Probing Stereoselectivity in Ring-Opening Metathesis Polymerization Mediated by Cyclometalated Ruthenium-Based Catalysts: A Combined Experimental and Computational Study RosebrughL. E. AhmedT. S. MarxV. M. HartungJ. LiuP. LópezJ. G. HoukK. N. GrubbsR. H. 2016 The microstructures of polymers produced by ring-opening metathesis polymerization (ROMP) with cyclometalated Ru-carbene metathesis catalysts were investigated. A strong bias for a <i>cis</i>,<i>syndiotactic</i> microstructure with minimal head-to-tail bias was observed. In instances where <i>trans</i> errors were introduced, it was determined that these regions were also syndiotactic. Furthermore, hypothetical reaction intermediates and transition structures were analyzed computationally. Combined experimental and computational data support a reaction mechanism in which <i>cis</i>,<i>syndio</i>-selectivity is a result of stereogenic metal control, while microstructural errors are predominantly due to alkylidene isomerization via rotation about the RuC double bond.