Main-Chain Stiffness of Cellulosic Bottlebrushes with Polystyrene Side Chains Introduced Regioselectively at the O-6 Position

摘要

In this article, we describe the conformational characteristics of a cellulosic bottlebrush, i.e., a cellulose derivative of densely substituted side chains, in a dilute solution in order to clarify the effect of cellulosic main-chain stiffness and side-chain crowdedness. Novel cellulosic bottlebrushes 1 with polystyrene (PSt) side chains and methyl groups at the O-6 and O-2,3 positions, respectively, were highly regioselectively synthesized via a protecting group strategy in combination with a copper-catalyzed azide alkyne coupling click reaction, yielding densely graft polymers where PS chains were introduced in every anhydroglucose repeating unit, i.e., at a "constant" distance of 0.5 nm along the cellulose backbone. Small-angle X-ray scattering measurements of 1 in dimethylformamide revealed that the cross-sectional radius of gyration depended on the degree of polymerization (DP) of the PSt side chains in accordance with the power law with an exponent of 0.50, indicating that the PSt side chains adopted extended conformation because of the inter-side-chain interaction, i.e., the excluded volume effect among the neighboring side chains. Size exclusion chromatography-multiangle light scattering experiments were conducted to determine the main-chain stiffness. Despite the use of a poor solvent for PSt, the stiffness parameter lambda(-1) was almost independent of the DP of the PSt side chains up to 60, even in the brush regime of side-chains with interchain interaction and hence somewhat stretched conformation. This behavior differs from those of previously reported bottlebrushes of flexible main chains and successfully demonstrated the cellulosic main chain intrinsically so stiff to form bottlebrushes of sufficiently long side chains with little effect on the main-chain conformation. This would expand the diversity in design of bottlebrushes along with the chirality of the cellulosic chain.