Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization

We report an efficient photoinduced living radical polymerization process that involves the use of chlorophyll as the photoredox catalyst, which allows the preparation of well-defined polymers.


Instrumentation
Gel Permeation Chromatography (GPC) was carried out on synthesized polymer with dimethylacetamide (DMAc) as the eluent. The GPC instrument consists of Shimadzu modular system with an autoinjector, a Phenomenex 5.0 μM bead sizeguard column (50 x 7.5 mm) followed by four Phenomenex 5.0 μM bead size columns (10 5 , 10 4 , 10 3 and 10 2 Å) for DMAc system, and a differential refractive-index detector and a UV detector (λ = 305 nm). The DMAc GPC system was calibrated based on narrow molecular weight distribution of polystyrene standards with molecular weights of 200 to 10 6 g mol -1 .
Nuclear Magnetic Resonance (NMR) spectroscopy was carried out with Bruker Avance III with SampleXpress operating at 300 MHz for 1 H using CDCl 3 as solvent and Bruker Avance III 500 operating at 500 MHz for 1 H using acetone-d6 as solvent. Tetramethylsilane (TMS) was used as a reference. The data obtained was reported as chemical shift (δ) measured in ppm downfield from TMS.
On-line Fourier Transform Near-Infrared (FTNIR) spectroscopy was used for determination of monomer conversion by mapping the decrement of the vinylic C-H stretching overtone of the monomer at ~ 6200 cm -1 . A Bruker IFS 66/S Fourier transform spectrometer equipped with a tungsten halogen lamp, a CaF 2 beam splitter and liquid nitrogen cooled InSb detector was used. Polymerizations in blue or red LED lights were carried out using FT-NIR quartz cuvette (1 cm × 2 mm). A spectrum composed of 16 scan with a resolution of 4 cm -1 was collected in the spectral region between 7000-4000 cm -1 by manually placing the sample into the holder at time intervals of 5, 10, or 30 minutes. The total collection time per spectrum was about 10 seconds and analysis was carried out with OPUS software.
UV-vis Spectroscopy spectra were recorded using a CARY 300 spectrophotometer (Varian) equipped with a temperature controller.
Fluorescence spectroscopy. Fluorescence spectra were recorded using Agilent fluorescent spectrometer.
Photopolymerization was carried out in the reaction vessel where the reaction mixtures are irradiated by RS Component PACK LAMP RGB blue/red LED lights (4.8 W, max = 461 nm (blue) and 635 nm (red) ) shown below. The distance of the samples to light bulb was 6 cm. The RGB multi-coloured LED light bulb with remote control was purchased from RS Components Australia. for 30 minutes. The degassed mixture was then irradiated in red LED light (4.8 W, max = 635 nm (red)) at room temperature. After 5 hours of irradiation, the reaction mixture was removed from the light source in order to be analysed by 1 H NMR (CDCl 3 ) and GPC (DMAc) to determine the conversions, numberaverage molecular weights (M n ) and polydispersities (M w /M n ).

General Procedures for Kinetic Studies of PET-RAFT Polymerization of Methyl Methacrylate (MMA) with Online Fourier Transform Near-Infrared (FTNIR) Spectroscopy.
A reaction stock solution consisting of DMSO (294 μL), MMA (0.358 g, 3.58 mmol), CPADB (5 mg, 17.90 μmol), and Chl a (64 μL of 224 μM of Chl a stock solution, 0.017 μmol) was prepared in a glass vial. Approximately 500 μL of stock solution was transferred into a 0.9 mL FTNIR quartz cuvette (1 cm × 2 mm) covered with aluminium foil. The reaction mixture in the cuvette was degassed for 30 minutes with nitrogen. The quartz cuvette was then irradiated in red LED light (4.8 W, max = 635 nm (red)) at room temperature. The cuvette was transferred to a sample holder manually for FTNIR measurements every 20 minutes. After 15 seconds of scanning, the cuvette was transferred back to the irradiation source.
Monomer conversions were calculated by taking the ratio of integrations of the wavenumber area 6250-taken at specific time points during the reaction to be analysed by 1 H NMR (CDCl 3 ) and GPC (DMAc) to determine the conversions, number average molecular weights (M n ) and polydispersities (M w /M n ).

General Procedures for Preparation of PMA-b-PDMA Diblock Copolymers by PET-RAFT.
In the synthesis of PMA-b-PDMA diblock copolymers, MA was polymerized in a 5mL glass vial containing DMSO (740 μL Chain extension of PMMA macroinitiator to DMA was carried out in a 5 mL glass vial in the presence of DMSO (495 μL), DMA (0.366 g, 3.69 mmol), PMMA macroinitiator (0.065 g, 7.38 μmol), and Chl a (66 μL of 224 μM of Chl a stock solution, 0.015 μmol) sealed with a rubber septum. Aluminium foil was used to cover the reaction mixture before degassing for 30 minutes with nitrogen. The reaction mixture was irradiated in red LED light (4.8 W, max = 635 nm (red)) at room temperature for 5 hours. The final reaction mixture was purified by precipitating in a mixture of methanol/petroleum spirit (1/1, v/v) with stirring. The pale yellow precipitate was collected and redissolved in minimum amount of dichloromethane before precipitating a second time in methanol/petroleum spirit (1/1, v/v) mixture. The precipitate was analysed in GPC and 1 H NMR: M n,GPC = 45 570 g/mol, M w /M n = 1.08 and 79 % monomer conversion. RAFT end group fidelity was determined by using UV-Vis spectroscopy.

The photostability test of Chl a.
A reaction stock solution consisting of DMSO (370 μL) and Chl a (75 μL of 224 μM of Chl a stock solution, 0.017 μmol) was prepared in a 0.9 mL FTNIR quartz cuvette (1 cm × 2 mm) covered with aluminium foil. The reaction mixture in the cuvette was degassed for 30 minutes with nitrogen. The quartz cuvette was then irradiated in red LED light (4.8 W, λ max = 635 nm (red)) at room temperature for 16 h. Another quartz cuvette containing the same formulation was degassed for 30 min with nitrogen, and then was kept in the dark as a parallel control.
After 16 h, MA (0.361 g, 4.19 mmol) and BTPA (5 mg, 20.97 μmol) was added into both cuvettes and sealed with rubber septa. The final reaction mixtures were degassed for 30 min with nitrogen. The cuvette was then irradiated under red light at room temperature. The monomer conversions were monitored by online FTNIR spectroscopy.  ND f ND f 20 000 1.21