclic lactones via photochemical [2 + 2] cycloaddition. Two bicyclic lactones, <b>M1</b> and <b>M2</b>, were obtained by the photochemical [2 + 2] cycloaddition of tetramethylethylene and <b>DPO</b>. Ring-opening polymerization (ROP) of <b>M1</b> and <b>M2</b> catalyzed by diphenyl phosphate (DPP), La[N(SiMe₃)₂]₃, and 1-<i>tert</i>-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris (dimethylamino) phosphoranylide-namino]-2λ⁵, 4λ⁵-catenadi(phosphazene) (^<i>t</i>Bu-P₄) were conducted. <b>M1</b> is highly polymerizable, either DPP or La[N(SiMe₃)₂]₃ could catalyze its living ROP under mild conditions, affording the well-defined <b>PM1</b> with a predictable molar mass and low dispersity. <b>M2</b> could only be polymerized with ^<i>t</i>Bu-P₄ as the catalyst, also generating the same polymer <b>PM1</b>. <b>PM1</b> has high thermal stability, with a <i>T</i>_d,5% being up to 376 °C. Ring-opening copolymerization (ROcP) of <b>M1</b> and δ-valerolactone (δ-VL) catalyzed by La[N(SiMe₃)₂]₃ afforded a series of random copolymers with enhanced thermal stabilities. Both <b>PM1</b> and the copolymer containing 10 mol % <b>M1</b> 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.