Synthesis of Poly(δ-valerolactone)-poly(N,N-dimethylacrylamide) Supermicelles in Concentrated Aqueous Media via...

Highly anisotropic diblock copolymer nanoparticles with poly-(δ-valerolactone) (PVL) cores are prepared via reverse sequence polymerization-induced self-assembly (PISA). A hydrophilic monomer, N,N-dimethylacrylamide (DMAC), is combined with a trithiocarbonate-capped PVL precursor and polymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization. This synthesis is initially conducted in the bulk; however, the reaction mixture is subsequently diluted with preheated deoxygenated water once a suitable intermediate DMAC conversion (37-42%) has been attained. Full DMAC conversion was achieved when targeting PVL-PDMAC diblock copolymer nanoparticles at 30% w/w solids, with a high chain extension efficiency and good control (M _w/M _n ≤ 1.29) being achieved for this RAFT polymerization. The initial copolymer morphology is ill-defined; however, depending on the target PVL-PDMAC composition, either anisotropic semiflexible rod-like particles or so-called supermicelles can be formed on aging such PVL-PDMAC diblock copolymer dispersions for up to 19 weeks at 20 °C. This is attributed to slow crystallization of the PVL block, which is initially amorphous immediately after synthesis, as judged by X-ray diffraction studies, but gradually becomes more semicrystalline on aging. Thus, the evolution in copolymer morphology is driven by crystallization-driven self-assembly (CDSA). The aged nanoparticles were characterized by transmission electron microscopy and small-angle X-ray scattering. The semiflexible rods were also subjected to shear-induced polarized light imaging analysis, with shear-induced alignment remaining for up to 1 h after cessation of shear.