Synthesis of thermoresponsive POSS-based eight-arm star poly(N-isopropylacrylamide): Comparison between Z-RAFT and R-RAFT strategies

POSS-based eight-arm star poly(N-isopropylacrylamide), POSS-(PNIPAM)8, was synthesized via solution reversible addition–fragmentation chain transfer (RAFT) polymerization employing the core-first method via both a Z-RAFT strategy and an R-RAFT strategy. Well-defined Z-type POSS-(PNIPAM)8 (POSS-(PNIPAM)8-Z) was obtained via employing the Z-RAFT strategy, in which side reactions, such as star-star coupling, linear-star coupling and intramolecular radical termination, were suppressed at moderate monomer conversion. However, when employing the general R-RAFT strategy, the RAFT polymerization was out of control at moderate monomer conversion, and polymers with a broad molecular weight distribution were obtained. The thermoresponse of star POSS-(PNIPAM)8 was investigated via turbidity analysis, variable-temperature 1H NMR analysis, DLS analysis, and TEM. It was found that the phase transition of star POSS-(PNIPAM)8 occurred within a narrow temperature window and less significant hysteresis in the cooling process existed than that found for linear PNIPAM. Star POSS-(PNIPAM)8 tended to form small-sized aggregates at temperatures above the phase transition temperature (PTT) of the PNIPAM arms. Star POSS-(PNIPAM)8 could greatly decrease the oil/water interfacial tension, and oil-in-water emulsions, e.g., toluene-in-water, n-dodecane-in-water, cyclohexane-in-water, and n-hexane-in-water, could be prepared just by employing highly diluted star POSS-(PNIPAM)8. Reversible emulsification-demulsification was triggered via increasing the temperature above the PTT or decreasing the temperature below the PTT.