Efficient Metal-Free Norbornadiene–Maleimide Click Reaction for the Formation of Molecular Bottlebrushes

A highly efficient metal-free grafting-onto strategy based on the cyclopentadiene (Cp)–maleimide click reaction was developed for the preparation of molecular bottlebrushes (MBBs) with diverse morphologies and compositions. First, the masked Cp units, norbornadiene (NBD) groups, were introduced onto poly(2-hydroxyethyl acrylate) (PHEA) as repeated anchoring groups to produce the PHEA–NBD backbone-to-be with different degrees of polymerization (DPs). Meanwhile, maleimide-terminated polyethylene glycol, poly(tert-butyl acrylate), poly(N,N-dimethylacrylamide), and poly(N,N-dimethylaminoethyl acrylate) were synthesized as side chains (SCs). Subsequently, the metal-free NBD–Mal click reaction under mild conditions was applied for preparing the MBBs with a tunable morphology and various functionalities. The grafting density was affected by the concentration and composition of the SCs. When the concentration of the SC was 0.06 M, the grafting density can reach up to 92% at the feed ratio of [Mal]/[NBD] = 1:1 and increase to 100% at the feed ratio of [Mal]/[NBD] = 1.5:1. Atomic force microscopy characterization indicated that the morphology of the obtained MBBs could be tuned from rodlike to wormlike by variation of the DP of the backbone from 105 to 419.