Enhanced Coil Expansion and Intrapolymer Complex Formation of Linear Poly(methacrylic acid) Containing Poly(ethylene glycol) Grafts

Poly(methacrylic acid) (PMA) and poly(ethylene glycol) (PEG) represent a polyacid/polybase pair capable of forming reversible, pH-responsive, hydrophobic complexes stabilized by hydrogen bonding in aqueous media. Linear PMA was modified with methoxy-terminated, long-chain (5000 M w) poly(ethylene glycol) (MPEG) via random coupling in dimethyl sulfoxide using dicyclohexylcarbodiimide as a coupling agent. The degree of ionization, α, was deteremined utilizing potentiometric titration. The presence of tethered MPEG markedly increased the pK a values over those of the PMA homopolymer or PMA/MPEG mixtures at α < 0.35. The dilute solution PMA−PEG intramolecular association was probed by monitoring the PEG nuclear magnetic resonance (NMR) spin−spin (T 2) relaxation as a function of pH. Two populations of MPEG exist at appropriate ionizations: those participating in hydrogen bonding with the PMA backbone and those not participating in hydrogen bonding. Polymer self-diffusion at varying degrees of ionization was measured using pulsed gradient spin echo (PGSE) NMR and dynamic light scattering. Acidic conditions produced coil collapse, apparently arising from the formation of intramolecular hydrogen-bonded complexes along the polymer backbone. Under basic conditions, coil expansion was observed due to conformational changes (polyelectrolyte effect), and the added volume of the tethered MPEG as well as the reduction of intramolecular complexation as the number of carboxylate anions increased.