Assemblies of Polyacrylonitrile-Derived Photoactive Polymers as Blue and Green Light Photo-Cocatalysts for Cu-Catalyzed ATRP in Water and Organic Solvents

Photoluminescent nanosized quasi-spherical polymeric assemblies prepared by the hydrothermal reaction of polyacrylonitrile (PAN), ht-PLPPAN, were demonstrated to have the ability to photo-induce atom transfer radical polymerization (ATRP) catalyzed by low, parts per million concentrations of CuII complex with tris(2-pyridylmethyl)amine (TPMA). Such photo induced ATRP reactions of acrylate and methacrylate monomers were performed in water or organic solvents, using ht-PLPPAN as the photo-cocatalyst under blue or green light irradiation. Mechanistic studies indicate that ht-PLPPAN helps to sustain the polymerization by facilitating the activation of alkyl bromide species by two modes: 1) green or blue light-driven photoreduction of the CuII catalyst to the activating CuI form, and 2) direct activation of dormant alkyl bromide species which occurs only under blue light. The photoreduction of the CuII complex by ht-PLPPAN was confirmed by linear sweep voltammetry performed under illumination. Analysis of the polymerization kinetics in aqueous media indicated even though CuI complexes comprised only 1–1.4% of all Cu species at equilibrium, they exhibited high activation rate constant and activated the alkyl bromide initiators five to six orders of magnitude faster than ht-PLPPAN.


INTRODUCTION
Photocatalytic reactions play a profound role in various areas of chemical research (Zhu and Wang, 2017;Melchionna and Fornasiero, 2020). As alternatives to traditionally used transition metal complexes, organic photocatalysts (OPC) have received great attention, due to their low cost, highly tunable structures and photophysical properties (Romero and Nicewicz, 2016). More recently, organic photoactive nanostructured objects (OPNO) have emerged as promising materials owing to the growing interest in combining organic chemistry and nanotechnology (Han et al., 2018;Zhang et al., 2020). OPNO can span the range from covalently bonded "carbon dots" (Sun et al., 2006) to "polymer dots" (Zhu et al., 2015a;Zhu et al., 2015b;Tao et al., 2019;Xia et al., 2019) held together via non-covalent interactions. The synthesis of the latter type of OPNOs often relies on the pre-assembly or self-assembly of polymeric substrates, which afford additional control over the structures and properties of OPNOs (Chen and Tseng, 2017;Jia et al., 2017). Because of the heterogeneous nature of their photoactive domains, many OPNOs absorb broadly from UV to near infrared (NIR) light, making them promising candidates as photocatalysts, due to the tunable irradiation wavelength (Zhu et al., 2015b).
Nonetheless, two main limitations remain for OPNO-based OPCs. First, most reports only focused on reactions in organic solvents.
Water-soluble photocatalysts and aqueous photochemical reactions were largely omitted. Second, reactions using long-wavelength irradiations (other than blue or UV light) were underexplored, despite the advantages of longwavelength irradiations, such as better penetration depths (Romero and Nicewicz, 2016;Han et al., 2018). Overcoming these limitations is necessary for expanding the applications of OPNO-based OPCs, obtaining deeper understandings of the photocatalytic mechanism, and economizing the photocatalytic processes (Zhang et al., 2020).
The key to solve these challenges is advancing the synthesis of OPNOs, which is traditionally limited to a narrow selection of substrates, such as citric acid/ethylenediamine (Zhu et al., 2013), conjugated polymers (Zhao et al., 2019) etc., Recently, polyacrylonitrile (PAN) has caught the attention of many researchers. PAN has been used as a substrate in the manufacturing of carbon fibers and heteroatom-doped nanocarbons (Tang et al., 2005;Zhong et al., 2012;Kopeć et al., 2017;Gottlieb et al., 2019;Kopeć et al., 2019;Yuan et al., 2019;Yuan et al., 2020). The photoluminescent properties of PAN were largely ignored, since untreated isolated nitrile groups do not form photoactive conjugated structures. However, recent reports point out that PAN can generate photoluminescence when dissolved at high concentrations or densely grafted from flat silica surfaces (Jang et al., 2006;Zhou et al., 2016;Kopeć et al., 2020). Additionally, there are recent reports of chemical conversion of PAN into photoactive species via microwave (Go et al., 2017), pyrolysis (Cao et al., 2020) and hydrothermal reactions (Ermakov et al., 2000;Sun et al., 2020), which all can induce the crosslinking or the hydrolysis of -C≡N, resulting in photoluminescent crosslinked nanoparticles or polymers. In particular, we recently reported the synthesis of visible light-absorbing photoluminescent polymers (PLPs) from PAN via a one-step hydrothermal reaction . In certain solvents (e.g., water), the resulting PLPs can self-assemble into polymer dot-like OPNOs. For example, the hydrothermally synthesized photoluminescent polymer (ht-PLP PAN ) assembles in water into spherical aggregates with an average diameter of ca. 20 nm, due to the presence of both hydrophilic (i.e., carboxylic) and hydrophobic (i.e., aliphatic carbons) moieties. The assembly behavior was studied mainly by dynamic light scattering (DLS) in multiple solvents, including water and dimethyl sulfoxide (DMSO) . The high water solubility (>100 mg/ml) makes ht-PLP PAN a promising candidate as an OPNO-based OPC in aqueous systems.
Nevertheless, photoinduced CRPs in the presence of OPNOs primarily employed organic solvents and oleophilic monomers (Jiang et al., 2018;Kütahya et al., 2020;Kütahya et al., 2021). Although photoinduced CRPs in aqueous media were reported using other types of catalysts Pan et al., 2015;Szczepaniak et al., 2020), extending these processes to new catalysts with different structures and photoluminescent mechanisms is needed to expand the understanding of the reaction mechanisms. Furthermore, ATRP using OPNOs under long-wavelength irradiations is underexplored compared to RAFT polymerization.
Herein, we expand the applicability of photoinduced ATRP by developing Cu-catalyzed ATRP in the presence of ht-PLP PAN as OPCs under blue (λ max 450 nm) and green (λ max 520 nm) light irradiation. During initial attempts, ht-PLP PAN did not initiate the polymerization when used as a photoinitiator in free radical polymerization (FRP) under inert atmosphere (N 2 ). Nonetheless, well-controlled polymerizations were successfully performed in water and in organic solvents (DMSO, dimethylformamide or DMF, and anisole) in the presence of alkyl bromide initiators and 25-500 ppm of Cu II complexes (Br-Cu II /L, L ligand). A low loading of ht-PLP PAN (<1 mg/ml) and moderate light intensities (4-6 mW.cm −2 ) were used. Aqueous Cu-catalyzed ATRP of a water-soluble monomer, oligo (ethylene glycol) methyl ether methacrylate (OEGMA), is shown in Scheme 1. Studies performed under different irradiation wavelengths revealed that under blue light irradiation ht-PLP PAN could both reduce the Cu II complexes and activate alkyl halides. In contrast, under green light irradiation it was only capable to reduce the Cu II complexes.

Materials
Methyl methacrylate (MMA, 99%, Sigma-Aldrich, United States), oligo (ethylene glycol) methyl ether methacrylate (OEGMA, average MW 500, Sigma-Aldrich, United States), acrylonitrile (AN, 99%, Sigma-Aldrich) and methyl acrylate (MA, 99%, Sigma-Aldrich, United States) were purified by passing the monomers through a column filled with basic alumina to remove the inhibitor. Deionized water (DI water) was obtained from Millipore-Sigma Milli-Q water purification system. Azobisisobutyronitrile (AIBN, 98%, Sigma-Aldrich, United States) was recrystallized in anisole and stored at 4°C in dark. Tris (2-pyridylmethyl)amine (TPMA) was synthesized based on previously reported methods (Xia and Matyjaszewski, 1999 Instruments 1 H nuclear magnetic resonance (NMR) was conducted on a Bruker 500 MHz AVANCE III NMR spectrometer. Polymerizations in organic solvents used DMSO-d 6 as the solvent for 1 H NMR, and D 2 O for aqueous ATRP. DMF GPC was equipped with a refractive index (RI) detector, an Agilent 1260 Infinity II pump and a Wyatt Optilab T-rEX RI detector, with a PSS GRAM analytical column set (10 µm particle size) and LiBr-containing HPLC grade DMF as the eluent (LiBr: 0.05 M). Agilent EasiVial poly (methyl methacrylate) (PMMA) calibration kit (part number: PL 2020-0200) was used to calibrate the RI detector of GPC. Blue (λ max 450 nm, 5.0 mW.cm −2 ) and green (λ max 520 nm, 4.7 mW.cm −2 ) LED lights were purchased from aspectLED. The light intensity was measured by a ThorLabs PM100D compact power and energy meter console. The photoreactor for polymerization was prepared by mounting LED strips inside a round glass container, and air flow was applied to limit temperature increase during the polymerization. Solvent evaporation was performed on a Biotage V10 rapid solvent removal system. UV-vis spectra were taken on an Agilent Cary 60 UV-vis spectrometer and recorded by Cary WinUV software. Linear sweep voltammetries were measured by an Autolab PGSTAT302N potentiostat/ galvanostat (Metrohm) run by a computer with NOVA 2.0 software. Dynamic light scattering (DLS) was measured by a Malvern Zetasizer Ultra light scattering system. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2000EX TEM.

Synthesis of ht-PLP PAN
ht-PLP PAN , was synthesized via previously reported procedures . PAN 165 (subscript defines the degree of polymerization) was used to prepare ht-PLP PAN and was synthesized by initiators for continuous activator regeneration (ICAR) ATRP using BPN as the initiator (Lamson et al., 2016;Kopeć et al., 2017). In a typical procedure, a 10 mg/ml suspension of ball-milled PAN 165 in DI water was prepared. 10 ml suspension was stirred for 10 min and then transferred to a 25 ml autoclave reactor with high-temperature resistant liner. The autoclave reactor was securely sealed and placed in a pre-heated oven (180°C). The heating was turned off after 12 h and the oven was let to cool down to room temperature naturally. The dark brown solution was passed through a 0.22 µm syringe filter with a polyethersulfone membrane. The solution was directly dried by vacuum to yield brown solid ht-PLP PAN .

General Procedure for Light-Mediated ATRP in the Presence of ht-PLP PAN
In a typical procedure, 5 mg ht-PLP PAN was added to a 10 ml Schlenk flask containing 0.3 ml DMF and 7.2 ml DI water with a magnetic stir bar. HO-EBiB (3.9 µL, 0.027 mmol, 1 eq), CuBr 2 (0.60 mg, 2.7 µmol, 0.1 eq), TPMA (2.35 mg, 8.1 µmol, 0.3 eq), NaBr (41.17 mg, 0.4 mmol), OEGMA (2.5 ml, 5.4 mmol, 200 eq) were subsequently added to the Schlenk flask. The Schlenk flask was then purged with N 2 for approx. 25 min, and 0.1 ml of the reaction was withdrawn and was used as the "t 0" sample. Finally, the Schlenk flask was placed in the photoreactor, and light was turned on to start the polymerization. The conversion of OEGMA was monitored by 1 H NMR by withdrawing samples (∼0.1 ml each time) from the reaction mixture at different time points.

General Procedures for Voltammetric Measurements
Linear sweep voltammetries were carried out in a 5-neck electrochemical cell placed inside the photoreactor, equipped with a 3-electrode system and connected to an Autolab PGSTAT302N potentiostat/galvanostat (Metrohm) controlled by NOVA 2.0 software. The 3-electrode system was composed by: 1) a Pt foil counter electrode; 2) a homemade quasi-reference electrode: Ag|AgI|(0.1 M n-Bu 4 NI in DMF); 3) a glassy carbon (GC) disk tip (3 mm dia, Metrohm), connected to a rotating disk electrode (RDE) system, as working electrode. Before each experiment, the GC disk was cleaned by polishing with a 0.25-μm diamond

RESULTS AND DISCUSSION
Photophysical Properties of ht-PLP PAN ht-PLP PAN was synthesized based on a previously reported hydrothermal reaction . Dynamic light scattering (DLS) measurement suggested that ht-PLP PAN formed aggregates in water (Supplementary Figure 1). Transmission electron microscopy (TEM) revealed that the average diameter of the quasi-spherical assemblies was ca. 20 nm (Supplementary Figure 2). At first, using ht-PLP PAN as a photoinitiator in FRP was attempted under blue or green light irradiation. However, no polymerization was observed under N 2 atmosphere in the absence of additional initiators ( Table 1, entries 5 and 9, discussed in detail later). Thus, our focus shifted to using ht-PLP PAN as a reducing agent for the CuBr 2 complex with a common ATRP ligand, tris(2-pyridylmethyl) amine (TPMA). (Xia and Matyjaszewski, 1999).
To evaluate the capability of ht-PLP PAN in photoreducing the CuBr 2 /TPMA complex, photophysical properties of ht-PLP PAN (Figure 1) were analyzed. Our previous study reported a broad UV-vis absorption profile ( Figure 1A) and a short lifetime of the excited state ht-PLP PAN (<5 ns), suggesting the singlet nature . Putative photophysical properties of ht-PLP PAN were modeled using density functional theory (DFT) calculations with model oligoimine-based structures of different conjugation lengths oI N (where N 3-10 denotes the number of nitrogen atoms along the backbone), which were deemed likely to arise in the course of hydrothermal treatment ( Figure 1B, see Supplementary Material for Cartesian coordinates).  In all instances, the energies of the lowest unoccupied molecular orbital (LUMO) of oI N were higher than the LUMO (β) of [Br-Cu II /TPMA] + , indicating conditions favorable for reduction of the Cu II complex upon photoexcitation ( Figure 1C). Cu-Catalyzed ATRP in Water Using ht-PLP PAN Blue (λ max 450 nm) and green light sources (λ max 520 nm) were chosen for photoinduced ATRP using ht-PLP PAN . Oligo (ethylene glycol) methyl ether methacrylate with an average molecular weight of 500 (OEGMA 500 ) was polymerized using ATRP catalyzed by a CuBr 2 -complex with TPMA as the ligand (molar ratio: CuBr 2 /TPMA 1/3) ( Table 1). 100 ppm or 500 ppm (relative to the monomer concentration) of CuBr 2 / TPMA were used, with 2-hydroxyethyl 2-bromoisobutyrate (HO-EBiB) as the ATRP initiator and 0.5 mg/ml ht-PLP PAN as the photo-cocatalyst. Additionally, 40 mM of NaBr was added to suppress the dissociation of the weak Cu II -Br bond in the ATRP deactivator (Simakova et al., 2012;Fu et al., 2018). SCHEME 1 | (A) Schematic representation of the synthesis of ht-PLP PAN , and an image of ht-PLP PAN dissolved in water under UV light excitation (365 nm) (B) Proposed assembled structure of ht-PLP PAN based on the previous report , and the use of ht-PLP PAN as the photo-cocatalyst in Cu-catalyzed lightmediated ATRP. Functional groups highlighted in blue represent typical hydrophilic fragments.
Frontiers in Chemistry | www.frontiersin.org August 2021 | Volume 9 | Article 734076 First, polymerizations were performed under blue light irradiation. Linear semilogarithmic kinetic plots for ATRP using both 500 ppm (entry 1, Table 1) and 100 ppm (entry 2, Table 1) of the Cu II complex are shown in Figure 2A. Polymer molecular weight and dispersity (Đ) were measured by gel permeation chromatography (GPC) using dimethylformamide (DMF) as the mobile phase ( Figure 2B). Increasing the loading of CuBr 2 /TPMA from 100 ppm (entry 2) to 500 ppm (entry 1) resulted in a decreased Đ from 1.69 (entry 2) to 1.35 (entry 1), due to the higher equilibrium concentration of the deactivator.  Frontiers in Chemistry | www.frontiersin.org August 2021 | Volume 9 | Article 734076 5 Clean shifts of the molecular weight distribution (MWD) traces are shown in Supplementary Figure 4. It is worth mentioning that the difference in the molecular weight measured from the light scattering (LS) detector of GPC (M n,GPC ) and the theoretical molecular weight (M n,theo ) was likely due to different polymercolumn interactions between poly (OEGMA) and the calibration standard (PMMA) used for the calibration of the LS detector. Figure 2A also illustrates that the rate of polymerization for entry 1 (500 ppm of Cu complex) was faster than that for entry 2 (100 ppm of Cu complex). This difference was attributed to the higher concentration of propagating radicals in entry 1 and faster reduction of the Cu II complex. Similar kinetic results were previously reported by several ATRP methods based on activator regeneration (Simakova et al., 2012;Mendonça et al., 2014). Despite the lower monomer conversion determined by 1 H NMR, entry 2 showed a higher M n,GPC than entry 1 ( Table 1). This difference indicated that the initiation efficiency of HO-EBiB was lower when a lower loading of CuBr 2 /TPMA was used in entry 1.
In order to confirm the role of ht-PLP PAN , a control experiment was performed in its absence (entry 3, Table 1). No substantial monomer conversion (<5% after 8 h) was observed, indicating that the ht-PLP PAN was required to generate the Cu I activator complex. In addition, the possibility of direct activation of the alkyl bromide (R-Br) initiator by ht-PLP PAN was considered. A control experiment (entry 4, Table 1) showed that under blue light irradiation HO-EBiB was indeed activated in the presence of ht-PLP PAN and in the absence of the Cu II complex. The polymerization was uncontrolled (Đ 2.74) due to the absence of Cu II deactivators. In comparison, no polymerization occurred when no HO-EBiB was added, indicating that ht-PLP PAN did not generate radicals directly from the monomer. A quantitative comparison between the activation of R-Br by the Cu I complex and by ht-PLP PAN is presented in the Photocatalytic Mechanism section.
Similar polymerizations were conducted under green light irradiation (entries 6-7, Table 1 and Figure 2). In general, polymerization rates under green light irradiation were slower than those under blue light irradiation, and proceeded only after an induction period ( Figure 2A). Finally, similar control experiments (entries 8 and 9, Table 1) were performed under green light. In contrast with the control experiments performed under blue light, no polymerization was observed under green light irradiation in the absence of Cu II , indicating that ht-PLP PAN luminophores excited by this range of wavelengths were not capable to activate the alkyl bromide initiator under green light, even after prolonged irradiation (48 h, entry 8, Table 1).
In addition to the already discussed lower "reducing power" of longer-wavelength luminophores inferred from DFT model calculations, two other factors that could be responsible for the slower polymerization and absence of ht-PLP PAN -driven activation of R-Br under green light are: 1) the lower light absorption of ht-PLP PAN at 520 nm (Figure 1), and 2) the slightly lower intensity of the green light photoreactor (4.7 mW.cm −2 , vs 5.0 mW.cm −2 of the blue light photoreactor). Nevertheless, green light irradiation still resulted in reasonably well-controlled polymerization of OEGMA 500 (Đ 1.38) using 500 ppm of Cu II /TPMA (entry 6, Table 1).

Cu-Catalyzed ATRP in Organic Solvents Using ht-PLP PAN
To demonstrate the versatility of ht-PLP PAN as OPCs, a mixed solvent containing 1:1 (v/v) of DMSO and DMF was first used to polymerize MMA with ethyl α-bromophenylacetate (EBPA) as the initiator. It is worth mentioning that ht-PLP PAN only partially dissolved when using DMSO/DMF due to the lower polarity of the reaction media, especially after the addition of MMA. Despite the weaker solubility of ht-PLP PAN , well-controlled polymerizations were obtained with ht-PLP PAN dispersed in the solvent using 100 ppm or 25 ppm of CuBr 2 /TPMA as the catalyst ( Table 2).
Similar to the case of aqueous media, better control was obtained for the polymerization of MMA with a higher loading of CuBr 2 /TPMA (entries 1 and 5, Table 2). However, 500 ppm of the Cu complex were required to control the process in aqueous media (entries 1 and 6, Table 1), while concentrations of Cu II complex as low as 25 ppm were sufficient in organic solvents (entry 6, Table 2). Linear semilogarithmic kinetic plots and the GPC traces observed under those conditions are shown in Figure 3. Clean shifts of GPC traces were also observed (Supplementary Figure 5). Additionally, the direct activation of EBPA by ht-PLP PAN was evaluated by control experiments in the absence of CuBr 2 /TPMA (entries 3 and 7, Table 1). Similar to results in aqueous media, ht-PLP PAN activated EBPA only under blue light irradiation. The supplemental activation of EBPA under blue light irradiation likely contributed to the higher dispersity when 25 ppm loading of CuBr 2 /TPMA was used (entry 2). The uncontrolled polymerization in the absence of any Cu complexes indicated that no deactivations occurred due to the absence of deactivators.
Finally, ATRP using ht-PLP PAN was extended to other monomers and solvents (Supplementary Table 1). For example, methyl acrylate (MA) was polymerized using 100 ppm of the CuBr 2 /TPMA complex and ethyl α-bromoisobutyrate (EBiB) as the initiator (entry 1, Supplementary Table 1). Good control over the polymerization was illustrated by the low Đ 1.16 of the polymer (Supplementary Figures 6A,B). Additionally, ATRP of MMA was performed using 100 ppm of the CuBr 2 /TPMA complex and anisole as a less polar solvent (entry 2, Supplementary Table 1). The rate of polymerization was slower in anisole compared to DMSO/DMF, as expected from the variation of the ATRP equilibrium constant, K ATRP , with solvent polarity (Ribelli et al., 2019). Nonetheless, well-controlled polymerization (Đ 1.19) was still observed (Supplementary Figures 6C,D).

Photocatalytic Mechanism and Comparison Between Activation by Cu I Complex and by ht-PLP PAN
In some ATRP methods with regeneration of the Cu I /L activator, the agent used for the activator regeneration can also contribute Frontiers in Chemistry | www.frontiersin.org August 2021 | Volume 9 | Article 734076 to the activation of dormant alkyl bromides to form propagating radicals, which generally occurs by reaction between the agent and the alkyl bromide (Konkolewicz et al., 2013;Konkolewicz et al., 2014a;Konkolewicz et al., 2014b;Zhang et al., 2011). To better understand the polymerization mechanism of the current system, the photoreduction of the Cu II complex by ht-PLP PAN was first monitored by linear sweep voltammetry (LSV). As shown in Figure 4, LSV was scanned from ca. 0.5 V (vs SCE) where the Cu I complex is oxidized if it exists. Indeed, the increased intensity of the anodic current under blue or green FIGURE 3 | Photoinduced ATRP in organic solvents. First-order kinetic plots corresponding to (A) entries 1 (blue, 100 ppm) and 2 (blue, 25 ppm) (B) entries 5 (green, 100 ppm) and 6 (green, 25 ppm) of Table 2. (C) GPC traces corresponding to the polymer product of entries 1, 2, 5 and 6 of Table 2.
FIGURE 2 | Photoinduced ATRP in water (A) First-order kinetic plots corresponding to entries 1 (blue, 500 ppm), 2 (blue, 100 ppm), 6 (green, 500 ppm) and 7 (green, 100 ppm) of  Figure 7A). The small extent of photoreduction in the latter case was likely caused by the presence of excess ligand that can act as the electron donor. Notably, the LSV traces acquired in water did not show a consistent increase of Cu I /L concentration (Supplementary Figure 7B), in contrast with polymerization results ( Table 1) that indicated ht-PLP PAN was capable of photoreducing the Cu II complex in water. This apparent discrepancy indicates that the amount of Cu I produced by ht-PLP PAN in water was below the reliable LSV detectability threshold but was still sufficient to effectively initiate polymerization owing to the higher K ATRP of Cu catalysts in water (Tang et al., 2008;Fantin et al., 2015). It should be pointed out that the apparent lower reducing efficiency of ht-PLP PAN in water in comparison with organic solvents could be caused by its nano-assembly, which, while important for allowing these mostly hydrophobic species to function in water, would inevitably decrease the number of accessible photocatalytic sites.
To further understand the composition of the catalytic system for aqueous ATRP under blue light irradiation, the following calculations were performed to quantify the concentrations of Cu I and Cu II species in polymerizations corresponding to conditions in Table 1. The activation rates of alkyl bromides by Cu I /L and ht-PLP PAN were estimated for the polymerization of OEGMA (entries 1, 2 and 4 of Table 1). First, the rate of activation by the Cu I complex (R a1 ) was given by Eq. (1), (Matyjaszewski 2012;Krys et al. 2016;Krys and Matyjaszewski 2017)  On the other hand, the activation rate of alkyl bromides by ht-PLP PAN (R a−pc ) was determined from Eq. 3: where, R p, without Cu refers to the rate of polymerization in the absence of Cu II complex. k t is the termination rate constant of OEGMA, (Smolne et al., 2016) and [R·] is the concentration of propagating radicals. Details of the calculation, including the scaling of kinetic parameters, are included in the Supplementary Material (Supplementary Tables 2-4) (Fantin et al., 2015;Fantin et al., 2017). Specifically, R p, without Cu was monitored by 1 H NMR (entry 4 of Table 1, Supplementary Figure 8). The polymerization was slower when no Cu complex was present, and polymers with high molecular weight (over seven times higher than M n,theo ) were observed by GPC, indicating a low initiation efficiency when ht-PLP PAN was the only activator.

CONCLUSIONS AND PERSPECTIVES
In conclusion, a dispersible PAN-derived photo-cocatalyst, ht-PLP PAN , was applied as OPCs in Cu-catalyzed ATRP under blue or green light irradiation. Hydrophilic and hydrophobic functional groups prompted ht-PLP PAN to assemble into spherical OPNOs in water. Compared to previously reported photoinduced CRPs using self-assembled photocatalysts, this work expanded the utilization of OPNO-based photocatalysts or photo-cocatalysts from organic solvents (DMF/DMSO, anisole) to aqueous media. Well-controlled polymerization of acrylate and methacrylate monomers were reported. Furthermore, analysis of the R-Br activation demonstrated how the irradiation wavelength affected the activation mechanism of Cu-catalyzed ATRP. Control experiments showed that both ht-PLP PAN and the Cu I complex could activate R-Br under blue light irradiation. While under green light irradiation, the Cu I complex was the only activator for R-Br. But the reaction between the Cu I complex and R-Br was five to six orders of magnitude faster than the activation of R-Br by ht-PLP PAN , due to the high activation rate constant of the Cu I complex, particularly in aqueous media.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
MS designed and performed the experiments. FL designed and performed the voltammetric analysis. RY prepared the polymer substrate. MS, FL, SD-S, TK and KM discussed the photocatalytic mechanism. TK performed the DFT calculations. All authors contributed to the preparation and revision of the manuscript.

FUNDING
Financial support from the Department of Energy (ER45998) is greatly appreciated.