Thermoresponsive behavior of amphiphilic diblock co-oligomers of ethylene glycol and styrene in aqueous solution

We report the thermoresponsive behavior in aqueous solution of amphiphilic diblock co-oligomers of ethylene glycol (EG) and styrene (S) obtained by reversible addition fragmentation chain transfer (RAFT) polymerization (mPEG₁₆-b-PS₂, mPEG₁₆-b-PS₄ and mPEG₁₆-b-PS₆). The block co-oligomers were obtained from the polymerization of styrene mediated by a PEG macro-RAFT agent, mPEG-PBTC. The precise control offered by RAFT polymerization over the length of the short hydrophobic chain permits tuning of the thermoresponsive behavior for all the studied methoxy poly(ethylene glycol)-block-polystyrene copolymers. mPEG ₁₆-b-PS₂ was shown to self-assemble into micelles at temperatures below the lower critical solution temperature (LCST) of the block co-oligomer, whilst no such critical temperature was observed for solutions of self-assembled mPEG₁₆-b-PS₄ and mPEG₁₆-b- PS₆. mPEG₁₆-b-PS₄ self-assembles into relatively mobile spherical particles which evolve into prolate spheroids with increasing temperature, whilst mPEG₁₆-b-PS₆ forms particle clusters, which have lower mobility than the mPEG₁₆-b-PS₄ particles. This work is the first study on the aggregation behavior of such short chain lengths of mPEG-b-PS as a function of temperature, and shows how tuning the PS oligomer length permits control of the thermoresponsive behavior of mPEG-b-PS assemblies in water.