Supramolecular Polymer Networks Made by Solvent-Free Copolymerization of A Liquid 2-Ureido-4[1h]-pyrimidinone Methacrylamide

Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy) side chains are of considerable interest due to the stimuli-responsive nature imparted by the reversible cross-links formed by dimerized UPy groups. Previously reported UPy-containing acrylic monomers are solid and show limited miscibility with comonomers, and this has stifled their (co)polymerization in bulk. We here report the synthesis of a liquid 2-ureido-4[1H]-pyrimidinone methacrylamide (UPy-OPG-MAA), which was made by connecting the UPy motif and methacrylamide (MAA) via an amine-terminated oligo(propylene glycol) (OPG) linker. The new monomer was miscible with conventional methacrylates. This permitted the photoinitiated free-radical bulk copolymerization with hexyl methacrylate (HMA) to afford a series of copolymers (poly(UPy-OPG-MAA-co-HMA)) in which the UPy-OPG-MAA content was varied between 0 and 20 mol %. The investigation of the mechanical properties of these copolymers by dynamic mechanical analysis and adhesion tests revealed that the introduction of the UPy groups leads to an increase of the stiffness in the glassy state, the formation of a rubbery plateau above the glass transition temperature, and a significant increase of the adhesive strength. Joints bonded with poly(UPy-OPG-MAA-co-HMA) could be debonded on demand using light or heat.