A2BA2 Block Copolymers of Poly(N-isopropylacrylamide) (A) and Poly(ethylene glycol) (B): Synthesis and Thermal Gelation Properties of Aqueous Solutions

A(2)BA(2) block copolymers of poly(N-isopropylacrylamide) (A, PNIPAM) and poly(ethylene glycol) (B, PEG) were synthesized by SET-LRP for the first time. The H-1-NMR and GPC analyses showed that the block copolymers prepared had in average 3.5-3.8 PNIPAM arms/macromolecule and low polydispersities (M-w/M-n = 1.25-1.35). The thermogelation behavior of their 20 wt% aqueous solutions was investigated by DSC, dynamic rheometry and the tube inversion method as a function of the molecular weight of both PEG block and PNIPAM arms. The results showed that both phase transition temperature, as determined by DSC, and gelation temperature, determined by the tube inversion method, increased with the PEG block length and decreased with the MW of the PNIPAM arms, while the forming gels were more stable for longer PEG chains. By comparing the thermoresponsive properties of the H-shaped block copolymers synthesized with those of some linear PNIPAM-PEG-PNIPAM triblock copolymers of similar molecular weights of both PNIPAM side blocks and PEG middle chain, almost identical results were obtained, indicating practically no influence of the shape of the PNIPAM blocks from this point of view.