Facile Approach to Modulating Stereochemistry in Ring‐Opening Metathesis Polymerization of Cyclooctadiene

ABSTRACT

Olefin trans/cis geometry strongly influences the properties of unsaturated polymers, yet controlling stereochemistry in ring‐opening metathesis polymerization (ROMP) often requires specialized catalysts or monomers. In ROMP, polymer stereochemistry is determined not only during propagation but also through subsequent secondary metathesis and, potentially, isomerization reactions with the backbone olefins. We therefore hypothesized that a broad range of trans/cis ratios could be achieved by kinetically controlling the balance between propagation and secondary metathesis reactions. To test this hypothesis, we investigated the ROMP of 1,5‐cyclooctadiene (COD) using commercially available catalysts and monitored monomer conversion and polymer trans/cis ratios over time. Variations in catalyst identity, additives (triphenylphosphine and copper(I) iodide), and temperature significantly influenced stereochemical outcomes, enabling access to polybutadienes with 28%–80% trans content. Hoveyda–Grubbs second‐generation catalyst promoted the fastest polymerization, while slower Grubbs catalysts enabled interception of high‐cis products. Triphenylphosphine slowed propagation, extending reaction times and favoring high cis content, whereas copper(I) iodide accelerated propagation, producing materials closer to thermodynamic equilibrium. Elevated temperatures similarly shifted product distributions toward higher trans content. Thermal analysis of the resulting polymers confirmed that stereochemical variations directly affect material properties. These results establish a practical kinetic strategy for tuning polymer stereochemistry in the ROMP of COD.