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Associative thickeners, such as hydrophobically modified ethoxylated urethanes (HEURs), are an important class of rheological modifiers allowing precise control and optimization of the rheology of waterborne coatings. In this work, we present a novel, comprehensive investigation of one-step and two-step HEUR synthesis processes, highlighting their impact on the final HEUR properties. In the conventional one-step process (current industrial practice), there are inherent limitations in producing high molecular weight polymers due to the complex competition between end-capping and polymerization. We show that the two-step method allows for much higher molecular weight polymers than the one-step method while using less amounts of toxic diisocyanates. Additionally, using the two-step method, the polymerization can be simply and efficiently controlled by the addition timepoint of the end-capping agent, which …