Tunable and Predictable Bimodal Metathesis Polymers Enabled by Fusion Catalysis-Controlled Polymerization of Norbornene.

The diversity of molecular weight distribution (MWD) is a cornerstone in materials property control within the polymer community. Modulating the shape of the MWD profile through an established polymerization process is challenging, as it requires overcoming inherent kinetic limitations. In this work, we report a dual-catalysis strategy to construct a tunable bimodal distribution via one-pot ring-opening metathesis polymerization (ROMP) of norbornene. A composite catalytic system, comprising two metathesis catalysts with distinct reactivities, is designed, and the catalysts interact cooperatively to mediate chain growth rather than proceeding in an isolated manner. Three-fold regulation of peak spacing (23.8-170.0 kg/mol), relative peak height (0.2-1.0), and molecular weight span is readily achieved by adjusting the catalyst composition ratio, polymerization time, and total catalyst loading. The rheological measurements of representative unimodal and bimodal samples show that the bimodal polymer has reduced storage modulus G' and complex viscosity (η*) compared with its unimodal counterpart. Additionally, an equation is derived to predict the contributions of the two catalytic populations to the resulting distribution by quantifying the effect of catalyst composition linked to the kinetic differences, which serves as a predictive framework integrated into the developed strategy. Kinetic experiments and theoretical computations reveal a fusion-catalysis mechanism behind bimodal distribution formation, in which propagating chains dynamically shuttle between two kinetic behaviors through the switching of ligand association states. This work establishes a robust strategy enabling previously inaccessible control over distribution shape via one-pot ROMP, advancing MWD engineering toward broader polymerization platforms.