S-PLLA was prepared via ring-opening polymerization of l -lactide. Typical procedures were employed for preparing the 4-arm
PLLA.26 (link) Pentaerythritol (3.93 mg, 2.9 ×
10–2 mmol), Sn(Oct)2 (17.6 mg, 4.34 ×
10–2 mmol), andl -lactide(5 g, 34.7 mmol)
were added to a dry glass ampule equipped with a magnetic stirrer.
After purging with dry nitrogen six times, the ampule was sealed under
vacuum and kept in an oil bath of 140 °C for 24 h. The products
were dissolved in chloroform and then transferred into excess methanol
for precipitation, and the process was repeated three times. After
drying in a vacuum oven overnight at room temperature, 4-arm S-PLLA
was obtained.
The S-PLLA was characterized by 1H
nuclear magnetic resonance (1H NMR, AVANCE III 500NMR,
Bruker) spectroscopy. Briefly, S-PLLA was dissolved in a deuterated
chloroform solution and characterized by 1H NMR. S-PLLA
powder was mixed with KBr uniformly and pressed into a flaky shape,
which was then characterized by Fourier transform infrared spectroscopy
(FT-IR, Equinox 55, Bruker) with total reflection scanning. The molecular
weight and distribution of S-PLLA were determined by gel permeation
chromatography (GPC, Water1515, Water), in which tetrahydrofuran was
the mobile phase with a flow rate of 1.0 mL/min, and the test temperature
was 40 °C.27 The narrow distribution
of polystyrene was used as the working curve for the standard GPC.
PLLA.26 (link) Pentaerythritol (3.93 mg, 2.9 ×
10–2 mmol), Sn(Oct)2 (17.6 mg, 4.34 ×
10–2 mmol), and
were added to a dry glass ampule equipped with a magnetic stirrer.
After purging with dry nitrogen six times, the ampule was sealed under
vacuum and kept in an oil bath of 140 °C for 24 h. The products
were dissolved in chloroform and then transferred into excess methanol
for precipitation, and the process was repeated three times. After
drying in a vacuum oven overnight at room temperature, 4-arm S-PLLA
was obtained.
The S-PLLA was characterized by 1H
nuclear magnetic resonance (1H NMR, AVANCE III 500NMR,
Bruker) spectroscopy. Briefly, S-PLLA was dissolved in a deuterated
chloroform solution and characterized by 1H NMR. S-PLLA
powder was mixed with KBr uniformly and pressed into a flaky shape,
which was then characterized by Fourier transform infrared spectroscopy
(FT-IR, Equinox 55, Bruker) with total reflection scanning. The molecular
weight and distribution of S-PLLA were determined by gel permeation
chromatography (GPC, Water1515, Water), in which tetrahydrofuran was
the mobile phase with a flow rate of 1.0 mL/min, and the test temperature
was 40 °C.27 The narrow distribution
of polystyrene was used as the working curve for the standard GPC.