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

Diblock copolymers consisting of a water-soluble non ionic block and either an an ionic 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 polyrnerization 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 (M(n)) increased linearly with the monomer conversion (up to conversions of 30% for MAPTAC and 50% for AMPS), whereas the polydispersity (M(w)/M(n)) remained nearly constant (M(w)/M(n) < 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 (1)H 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 in open question, but it is speculated to be a multicore intermicellar aggregate on the basis of various characterization data.