Miktoarm Stars via Grafting-Through Copolymerization: Self-Assembly and the Star-to-Bottlebrush Transition
2019
Levi, Adam E. | Lequieu, Joshua | Horne, Jacob D. | Bates, Morgan W. | Ren, Jing M. | Delaney, Kris T. | Fredrickson, Glenn H. | Bates, Christopher M.
The grafting-through copolymerization of two distinct macromonomers via ring-opening metathesis polymerization is typically used to form statistical or diblock bottlebrush polymers with large total backbone degrees of polymerization (NBB) relative to that of the side-chains (NSC). Here, we demonstrate that Grubbs-type chemistry in the opposite limit, namely NBB ≪ NSC, 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 supports the notion that two-component bottlebrush copolymers with small NBB behave like miktoarm star polymers. The star-to-bottlebrush transition is quantifiable for both statistical 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 the number of arms and composition can be tolerated. The analytical approach introduced to distinguish chain conformations in complex macromolecules also complements previous methods, for example, form factor scattering and rheology.
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