The ability of highly cross-linked thermoset materials to relax stresses by network chain segment mobility above or just below T-g is generally limited. We describe materials in which some of the cross-links have been replaced by dimers of quadruple hydrogen-bonding ureidopyrimidinone (UPy) moieties, which act as reversible cross-links. Materials based on mixtures of E-caprolactone and L-lactic acid, and containing different ratios of covalent and UPy dimer cross-links, were synthesized, and their thermal and mechanical properties were tested by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The presence of reversible cross-links results in superior relaxation of stresses even below T-g, as witnessed by the creep and stress relaxation behavior. These properties bode well for potential applications, e.g., in coatings technology.