Physically Cross-Linked Polybutadiene by Quadruple Hydrogen Bonding through Side-Chain Incorporation of Ureidopyrimidinone with Branched Alkyl Side Chains

The synthesis of a supramolecular polymer network based on ureidopyrimidinone (UPy) is demonstrated starting from a low-molecular weight polybutadiene (PB) polymer, which was modified with UPy quadruple hydrogen-bonding motifs through UV-initiated thiol–ene chemistry under ambient conditions. Here, the UPy units contain a branched alkyl side chain rather than a methyl group, as is the case in most literature examples. The sterically demanding aliphatic 2-ethylpentyl side chain was introduced to prevent stacking of the UPy dimers and to enhance the compatibility of the UPy unit with the apolar PB polymer matrix. The UPy units were grafted onto PB with different functionalization degrees yielding materials with a relatively broad range of glass transition temperatures (Tg) and material properties, as evaluated by thermogravimetric analysis, differential scanning calorimetry (DSC), and dynamic mechanical thermoanalysis. Importantly, only the system with 13 mol % UPy functionalization showed some macroscopic phase separation as indicated by its partial opaque appearance, even though this was not detected by DSC. Previous reports on PB end functionalized with methyl-functional UPy revealed phase separation through DSC, indicating that the branched side chains indeed suppress the phase separation. The network dynamics were assessed by rheological measurements at different temperatures, which were subsequently fitted to the sticky Rouse model and creep experiments. We found that the sticky Rouse model satisfactorily fits the linear response of the systems in the terminal regime. However, significant discrepancy from theory still persists in the rubbery plateau regime. This deviation is attributed to the presence of structural defects in the systems. Altogether, this work demonstrates the importance of designing the supramolecular physical cross-linker unit with a branched aliphatic side chain to enhance the compatibility with the polymer matrix for the formation of supramolecular polymer networks.