Synthesis of homo- and copolymer containing sulfonic acid via atom transfer radical polymerization

ABSTRACT Well-defined functional poly(p-phenyl styrenesulfonate) and poly(p-phenyl styrene-sulfonate-co-styrene) were successfully synthesized by the atom transfer radical polymerization (ATRP) using CuBr/bpy(PMDETA) catalyst and 1-phenylethyl bromide (1-PEBr) as an ATRP initiator in diphenyl ether (DPE) or dimethyl formamide (DMF). In both homo- and copolymers, the CuBr/PMDETA catalytic system in DPE or DME showed higher yield than CuBr/bpy and the polydispersity index (PDI) of polymer was low. Using PMDETA or bpy as a ligand in DMF, the high yield with high PDI was obtained than in DPE. We found that the CuBr/PMDETA catalyzed ATRP of p-phenyl styrenesulfonate and copolymerization with styrene comonomer in DPE proceeded in a controlled manner. The polymers containing sulfonic acid were obtained by the chemical deprotection of protecting group, followed by acidification. The molecular structure, molecular weights and thermal properties of the copolymers were determined by nuclear magnetic resonance (1H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively.


Introduction
The controlled radical polymerizations (CRPs) such as nitroxide-mediated radical polymerization (NMP) [1,2], atom transfer radical polymerization (ATRP) [3][4][5][6][7][8], reversible addition-fragmentation chain transfer (RAFT) polymerization [9][10][11], etc. have been used as synthetic tools to prepare well-defined polymers or copolymers with predetermined molecular weights, precise chain-end functionalities, and controlled topologies. ATRP has become a promising one of the different CRPs due to its mild reaction conditions, broad applicability to a variety of monomers, and good control over the polymer chain length and chain-end functionalities [3,4,. In ATRP, the molecular weights of the resultant polymer can be calculated using the ratio of monomer to the initiator in the polymer chain. The different tailor-made polymers with complex architectures like block copolymers, star polymers, dendrimers and hyperbranched polymers can be synthesized by varying the composition and topology of the polymer chains [37][38][39][40][41][42].
Homo-and copolymers of styrene sulfonate [43] have much interest in the field of renewable energy.
Coughlin and coworkers have reported the sulfonation of polystyrene (PS) and the emulsion polymerization of styrene sulfonate (SS) monomer [55]. They found that small shoulders appeared in the 1 H NMR spectrum of poly(styrene sulfonate) (PSS) obtained through sulfonation. The shoulders were not observed in PSS synthesized by polymerizing of styrene sulfonate (SS) monomer. This result indicated that the side reactions such as incomplete sulfonation, sulfonation in different positions, or other side products during sulfonation of PS were occurred.
The ATRP of monomers with sulfonic acids are difficult because of their strong reacting with either the ATRP catalyst, alkyl halide-type initiator or the polymeric dormant species [56]. In these cases, the sulfonic acid group in a monomer can be protected as an alkyl ester, salts formed at high pH or by neutralization with alkyl amines before the polymerization. Examples of protective groups include t-butyl, benzyl, tetrahydropyrannyl, 4-nitro-phenyl, and 1-ethoxyethyl [25,57]. The protecting groups can control the polymer properties [58] and the obtained polymers can be more functionalized by transformation reaction of these groups.
There have been few reports on the synthesis of PSS and copolymerization of SS with a comonomer by the controlled radical polymerizations. Mitsukami et al. have prepared homopolymer PSS and block copolymer with sodium 4-vinylbenzoate in aqueous media by RAFT [43]. They observed that block copolymer was pH responsive.
To find out a suitable ATRP condition is very important for the polymerization of a monomer especially with a polar functional group. In our previous work, we reported the ATRP conditions for the homoplymerization of styrene and acrylates [59].
In the present work, we have successfully synthesized well-defined sulfonated homopolymers (PSS) and copolymers (CP-SS) with chain-end functionalities by the ATRP of p-phenyl styrene-sulfonate (SS) and copolymerization with styrene (S) comonomer, respectively. The effects of ATRP ligand and solvent on the polymer yields, molecular weights (M n ), and polydispersity index (PDI) were investigated in detail. These polymers were characterized by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic spectroscopy ( 1 H NMR) and size exclusion chromatography (SEC). Thermal properties of the copolymers were investigated by the thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC).

Materials and instrumentation
In this study, styrene monomer (S), copper bromide (CuBr) and solvents [diphenyl ether (DPE) and dimethyl formamide (DMF)] used were purified according to a standard procedure. The ATRP ligands (2,2'-bipyridine (bpy), and N,N,N',N",N"pentamethyldiethylenetriamine (PMDETA)) and 1-phenylethyl bromide (1-PEBr) as an initiator were used as received without further purification. The 1 H NMR of polymers were measured in CDCl 3 at 25 °C operating at 400 MHz using the JEOL JNM-ECS 400SS spectrometer and the chemical shits were expressed in δ ppm. FT-IR spectra were recorded with a JEOL JIR-7000 FT-IR spectrometer and reported in reciprocal centimeter (cm −1 ). The molecular weights and polydispersity index (PDI) of polymers were determined by size exclusion chromatography (SEC) with Tosoh instrument with HLC 8020 UV (254 nm) detection. DMF was used as a carrier solvent at a flow rate of 1.0 mL.min −1 at 40 °C. Two polystyrene gel columns of bead size 10 μm (Shodex KF-806 L, Showa Denko K. K.) were used. Differential scanning calorimetry (DSC) analysis was performed on an SII EXSTER 600 (Seiko Instruments Inc., Japan) system under a nitrogen atmosphere. Differences in the thermal history of the polymers were eliminated by first heating the specimen to 200 °C, cooling it to 20 °C, and then recording the second DSC scan (all heating rates: 10 °C/min). Thermal gravimetric analysis (TGA) was carried out using a TG/DTA 6000 analyzer (Seiko Instruments Inc., Japan) under a flow of nitrogen (constant heating rate: 10 °C/min; 25-550 °C).

Synthesis of functional poly(p-phenyl styrene-sulfonate-co-styrene) CP-SS by ATRP
CuBr (15 mg, 0.10 mmol), 70 mol% of S (366 mg, 3.51 mmol), 30 mol% of SS (390 mg, 1.50 mmol), and 1.25 mL of DPE were added to a 6 mL vial successively. The reaction mixture was purged with argon for 5 min and then PMDETA (54 mg, 0.31 mmol) was added. 1-PEBr (20 mg, 0.11 mmol) as an initiator was added into the mixture after an additional 5 min of argon bubbling. The polymerization was carried out in an oil bath at 110 °C for 24 h with a stirring at 400 rpm. The resultant copolymers were precipitated by dropwise addition in 125 mL of methanol. The CP-SS was collected by filtration and then dried at 40

Synthesis of PSSNa and CP-SSNa
General synthesis procedure: To a flask with a magnetic stir bar inside, three equivalents of NaOH to SS moiety in the polymer or copolymer was added in a mixed solvent (THF/MeOH/H 2 O (50/10/1) = 16.8/3.36/ 0.336) was added. The reaction was conducted in an oil bath at 50 °C for 24 h.
PSSNa: After 24 h, the reaction mixture was cooled to room temperature, and the insoluble fraction was collected by centrifugation and washed with THF, MeOH, and CH 3

Synthesis of CP-SSH
To a flask with a magnetic stir bar, CP-SSNa (459 mg, 0.100 mmol of SSNa moiety) and 53 mL of THF were added. 1.10 mL (20 equivalents to SSNa moiety) of the diluted H 2 SO 4 in THF was added slowly into the mixture. The reaction was performed at room temperature for 24 h. The polymers were precipitated by dropwise addition in Et 2 O. The insoluble fraction was collected by centrifugation and washed with a small amount of MeOH and CH 3

Synthesis of poly(p-phenyl styrene-sulfonate) PSS by ATRP
PSS was prepared by the ATRP of SS, as illustrated in Scheme 1. The effects of ligand and solvent on the polymer yield and the polydispersity were summarized in Table 1. The ATRP conditions for SS were set as same as S polymerization [59]. PSS was obtained in low yield than PS because of the bulky functional group (C 6 H 5 OO 2 S ̶ ) present in the SS monomer (entry I vs. 1; entry II vs. 2). On the other hand, the M n of PSS with high PDI compared to PS were higher than the calculated because of the chain termination at the early stage of polymerization. S polymerization showed high PDI values when PMDETA was used as an ATRP ligand instead of bpy, probably due to the more initiating species formed firstly (entry I vs. II), whereas SS polymerization exhibited low polydispersity (entry 1 vs. 2). DPE was changed to DMF for checking the solvent effect on the ATRP of SS. The yield of PSS was increased when DMF with bpy or PMDETA was used as a solvent in lieu of DPE and the PDI values were relatively high (entry 1 vs. 3; entry 2 vs. 4). A similar rate enhancement in polar media was observed from different studies [62]. A faster initiation might have occurred due to high solubility of catalyst and polar monomer in DMF. The ATRP of SS with PMDETA in DPE or DMF proceeded faster than bpy and the PDI values were also low (entry 1 vs. 2; entry 3 vs. 4). Figure 1 shows the SEC traces of PSS synthesized by the ATRP using CuBr/ bpy(PMDETA) catalytic system of SS in DPE or DMF. In the SEC traces, the molecular weight distributions (MWD) of PSS were moderate and unimodal (PDI1.5).
The structure of PSS was characterized by 1 H NMR and FT-IR spectra. From the 1 H NMR spectrum of PSS in Figure 2, the peaks appeared at 1.3-1.8 ppm and 6.5-7.7 ppm confirmed the present of aliphatic and aromatic protons, respectively. The peak positions of aromatic protons in PSS chains were shifted to down field than in PS due to the electronic effect of sulfonate group. In FT-IR spectrum of PSS in Figure 3, the typical stretching bands for C = C bonds in aromatic rings were found at 1560, 1489 and 1452 cm −1 . The asymmetrical and symmetrical stretching vibration peaks for S = O bond were observed at 1374 (strong) and 1181 (weak) cm −1 , respectively.
These results indicated that well-defined function-nal PSS successfully synthesized by the CuBr/ PMDETA catalyzed ATRP in DPE.

Synthesis of poly(p-phenyl styrene-sulfonateco-styrene) CP-SS by ATRP
The properties of PS can be regulated by the copolymerization with a commoner [63,64]. After the successful synthesis of PSS, CP-SS was prepared by the copolymerization of 30 mol% SS with 70 mol% S using the same ATRP conditions that was employed in PSS, as illustrated in Scheme 1.
When ATRP was employed using bpy or PMDETA as a ligand in DPE or DMF, the yield of CP-SS was lower than Scheme 1. Synthesis of PSS and CP-SS.

Scheme 2. Synthesis of CP-SSH.
PSS because ATRP catalyst may also coordinate with S = O of the SS comonomer which reduced the catalytic activity (entries 5-8 vs. 1-3). The M n of CP-SS was higher than the calculated. The M n was in good agreement with M n , th when bpy was used in DPE. Copolymerization of SS and S with PMDETA in DMF showed slightly high yield than in DPE. It may be possible that the polarity of DMF increases the solubility of catalysts and monomers that might have assisted the initiation. By contrast, high M n was obtained due to the chain termination at the end of polymerization and the PDI (=1.79) values were also high. The rate of copolymerization was also increased when bpy was used in DMF and the M n of CP-SS with high polydispersity (PDI = 2.26) was high probably owing to the chain termination at the end of polymerization. Figure 4 exhibits the SEC traces of CP-SS prepared by the CuBr/bpy(PMDETA) catalyzed ATRP of SS with S in DPE or DMF. In SEC traces, the MWDs of CP-SS were broader when bpy or PMDETA was used in DMF than in DPE (entries 7 and 8).
In 1 H NMR spectrum of CP-SS in Figure 2, the peaks for aliphatic and aromatic protons of SS and S moieties in CP-SS chains were observed at 1.3-1.8 ppm and 6.5-7.5 ppm, respectively, which were found in PSS. The peak appeared at 1.10 ppm confirmed the present of chain-end methyl protons from 1-PEBr initiator in the PSS and CP-SS chains. The molar ratio of SS and S measured from the 1 H NMR spectrum was 60:30 that was close to the calculated one.  In FT-IR spectrum of CP-SS in Figure 3, the asymmetrical and symmetrical stretching vibrational bands for S = O bond were found at 1176 and 1376 cm −1 , respectively, that were exhibited in PSS. These results indicated that copolymerization of SS with S was successfully proceeded.
These above results confirmed that well-defined CP-SS was successfully synthesized using the suitable ATRP conditions as same as the SS polymerization.

Synthesis of poly(styrenesulfonate-co-styrene) CP-SSNa and poly(styrene sulfonic acid-co-styrene) CP-SSH by deprotection
PSSNa or CP-SSNa was obtained by the deprotecttion of phenyl group in the SS moiety of polymer chain using three equivalents NaOH to SS moiety of PSS or CP-SS in a mixed solvent (THF/MeOH/H 2 O = 50/10/1) at 50 °C for 24 h (Scheme 2). The yield and SSNa content in PSS and CP-SS were 89%, 4.78 mmol g −1 and 90%, 2.18 mmol g −1 , respectively.
The broad absorption peaks found at 1189 cm −1 and 1190 cm −1 in the FT-IR spectra of PSSNa and CP-SSNa ( Figure 3), respectively, confirmed the S = O bond in SO 3 Na in both the homopolymer and copolymer. CP-SSH was prepared by treating of twenty equivalents conc. H 2 SO 4 to SSNa moiety of CP-SSNa in THF. The yield and sulfonic acid content were in quantitative. FT-IR of CP-SSH in Figure 3 exhibited that broad absorption band assigned to stretching vibration of S = O bond in SO 3 H. This result confirmed that CP-SSH was successfully prepared from the deprotection of CP-SS, followed by acidification.

Thermal properties
The thermal properties of CP-SS and CP-SSH were studied by TGA and DSC analysis. TGA and DSC thermograms for CP-SS and CP-SSH are shown in Figures 5(a,b). TGA of CP-SS showed that the degradation temperatures (T d ) of CP-SS (345 °C) were very close to T d of polystyrene (PS) [65]. The T d of CP-SSH were shifted to higher temperatures as a function of sulfonic acid ( ̶ SO 3 H) groups, as compared to PS and   CP-SS (from 345 °C to 375 °C). In endothermic DSC thermograms, the similar results were observed i.e., the glass transition temperatures (T g ) of CP-SSH were also shifted to higher temperatures due to the presence of polar ̶ SO 3 H groups in the polymer chains, as compared to PS and CP-SS (from 100 to 155 °C). The high T g values of CP-SSH can be considered as an indicator of the increase of structural rigidity of CP-SSH chains.

Conclusion
Well-defined functional poly(p-phenyl styrene-sulfonate) PSS and poly(p-phenyl styrenesulfonate-co-styrene) CP-SS were successfully synthesized by the CuBr/bpy-(PMDETA) catalyzed atom transfer radical polymerization (ATRP) of p-phenyl styrene-sulfonate (SS) and copolymerization with styrene (S) comonomer using 1-PEBr as an initiator in DPE or DMF. The effects of ligand and solvent on the polymer yield, the molecular weights (M n ), and the polydispersity (PDI) were investigated. In PSS and CP-SS, the CuBr/PMDETA catalytic system in DPE or DME showed higher yield than CuBr/bpy and the polydispersity of polymer was low. Using PMDETA or bpy as a ligand in DMF, the high yield was obtained than in DPE and the PDI values were high. We found that the CuBr/PMDETA catalyzed ATRP of SS and copolymerization of SS with S in DPE proceeded in a controlled manner. CP-SSH containing sulfonic acid was obtained by the chemical deprotection of protecting group, followed by the acidification reaction. The molecular structure and molecular weights of the polymers were determined by nuclear magnetic resonance ( 1 H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and size exclusion chromatography (SEC), respectively. CP-SSH copolymers showed higher degradation temperatures with high T g values, as compared to PS and CP-SS.

Disclosure statement
No potential conflict of interests was reported by the author(s).