Synthesis and 'Schizophrenic' micellization of double hydrophilic AB4 miktoarm star and AB diblock copolymers:: Structure and kinetics of micellization

Poly(N-isopropylacrylamide) (PNIPAM)-based tetrafunctional atom transfer radical polymerization (ATRP) macroinitiator (1b) was synthesized via addition reaction of mono-amino-terminated PNIPAM (1a) with glycidol, followed by esterification with excess 2-bromoisobutyryl bromide. Well-defined double hydrophilic miktoarm AB(4) star copolymer, PNIPAM-b-(PDEA)(4), was then synthesized by polymerizing 2-(diethylamino)ethyl methacrylate (DEA) via ATRP in 2-propanol at 45 degrees C using 1b, where PDEA was poly(2-(diethylamino)ethyl methacrylate). For comparison, PNIPAM-b-PDEA linear diblock copolymer with comparable molecular weight and composition to that of PNIPAM-b-(PDEA)(4) was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The pH- and thermoresponsive 'schizophrenic' micellization behavior of the obtained PNIPAM(65)-b-(PDEA(63))(4) miktoarm star and PNIPAM(70)-b-PDEA(260) linear diblock copolymers were investigated by H-1 NMR and laser light scattering (LLS). In acidic solution and elevated temperatures, PNIPAM-core micelles were formed; whereas at slightly alkaline conditions and room temperature, structurally inverted PDEA-core micelles were formed. The size of the PDEA-core micelles of PNIPAM(65)-b-(PDEA(63))(4) is much smaller than that of PNIPAM(70)-b-PDEA(260). Furthermore, the pH-induced micellization kinetics of the AB(4) miktoarm star and AB block copolymers were investigated by the stopped-flow light scattering technique upon a pH jump from 4 to 10. Typical kinetic traces for the micellization of both types of copolymers can be well fitted with double-exponential functions, yielding a fast (tau(1)) and a slow (tau(2)) relaxation processes. tau(1) for both copolymers decreased with increasing polymer concentration. tau(2) was independent of polymer concentration for PNIPAM(65)-b-(PDEA(63))(4), whereas it decreased with increasing polymer concentration for PNIPAM(70)-b-PDEA(260). The chain architectural effects on the micellization properties and the underlying mechanisms were discussed in detail.