The castor oil‐derived fatty acid (thioether‐containing ω‐hydroxyacid) is obtained using the UV‐catalyzed thiol‐Michael addition and is used for the first time as a monomer, for the development of biorelevant copolymers based on ε‐caprolactone, methoxy‐poly(ethylene glycol), and poly(ethylene glycol). Proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, and size exclusion chromatography, confirm the successful synthesis of diblock and triblock copolymers with very narrow molecular weight distributions. X‐ray diffractometry, polarizing light microscopy, and differential scanning calorimetry, suggest that crystallization of the crystalline block chains is restricted within the microphase‐separated morphology and that it is strongly affected by the competition between the two semicrystalline blocks. Moreover, thermogravimetric analysis reveals that the thermal properties of the copolymers are intermediate between those of their parent homopolymers. Regardless of their block composition, the biorelevant copolyesters do not exhibit apparent cytotoxicity, according to the (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide) (MTT) and fluorescence microscopy assays.

In the present study, the newly synthesized castor oil-derived thioether-containing ω-hydroxyacid (TEHA) block copolymers with polycaprolactone (TEHA-b-PCL), with methoxypoly(ethylene glycol) (mPEG), (TEHA-b-mPEG) and with poly(ethylene glycol) (PEG) (TEHA-b-PEG-b-TEHA), were investigated as polymeric carriers for fabrication of naltrexone (NLX)-loaded microspheres by the single emulsion solvent evaporation technique. These microspheres are appropriate for the long-term treatment of opioid/alcohol dependence. Physical properties of the obtained microspheres were characterized in terms of size, morphology, drug loading capacity, and drug release. A scanning electron microscopy study revealed that the desired NLX-loaded uniform microspheres with a mean particle size of 5–10 µm were obtained in all cases. The maximum percentage encapsulation efficiency was found to be about 25.9% for the microspheres obtained from the TEHA-b-PEG-b-TEHA copolymer. Differential scanning calorimetry and X-ray diffractometry analysis confirmed the drug entrapment within microspheres in the amorphous state. In vitro dissolution studies revealed that all NLX-loaded formulations had a similar drug release profile: An initial burst release after 24 h, followed by a sustained and slower drug release for up to 50 days. The analysis of the release kinetic data, which were fitted into the Korsmeyer–Peppas release model, indicated that diffusion is the main release mechanism of NLX from TEHA-b-PCL and TEHA-b-mPEG microspheres, while microspheres obtained from TEHA-b-PEG-b-TEHA exhibited a drug release closer to an erosion process.