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Supramolecular polymers (SMPs) have quite different crystallization behavior from the conventional polymers, because of the confinement effects of supramolecular units. Crystallization of SMPs undergoes in a "confined" and "dynamic" manner. Herein, we selected the 2-ureido-4[1H]-pyrimidione (UPy)-bonded poly(l-lactic acid) (PLLA) as a model SMP and investigated the crystallization kinetics, polymorphic crystalline structure and phase transition of supramolecular PLLAs (SM-PLLAs). SM-PLLAs were synthesized by the end functionalization of hydroxyl-terminated 2- and 3-arm PLLA precursors with different molecular weights. Crystallization rate and crystallinity of SM-PLLAs are strongly depressed in both nonisothermal and isothermal crystallizations, as compared to the nonfunctionalized PLLA precursors. Crystalline structure of SM-PLLAs is sensitive to the crystallization temperature (Tc). A low Tc (75-95 °C) facilitates the formation of metastable β crystals of PLLA in SM-PLLAs; while a high Tc (100-130 °C) favors the generation of α (or α') crystals. The β crystals formed in SM-PLLAs transform into the more stable α crystals in the following heating process. We propose that the preferential formation of β crystals is ascribed to the "strong" confinements of UPy motifs and UPy-UPy interactions, which may exert an orientation and stretching effects to the linked PLLA chains during crystallization. This study has shed light on the unique "confined" and "dynamic" crystallization behavior of SMPs and also paved a way to obtain the PLLA β-form from SM-PLLA under the crystallization conditions free of pressure, stretching, and shearing. [ABSTRACT FROM AUTHOR]

Precipitation (or solvent displacement) method has been a simple yet efficient way to prepare the micro- and nanoparticles of polymers. However, control over the particle morphology of semicrystalline polymer is extremely challenging in the precipitation method, due to the interplay of polymer crystallization with liquid-liquid phase separation in solution. This limits the preparation of polymer particles with well-controlled morphology. Herein we report on the preparation of flower-shaped and spherical biodegradable polymer particles by precipitating the ureido-4[1H]-pyrimidione-functionalized supramolecular poly(lactic acid) (PLA) from a good solvent to an antisolvent. Morphology of PLA particles was successfully manipulated by the solution crystallization, molecular weight, and intermolecular interactions of polymer precursors. Homocrystallization of supramolecular poly(l-lactic acid) yielded the flower-shaped particles in precipitation; yet stereocomplex crystallization of PLA supramolecular copolymers led to the formation of sphere particles. The underlying mechanism for crystallization-driven formation of various particles was proposed. The prepared sphere particles can be used as the carriers of hydrophobic drug. Degradation and drug release rates of the prepared PLA particles decreased with increasing the content of stereocomplexes. Our study paves a way to the biodegradable PLA particles with controllable morphologies that may find potential applications in biomedical field. [ABSTRACT FROM AUTHOR]