Bifunctional Ruthenium Alkylidene Initiator for Outward Growth of Block Copolymers via Ring‐Opening Metathesis Polymerization

ABSTRACT Ring‐opening metathesis polymerization (ROMP) offers a rapid, mild, and controlled route to prepare block copolymers via sequential monomer addition, where the number of blocks equates to the number of additions. However, block extension is hampered once control is lost, which can occur from cross‐metathesis and polymer precipitation during growth, precluding the ability to make certain functional triblock copolymers via unidirectional chain‐growth. Middle‐out growth from a bifunctional initiator cuts the number of additions in ~half, which has been extensively leveraged for controlled radical polymerizations but remains scarce for ROMP. Herein, a novel bifunctional Hoveyda‐Grubbs type ruthenium‐alkylidene initiator is presented and systematically examined for its use in ROMP of cyclic olefin monomers. The initiator is air‐stable, provides polymers with narrow molecular weight distributions ( Đ < 1.20) and enables the preparation of functional ABA triblock and ABABA pentablock copolymers. Exemplars include norbornene‐based thermoplastic elastomers that are soft yet strong, and those that contain outer blocks from low ring strain cyclic olefins, namely cis ‐cyclooctene to form poly(cyclooctene) that readily undergoes cross‐metathesis and cyclooctatetraene to form (semi)conductive yet poorly soluble poly(acetylene). This platform represents a valuable synthetic tool for the rapid generation of functional block copolymers with potential applications in soft robotics and electronics.