Substituent effect on structure and physical properties of semicrystalline Diels-Alder network polymers

Semicrystalline cross-linked polymers were synthesized using the Diels-Alder (DA) reaction between furyl-telechelic poly(ε-caprolactone) prepolymers (PCLF2-x. x (substituent in the vicinity of furan) = A (amide), E (ester) and U (urethane)) and a tris-maleimide linker (M3). The equilibrium of the thermoreversible DA reaction shifted by changing the substituent, that is, the dissociation of PCLF2-x and M3 during the retro-DA reaction was significant in the order of PCLF2-E-M3 > PCLF2-U-M3 > PCLF2-A-M3, while the association during the DA reaction was effective in the reversed order. This can be attributed not only to the difference in electron density in furan rings (PCLF2-A ≈ PCLF2-U > PCLF2-E) but also to the presence of hydrogen-bonding between CO in maleimide and N-H in the amide or urethane bond in the vicinity of furan and its strength (PCLF2-A-M3 > PCLF2-U-M3) and the absence for PCLF2-E-M3. The shift of reaction equilibrium resulted in effective modulation of structures and physical properties of crystallization-first (CRY1st) and cross-linking-first (CL1st) network polymers.