A supramolecular polymer system is presented in which relatively short trimethylene carbonate (TMC) prepolymers are linked via reversible quadruple hydrogen bonding ureidopyrimidinone (UPy) moieties. Several UPy-modified bifunctional and trifunctional TMC polymers were synthesized. Tunability of the mechanical and thermal properties was achieved by mixing different trifunctional UPy−TMC with bifunctional UPy−TMC polymers. The concentration of UPy groups influenced the crystallization behavior of the materials. The UPy-modified TMC polymers displayed melting endotherms, which are caused by aggregation of the UPy units into small crystallites. It is assumed that the UPy dimers stack in the lateral direction due to urethane hydrogen bonding. Atomic force microscopy confirmed the presence of fiberlike stacks. These crystallites give the material its mechanical strength and prevent flowing of the material after processing into 3D scaffolds. More importantly, these polymers can be processed quite easily at slightly elevated temperatures. Because of the strong temperature dependence of the reversible nature of the hydrogen bonds, their melt viscosities are low, while at temperatures below 50 °C excellent mechanical properties are found as a result of supramolecular, physical cross-linking. The UPy−TMC polymers are shown to be biocompatible and fibroblasts proliferate well on drop cast films of UPy−TMC. Thus, these novel supramolecular UPy−TMC polymers are very promising, biocompatible, tunable, and easily processable biomaterials for applications such as tissue engineering.