Synthesis of Triblock Bottlebrush Copolymer and Its Solution Self-assembly to Form Porous Nanoparticles

A triblock bottlebrush copolymer, poly(N,N-dimethyl acrylamide)-block-(polyacrylate-graft-(polystyrene-alternate-poly(L-lactide)))-block-poly(N,N-dimethyl acrylamide) (PDMA-b-(PA-g-(PS-alt-PLLA))-b-PDMA), was prepared with an amphiphilic coil-rod-coil macromolecular structure based on the combination of reversible addition-fragmentation chain transfer polymerization (RAFT), copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC), and ring-opening polymerization (ROP). For the formation of the triblock bottlebrush copolymer, RAFT was used to prepare the well-defined triblock copolymer main chain of PDMA-b-PA-b-PDMA. CuAAC was then employed to graft the PS side chains onto the reactive PA block to form the triblock bottlebrush copolymer PDMA-b-(PA-g-PS)-b-PDMA with PS side chains The in situ ROP of L-lactide to graft PLLA from PA block finally produced the triblock bottlebrush copolymer of PDMA-b-(PA-g-(PS-alt-PLLA))-b-PDMA with V-shaped side chains of PS and PLLA. In this macromolecule, PDMA formed the hydrophilic coil blocks while the bottlebrush polymer of PA-g-(PS-alt-PLLA) formed the hydrophobic rod block. Self-assembly of the amphiphilic PDMA-b-(PA-g-(PS-alt-PLLA))-b-PDMA was subsequently investigated in selective solvents. Due to the unique coil-rod-coil molecular structure, this triblock bottlebrush copolymer self-assembled into sheet-like micelles or vesicles in the selective solvents of THF/methanol or THF/ethanol, respectively. In these self-assemblies, the coil hydrophilic PDMA block formed the dispersing shell to keep their dispersion while the rigid hydrophobic bottlebrush block aggregated into the sheet-like cores to maintain stability of the self-assemblies. The PS and PLLA side chains with V-shaped structure along the middle blocks microphase-separated inside the aggregated micellar sheet or vesicle wall, in which the isolated PLLA microdomain was surrounded by the continuous PS microdomain Furthermore, the selective hydrolysis of PLLA microdomain left the nanopores inside the cores of micelles and vesicles. It has been demonstrated the hydrolysis rate was heavily depended on the morphology of the self-assemblies, where the hydrolysis of PLLA microdomains of sheet-like micelles was much faster than that of the corresponding vesicles.