PET-RAFT Polymerization Catalyzed by Small Organic Molecule under Green Light Irradiation

Photocatalyzed polymerization using organic molecules as catalysts has attracted broad interest because of its easy operation in ambient environments and low toxicity compared with metallic catalysts. In this work, we reported that 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) can act as an efficient photoredox catalyst for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization under green light irradiation. Well-defined (co)polymers can be obtained using this technique without any additional additives like noble metals and electron donors or acceptors. The living characteristics of polymerization were verified by kinetic study and the narrow dispersity (Đ) of the produced polymer. Excellent chain-end fidelity was demonstrated through chain extension as well. In addition, this technique showed great potential for various RAFT agents and monomers including acrylates and acrylamides.


Introduction
Photo-polymerization has received great attention because of its easy operation, temporal control, ambient condition, and so on. It has unique advantages in the synthesis of complex polymers, in bio-systems, and in microelectronics. Generally, visible light-driven polymerization is favored for its environmental friendliness and non-invasiveness. However, because conventional monomers have almost no absorption in visible region, a visible light-driven polymerization system generally requires photocatalysts or photosensitizers to harvest energy from visible light to trigger polymerization.
In recent years, visible light-driven polymerization has been rapidly developed, as many efficient photocatalysts or photosensitizers have been found, including metallic and non-metallic materials [1,2]. Owing to long excited lifetimes (microsecond) and high reductive abilities, transition metal compounds like Ir(ppy) 3 and Ru(bpy) 3 Cl 2 have exhibited high capabilities in photocatalyzed polymerization. Boyer et al. [3] reported photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization using Ir(ppy) 3 as a photocatalyst, which can be performed even in the presence of air. In addition, as a useful tool, this technique played an important role in the synthesis of multi-block copolymers [4,5], various stereopolymers [6], copolymers with well-defined sequence structures and sophisticated microstructures [7], and discrete oligomers [8]. Hawker et al. reported that Ir(ppy) 3 could be utilized in visible light-induced atom transfer radical polymerization of methacrylates [9], acrylates [10], and to fabricate a 3D polymer brush [11]. Besides, many other metallic materials [12][13][14][15][16][17][18][19] have been developed to achieve living/controlled polymerization under irradiation by visible light as well.
Small organic molecules with donor-acceptor-donor (D-A-D)-type π-conjugated structures play a significant role in organic photovoltaic devices because of efficient light-generated electron transfer processes [45]. With a high efficiency of charge separation and a long exciton lifetime, some of these organic molecules have great potential in the field of photocatalyzed polymerization. But few successful examples have been published [46,47]. In this study, we found an organic molecule comprising the thiophene as a donor unit (D), benzothiadiazole as an acceptor unit (A), and 4,7di(thiophen-2-yl)benzo[c] [1,2,5]thiadiazole (DTBT) (Scheme 1), which can catalyze PET-RAFT polymerization for synthesis of well-defined (co)polymers with low molecular weight distribution and high end-group fidelity of product under green light. In addition, this system has great compatibility in a variety of RAFT agents and monomers. Scheme 1. Diagram of photoredox catalyst for photoinduced electron transfer-reversible additionfragmentation chain transfer (PET-RAFT), chemical structure of 4,7-di(thiophen-2yl)benzo[c] [1,2,5]thiadiazole (DTBT), and RAFT agents and monomers.

Characterization
All chemical structures were measured by a 400 MHz nuclear magnetic resonance (NMR) spectrometer (Bruker, Karlsruhe, Germany). The number-average molar mass (M n,GPC ) and Đ were determined by a gel permeation chromatography (Waters, Milford, MA, USA) using DMF (1.0 mL/min) as the eluent with polystyrene standards. The absorption spectrum was measured by UV-Visible Spectroscopy (Sahimadzu, Kyoto, Japan).

Polymerizations
The polymerizations were conducted in a sealed glass tube, in a typical experiment, a 3 mL glass vial charged with monomer, chain transfer agent (CTA), catalyst and DMF as solvent. The mixture was deoxygenated by three freeze-pump-thaw cycles and sealed under vacuum. Then, it were irradiated under LED light at ambient temperature. Monomer conversions were measured by 1 H-NMR, the final polymers were obtained by precipitation and analyzed by 1 H NMR and GPC.

Results and Discussion
According to the reported PET-RAFT polymerization, the redox property and lifetime of the excited state of the photocatalyst are important [20]. DTBT is a small organic molecule semiconductor. It has a broad absorption area in visible light, even to 550 nm ( Figure 1A), and can be excited by blue or green light. It has a low reduction potential value of -1.15 V and a long-living photogenerated exciton, [48] which is comparable with the mostly used transition metal photocatalyst (Ru(III)/Ru(II)* = -0.8 V) [12]. Therefore, we can envision that DTBT will be a promising photocatalyst for PET-RAFT polymerization.
In order to verify whether DTBT was effective for PET-RAFT polymerization, we chose N, N-dimethylacrylamide (DMA) as the model monomer, 2-((((2-carboxyethyl)thio)carbonothioyl) thio)-2-methylpropanoic acid (CEMP) ( Figure S1) as the RAFT agent, N, N-dimethylformamide (DMF) as the solvent, and a blue (λ max = 460 nm) or green (λ max = 530 nm) LED as the light source. When in the presence of DTBT, poly-(N,N-dimethylacrylamide) (PDMA) with an M n of 11600 Da and a narrow dispersity (Đ = 1.23) formed after 12.0 h irradiation (Table 1, Entry 1, Figure S2), and higher conversion would be reached if we increased the amount of catalyst (Table 1, Entry 2-3, Figure S3). In 1 H-NMR spectroscopy of the formed PDMA, the signal peak at 0.8 ppm was assigned to the proton of -CH 3 from the RAFT agent of CEMP ( Figure S2). Also, several control experiments were set to prove the roles of light, RAFT agent, and photocatalyst. As shown in Table 1, when we operated the experiment in dark conditions, no monomer consumption was detected over 12.0 h, indicating that polymerization was triggered by light. If we conducted polymerization under irradiation by green light without DTBT, no polymer was obtained because of very low monomer conversion (2.0%). But if blue light was used instead of green light, some polymers were obtained, because CEMP had an absorption in the blue region and a maximal absorption peak at 441 nm [49,50]. CEMP can be excited into the excitation state through the n→π* forbidden transition of the C=S group under blue light irradiation, then generate radicals, which could initiate polymerization [51][52][53]. Furthermore, a control experiment without the RAFT agent was also carried out, but no polymerization occurred either. All these results indicated that DTBT can act as a photocatalyst for PET-RAFT polymerization, and the trithiocarbonate molecule acted as both initiator and chain transfer agent. Based on the above study, we proposed the polymerization procedure in Scheme 2. DTBT generated conduction-band electrons (e − ) and valence-band holes (h + ) under green light. The photo-generated electron (e − ) had stronger reduction activity than the RAFT agent. Hence, the RAFT agent was deoxidized to produce a radical, which provided a radical source to initiate RAFT polymerization The thiocarbonylthio compound acted as both initiator and chain transfer agent, which was why DTBT became indispensable in polymerization. At the same time, the radical (P n •) may also be deactivated by holes (h + ) to regenerate the thiocarbonylthio compound, and the whole catalytic cycle would be restarted again.  Based on the above study, we proposed the polymerization procedure in Scheme 2. DTBT generated conduction-band electrons (e − ) and valence-band holes (h + ) under green light. The photogenerated electron (e − ) had stronger reduction activity than the RAFT agent. Hence, the RAFT agent was deoxidized to produce a radical, which provided a radical source to initiate RAFT polymerization The thiocarbonylthio compound acted as both initiator and chain transfer agent, which was why DTBT became indispensable in polymerization. At the same time, the radical (Pn•) may also be deactivated by holes (h + ) to regenerate the thiocarbonylthio compound, and the whole catalytic cycle would be restarted again.  Figure 1C) were observed. The plot of Mn,NMR ( Figure 1C and Figure S4) versus monomer conversion gave a linear relationship in good agreement with that of the Mn,th (calculated by NMR). It demonstrated that almost constant active radicals existed in this system, and DTBT had remarkable activity in PET-RAFT polymerization of acrylamide driven by low-energy green light in ambient conditions. Moreover, GPC curves showed a single peak of produced polymer with narrow dispersity (Đ < 1.30), and it shifted to a short retention time (high molecular weight) region along with an increased polymerization time.  Figure 1C) were observed. The plot of M n,NMR ( Figure 1C and Figure S4) versus monomer conversion gave a linear relationship in good agreement with that of the M n,th (calculated by NMR). It demonstrated that almost constant active radicals existed in this system, and DTBT had remarkable activity in PET-RAFT polymerization of acrylamide driven by low-energy green light in ambient conditions. Moreover, GPC curves showed a single peak of produced polymer with narrow dispersity (Đ < 1.30), and it shifted to a short retention time (high molecular weight) region along with an increased polymerization time. Chain extension is an available path to synthesize block copolymers to further attest fidelity of the polymeric chain, although the corresponding functional group has already been identified using 1 H NMR spectroscopy. Chain extension of PDMA was performed using N, N-dimethylaminoethyl acrylate (DMAEA) as the second monomer. The shift of trace GPC product to the higher molecular weight side (Figure 2A  Chain extension is an available path to synthesize block copolymers to further attest fidelity of the polymeric chain, although the corresponding functional group has already been identified using 1 H NMR spectroscopy. Chain extension of PDMA was performed using N, N-dimethylaminoethyl acrylate (DMAEA) as the second monomer. The shift of trace GPC product to the higher molecular weight side (Figure 2A Chain extension is an available path to synthesize block copolymers to further attest fidelity of the polymeric chain, although the corresponding functional group has already been identified using 1 H NMR spectroscopy. Chain extension of PDMA was performed using N, N-dimethylaminoethyl acrylate (DMAEA) as the second monomer. The shift of trace GPC product to the higher molecular weight side (Figure 2A  To take full advantage of light stimulus, we performed "On" (in the presence of light) and "Off" (in the absence of light) experiments to investigate the temporal control of this DTBT-catalyzed PET-RAFT system. When the light was switched to the "On" state, polymerization occurred. When the light was in the "Off" state, no polymerization occurred based on 1 H-NMR results ( Figure 3A). The final product with a low Đ of 1.20 and an expected molecular weight in line with the trait of living/controlled polymerization was obtained ( Figure 3B), which showed good ability of this system in the temporal control process. To take full advantage of light stimulus, we performed "On" (in the presence of light) and "Off" (in the absence of light) experiments to investigate the temporal control of this DTBT-catalyzed PET-RAFT system. When the light was switched to the "On" state, polymerization occurred. When the light was in the "Off" state, no polymerization occurred based on 1 H-NMR results ( Figure 3A). The final product with a low Đ of 1.20 and an expected molecular weight in line with the trait of living/controlled polymerization was obtained ( Figure 3B), which showed good ability of this system in the temporal control process. In light of the versatility of PET-RAFT polymerization, it could be used in broad monomers and a variety of RAFT agents. Therefore, we envisioned that our DTBT-catalyzed PET-RAFT system had the same capabilities. As shown in Table 2, S,S'-bis(α,α'-dimethyl-α''-acetic acid)trithiocarbonate (BDMAT) ( Figure S5), 2-(n-butyltrithiocarbonate)-propionic acid (BTPA) (Figure S6), and bis(carboxymethyl) trithiocarbonate (BCMT) were used as RAFT agents in this system instead of CEMP. As expected, all polymerizations ( Figure S7-S9) were carried out successfully using these trithiocarbonates as chain transfer agents and initiators, and as a result, polymers with narrow dispersity and unimodal peaks in GPC curves were obtained.
Subsequently, some other monomers, such as N- (3-(dimethylamino)propyl)acrylamide (DPAA), N, N-dimethylaminoethyl acrylate (DMAEA), N, N-diethylacrylamide (DEA) and N- (3-(dimethylamino)propyl)acrylamide (DPAA), were applied in this system. Polymerizations of DPAA, DMAEA, and DEA were carried out under the same conditions as those of DMA. Polymerization of DEA resulted in a conversion of 85.9% with a low Đ of 1.31, which was similar to that of DMA because of their similar structures. Polymerization of DMAEA and DPAA had lower monomer conversions than DMA and DEA, but still produced polymers (Figure S10-S13) with low Đ, revealing the generality and compatibility of DTBT as a photocatalyst for the polymerization of these two monomers.  In light of the versatility of PET-RAFT polymerization, it could be used in broad monomers and a variety of RAFT agents. Therefore, we envisioned that our DTBT-catalyzed PET-RAFT system had the same capabilities. As shown in Table 2, S,S -bis(α,α -dimethyl-α"-acetic acid)trithiocarbonate (BDMAT) ( Figure S5), 2-(n-butyltrithiocarbonate)-propionic acid (BTPA) (Figure S6), and bis(carboxymethyl) trithiocarbonate (BCMT) were used as RAFT agents in this system instead of CEMP. As expected, all polymerizations ( Figure S7-S9) were carried out successfully using these trithiocarbonates as chain transfer agents and initiators, and as a result, polymers with narrow dispersity and unimodal peaks in GPC curves were obtained.  in a conversion of 85.9% with a low Đ of 1.31, which was similar to that of DMA because of their similar structures. Polymerization of DMAEA and DPAA had lower monomer conversions than DMA and DEA, but still produced polymers (Figure S10-S13) with low Đ, revealing the generality and compatibility of DTBT as a photocatalyst for the polymerization of these two monomers.

Conclusions
In conclusion, we developed a small organic molecule, DTBT, as photocatalyst for PET-RAFT polymerization. DTBT had noticeable activity in catalyzing PET-RAFT polymerization of common monomers (acrylamides and acrylates), and applying in several chain transfer agents without any help of sacrificial reagents or precious metal co-catalysts. Furthermore, this DTBT-catalyzed system showed excellent control over molecular weight, dispersity, and high group fidelity with successful chain extension.