Synthesis of Diblock Copolymer Nanoparticles via RAFT Alcoholic Dispersion Polymerization: Effect of Block Copolymer Composition, Molecular Weight, Copolymer Concentration, and Solvent Type on the Final Particle Morphology

Various poly(2-hydroxyethyl methacrylate-b-benzyl methacrylate) (PHEMA(n)-PBzMA(m)) and poly(2-hydroxypropyl methacrylate-b-benzyl methacrylate) (PHPMA(n)-PBzMA(m)) nano-objects have been prepared via reversible addition-fragmentation chain transfer (RAFT) alcoholic dispersion polymerization. Using either a PHPMA or PHEMA macro-CTA as a steric stabilizer, chain extension with BzMA was conducted in methanol, ethanol, or isopropanol. In each case, in situ self-assembly is driven by the growing PBzMA chains, which become insoluble in lower alcohols above a certain critical chain length. Empirically, PHPMA macro-CTA proved to be much more effective than PHEMA macro-CTA in such syntheses, since the former conferred higher colloidal stability in alcohol. By constructing two detailed phase diagrams, the final nanoparticle morphology is shown to be sensitive to the DP of the core-forming block (PBzMA), the total solids content, and also the mean DP of the stabilizer block (PHPMA). The latter effect is readily demonstrated for PHPMA macro-CTAs possessing mean DPs of 48 and 63. Using PHPMA(48) as a steric stabilizer, a range of nano-objects (spheres, worms or vesicles) can be accessed simply by tuning the DP of the core-forming PBzMA block. In contrast, using the PHPMA(63) stabilizer only produces spherical morphologies. Presumably this is because the latter confers more effective steric stabilization, which prevents the efficient fusion of spheres to form worms. Nevertheless the PHPMA(63)-PBzMA(n) formulation may still be useful, since it allows access to spherical nanoparticles with tunable mean diameters of 29-100 nm. Such phase diagrams are essential for the reproducible targeting of copolymer morphologies, since they enable mixed phase regions to be avoided and allow the predictable synthesis of pure spheres, worms, or vesicles at a given concentration. Finally, a block copolymer jellyfish was observed during these PISA syntheses, which suggests that such intermediates are most likely a generic feature of the in situ conversion of worms into vesicles.