Interplay between Molecular Packing, Drug Loading, and Core Cross-Linking in Bottlebrush Copolymer Micelles

Amphiphilic copolymers with bottlebrush architecture provide opportunities for the refinement of materials properties that may not be attainable from their linear analogues. In this study, we investigated the effect of polymer architecture on an interplay between molecular packing inside micelle cores, cargo loading, and core cross-linking. Four families of polylactide-b-poly(ethylene oxide) (PLA-PEO) bottlebrush block copolymers with different side-chain arrangements were synthesized by a combination of grafting-through and grafting-from methods. Copolymers with double-graft PLA side chains produced smaller and more uniform micelles than those with single-graft PLA branches. Photoactive coumarin groups, installed at PLA side chain ends, improved paclitaxel loading efficiencies of the copolymer micelles and allowed for the preparation of uniform, core-cross-linked PLA nanoparticles. The highest paclitaxel uptake (up to 30 wt % of the micelle core) was observed for micelles prepared from bottlebrush copolymers with branched PEO side chains, with paclitaxel uptake increasing with the size of PEO side chains. On the other hand, micelle photo-cross-linking efficiency was the highest (up to ∼90%) for copolymers with linear PEO side chains and decreased with increasing size of the hydrophilic headgroup. These trends were attributed to the decrease in molecular packing efficiency inside micelle cores for copolymers with larger and more rigid hydrophilic headgroups. For poorly packed micelles, paclitaxel loading improved core photo-cross-linking efficiencies, suggesting structural rearrangements inside micelles with cargo uptake. Preliminary results also showed that paclitaxel release from bottlebrush micelles was slowed down with increasing degree of core cross-linking.