2.2. Characterization

The morphology of samples was investigated by means of a field-emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectrometer (Hitachi JEOL-JSM-5 6700F system, Tokyo, Japan). Dynamic light scattering (DLS) measurements were performed using an electrophoretic light scattering instrument (Zetasizer Nano ZS, Malvern Panalytical Ltd, Malvern, UK), equipped with a He–Ne laser and ELS controller at a wavelength of 633 nm, and the scattering light intensity was detected at 90° to an incident beam. UV/Vis spectra were recorded on a UV/Vis spectrophotometer (Optizen POP, Daejeon, Korea). The molecular weight distribution (Ð) and number average molecular weight (M_n) were determined at 25 °C using a gel permeation chromatograph (GPC: Agilent Technologies, USA) equipped with an RID detector, PL gel column (5 μm; 102–104 Å), and HP 1100 pump. THF was used as an eluent (flow rate: 1 mL/min), and a calibration curve was constructed using polystyrene standards. NMR spectra were recorded on a JNM-ECP 400 MHz (JEOL, Akishima-shi, Japan) NMR spectrometer.

1-(2-Hydroxyethyl)-pyrrole-2, 5-dione (HEMI) and 3, 3′-diselanediyldipropionic acid (DSeDPA) were prepared according to the procedure reported earlier [41]. DMAP (24 mg, 0.2 equiv.) and DCC (472 mg, 1.7 equiv.) were added to a stirred solution of HEMI (588 mg, 2 equiv.) in 6 mL of THF at 0 °C, followed by the addition of DSeDPA (640 mg, 1 equiv.) in 10 mL of THF. The mixture was stirred for 30 min at 0 °C and further stirred at room temperature (RT) for 24 h. The precipitated dicyclohexylurea as a byproduct was filtered off. The filtrate was concentrated and extracted with DCM, and the combined organic layers were washed with a saturated aq. NaCl solution, dried over anhydrous NaSO₄, and filtered. The solvent was evaporated, and the residue was purified using column chromatography (50% EtOAc/hexane) to get a white solid of BMEDSeDP (980 mg, 84%). ¹H-NMR (400 MHz, CDCl₃): δ = 2.78 (t, J = 7.2 Hz, 4H), 3.05 (t, J = 7.2 Hz, 4H), 3.82–3.77 (m, 4H), 4.26 (t, J = 4.5 Hz, 4H), 6.73 (s, 4H) ppm.

HMPPOT was synthesized in two steps. In the first step, 4-hydroxy benzyl alcohol (5 g, 1 equiv.) and trimethylamine (6.9 mL, 1.2 equiv.) were dissolved in dry THF (200 mL) with constant stirring under N₂ atmosphere. 2-Chloropropionyl chloride (3.9 mL, 1 equiv.) was added dropwise to the above solution at 0 °C and stirred for 48 h at RT. Residues from the reaction were removed by filtration, and the filtrate was evaporated. The resulting solid was purified by column chromatography to obtain 4-(hydroxymethyl)phenyl 2-chloropropanoate as a white solid (6.5 g, 75%). ¹H-NMR (400 MHz, CDCl₃): δ = 1.3 (d, 3H), 4.45 (s, 2H), 4.6 (q, 1H), 6.9 and 7.23 (dd, 4H) ppm.

In the second step, 4-(hydroxymethyl)phenyl 2-chloropropanoate (3.5 g, 1 equiv.) dissolved in acetone (40 mL) was added dropwise to the solution of potassium octyl-trithiocarbonate (5.2 g, 1.2 equiv.) in acetone (200 mL), and the mixture was stirred for 14 h at RT. The precipitated byproduct was removed by filtration, and the filtrate was evaporated. The residue was dissolved in ethyl acetate and washed with water (3 × 150 mL), dried over anhydrous NaSO₄, filtered, and evaporated. The product was purified by column chromatography to obtain HMPPOT as a pale-yellow solid (5.5 g, 80%). ¹H-NMR (400 MHz, CDCl₃): δ = 1.7 (d, 3H), 4.65 (s, 2H), 4.9 (q, 1H), 7.1–7.7 (dd, 4H), 3.4 (d, 2H), 0.9 (t, 3H), 1.2–1.5 (m, 12H) ppm.

The synthesis of PLA₂₀-CTA was carried out via ring-opening copolymerization (ROP) of LA using a dual initiator, HMPPOT, and a catalyst, 1, 8-diazabicycloundec-7-ene (DBU). In brief, under N₂ atmosphere in a glove box, LA (2.0 g, 26 equiv.) was dissolved in 8.0 mL of dry DCM, and then HMPPOT (200 mg, 1 equiv.) and DBU (40 μL, 42.2 mg, 0.5 equiv.) were added to the mixture and stirred for 20 min at RT to carry out the polymerization. After 20 min, benzoic acid (39 mg) was added to stop the polymerization. The resulting PLA₂₀-CTA was purified by precipitating in cold CH₃OH and dried under vacuum to get the product as a white solid (2.05 g, 92%). ¹H-NMR (400 MHz, CDCl₃): δ = 7.08 (d, 2H), 7.33 (d, 2H), 5.06 (d, 2H), 4.36 (q, 1H), 3.39 (m, 2H), 1.71 (d, 3H), 0.87 (m, 3H) ppm.

PLA₂₀-CTA (100 mg, 1 equiv.), FMA (98 mg, 20 equiv.), NAM (332 mg, 80 equiv.), and AIBN (1 mg, 0.2 equiv.) were dissolved in dry 1, 4-dioxane (4 mL) and purged with N₂ for 40 min. The polymerization was carried out at 70 °C for 12 h, and then the solution was frozen with liquid nitrogen to stop the polymerization. The polymer was precipitated twice in cold diethyl ether and dried under vacuum to get the PLA₂₀-b-P(FMA₁₀-co-NAM₇₈) copolymer as a white solid (Yield = 400 mg, 80%). ¹H-NMR (400 MHz, CDCl₃): δ = 7.45 (s, 1H), 6.36 (d, 2H), 5.02 (m, 2H), 1.24 (m, 3H), 0.96 (s, 1H) ppm.

Several aqueous concentrations of PLA₂₀-b-P(FMA₁₀-co-NAM₇₈) (0.0005–1.0 mg/mL) were prepared using deionized water. A stock solution of pyrene in acetone was transferred to all the copolymer concentrations to get a final pyrene concentration of 6 × 10⁻⁷ M in each sample, and acetone was evaporated under nitrogen bubbling. The excitation spectra of all the samples (300–360 nm) were recorded at 394 nm emission wavelength using 5 nm slit width. The excitation spectrum peak intensity ratio of pyrene at 337 and 333 nm (I₃₃₇/I₃₃₃) was plotted against the function of copolymer concentration. CMC value was determined considering the interception point of both tangent straight lines.

The crosslinking reaction of PLA₂₀-b-P(FMA₁₀-co-NAM₇₈) was conducted by dissolving the copolymer (20 mg, 1 equiv.) and BMEDSeDP crosslinker (2 mg, 1.5 equiv.) in ACN (1 mL), followed by the dropwise addition of H₂O (5 mL) into the solution under vigorous stirring for 1 h. The micellar solution was subsequently heated at 60 °C for 14 h to induce the crosslinking reaction in the micellar shell.