Synthesis of Oppositely Charged Block Copolymers of Poly(ethylene glycol) via Reversible Addition−Fragmentation Chain Transfer Radical Polymerization and Characterization of Their Polyion Complex Micelles in Water

Abstract

Diblock copolymers consisting of a water-soluble nonionic block and either an anionic or cationic block were prepared from sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) or (3-(methacryloylamino)propyl)trimethylammonium chloride (MAPTAC) via reversible addition−fragmentation chain transfer (RAFT)-controlled radical polymerization using poly(ethylene glycol) (PEG)-based chain transfer agent (PEG-CTA) in water. The RAFT polymerization proceeded in a living fashion, as suggested by the observation that the number-average molecular weight (Mn) increased linearly with the monomer conversion (up to conversions of 30% for MAPTAC and 50% for AMPS), whereas the polydispersity (Mw/Mn) remained nearly constant (Mw/Mn < 1.05 for MAPTAC and <1.2 for AMPS) independent of the conversion. The mixing of aqueous solutions of the oppositely charged diblock copolymers, PEG-b-PAMPS and PEG-b-PMAPTAC, led to the spontaneous formation of polyion complex (PIC) micelles. The PIC micelles were characterized by 1H NMR spin−spin relaxation time, static light scattering (SLS), dynamic light scattering (DLS), and scanning electron microscopy (SEM) techniques. The hydrodynamic size of the micelle depended on the mixing ratio of PEG-b-PAMPS and PEG-b-PMAPTAC with the size maximizing at the mixing ratio of stoichiometric charge neutralization. The mixing of the oppositely charged diblock copolymers with shorter charged blocks formed a core−shell PIC micelle. In contrast, a complicated aggregate was formed from a pair of longer blocks. The exact structure of the aggregate is still an open question, but it is speculated to be a multicore intermicellar aggregate on the basis of various characterization data.