Synthesis and 'schizophrenic' micellization of double hydrophilic AB ₄ 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 AB₄ star copolymer, PNIPAM-b-(PDEA)₄, was then synthesized by polymerizing 2-(diethylamino)ethyl methacrylate (DEA) via ATRP in 2-propanol at 45°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-(PDEA₆₃)₄ miktoarm star and PNIPAM ₇₀-b-PDEA₂₆₀ linear diblock copolymers were investigated by ¹H 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-(PDEA₆₃)₄ is much smaller than that of PNIPAM₇₀-b-PDEA₂₆₀. Furthermore, the pH-induced micellization kinetics of the AB₄ 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 (τ₁) and a slow (τ₂) relaxation processes. τ₁ for both copolymers decreased with increasing polymer concentration. τ₂ was independent of polymer concentration for PNIPAM₆₅-b-(PDEA₆₃)₄, whereas it decreased with increasing polymer concentration for PNIPAM ₇₀-b-PDEA₂₆₀. The chain architectural effects on the micellization properties and the underlying mechanisms were discussed in detail.