The physical properties of polyalkenamers and conjugated polymers such as poly(phenyl vinylene)s (PPVs) are dramatically affected by the geometry of the repeating alkenes (Z/cis or E/trans). For example, cis polyisoprene is amorphous, while trans polyisoprene—also known as gutta percha—is a hard, brittle semi-crystalline material. The former is used to make laboratory gloves, while the latter used to be in golf balls. Similarly, the optical properties of PPVs highly depend on the content of cis and trans linkages. High cis content translates into a more soluble material, but typically a lower effective conjugation length. While alkene stereochemistry has a profound influence on the mechanical, thermal, and optoelectronic properties of polymers containing such linkages, very few methods can dictate the configuration of alkenes in macromolecules with high precision. In this presentation, we will demonstrate how stereoselective olefin metathesis can be harnessed to provide materials with highly precise structures and tailored properties. The versatility of two recently discovered classes of Ru carbene catalysts—namely stereoretentive and Z-selective catalysts—will be highlighted throughout the talk with examples of ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET). A new approach to all-cis PPVs with living characteristics through stereoretentive ROMP will serve as entry point to discuss the impact of olefin stereochemistry on their optical properties and processability. The photoresponsive behavior of all cis-PPVs will be illustrated with examples including homopolymers and diblock copolymers. Subsequently, we will discuss the implementation of stereoselective olefin metathesis into the ADMET process. Optimization of the polymerization through careful monomer and catalyst selection allowed the synthesis of several classes of polymers with exquisite cis selectivity. Impressively, this unique ADMET only requires low catalyst loadings, tolerates many Lewis basic functional groups, and most importantly, displays predictable stereocontrol through modulation of the reaction conditions. The cis selectivity was shown to highly depend on the reaction temperature thereby offering a practical handle to tune the stereochemistry of the backbone of the polymer. The impact of this structural parameter will be highlighted with various polymers including polyesters, polycarbonates, polysulfites, and polyethers.

Stereocontrolled olefin metathesis: A unique handle to tune the properties of soft materials

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ACS Fall 2022

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