Stereoselective photoredox ring-opening polymerization of O-carboxyanhydrides

Biodegradable polyesters with various tacticities have been synthesized by means of stereoselective ring-opening polymerization of racemic lactide and β-lactones but with limited side-chain groups. However, stereoselective synthesis of functional polyesters remains challenging from O-carboxyanhydrides that have abundant pendant side-chain functional groups. Herein we report a powerful strategy to synthesize stereoblock polyesters by stereoselective ring-opening polymerization of racemic O-carboxyanhydrides with the use of photoredox Ni/Ir catalysts and a selected Zn complex with an achiral ligand. The obtained stereoblock copolymers are highly isotactic with high molecular weights ( > 70 kDa) and narrow molecular weight distributions (Mw/Mn < 1.1), and they display distinct melting temperatures that are similar to their stereocomplex counterparts. Furthermore, in one-pot photoredox copolymerization of two different O-carboxyanhydrides, the use of such Zn complex mediates kinetic resolution of the comonomers during enchainment and shows a chirality preference that allows for the synthesis of gradient copolymers.


Ni catalysts
The Ni complex solution was prepared freshly prior to the reaction. In a glove box, Ni(COD)2 (5.5 mg, 0.02 mmol) was mixed with bipyridyl ligand (e.g., bpy-1, 3.1 mg, 0.02 mmol) or or tricyclophosphine (11.2 mg, 0.04 mmol) in THF solution (300 μL) at room temperature and stirred for 1-2 hour to ensure all Ni(COD)2 dissolved. The Ni complex solution could be stored in the glove box freezer (-30 o C) no longer than seven days.

NMR
All room temperature NMR and homodecoupling 1 H NMR spectra were recorded on Agilent U4-DD2 (400 MHz) or Bruker Avance II (500 MHz). Low temperature 1 H and 13 C NMR spectra were measured on Bruker Avance III (600 MHz) after the inner temperature of the NMR machine reached -20 o C for 20 min. The samples were kept at -20 o C ± 5 o C with a dry ice/ethylene glycol bath before the low-temperature NMR acquisition.

FTIR
Fourier-transform infrared spectra were recorded on an Agilent Cary 630 FT-IR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with Diamond ATR and transmission sampling accessory.

Monomer conversion measurement:
A small aliquot of polymer solution (20 µL) was removed out of the glove box and quenched with 5% acetic acid / THF solution (20 µL). The mixture (~10 µL) was immediately dropped onto the FTIR-ATR diamond sampler and formed a film within 10-20 seconds for the spectra measurement. The peak at 1800 cm -1 is assigned as the anhydride bond stretch in OCA; the peak at 1760 cm -1 corresponds to the formation of the ester bond in the polymer. The monomer conversion was determined by the intensity ratio between 1760 cm -1 and 1800 cm -1 : conversion% = I1760 / (I1760+ I1800). 1, 9

Gel permeation chromatography (GPC)
GPC experiments were performed on a system equipped with an isocratic pump with degasser were obtained and reported from the second heating run.

Ir-1
In a glove box, prior to the polymerization, all reagents were cooled in the cold trap equipped with a thermometer at -20-30 o C, which was cooled by the liquid nitrogen and ethanol in dewar.

1)ZnEt / BnOH / Ir-1
In a glove box, prior to the polymerization, all reagents were cooled in the cold trap equipped with a thermometer at -20-30 o C, which was cooled by the liquid nitrogen and ethanol in dewar.
c The α-methine peaks are overlapped with CH2 peaks in Cbz groups in 1 H NMR spectrum, preventing from the homodecoupling analysis (see Supplementary Figure 9). The Pm of poly(rac-3) was determined by 13 C NMR spectrum. d The integration of the tetrad rmr is 0.02 (see Supplementary Figure 10).