Precise Synthesis of Ultra-Dense Bottlebrush Polymers Unearths Unique Trends in Lyotropic Ordering

Biomacromolecular networks with multiscale fibrillar structures are characterized by exceptional mechanical properties, mak-ing them attractive architectures for synthetic materials. However, there is a dearth of synthetic polymeric building blocks capable of forming similarly structured networks. Bottlebrush polymers (BBPs) are anisotropic graft polymers with the po-tential to mimic and replace biomacromolecules such as tropocollagen for the fabrication of synthetic fibrillar networks; however, a longstanding limitation of BBP’s has been the lack of rigidity necessary to access the lyotropic ordering that underpins the formation of collagenous networks. While the correlation between BBP rigidity and grafting density is well-established, synthetic approaches to rigidify BBPs by increased grafting density are underdeveloped. To address this gap in synthetic capability, we report the synthesis of novel macroinitiators that provide well-defined BBPs with unprecedentedly high grafting density. A suite of light scattering techniques are used to correlate macromolecular rigidity with grafting archi-tecture and density, and demonstrate for the first time that poly(norbornene) BBPs exhibit long-range lyotropic ordering as a result of their rod-like character. Specifically, the newly reported ultra-densely grafted structures, preparable on multigram scale, form hexagonal arrays while conventional BBPs do not, despite showing long range spatial correlations. These results implicate the central role of density and entanglement in the solution phase assembly of BBPs and provides new fundamen-tal insight that is broadly relevant to the fabrication and performance of BBP derived materials, spanning biomedical research to photonic materials and thermal management technologies. Furthermore, these newly reported liquid crystalline BBPs pro-vide a structural template to explore the untapped potential of the bottom-up assembly of semiflexible networks and are ul-timately intended to provide a modular route to hierarchically structured biomimetic materials.