Miktoarm Stars via Grafting-Through Copolymerization: Self-Assembly and the Star-to-Bottlebrush Transition

The grafting-through copolymerization of two distinct macromonomers via ring-opening metathesis polymerization is typically used to form random or diblock bottlebrush polymers with large total backbone degrees of polymerization (N_BB) relative to that of the side-chains (N_SC). Here, we demonstrate that Grubbs-type chemistry in the opposite limit, namely N_BB << N_SC, produces well-defined materials with excellent control over ensemble-averaged properties, including molar mass, dispersity, composition, and number of branch points. The dependence of self-assembly on these molecular design parameters was systematically probed using small angle X-ray scattering and self-consistent field theoretic simulations. Our analysis reveals that two-component bottlebrush copolymers with small N_BB behave like miktoarm star polymers. The star-to-bottlebrush transition is quantifiable for both random and diblock sequences by unique signatures in the experimental scaling of domain spacing and simulated distribution of backbone/side-chain density within lamellar unit cells. These findings represent a conceptual framework that simplifies the synthesis of miktoarm star polymers when dispersity in molar mass and composition can be tolerated.