Photoinduced Cu(II)-Mediated RDRP to P(VDF-co-CTFE)-g-PAN

Photoinduced Cu(II)-mediated reversible deactivation radical polymerization (RDRP) was employed to synthesize poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-polyacrylonitrile (P(VDF-co-CTFE)-g-PAN). The concentration of copper catalyst (CuCl2) loading was as low as 1/64 equivalent to chlorine atom in the presence of Me6-Tren under UV irradiation. The light-responsive nature of graft polymerization was confirmed by “off-on” impulsive irradiation experiments. Temporal control of the polymerization process and varied graft contents were achieved via this photoinduced Cu(II)-mediated RDRP.

Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) is one of the most used high-performance fluoropolymers with various applications [52]. Several kinds of chemical and

Results and Discussion
Poly(acrylonitrile) (PAN) and its copolymers have been widely used as precursors for novel carbon materials with outstanding properties and performances [43]. P(VDF-co-CTFE)-g-PAN copolymers could be synthesized via Cu(0)-mediated reversible deactivation radical polymerization (RDRP) in batch and flow reactors [57,60]. Comparing with traditional ATRP protocol, copper catalyst loading was as low as about 1/4 equivalent to the chlorine atoms in the batch reactor. Copper catalyst, however, is highly desirable to decrease in order to reduce the amount of metal residue, which would potentially influence the application of final material. Inspired by the work about photoinduced Cu(II)-mediated RDRP [25,26], CuCl2/Me6-Tren was employed to promote the polymerizations of AN with P(VDF-co-CTFE) as macroinitiator. The polymerization results are listed in Table 1   Controlled experiments in the absence of light source or any reagent were conducted. P(VDF-co-CTFE) with no PAN graft chains was obtained by the removal of UV irradiation (Table 1,  Controlled experiments in the absence of light source or any reagent were conducted. P(VDF-co-CTFE) with no PAN graft chains was obtained by the removal of UV irradiation (Table 1, Figure 1, the graft content reached 2.1 mol % after 30 min and stayed almost unchanged during 30 min dark. The re-exposure to UV enabled the polymerization to start again. These cycles were repeated several times. The omission of P(VDF-co-CTFE) afforded no product (Table 1, run 2), which indicated that autopolymerization of AN did not occur under the current condition. In the absence of catalyst/ligand, Polymers 2018, 10, 68 4 of 10 no graft content was observed (Table 1, run 3), which elucidated the control of activation/deactivation equilibrium by CuCl 2 /Me 6 -Tren.
The influence of [Cu]:[L] on graft polymerization was investigated. It was supposed that 1:6 would be better to yield a higher graft content (Table 1, runs 4, 5, and 6), which was consistent with previous reports [25,26]. Kinetics study showed a linear dependence between −ln(1-conversion) and reaction time. This confirmed that the polymerization rate was first-order with respect to the monomer concentration ( Figure 2). Under the optimized reaction conditions, polymerizations with different monomer feed ratio were carried out to fabricate P(VDF-co-CTFE)-g-PAN with varied graft contents ( Table 1, Figure 1, the graft content reached 2.1 mol % after 30 min and stayed almost unchanged during 30 min dark. The re-exposure to UV enabled the polymerization to start again. These cycles were repeated several times. The omission of P(VDF-co-CTFE) afforded no product (Table 1, run 2), which indicated that autopolymerization of AN did not occur under the current condition. In the absence of catalyst/ligand, no graft content was observed (Table 1, run 3), which elucidated the control of activation/deactivation equilibrium by CuCl2/Me6-Tren.
The influence of [Cu]:[L] on graft polymerization was investigated. It was supposed that 1:6 would be better to yield a higher graft content (Table 1, runs 4, 5, and 6), which was consistent with previous reports [25,26]. Kinetics study showed a linear dependence between −ln(1-conversion) and reaction time. This confirmed that the polymerization rate was first-order with respect to the monomer concentration ( Figure 2). Under the optimized reaction conditions, polymerizations with different monomer feed ratio were carried out to fabricate P(VDF-co-CTFE)-g-PAN with varied graft contents ( Table 1, Figure 1, the graft content reached 2.1 mol % after 30 min and stayed almost unchanged during 30 min dark. The re-exposure to UV enabled the polymerization to start again. These cycles were repeated several times. The omission of P(VDF-co-CTFE) afforded no product (Table 1, run 2), which indicated that autopolymerization of AN did not occur under the current condition. In the absence of catalyst/ligand, no graft content was observed (Table 1, run 3), which elucidated the control of activation/deactivation equilibrium by CuCl2/Me6-Tren. The influence of [Cu]:[L] on graft polymerization was investigated. It was supposed that 1:6 would be better to yield a higher graft content (Table 1, runs 4, 5, and 6), which was consistent with previous reports [25,26]. Kinetics study showed a linear dependence between −ln(1-conversion) and reaction time. This confirmed that the polymerization rate was first-order with respect to the monomer concentration (Figure 2). Under the optimized reaction conditions, polymerizations with different monomer feed ratio were carried out to fabricate P(VDF-co-CTFE)-g-PAN with varied graft contents ( Table 1,     The chemical structure of P(VDF-co-CTFE)-g-PAN was characterized by FTIR, 1 H NMR, and 19 F NMR. In Figure 3 head-head and head-tail connections of VDF units (2.2-2.4 ppm and 2.7-3.2 ppm). The shoulder peak at 3.0-3.3 ppm corresponded to the proton signal of VDF adjacent to CTFE (-CF 2 CH 2 CFClCF 2 -). A new peak appearing around 1.9-2.1ppm was attributed to the proton on the methylene group of the AN unit (-CH 2 CHCN-). Another new peak appeared at 3.0-3.3 ppm and overlapped with the proton signals of head-to-tail connections of VDF in P(VDF-co-CTFE), which was assigned to the methine proton (-CH 2 CHCN-). The signals of -CH 2 CHFCF 2 -from hydrogenation of CTFE unites and -CH=CFlCF 2 -from elimination of HCl from main chains were not observed. This indicated that the typical side reactions did not happen in this photoinduced Cu(II)-mediated RDRP process. The 19 F NMR spectrum provided more information about the structure of graft copolymers. In Figure 5, the new peak appearing at 112.3 ppm was attributed to the AN units inserting into the C-Cl bond, where the C-C bond took the place of the C-Cl bond, which was consistent with the previous report [57]. No other new peaks were observed, which illustrated that the typical side reactions in traditional ATRP were avoided in this process. The chemical structure of P(VDF-co-CTFE)-g-PAN was characterized by FTIR, 1 H NMR, and 19 F NMR. In Figure 3, the characteristic absorption peak at 2247 cm −1 (-CN) indicated the presence of PAN segments in the copolymer. The 1 H NMR spectrum (Figure 4) displayed two multiple peaks of head-head and head-tail connections of VDF units (2.2-2.4 ppm and 2.7-3.2 ppm). The shoulder peak at 3.0-3.3 ppm corresponded to the proton signal of VDF adjacent to CTFE (-CF2CH2CFClCF2-). A new peak appearing around 1.9-2.1ppm was attributed to the proton on the methylene group of the AN unit (-CH2CHCN-). Another new peak appeared at 3.0-3.3 ppm and overlapped with the proton signals of head-to-tail connections of VDF in P(VDF-co-CTFE), which was assigned to the methine proton (-CH2CHCN-). The signals of -CH2CHFCF2-from hydrogenation of CTFE unites and -CH=CFlCF2-from elimination of HCl from main chains were not observed. This indicated that the typical side reactions did not happen in this photoinduced Cu(II)-mediated RDRP process. The 19 F NMR spectrum provided more information about the structure of graft copolymers. In Figure 5, the new peak appearing at 112.3 ppm was attributed to the AN units inserting into the C-Cl bond, where the C-C bond took the place of the C-Cl bond, which was consistent with the previous report [57]. No other new peaks were observed, which illustrated that the typical side reactions in traditional ATRP were avoided in this process.   The chemical structure of P(VDF-co-CTFE)-g-PAN was characterized by FTIR, 1 H NMR, and 19 F NMR. In Figure 3, the characteristic absorption peak at 2247 cm −1 (-CN) indicated the presence of PAN segments in the copolymer. The 1 H NMR spectrum (Figure 4) displayed two multiple peaks of head-head and head-tail connections of VDF units (2.2-2.4 ppm and 2.7-3.2 ppm). The shoulder peak at 3.0-3.3 ppm corresponded to the proton signal of VDF adjacent to CTFE (-CF2CH2CFClCF2-). A new peak appearing around 1.9-2.1ppm was attributed to the proton on the methylene group of the AN unit (-CH2CHCN-). Another new peak appeared at 3.0-3.3 ppm and overlapped with the proton signals of head-to-tail connections of VDF in P(VDF-co-CTFE), which was assigned to the methine proton (-CH2CHCN-). The signals of -CH2CHFCF2-from hydrogenation of CTFE unites and -CH=CFlCF2-from elimination of HCl from main chains were not observed. This indicated that the typical side reactions did not happen in this photoinduced Cu(II)-mediated RDRP process. The 19 F NMR spectrum provided more information about the structure of graft copolymers. In Figure 5, the new peak appearing at 112.3 ppm was attributed to the AN units inserting into the C-Cl bond, where the C-C bond took the place of the C-Cl bond, which was consistent with the previous report [57]. No other new peaks were observed, which illustrated that the typical side reactions in traditional ATRP were avoided in this process.   The thermal properties of P(VDF-co-CTFE)-g-PAN was explored by using DSC and TGA. In Figure 6, one endothermic peak was observed on each curve below 180 °C, which was assigned to the melting of the crystalline fluoropolymers. The melting temperature decreased from 149 to 137 °C with the increase of PAN graft content. This suggested that the crystalline degree of P(VDF-co-CTFE) and the crystal domain size were influenced by the introduction of PAN graft segments. The TGA (Figure 7) showed that the pristine P(VDF-co-CTFE) began to decompose at about 400°C, and about 40 wt % remained at 500 °C. Meanwhile, two polymer degradation stages were observed for the P(VDF-co-CTFE)-g-PAN copolymers. The first stage started from about 300 °C, which corresponded to the decomposition of PAN segment. The second stage began from about 450 °C, which was attributed to the degradation of P(VDF-co-CTFE) backbone chain. It was noteworthy that the weight remaining over 500 °C was enhanced with the increase of PAN content. It was supposed that the PAN was carbonized to carbon material with higher thermal stability.  The thermal properties of P(VDF-co-CTFE)-g-PAN was explored by using DSC and TGA. In Figure 6, one endothermic peak was observed on each curve below 180 • C, which was assigned to the melting of the crystalline fluoropolymers. The melting temperature decreased from 149 to 137 • C with the increase of PAN graft content. This suggested that the crystalline degree of P(VDF-co-CTFE) and the crystal domain size were influenced by the introduction of PAN graft segments. The TGA (Figure 7) showed that the pristine P(VDF-co-CTFE) began to decompose at about 400 • C, and about 40 wt % remained at 500 • C. Meanwhile, two polymer degradation stages were observed for the P(VDF-co-CTFE)-g-PAN copolymers. The first stage started from about 300 • C, which corresponded to the decomposition of PAN segment. The second stage began from about 450 • C, which was attributed to the degradation of P(VDF-co-CTFE) backbone chain. It was noteworthy that the weight remaining over 500 • C was enhanced with the increase of PAN content. It was supposed that the PAN was carbonized to carbon material with higher thermal stability. The thermal properties of P(VDF-co-CTFE)-g-PAN was explored by using DSC and TGA. In Figure 6, one endothermic peak was observed on each curve below 180 °C, which was assigned to the melting of the crystalline fluoropolymers. The melting temperature decreased from 149 to 137 °C with the increase of PAN graft content. This suggested that the crystalline degree of P(VDF-co-CTFE) and the crystal domain size were influenced by the introduction of PAN graft segments. The TGA (Figure 7) showed that the pristine P(VDF-co-CTFE) began to decompose at about 400°C, and about 40 wt % remained at 500 °C. Meanwhile, two polymer degradation stages were observed for the P(VDF-co-CTFE)-g-PAN copolymers. The first stage started from about 300 °C, which corresponded to the decomposition of PAN segment. The second stage began from about 450 °C, which was attributed to the degradation of P(VDF-co-CTFE) backbone chain. It was noteworthy that the weight remaining over 500 °C was enhanced with the increase of PAN content. It was supposed that the PAN was carbonized to carbon material with higher thermal stability.

Conclusions
In summary, poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-polyacrylonitrile (P(VDF-co-CTFE)-g-PAN) with varied graft contents were prepared via photoinduced Cu(II)-mediated reversible deactivation radical polymerization (RDRP). Upon UV irradiation, CuCl2/Me6-Tren enabled low catalyst loading and temporal control of the polymerization process. This protocol might have potential application in the area of novel dielectric materials.

Conclusions
In summary, poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-polyacrylonitrile (P(VDFco-CTFE)-g-PAN) with varied graft contents were prepared via photoinduced Cu(II)-mediated reversible deactivation radical polymerization (RDRP). Upon UV irradiation, CuCl 2 /Me 6 -Tren enabled low catalyst loading and temporal control of the polymerization process. This protocol might have potential application in the area of novel dielectric materials.