Temperature-Dependent Ring-Opening Polymerization-Induced Self-Assembly Using Crystallizable Polylactones as Core-Forming Blocks

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Ring-opening polymerization-induced self-assembly (ROPISA) was achieved by employing hydroxyl-terminated polyisobutylene (PIB-OH), diphenyl phosphate (DPP), cyclohexane, and ε-caprolactone (ε-CL) and/or δ-valerolactone (δ-VL) as the stabilizer/macroinitiator, catalyst, solvent, and monomers, respectively. ROPISA has enabled the introduction of biodegradable, biocompatible, and crystallizable polylactones to nanoparticles and related materials. The polymerization temperatures (20, 50, or 80 °C) had an important effect on the crystallization behavior and corresponding self-assembly of PIB- b -PCL and PIB- b -PVL. The highest temperature (80 °C) interfered with the crystallization and in situ fixation, which readily reorganized nanoparticles and formed either spherical micelles or a precipitate. However, at the lowest temperature (20 °C), fibrillar nanoparticles were gradually formed and stabilized according to the classical polymerization-induced self-assembly (PISA) mechanism. Furthermore, temperature adjustment altered the monomer sequence in the P(CL- co -VL) block and the subsequent crystallization and self-assembly. At 80 °C, the P(CL -co- VL) block formed a weakly crystallized random structure, which generated spherical micelles. By contrast, at 20 °C, ROPISA favored the generation of P(CL -co- VL) containing a quasi-block sequence, resulting in the acquisition of fibrillar micelles. Interestingly, at different polymerization temperatures, nanoparticles exhibited different morphologies, even when the compositions were similar. The results of this study revealed that ROPISA provided a feasible alternative to PISA for synthesizing nanoparticles.

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