Protein-like Behavior of Statistical VCL-VI Block Copolymers

Amphiphilic copolymers of N-vinylcaprolactam (VCL) and N-vinylimidazole (VI) were synthesized via free-radical copolymerization in bulk. The phase behavior was systematically investigated using a combination of nephelometric methods, dynamic light scattering, UV spectroscopy, and high-sensitivity differential scanning calorimetry. These copolymers undergo a coil-to-globule conformational transition in aqueous solutions upon heating to 65 °C, while maintaining complete optical homogeneity and avoiding macroscopic phase separation. To better understand our results, model copolymers were obtained within the framework of reactive dissipative particle dynamics simulations under conditions reproducing the experimental synthesis. Overall, the model copolymers exhibit, on the one hand, the monomer unit distribution statistics characteristic of random copolymers and, on the other hand, a nonuniform gradient distribution type. In a selective solvent mimicking water above the lower critical solution temperature of poly-VCL, they form core–shell nanostructures, with solvophilic VI units comprising the outer shell. A strikingly unexpected result of the modeling was the conclusion that the distinct protein-like behavior of the experimentally synthesized polymers can be explained using models of copolymer chains with an apparently statistical nature and a gradient distribution of monomer units. Thus, the biomimetic behavior in our experimentally synthesized VI–VCL copolymers may be rationalized by a finely tuned balance between the average lengths of VCL- and VI-rich sequences along the chain, which can be governed by the reactivity ratios of the comonomers and the initial monomer feed composition.