Reversible Addition-Fragmentation Chain Transfer Polymerization of Acrylonitrile under Irradiation of Blue LED Light

Compared to unhealthy UV or γ-ray and high-energy-consumption thermal external stimuli, the promising light emitting diode (LED) external stimulus has some outstanding technological merits such as narrow wavelength distribution, low heat generation and energy consumption, and safety for human beings. In this work, a novel reversible addition-fragmentation transfer (RAFT) polymerization system for acrylonitrile (AN) was developed under the irradiation of blue LED light at room temperature, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a novel radical initiator and 2-cyanoprop-2-yl-1-dithionaphthalate (CPDN) as the typical chain transfer agent. Well-defined polyacrylonitrile (PAN) with a controlled molecular weight and narrow molecular weight distribution was successfully synthesized. This strategy may provide another effective method for scientific researchers or the industrial community to synthesize a PAN-based precursor of carbon fibers.


Introduction
It is generally known that carbon fibers [1][2][3][4] are an important material for the development of advanced technology. Due to the outstanding properties of carbon fibers, among which are a low density, high axial strength, high modules, excellent fatigue resistance, high temperature resistance and corrosion resistance, they have been utilized in varieties of military and civilian industries, including aerospace, aviation, machineries, textiles, sports equipment and leisure products. Compared with carbon fibers based on other precursors, polyacrylonitrile (PAN)-based carbon fibers [5][6][7][8] with impressive tensile strength and mechanical properties have occupied the greatest market share in the carbon fiber field. However, the experimental measured values of the tensile strength of PAN-based carbon fibers are far below the theoretically predicted values due to the fact that conventional PAN precursors polymerized by solution, suspension or emulsion polymerization have impurities and defects [1]. Therefore, a high-quality PAN precursor with a low molecular weight distribution is essential to prepare PAN-based carbon fibers.

Characterizations
The number-average molecular weight (M n,GPC ) and molecular weight distribution (M w /M n ) values of the obtained polymers were determined by a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (Tosoh Bioscience Shanghai Co. Ltd., Shanghai, China), using TSK gel guardcolumn superMP-N (4.6 mm × 20 mm) and two TSK gel supermultipore HZ-N (4.6 mm × 150 nm) with measurable molecular weights ranging from 10 2 to 10 6 g·mol −1 . DMF with 10 mmol·L −1 of LiBr was used as eluent at a flow rate of 1.0 mL·min −1 at 40 • C. GPC samples were injected using a TOSOH plus autosampler and calibrated with PS standards purchased from TOSOH (Tosoh Bioscience Shanghai Co. Ltd., Shanghai, China). The 1 H Nuclear Magnetic Resonance (NMR) spectra of polymers was recorded on a Bruker 300 MHz nuclear magnetic resonance instrument (Bruker Daltonics Inc., Billerica, MA, USA) using DMSO-d 6 as the solvent and tetramethylsilane (TMS) as the internal standard. Matrix assisted laser desorption/ionization time-of-flight mass spectra were recorded with an UltrafleXtreme MALDI-TOF mass spectrometer (Bruker Daltonics Inc., Billerica, MA, USA) equipped with a 1000 Hz smart beam-II laser.

Results and Discussion
Based on our previous work, 4CzIPN, as a novel organic photocatalyst in metal-free ATRP, can also solely initiate polymerization under blue LED irradiation [44]. Therefore, we employed 4CzIPN as a photoinitiator in the RAFT polymerization of AN mediated by CPDN in this work for the first time. Well-defined PANs with controlled molecular weights and molecular weight distributions were synthesized by a series of polymerizations under varied conditions.
The results are shown in Table 1. In the absence of 4CzIPN (entry 1 in Table 1) or blue LED irradiation (entry 3 in Table 1), no polymers were obtained even after 24 h of polymerization time, while 55.1% of the monomer conversion was achieved in 3.5 h to yield PAN with a high-number-average molecular weight and a broad molecular weight distribution (M n,GPC = 155,700 g·mol −1 , M w /M n = 2.26) under blue LED irradiation (entry 2 in Table 1) in the absence of CPDN. It indicates that 4CzIPN can indeed serve as a novel photoinitiator and CPDN as a chain transfer agent, making them great choices for well-controlled RAFT polymerization of AN. The polymerizations in the presence of 4CzIPN resulted in PANs with calibrated number-average molecular weights [28] close to the corresponding theoretical ones and narrow molecular weight distributions. Although the level of molecular weight control with 4CzIPN was similar to that of unhealthy UV-sensitive TPO (entry 5 in Table 1) and thermally unstable AIBN (entry 6 in Table 1), visible-light-sensitive 4CzIPN (entry 4 in Table 1) as a novel initiator is more suitable for CPDN-mediated RAFT polymerization of AN, due to the lower energy consumption and being environmentally friendly. Additionally, the effect of the degree of polymerization targets for blue-LED−photoinduced RAFT polymerization of AN was further investigated (entries 7−10 in Table 1). Moreover, with the increase of the molecular weights of the PANs, the trend of higher molecular weight distribution was also observed. It is well known that the establishment of reversible dynamic equilibrium between active species with dormant species in RAFT polymerization would be retarded due to the diffusion control [49] of free radicals in a high-viscosity mixture of high-molecular-weight PANs.  that the establishment of reversible dynamic equilibrium between active species with dormant species in RAFT polymerization would be retarded due to the diffusion control [49] of free radicals in a high-viscosity mixture of high-molecular-weight PANs. To deeply understand the polymerization behaviors of AN, the polymerization kinetics with a molar ratio of [AN]0:[CPDN]0:[4CzIPN]0 = 400:1:0.01 were carried out under blue LED irradiation at room temperature. An obvious induction period was observed (Figure 1a), which may be attributed to the slow decomposition of intermediate radical species. The pseudo-first-order polymerization kinetic plot indicated that the radical concentration remained approximately constant during the RAFT polymerization period. The calibrated number-average molecular weights which approached the corresponding theoretical ones almost linearly increased with monomer conversion (Figure 1b). In addition, the obtained PANs all have moderately narrow molecular weight distributions (Mw/Mn < 1.4). All evidence above confirmed the "living"/controlled features of this novel blue-LED−photoinduced RAFT polymerization of AN. For further insight into the "living" feature of the blue-LED−photoinduced RAFT polymerization of AN, the chain-end group of PAN was analyzed by 1 H NMR in DMSO-d6 as shown in Figure 2a. The signals at chemical shifts δ = 7.6-8.2 ppm (a in Figure 2a) were assigned to the protons of naphthyl from CPDN. The signals at chemical shifts δ = 2.1 ppm (b in Figure 2a) and δ = 3.1 ppm (c in Figure 2a) were assigned to the protons of methylene and methine in the PAN backbone, For further insight into the "living" feature of the blue-LED−photoinduced RAFT polymerization of AN, the chain-end group of PAN was analyzed by 1 H NMR in DMSO-d 6 as shown in Figure 2a. The signals at chemical shifts δ = 7.6-8.2 ppm (a in Figure 2a) were assigned to the protons of naphthyl from CPDN. The signals at chemical shifts δ = 2.1 ppm (b in Figure 2a) and δ = 3.1 ppm (c in Figure 2a) were assigned to the protons of methylene and methine in the PAN backbone, respectively. In addition, well-defined PAN prepared by blue-LED−photoinduced RAFT polymerization was analyzed by MALDI-TOF MS (Figure 2b). The main series of peaks with the same interval (53.05 of m/z, representing an AN repeat unit) were assigned to PAN with an isobutyronitrile group at the α chain end and vinyl at the ω chain end. Loss of the CPDN fragment at the ω chain end of PAN was attributed due to a weak carbon-sulfur bond cleavage and removal from the PAN chain end during MALDI-TOF MS testing, as previously report [50]. Therefore, it clearly demonstrated that the blue-LED−photoinduced RAFT polymerization of AN was successfully mediated by CPDN and initiated by 4CzIPN. In addition, according to the results discussed above, the possible polymerization mechanism of this system was proposed. In this system, 4CzIPN acted as a free radical "initiator". Namely, the initial free radicals should be generated from 4CzIPN under blue LED irradiation to initiate the typical RAFT polymerization of AN in the presence of CPDN, although the precise structure of the free radicals is unclear so far. respectively. In addition, well-defined PAN prepared by blue-LED−photoinduced RAFT polymerization was analyzed by MALDI-TOF MS (Figure 2b). The main series of peaks with the same interval (53.05 of m/z, representing an AN repeat unit) were assigned to PAN with an isobutyronitrile group at the α chain end and vinyl at the ω chain end. Loss of the CPDN fragment at the ω chain end of PAN was attributed due to a weak carbon-sulfur bond cleavage and removal from the PAN chain end during MALDI-TOF MS testing, as previously report [50]. Therefore, it clearly demonstrated that the blue-LED−photoinduced RAFT polymerization of AN was successfully mediated by CPDN and initiated by 4CzIPN. In addition, according to the results discussed above, the possible polymerization mechanism of this system was proposed. In this system, 4CzIPN acted as a free radical "initiator". Namely, the initial free radicals should be generated from 4CzIPN under blue LED irradiation to initiate the typical RAFT polymerization of AN in the presence of CPDN, although the precise structure of the free radicals is unclear so far.

Conclusions
The organic photocatalyst 4CzIPN was successfully used in the RAFT polymerization of AN under blue LED irradiation at room temperature with CPDN as the chain transfer agent. It is worth mentioning that this strategy may provide an approach for scientific researchers and the industrial community to easily synthesize well-defined PAN-based precursors for carbon fibers.

Conclusions
The organic photocatalyst 4CzIPN was successfully used in the RAFT polymerization of AN under blue LED irradiation at room temperature with CPDN as the chain transfer agent. It is worth mentioning that this strategy may provide an approach for scientific researchers and the industrial community to easily synthesize well-defined PAN-based precursors for carbon fibers.