Ring-opening metathesis polymerization (ROMP) has emerged as a convenient route for creating unsaturated polyolefin precursors with controllable molecular weight and stereoselectivity. ROMP is governed in part by the ring-strain of the monomer: highly strained cyclic species such as norbornene or cyclobutene have a favorable forward ROMP reaction, yielding polymers with controllable molecular weight and narrow dispersity. Cyclooctenes and cyclooctadienes, which exhibit a much lower ring strain than norbornene and cyclobutene monomers, pose a greater challenge when narrow dispersity (D &lt; 1.3), high conversion, and well-defined polymer architectures are desired. These 8-membered rings are prone to side reactions such as back-biting and cross metathesis of growing polymer chains. Here, we present a strategy for reducing the propensity for side reactions by performing ROMP of cis,cis-1,5-cyclooctadiene, trans-cyclooctene, and 1-butyl-trans-cyclooctene 8-membered rings in the presence of excess organic ligand; the presence of these excess ligands serve to compete with monomer for occupation of the active catalyst site of Grubbs 3rd Generation Ruthenium catalyst and favor the propagation of polymer chains as opposed to side reactions such as cross-metathesis. Stereoselectivity and molecular weight control are explored by varying the strength and concentration of organic ligands and how they interact with 8-membered rings exhibiting different ring strains. Controlled polymerization of these 8-membered cycloalkenes is investigated as a pathway to synthesizing architecturally diverse model poly(ethylene) compounds with tunable branch length, branch content, and branch distribution.

Controlled Ring-Opening Metathesis of 8-Membered Cyclic Olefins Through the Addition of Excess Ligand

ACS Fall 2021

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