Site‐Selective Dynamic Covalent Exchange of Acyl Hydrazone Moieties in Copolymers: Substituent Effects and Thermal Stability

ABSTRACT

A dynamic covalent exchange reaction of acyl hydrazone moieties provides a versatile platform for post‐polymerization modification and compositional control in polymer systems. Here, we report the site‐selective exchange behavior of a copolymer bearing electron‐rich 4‐(dimethylamino)phenyl (DMA) and electron‐deficient 4‐cyanophenyl (CN) units (polyDMA‐ co ‐CN). Systematic investigations with para ‐substituted benzaldehydes revealed that DMA units underwent selective exchange, whereas CN units remained largely inert. Aldehydes with electron‐withdrawing groups (H and Cl) exhibited higher exchange efficiencies than those with electron‐donating groups (MeO). Density functional theory calculations supported the observed substituent effects. The higher electron density of the DMA imine nitrogen enhances its susceptibility to protonation and hydrolysis, while the lower electron density at the carbonyl carbon of the electron‐withdrawing aldehyde facilitates nucleophilic attack. Reaction temperatures and reagent ratios were exploited to achieve site‐selective exchange, enabling the programmable synthesis of three‐component copolymers with approximately equal amounts of BA, DMA, and CN units or MeO, DMA, and CN units [poly(BA ₂₂ ‐ co ‐MeO ₂₇ ‐ co ‐CN ₅₁ ) and poly(MeO ₂₃ ‐ co ‐DMA ₂₅ ‐ co ‐CN ₅₂ )]. Thermogravimetric analysis revealed a stability order of poly(DMA ₅₀ ‐ co ‐CN ₅₀ ) > poly(MeO ₅₁ ‐ co ‐DMA ₁ ‐ co ‐CN ₄₈ ) > poly(BA ₅₂ ‐ co ‐DMA ₁ ‐ co ‐CN ₄₇ ), consistent with the calculated energies for the C–C bonds between the imine carbon and aryl ring. These findings provide molecular‐level insights into substituent‐controlled reactivity and stability, establishing new guidelines for the design of dynamic covalent polymers with programmable composition and enhanced thermal robustness.