Stereoretentive Olefin Metathesis: A New Avenue for the Synthesis of All-cis Poly(p-phenylene vinylene)s and Stereo­defined Polyalkenamers

Abstract

Olefin metathesis has tremendously impacted all fields of synthetic chemistry. However, the control of the olefin stereochemistry during this process remains a grand challenge. Recent innovations in catalyst design have permitted control of the stereochemistry of the olefin product. Here, we discuss the development of stereoretentive olefin metathesis, with an emphasis on the synthesis of stereodefined polyalkenamers through ring-opening metathesis polymerization (ROMP). We then present our application of this unique reaction manifold to the preparation of all-cis poly(p-phenylene vinylene)s (PPVs). A dithiolate Ru catalyst was found to deliver perfect cis selectivity for the polymerization of a paracyclophane diene monomer. By using optimized conditions, all-cis PPVs with narrow dispersities and predictable molar masses were obtained by varying the ratio of monomer to catalyst. The high chain fidelity of the stereoretentive ROMP with a paracyclophane diene monomer enabled the preparation of well-defined diblock copolymers with a norbornene co-monomer. Photochemical isomerization of all-cis to all-trans PPVs was effected with both homopolymers and diblock copolymers. This process was shown to be selective for the PPV block, and resulted in changes in optical properties, polymer size, and solubility. Stereoretentive ROMP provides a promising platform for synthesizing polymers with unique properties, including photoresponsive all-cis PPVs with living characteristics.1 Introduction2 Synthetic Applications of Stereoretentive Olefin Metathesis3 Stereocontrol of Polyalkenamers through Stereoretentive ROMP4 Stereoretentive ROMP To Access All-cis Poly(p-phenylene vinylene)s5 Conclusion

Linear and Nonlinear Optical Properties of All-cis and All-trans Poly(p-phenylenevinylene)

“…To access stereodefined PPVs with either >99% cis or >99% trans alkenes, we relied on a two-step approach: First, stereoretentive ring-opening metathesis polymerization (ROMP) − of a paracyclophane-1,9-diene monomer − would deliver an all- cis PPV segment, and then selective photoisomerization of the PPV block would provide the all- trans congener . To optimize the solubility in organic solvents of both cis and trans PPVs, we designed block copolymers containing a solubilizing poly­(norbornene imide) segment stitched to the stereodefined PPV.…”

cis-Selective Acyclic Diene Metathesis Polymerization of α,ω-Dienes

“…Polymerizations based on olefin metathesis, such as acyclic diene metathesis (ADMET) , and ROMP, represent a promising and versatile strategy to access a diverse pool of stereodefined polyalkenamers because of the robustness, functional-group tolerance, and structural diversity of metathesis catalysts. , Specifically designed W or Mo alkylidenes were found to overcome the thermodynamic preference of ROMP and to deliver high cis selectivity mostly with nonpolar monomers via kinetic control. , The recent development of Z -selective − and stereoretentive − Ru catalysts has allowed the expansion of the scope of cis -selective ROMP processes. − Interestingly, while ADMET is a powerful tool for the precise synthesis of polymers, , control over the stereochemistry of the repeating alkenes has long escaped this versatile polymerization.…”