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Monomer composition, cross-link density, and cross-link distribution are the primary determinants of material properties in thermosetting networks. Here, we investigate the effect of regioisomerism and composition in polymeric networks via a recently developed scalable sequence-defined polyurethane macromer (SD-PUM) platform. The iterative, support-free SD-PUM platform facilitates the assembly of macromers with tunable backbones and pendant groups at the gram-scale. A series of SD-PUMs made with different end-groups and vanillin positional isomers were synthesized and cross-linked into thermosetting networks via multivalent thiol cross-linkers. The results show that the thermomechanical properties of the ensuing networks are sensitive to the choice of regioisomer embedded in the SD-PUM backbone as well as the type of macromer end-group. When a cross-linker with higher functionality is employed, regioisomeric differences result in more pronounced changes in material properties. This work showcases the robustness of the SD-PUM platform and its use of backbone and end-group regioisomers to modulate material properties.