Dynamic covalent chemistries (DCC) continue to exhibit great potential as mechanisms to access functional, recyclable, and reprocessable materials. This work focuses on the use of the reversible thia-Michael reaction to create a dynamic covalent network whose functionality and mechanical performance can be tuned through chemical modification of the electrophilic species. A series of ditopic electrophiles with varied electron withdrawing capabilities were synthesized through the addition of benzalcyanoesters to either side of a triethylene glycol chain. These compounds readily react with thiols under ambient conditions and the addition of equi-molar (by reactive group) quantities of a tetrathiol crosslinker, pentaerythritol tetrakis(3-mercaptopropionate) (PTMP), resulted in a highly crosslinked polymer network. Equilibrium studies performed in solution with model systems indicated that the equilibrium properties of the thia-Michael reaction follow Hammett with reference to the electron withdrawing capabilities of the para-position substituent on the benzalcyanoester. With this in mind, we sought to probe the effect of different equilibrium profiles within the solid-state dynamic polymer networks. Rheological analysis revealed trends in the dynamic behavior of the networks including the characteristic drop in modulus indicative of flow at lower temperatures with decreased electron withdrawing capabilities of the ditopic components. Furthermore, hybrid systems containing two electrophiles with different thermodynamic properties resulted in thermally responsive shape-memory films capable of reprogramming as a result of the different dynamic temperatures of the two ditopic compounds. This work demonstrates the viability of the benzalcyanoester-based thia-Michael reaction as a reversible mechanism to access robust dynamic polymer networks that can be easily tuned to exhibit desired functionality and material properties.

Reprocessable, reprogrammable shape-memory polymer networks via dynamic thia-Michael addition

255th National Meeting (2018)

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