Tuning the Architecture of High-Density Bottlebrush Block Copolymers to Control Their Photonic Properties

The backbone rigidity of bottlebrush block copolymers (BBCPs) induced by the dense branches distributed along their polymeric backbones has made them materials of interest for fast self-assembly of photonic materials. Today, the rigidity of BBCPs is limited by the maximum number of branches per backbone repeating unit synthetically accessible (≤2). Here, we report a methodology to yield polystyrene-b-poly(lactic acid) BBCP with five PLA branches per backbone repeating unit (PS-b-PLA5). A graft-through ring-opening metathesis polymerization (ROMP) of PS macromonomer followed by the addition of a norbornene-based monomer bearing five hydroxyl groups (M(OH)5) yields a block copolymer with a bottlebrush block and a linear polyol block (PS-b-PM(OH)5). A graft-from ring-opening polymerization of lactide initiated by the polyol macroinitiator then affords the desired PS-b-PLA5. Monitoring the monomer consumptions for the three polymerizations supports the successful synthesis of the targeted high-branch-density BBCP. The low solubility of the growing PS-b-PM(OH)5 favored a termination of the ROMP via recombination, which results in the formation of triblock PS-b-PM(OH)5-b-PS (that can subsequently be converted into PS-b-PLA5-b-PS). Simple manipulation of the solvent composition for the ROMP provides control over the fraction of triblock BBCPs present in the final material. The self-assembly structures of the polymers containing different ratios of diblock to triblock were compared experimentally by SEM and UV–vis diffusion reflectance spectroscopy. All materials showed similar domain spacings, which is consistent with predictions obtained from coarse-grained simulation. Overall, this work showcases the broad application of the GT-GF methodology in the synthesis of high-density BBCPs and the control we have over the synthesized product.