An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization

Abstract

Stereochemistry can have ap rofound impact on polymer and materials properties.U nfortunately,s traightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve.Inadeparture from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered ar emarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP.I on-pairing, initiator sterics,a nd solvation effects eachhad profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to > 98 %. High cis content was obtained from relatively larger counterions,t oluene as solvent, low temperatures (À78 8 8C), and initiators with low Charton values.C onversely, smaller counterions,d ichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from ac ombined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.

Pyrylium salts acting as both energy transfer and electron transfer photocatalysts for E → Z isomerization of activated alkenes and cyclization of cinnamic or biaryl carboxylic acids

“…However, all of these reactions used their SET oxidation ability, and relatively little is known about their EnT property. 7 As our ongoing interest in exploring visible-light-induced, transition metal-free radical reactions, 8 we herein report the application of photocatalytic pyrylium salts in EnT and SET mediated transformations (Scheme 1C).…”

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

“…[244] Nevertheless, MF-ROMP has been demonstrated to be effective for the preparation of block copolymers in combination with ROP and a bifunctional initiator [245] and to perform stereoselective ROMP. [246] Although in Section 2.1 we discussed the contribution of organocatalysis mediation in O-ATRP, other important contributions have been made in living radical polymerization. We would especially like to highlight the work by Goto and co-workers, whereby controlled living radical polymerization of a variety of chemically distinct monomers was achieved via an organocatalyzed process involving halogen exchange.…”

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

“…This approach delivered excellent temporal control over the polymerization reaction, although with considerably lower functional group tolerance than conventional metal‐catalyzed ROMP [244] . Nevertheless, MF‐ROMP has been demonstrated to be effective for the preparation of block copolymers in combination with ROP and a bifunctional initiator [245] and to perform stereoselective ROMP [246] . Although in Section 2.1 we discussed the contribution of organocatalysis mediation in O‐ATRP, other important contributions have been made in living radical polymerization.…”