Alternating Copolymerization of Cyclobutenes and Norbornenes by a Dual-Site Grubbs Catalyst

Catalytic alternating copolymerization is an appealing yet challenging sequential synthetic strategy that requires catalysts to have uninterrupted alternating recognition capabilities for different monomers. Chemists usually utilize monomers that are difficult to self-polymerize, ensuring that the rate of alternating copolymerization is far higher than the rate of homopolymerization of each monomer. However, reports of alternating copolymerization between highly self-polymerizable monomers are exceedingly uncommon. This work employs a sterically demanding chiral Grubbs catalyst featuring an electron-donating C-Ru bond, which facilitates the alternation of each productive metathesis to occur on both sides. Therefore, by leveraging the steric differences on its right and left sides, the catalyst can effectively distinguish between two cycloalkenes that both demonstrate high ring-opening metathesis polymerization (ROMP) reactivity, albeit with slightly different steric hindrances. Efficient alternating copolymerization of cyclobutene and norbornene was achieved at a 1:1 monomer feed ratio, providing the resultant polymers with high molecular weights and low dispersity. In contrast to random copolymers or polynorbornenes, these alternating copolymers exhibit notably improved mechanical properties, characterized by high strength and high toughness without sacrificing elasticity. This study illustrates the wide range of potential uses for chiral catalysts in precision polymer synthesis.