Phase Separation and pH-Dependent Behavior of Four-Arm Star-Shaped Porphyrin-PNIPAM4 Conjugates

Star-shaped porphyrin-PNIPAM4 (PP) conjugates having four PNIPAM arms connected to a central tetraphenylporphyrin unit were synthesized using reversible addition–fragmentation chain-transfer polymerization. Temperature-induced phase-separation behavior of the conjugates was investigated, and the lower critical solution temperature (type II)–composition phase diagram was constructed using Flory–Huggins theory. Interestingly, in contrast to PNIPAM homopolymers, the shorter PNIPAM arms of PP conjugates lead to a lower phase-separation temperature (Tp). The concentration dependency of the size of the cooperative domain was also determined. Below Tp, experimental data indicate that PP behaves as a 1D supramolecular polymer with a concentration-dependent length, while above Tp, PP globules adopt a larger spherical shape. Various temperature–pH reversible and irreversible interdependencies (“cross-effects”) between phase separation and protonation were observed. The PP conjugates represent a dual temperature–pH-responsive model system possessing various aggregated states, making them candidates for visual indicators, pH or temperature sensors, or singlet oxygen generators for biomedical applications.