Rigid-rod polymers are a class of high-performance lightweight materials that are made up of a highly conjugated heteroaromatic backbone. Unfortunately, synthesis and processing of these materials is difficult because their chain stiffness leads to low solubility and high thermal transitions. We hypothesize that by incorporating ‘shape shifting’ molecular cages into the rigid-rod backbone we can achieve high thermal stability alongside tunable physical properties of these materials. Bullvalene is the ideal fluxional molecule for this application because undergoes low barrier Cope rearrangements to sample over one million possible degenerate isomers. In this work, we incorporate bullvalene into the rigid-rod backbone by copolymerizing a bis-boronate ester bullvalene with 1,4-dibromo-2,5-dihexyl-benzene using a Suzuki polycondensation resulting in a molar mass (Mn) of up to 27 kDa and dispersities (Mw/Mn) of ~1.2. These polymers retain their thermal stability with a Td of 450 oC, but exhibit an abnormally low glass transition temperature (Tg) of 35 oC compared to typical rigid rods with a Tg of &gt;150 oC. Variable temperature NMR spectroscopy experiments were performed on this polymer along with a static analog to assess fluxionality. Broadening of 1H-NMR peaks in the fluxional polymer at higher temperature indicates that the Cope rearrangement is occurring in the polymer chain and that bullvalene can act as a molecular “Rubiks’ cube” and induce chain flexibility into the polymer backbone.

Shape shifting polymers from bullvalene

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