Avance 3 400

NMR spectra were recorded on Bruker Avance III/400 or Bruker Avance III/700 (Karlsruhe, Germany) for ¹H and 176.1 MHz or 100.6 MHz for ¹³C (see ESI†). Chemical shifts were recorded relative to SiMe₄ (δ 0.00) or solvent resonance (CDCl₃δ 7.26, CD₃OD δ 3.31). Multiplicities were given as: s (singlet), d (doublet), dd (double doublet), ddd (double double doublet), t (triplet), dt (double triplet), and m (multiplet). The ⁷⁷Se NMR spectra were recorded on Bruker Avance III/400 or Bruker Avance III/700 with diphenyl diselenide as an external standard. NMR spectra were carried out using ACD/NMR Processor Academic Edition. Melting points were measured with a Büchi Tottoli SPM-20 heating unit (Büchi Labortechnik AG, Flawil, Switzerland) and were uncorrected. Elemental analyses were performed on a Vario MACRO CHN analyzer. Optical rotations were measured in 10 mm cells with a polAAr 3000 polarimeter. Column chromatography was performed using Sigma Aldrich 60 Å (52–73 A) 63–200 μm silica gel (Merck, Darmstadt, Germany).

The MPEG with a molecular weight of 2000D was dried and placed in a flask, after magnetic stirring at a constant temperature of 105°C for 90 mins under a vacuum condition.
The preprocessed MPEG and ε-caprola were simultaneously put in a dry glass ampoule filled with nitrogen; then, (Sn(Oct)₂) was added, stirred and mixed and reacted at 130°C for 6 hrs. MPEG will open the cyclic structure of ε-caprolactone to form MPEG-PCL diblock polymer, in which both MPEG and PCL have a molecular weight of 2000D. The purified MPEG-PCL copolymer was placed in a desiccator to be reserved.
First, folic acid (100 mg) and NH₂-PEG-PCL (200 mg) were dissolved in 4 mL of dimethylsulfoxide (DMSO), respectively. Then, the solutions were introduced to a flask, and DMAP (150 mg) and DCC (130 mg) were added as catalyst. After stirring at room temperature under nitrogen for 10 hrs, the resulting solution was dialyzed (MWCO =1000 Da) against water. The purified Fa-PEG-PCL was freeze-dried and stored at 4°C before further use. ¹H nuclear magnetic resonance spectroscopy (¹H-NMR, Bruker Avance III 400, Bruker, Germany) and Fourier transform infrared spectroscopy (FTIR, Nicolet 200 SXV, Thermo Fisher Scientific, Waltham, MA, USA) were performed to study the chemical structure of the obtained polymers. UV-vis absorption spectroscopy of the samples was recorded using a spectrophotometer (UV-2600, SHIMADZU, Japan).

The biodegradable PLA–10R5–PLA block copolymers were synthesized by ring-opening copolymerization method. In brief, calculated amount of _L-LA (30 g for sample S1 and 45 g for S2) and 10R5 (20 g for sample S1 and 5 g for S2) were added into a dry glass ampoule under nitrogen atmosphere, and catalyzed by 0.15 g of Sn(Oct)₂. The reaction system was kept at 130°C for 10 hours and rapidly heated to 140°C under vacuum for another 1 hour. After being cooled to room temperature under nitrogen atmosphere, the PLA–10R5–PLA copolymer was firstly dissolved in methylene chloride and then precipitated using excess cold petroleum ether. The mixture was filtered and dried at 40°C under vacuum for 48 hours. The purified copolymers were kept in a desiccator before use. ¹H nuclear magnetic resonance spectroscopy (¹H NMR; Bruker Avance III 400, Bruker Optik GmbH, Ettlingen, Germany), Fourier transform infrared spectroscopy (FTIR; Nicolet 200 SXV, Thermo Fisher Scientific) and gel permeation chromatography (GPC; Agilent 110 HPLC, Agilent Technologies, Santa Clara, CA, USA) were used to characterize the obtained copolymers.