Introduction

Atom transfer radical polymerization (ATRP), discovered in 1995, is an efficient way for living radical polymerization [1,2,3]. In ATRP, transition metal salts or complexes [4, 5] have been employed to maintain a dynamic equilibrium between active species and dormant species. One major shortcoming of classical ATRP was due to the use of relatively high catalyst concentration relative to monomer in contaminating the final polymer. Some efficient technologies have been developed to reduce the catalyst concentration [6,7,8], even ATRPs were performed without catalyst [9,10,11].

Recently, light-induced polymerization techniques have been a dramatic resurgence of interest in the field of photomediated ATRP due to its easy operation at ambient temperature and atmospheric pressure, and they offered spatial and temporal control over the polymerization [12,13,14]. In this system, photoinitiator played a key role in the photopolymerization. Many low-molecular weight photoinitiators and their derivatives, for example, benzophenone [15], phenothiazine [16], and phenoxazine [17], have been reported in photomediated ATRP. In addition, some semiconductor nanoparticles and transition metal-based photoredox catalysts, such as ZnO [18], Fe/ZnO [18], TiO2 [19], and Fe2O3 [20], have also been used in photomediated ATRP systems. However, the photomediated ATRP described above were carried out under UV, visible, and blue light [21]. There are some limits to use the short-wavelength light, for example, undesired side reactions [22]. Some dyes absorbed in the near-infrared (NIR) and red region have attracted more and more attention. Excellent efficiencies were reported using chlorophyll a [22,23,24] in living radical polymerization, perylene derivative in cationic photopolymerization [25], etc. In the case of bacteriochlorophyll a as photoinitiator, far-red (λ = 780 nm) and near-infrared light (λ = 850 nm) have been successfully employed for the activation of photoinduced electron transfer/reversible addition-fragmentation chain transfer (PET–RAFT) polymerization, respectively [22].

Recently, photoinduced metal-free ATRPs have been increasingly attracting attention [26,27,28]. Fluorescein (FL) [29] and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [30] have been also reported in photoinduced metal-free ATRP. Miyake et al. have reported perylene as a photoorganocatalyst for the metal-free ATRP of (meth)acrylates and styrene through direct reduction of alkyl bromides in its photo-excited state to induce the polymerization [31]. Our group reported that perylene derivatives 3,4,9,10-tetra-(12-alkoxycarbonyl)-perylene was used as photocatalyst in metal-free ATRP under blue light irradiation [32]. Some perylene derivatives have been reported to absorb in this region of the NIR spectrum. For example, pentarylenebis(dicarboximide) dye: (1,6,13,18-tetra(4-(2,3,3-trimethylbuty-2-yl)phenoxy)-N,N’-(2,6-diisopropylphenyl)-pentarylene-3,4,15,16-tetracarboxidiimide) (TTPDPT), which was first synthesized in 2006 [33], absorbed in near-infrared region.

In this work, we employed this photoinduced living polymerization technique for the controlled polymerization of methyl methacrylate (MMA) with TTPDPT as photocatalyst in the presence of prior deoxygenation processes under NIR light irradiation. The block copolymers were synthesized through successive chain extensions. Furthermore, the effect of light on this photoinduced metal-free ATRP was investigated in this article.

Experimental

Materials

The chemical structure of the studied TTPDPT is shown in Scheme 1. MMA and styrene (St) were purchased from Tianjin Bodi Chemical Co. (Tianjin, China) and purified by vacuum distillation before use. Methyl α-bromoisobutyrate (MBI) was purchased from Shanghai Aladdin Reagent Co. (Shanghai, China) and used as received. Other reagents were commercially purchased. The synthesis of the TTPDPT was conducted according to the literature procedures [33]. The detailed procedure is seen in Supplementary materials.

Scheme 1
scheme 1

Chemical structure of TTPDPT

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Photopolymerization

The photoinduced metal-free ATRP of MMA was subsequently adopted for the following photopolymerization reactions. A deoxygenated mixture containing methyl methylacrylate (MMA, 10 mmol, 1 g), dichloromethane (CH2Cl2, 20 mL), methyl α-bromoisobutyrate (MBI, 1 mmol, 0.1813 g) and TTPDPT (20 ppm) was introduced into 50-mL three-neck round-bottom flask equipped with a magnetic stirrer, which was subjected to NIR light irradiation (830–890 nm, 35 W/cm2) in a water bath at room temperature for a period of time. After the polymerization, the reaction mixture was diluted with tetrahydrofuran (THF) and then it was poured into methanol. The product was filtered from the solution and dried under vacuum for 24 h at 60 °C.

Light sources

Photomediated ATRP of MMA was carried out in the flask where the reaction mixtures were irradiated by near-infrared (NIR) laser irradiation (Shenzhen Xinkun Yang Science and Technology Co.). The distance of the flask to light source was 10 cm.

Chain extension polymerization

A PMMA macroinitiator was prepared with [MMA]0/[MBI]0 = 100/1 under NIR light irradiation. The concentration of TTPDPT is 20 ppm. Chain-end functionality of the PMMA macroinitiator was confirmed using 1H NMR and chain extension. The initial PMMA macroinitiator was successfully chain extended with St monomer to yield P(MMA-b-St).

Characterization

1H nuclear magnetic resonance (1H NMR) measurements were recorded in CDCl3 on a Bruker 400 MHz spectrometer (Bruker, Germany).

Molecular weight properties of the polymers were determined by gel permeation chromatography (GPC). The GPC instrument (Waters, America) has a Waters 515 pump and a Waters 2414 differential refractometer using Waters Styragel columns (HR-1, HR-2, and HR-3) with THF as eluent at 35 °C and a flow rate of 1 mL/min. Linear PMMA standards were used for calibration. The Mark–Houwink–Sakurada equation (MHS) was used to convert the PMMA molecular weight with MHS parameters: KPMMA = 12.8 × 10− 5 dL/g and αPMMA = 0.69 [34].

UV spectrum was collected using TU-1901 double beam UV–Vis spectrophotometer (Beijing Purkinje General Instrument Co., China).

Results and discussion

UV spectrum of TTPDPT

UV spectrum of TTPDPT in CH2Cl2 is shown in Fig. 1. As shown, a CH2Cl2 solution of TTPDPT (0.001 mmol/L) exhibits wide-range optical absorption in the NIR region with a strong absorption at λmax = 876 nm. Two relatively weak absorption peaks at λmax = 257 nm and 833 nm were detected [33], respectively.

Fig. 1
figure 1

The UV spectrum of TTPDPT in CH2Cl2

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Kinetics studies of photoinduced metal-free ATRP of MMA under NIR light irradiation

The UV spectrum of TTPDPT showed strong absorption at 876 nm (Fig. 1) which belonged to the NIR region of the solar spectrum. The photoinduced metal-free ATRP reduced the side reaction with wavelength of lower energy such as NIR light. Initial photoinduced metal-free ATRPs of MMA with TTPDPT were carried out with a molar ratio of [MMA]0/[MBI]0/[TTPDPT]0 of 150/1/0.0001 under NIR (λmax = 876 nm) LED irradiation. Kinetic analysis showed a linear increase of ln([M]0/[M]) vs. irradiation time (Fig. 2). Interestingly, we observed a typical induction period of around 2.4 h in the polymerization, indicating that the propagating radicals were almost unchanged throughout the polymerization process. The induction period was due to the generation of free radicals which initiated the polymerization rests with the electron transfer process from the photo-excited TTPDPT to MBI [17]. A similar phenomenon was observed in the other photoinduced ATRP [27, 35, 36]. It indicated that there needed some time to establish the dynamic equilibrium between the active propagating radicals and the dormant species, which resulted in the slow formation of the carbon-centered radicals [31]. As shown in Fig. 3, the molecular weight of Mn,GPC increased linearly with monomer consumption. The good correlation between the theoretical values (Mn,th) and the experimental values (Mn,GPC) further confirmed the controlled character of the polymerization. Furthermore, lower Mw/Mn values (< 1.5) were obtained. The corresponding GPC curves are shown in Fig. S1 (Supplementary Materials).

Fig. 2
figure 2

Kinetic studies of photoinduced metal-free ATRP of MMA vs. irradiation time catalyzed by TTPDPT under NIR light irradiation at room temperature in CH2Cl2

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Fig. 3
figure 3

Number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) vs. conversion for photoinduced metal-free ATRP reaction

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The effect of [TTPDPT]0 content

To investigate the effect of TTPDPT in the photoinduced metal-free ATRP reaction, a range of experiments was performed with the molar ratio of [MMA]0/[MBI]0 of 150:1 (Table 1). The photopolymerization was first carried out in the absence of TTPDPT, and no conversion was observed (Entry 1, Table 1) for 24 h under NIR irradiation. When TTPDPT was added, PMMA at 10.46% monomer conversion was obtained after 10 h irradiation (Entry 2, Table 1), indicating that the TTPDPT is essential in this system. Furthermore, the conversion increased with increasing the amount of TTPDPT (Entries 2, 3, 4, 5, and 6, Table 1). The reaction mixture was placed in dark conditions for 24 h where no polymers were yielded (Entry 7, Table 1). More importantly, Mn,GPC increased with monomer conversion and were in accordance with the values of Mn,th. In addition, the resulting PMMA displayed a well-controlled molecular weight distribution (Mw/Mn<1.50), demonstrating the feasibility of TTPDPT to conduct the photoinduced ATRP reaction.

Table 1 Effect of [TTPDPT]0 content on photoinduced metal-free ATRP of MMA at 25 °C in CH2Cl2
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Effect of light on photoinduced metal-free ATRP of MMA

To investigate the photo-regulating effect of TTPDPT for the photoinduced metal-free ATRP reaction, the light was switched “on” and “off” periodically during the photoinduced metal-free ATRP process (Fig. 4). The reaction mixture was first exposed to NIR light for 5 h, affording 13.9% conversion of monomer (0–5 h, Fig. 4). The polymerization ceased immediately when the light source was removed (5–6 h, Fig. 4), indicating that the amount of active radical was ignored under dark conditions. When the reaction mixture was re-exposed to NIR light source (6–7 h, Fig. 4), the polymerization continued with the same polymerization rate as observed in the former light-on process. Subsequently, such cycle with “on–off” light switch could be repeated multiple times. These experiments successfully verified that the growth of polymer chains can be well regulated by periodic light control process.

Fig. 4
figure 4

Photoinduced metal-free ATRP of MMA using TTPDPT as photocatalyst with repeated ‘on–off’ cycling

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Analysis of chain end and chain extension of PMMA macroinitiator

The chain end of PMMA-Br macroinitiator prepared by the photoinduced metal-free ATRP method was analyzed by 1H NMR (Fig. 5).

Fig. 5
figure 5

1H NMR spectrum for PMMA-Br macroinitiator prepared by photoinduced metal-free ATRP

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As shown in Fig. 5, the signal at 3.76 ppm (a in Fig. 5) corresponded to the protons of methyl groups (–CH3) next to the halogen chain end, which showed deviation from the chemical shift 3.61 ppm (b in Fig. 5) because of the electron-attracting function of ω-Cl atom. The chemical shifts at 0.51–1.27 ppm (d in Fig. 5) and 1.54–2.05 ppm were attributed to the methyl group (–CH3) and methylene group (–CH2–), respectively. The chain extension experiments were carried out with the prepared above PMMA-Br as macroinitiator (Fig. 6). As shown in Fig. 6, the Mn,GPC increased from 5600 to 18,600 g/mol. The Mw/Mn changed from 1.34 to 1.52. The chain extension experiments demonstrated the process in a living radical polymerization way. 1H NMR spectrum of P(MMA-b-St) block copolymer is shown in Fig. 7, which demonstrated further the formation of P(MMA-b-St) block copolymer.

Fig. 6
figure 6

GPC curves of PMMA-Br before and P(MMA-b-St) after chain extension via photoinduced metal-free ATRP

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Fig. 7
figure 7

1H NMR spectrum of P(MMA-b-St)

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Analysis of mechanism

The proposed mechanism for photoinduced metal-free ATRP of MMA is provided in Scheme 2. As shown in Scheme 1, TTPDPT (PCn) was first excited under NIR light irradiation to generate the excited TTPDPT (PCn*). Then, a MBI molecule was reduced by the electron transfer from a photo-excited PC (PCn*) and provided a radical (Pn-R•) and PCn+1X−. The Pn-R·radical can initiate MMA monomer to generate Pn-R-MMA. PCn+1X− can deactivate the propagating polymer chain Pn-R-MMA·to generate Pn+1-R-X and ground state PCn. The PCn can re-enter the catalytic cycle under NIR light irradiation.

Scheme 2
scheme 2

Mechanism of photoinduced metal-free ATRP of MMA using TTPDPT as photocatalyst under NIR light irradiation

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Conclusion

Photoinduced metal-free ATRP of MMA was successfully carried out in CH2Cl2 at 25 °C with TTPDPT as photocatalyst and MBI as ATRP initiator under NIR light irradiation. The MBI can be reduced by the excited TTPDPT through electron transfer process, which generated radicals to initiate the polymerization. The ATRP of MMA followed good first-order kinetics throughout the reaction. The Mn,GPC values increased with monomer conversion and were well in agreement with the theoretical values (Mn,th). Lower Mw/Mn values were obtained. The polymerization rate increased with increasing the amount of TTPDPT. However, the polymerization stopped in the absence of TTPDPT and continued in the presence of TTPDPT under light irradiation. Furthermore, the polymerization can be easily governed by on/off switching light. It is noteworthy that NIR light source, which was easy, convenient, and inexpensive, can be used to initiate the polymerization reaction. A successful well-defined di-block copolymer was obtained through chain extension experiments.