Molecular Frustration-Governed Reentrant Sol–Gel–Sol Behavior in Gradient Terpolymers

Controlling the self-assembled morphology and viscoelastic properties of multicomponent copolymers remains a central challenge in polymer science. Here, we report two coupled phenomena discovered in a terpolymer system comprising poly(ethylene glycol) (PEG), butyl methacrylate (BuMA), and 2,2,2-trifluoroethyl methacrylate (TFEMA). The terpolymer was synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization in an imidazolium-type ionic liquid, in which polymerization-induced self-assembly (PISA) takes place. We found that the presence of BuMA significantly accelerates the polymerization rate of TFEMA. This “comonomer-accelerated” behavior spontaneously generates a steep compositional gradient despite simultaneous monomer feeding. Moreover, by varying the monomer feed composition while maintaining a constant degree of polymerization, we observed a sol–gel–sol reentrant behavior, originating from a morphological transformation from vesicles to wormlike micelles and back to vesicles. This behavior was successfully captured by dissipative particle dynamics (DPD) simulations despite their inherent simplification. This behavior is governed not by traditional packing parameters, but by the stability of chain interdigitation modulated by molecular frustration. This frustration-governed mechanism offers a non-packing-parameter principle for controlling polymer self-assembly and viscoelastic properties in multicomponent systems.