Synthesis and Stimuli Responsivity of Diblock Copolymers Composed of Sulfobetaine and Ionic Blocks: Influence of the Block Ratio

Ionic diblock copolymers having sulfobetaine, poly­(sodium styrene­sulfonate)-b- poly­(sulfopropyl di­methyl­ammonium propylacrylamide) (PSSNa-b-PSPP), and poly­[3-(meth­acryl­amido)­propyl trimethylammonium chloride])-b-poly­(sulfobetaine) (PMAPTAC-b-PSPP) were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. Polysulfobetaine has the temperature responsivity of the upper critical solution temperature (UCST) type. However, sulfobetaine/PSSNa and sulfobetaine/PMAPTAC with block ratios of 1:1.8 (36-b-66) and 1:1.3 (50-b-66), respectively, did not show any temperature responsivity. This is probably due to the interaction between sulfobetaine and ionic polymer (anionic or cationic) to form some complex. Therefore, we investigated the effect of the block ratio on the temperature response and interaction between sulfobetaine and ionic polymers. The UCST behavior of the block copolymer composed of a sulfobetaine chain and ionic chain was investigated by changing the block ratio by turbidimetry. PSSNa-b-PSPP and PMAPTAC-b-PSPP with block ratios of 1:42.5 (6:255) and 1:4 (16:61), respectively, showed temperature responsivity. The expression of temperature responsivity was found to be very sensitive to the chain length of the ionic chain block. The temperature responsivity was considered to disappear because of the interaction between the sulfobetaine chain and the ionic chain. The interaction was investigated by adding the ionic polymer to the sulfobetaine homopolymer. UCST behavior was confirmed by adding 0.1% PSSNa and 1% PMAPTAC, respectively. The results suggested that the sulfobetaine chain and the ionic chain interacted with each other and that PSSNa was more sensitive than PMAPTAC. In addition, it was confirmed by a ¹H NMR measurement that the sulfobetaine chain and ionic chain in the homopolymer mixture system and a block copolymer interact with each other.