The Precise Synthesis\nof Ultradense Bottlebrush Polymers\nUnearths Unique\nTrends in Lyotropic Ordering

Biomacromolecular networks with multiscale fibrillar\nstructures\nare characterized by exceptional mechanical properties, making them\nattractive architectures for synthetic materials. However, there is\na dearth of synthetic polymeric building blocks capable of forming\nsimilarly structured networks. Bottlebrush polymers (BBPs) are anisotropic\ngraft polymers with the potential to mimic and replace biomacromolecules\nsuch as tropocollagen for the fabrication of synthetic fibrillar networks;\nhowever, a longstanding limitation of BBPs has been the lack of rigidity\nnecessary to access the lyotropic ordering that underpins the formation\nof collagenous networks. While the correlation between BBP rigidity\nand grafting density is well established, synthetic approaches to\nrigidify BBPs by increased grafting density are underdeveloped. To\naddress this gap in synthetic capability, we report the synthesis\nof novel macroinitiators that provide well-defined BBPs with an unprecedentedly\nhigh grafting density. A suite of light scattering techniques are\nused to correlate macromolecular rigidity with grafting architecture\nand density and demonstrate for the first time that poly(norbornene)\nBBPs exhibit long-range lyotropic ordering as a result of their rodlike\ncharacter. Specifically, the newly reported ultradensely grafted structures,\npreparable on multigram scale, form hexagonal arrays while conventional\nBBPs do not, despite showing long-range spatial correlations. These\nresults implicate the central role of density and entanglement in\nthe solution phase assembly of BBPs and provide new fundamental insight\nthat is broadly relevant to the fabrication and performance of BBP-derived\nmaterials, spanning biomedical research to photonic materials and\nthermal management technologies. Furthermore, these newly reported\nliquid crystalline BBPs provide a structural template to explore the\nuntapped potential of the bottom-up assembly of semiflexible networks\nand are ultimately intended to provide a modular route to hierarchically\nstructured biomimetic materials.