The main chain structure of polystyrene only contains chemically stable and rigid phenyl groups, so it has high chemical stability and has attracted wide attention of researchers^{[31,32,36,37]}. Although the phenyl-phenyl structure endows these polymers with excellent oxidation stability, it also brings some disadvantages, such as poor solubility, toughness and film-forming ability, which limit the development of these polymers^[38,39]. People usually introduce flexible or bulky side chains into the polystyrene structure to improve the properties of these polymers. In addition, some researchers have essentially introduced some flexible or curved structures into the main chain structure of these polymers to prepare polymer films with better toughness.

Metal coupling reaction is a common method to prepare polystyrene polymers. In 1978, Yamamoto et al. Reported the coupling reaction of monomer 1,4-dibromobenzene in the presence of magnesium and nickel catalysts, but the precipitation produced in the reaction process hindered the polymerization, and the degree of polymerization of the polystyrene could only reach 5 ~ 15^[40]. Since then, there have been many reports on the synthesis of polystyrene by metal coupling, but most of them failed to solve the problem of low degree of polymerization of the product^[38,39,41]. The metal coupling reaction using dichlorobenzene or its derivatives as monomers and nickel catalyst is easy to operate, and can obtain polystyrene polymers with large molecular weight, which is favored by researchers, as shown in Figure 1^[36,42,43]. Theoretically, all dichlorobenzene monomers can be polycondensed with nickel catalyst, but in molecular design, we often need to consider the actual situation. For example, rigid poly (p-phenylene) is insoluble, and the introduction of m-phenyl structure in molecular design can increase the flexibility of the chain, thereby improving solubility. Nickel catalyst-induced metal coupling reaction is evolved from Suzuki coupling reaction. In the process of preparing polystyrene by Suzuki coupling reaction, dibromobenzene or its derivatives and diboronic acid or its derivatives are often used as reaction monomers. The Ullmann coupling reaction is another commonly used reaction for the preparation of polystyrene polymers, as shown in Figure 2. The reaction uses a copper catalyst, uses bromobenzene as a monomer, and needs to replace the hydrogen ion of the sulfonic acid with an organic cation or a metal ion^[44].

Although poly (p-phenylene) polymer has good chemical stability, its solubility is poor. When the degree of polymerization increases, it is insoluble in general organic solvents, which limits the growth of its molecular weight in the polymerization process^[31]. The solubility of poly (p-phenylene) polymers can be improved by introducing side chains into the polymers. In addition, the introduction of the functional side chain can also endow the polymer with certain functions, for example, the introduction of the sulfonic acid group can enable the polymer to have the ability to conduct protons. Litt et al. prepared a series of sulfonated poly (p-phenylene) PBPDSA, PPDSA and BXPY by Ullmann reaction, as shown in Fig. 3A – C^[44,47].

The polymer electrolyte membrane has high proton conductivity at low humidity, and the proton conductivity can reach 100 mS·cm^-1 at 75 deg C/15% RH. This study demonstrates the potential of poly (p-phenylene) polymer as a proton exchange membrane. However, this kind of polymer electrolyte membrane has poor toughness and is easily swollen by water, resulting in a sharp decline in mechanical strength with the increase of humidity. In order to solve the problems of PBPDSA, PPDSA and BXPY membranes, Litt et al. used strategies such as grafting hydrophobic alkyl chains and crosslinking to reduce the degree of water swelling of such polymer electrolyte membranes, as shown in Figure 3D and e^[43,44,48]. The modified polymer electrolyte membrane still has high proton conductivity at low humidity, and its conductivity can reach 100 mS·cm^-1 at 120 ℃/30% RH. However, the toughness of the polymer film was still poor, and the elongation at break was less than 10%. Miyatake et al. Introduced trifluoromethyl and methyl side chains to prepare a series of sulfonated poly (p-phenylene) polymers SPP-BP-CH₃ and SPP-BP-CF₃ with different degrees of sulfonation in order to reduce the linearity of poly (p-phenylene) polymers, as shown in Figure 4A^[49]. The prepared polymer has a large molecular weight (M_n=28~149 kg·mol^-1) and can be dissolved in dimethyl sulfoxide (DMSO) solvent, which provides a guarantee for the preparation of polymer membranes. However, the toughness of the prepared polymer electrolyte membrane is still very poor, and its elongation at break can only reach 5.2%. Rikukawa et al. Synthesized a series of poly (p-phenylene) polymers using 2,5-dichlorobenzophenone and its derivatives, and then sulfonated them to prepare a series of sulfonated poly (p-phenylene) polymer electrolytes^[50]. The introduction of the bulky aromatic ketone side chain changes the conformation of the poly (p-phenylene) main chain, so the polymer has good solubility and film-forming ability. However, the high proton conductivity of these polymer membranes depends on their high degree of sulfonation, which leads to severe swelling of these polymers in water or methanol.

The solubility of poly (p-phenylene) polymer can be effectively improved by grafting functional side chains on the main chain of poly (p-phenylene), and poly (p-phenylenepolymer) with high molecular weight can be prepared. However, this method is less effective in improving the mechanical properties of the polymer, and the prepared polymer electrolyte membrane still has poor toughness and low elongation at break. Therefore, there is still a need to develop new methods to improve the mechanical properties of such polymer electrolyte membranes. Miyatake et al. introduced a hexafluoroisopropyl group into the poly (p-phenylene) backbone to prepare a sulfonated poly (p-phenylene) polymer electrolyte SBAF, as shown in Figure 4B^[51]. The introduction of hexafluoroisopropyl group reduces the rigidity of the main chain of the polymer and increases the mechanical properties of the polymer. The elongation at break is 28% ~ 31% and the strength is 34 ~ 79 MPa. The trifluoromethyl group has strong electron-withdrawing ability, so that the polymer still has high oxidation resistance stability. The ion exchange capacity (IEC) and mass of the polymer membrane did not change after soaking in Fenton's reagent at 80 ℃ for 1 H. This study improves the mechanical properties of polymers without reducing the properties of linear sulfonated polyphenylene, and provides a new strategy for the modification of polyphenylene polymers.

The introduction of m-phenyl structure into the rigid main chain of poly (p-phenylene) can increase the flexibility and bendability of the polymer chain, which is a common method to improve the flexibility of poly (p-phenylene)^[42,52]. The introduction of m-phenyl does not change the all-aryl structure of the main chain of the polymer, so it still has high chemical stability and thermal stability. Miyatake et al. studied the effect of different m-phenyl content on the persistence length of polystyrene polymer, designed a phenyl monomer QP with the optimal m-phenyl content (m-phenyl: p-phenyl = 4:1), and then prepared a sulfonated polystyrene polymer SPP-QP containing m-phenyl by metal coupling reaction induced by nickel catalyst, as shown in Figure 5A^[42]. The SPP-QP polymer proton exchange membrane has excellent proton conductivity, which can reach 220 mS·cm^-1 at 80 ° C/95% RH. The introduction of m-phenyl group reduces the persistent length of the polystyrene polymer, and the main chain of the polymer changes from a rigid rod to a flexible coil, which is beneficial to the entanglement between the polymer chains and the formation of a polymer film with better toughness. Therefore, the prepared polymer membrane has good mechanical properties, and its breaking strength and elongation at break are 32 MPa and 68%, respectively, which are much higher than those of other polyphenylene polymer proton exchange membranes. In addition, the polymer film has high antioxidant stability, and the mass, IEC and molecular weight of the polymer are almost unchanged after soaking in Fenton's reagent (3%H₂O₂, 2 ppm Fe²⁺) at 80 ° C for 1 H. In order to further improve the mechanical properties of SPP-QP, Miyatake et al. Prepared a series of sulfonated polystyrene composite membranes by blending SPP-QP-BA polymer with alkyl diols of different lengths^[53]. The introduction of a certain length of alkyl glycol can effectively improve the mechanical properties of the polymer membrane. For example, the breaking strength and elongation at break of the SPP-QP-BA (C4) membrane are 43 MPa and 99%, respectively, and the other properties of the composite membrane are not reduced, which is similar to the original SPP-QP membrane. Although the introduction of QP monomer can effectively improve the mechanical properties of polyphenylene polymer, the preparation of QP monomer is complex, which requires a two-step Suzuki-Miyaura coupling reaction to synthesize, increasing the complexity of the preparation process of polyphenylene polymer, as shown in Figure 6. Subsequently, Miyatake et al. Reported a new sulfonated polystyrene polymer SPP-BP, as shown in Figure 5B^[54]. In the polymerization process, 3.3 '-dichlorobenzene and 4,4' -dichlorobenzene monomers are used to replace QP monomers, so that multi-step synthesis of QP monomers is not needed, the synthesis process is simplified, and the material cost is reduced. However, only the polymer SPP-BP (NO.4) with a proper feed ratio (3,3 '-dichlorobenzene: 4,4' -dichlorobenzene: 2,5-dichlorobenzenesulfonic acid = 0.88: 0.22: 1) can form a thin film.SPP-BP (NO.1), SPP-BP (NO.2), and SPP-BP (NO.3), which have less p-phenyl content, have poor solubility and cannot be completely dissolved in DMSO or DMF. The authors believe that the reason why SPP-BP (NO.5) and SPPbp (NO.6) cannot form a film may be that their molecular weights are too small to form a polymer film. SPP-BP (NO.4) has better mechanical properties than SPP-QP membrane. At 60% RH, its breaking strength and elongation at break are 42.6 MPa and 68%, respectively. The authors believe that this may be caused by the smaller size of the hydrophobic region of SPP-BP (NO.4).

The main chain of phenylpolystyrene contains only chemically stable phenyl structures, and each repeating unit contains six substituted phenyl groups, which endows phenylpolystyrene polymers with excellent chemical stability and mechanical properties^[55⇓~57]. Because of the regioselectivity of Diels-Alder polymerization, these polymers usually contain two mixed configurations of meta and para positions, so they have good solubility and are easy to process into films with good toughness^[58⇓~60]. Due to the high hydrophobic structure, the polymer is still insoluble in water under the condition of high sulfonation degree, which provides a guarantee for the preparation of polymer electrolyte membranes with high proton conductivity.

Phenyl polyphenylene is usually synthesized by the [4 + 2] Diels-Alder cycloaddition method, so it is also called Diels-Alder polyphenylene. The reaction process involves two monomers, one is the dicyclopentadienone monomer [5-tetraphenylcyclopentadienone) benzene] or its sulfonate, and the other is diynyl benzene or its derivatives, as shown in Figure 7A^[58,59,61]. In the reaction process, the bicyclic dienone is decarbonylated to generate phenyl in situ, and carbon monoxide gas is generated to prevent the reaction from proceeding in the reverse direction. The structure of this kind of polymer is uncertain, on the one hand, because the alkynyl group has two attack directions during the reaction, forming m-aryl bond and p-aryl bond with different configurations, on the other hand, because the distribution of sulfonic acid groups after sulfonation is very random, the specific position of sulfonic acid groups can not be determined^{[58,60,62,63]}. This reaction is a thermal addition reaction, which does not require the addition of catalysts or other substances, which is an incomparable advantage of many polymerization reactions. In addition, the microwave-assisted Diels-Alder method can also be used to synthesize this kind of polymer. The synthesized polymer has similar chemical properties with the polymer synthesized by thermal addition method, but the mechanical properties of the synthesized polymer are poor, which may be due to the fact that more para-configuration is produced in the reaction process^[64]. Although the polymerization reaction is relatively simple, the synthesis steps of the dicyclopentadienone monomer are relatively complex, as shown in Figure 7B, which increases the difficulty in the preparation of this type of polymer.

There are two methods for the sulfonation of phenyl polyphenylene. One is the post-sulfonation method: the phenyl polystyrene polymer is directly sulfonated, so the distribution of sulfonic acid groups on many phenyl groups is random, resulting in an unclear molecular structure. The other is a pre-sulfonation monomer method: firstly, a dicyclopentadienone monomer is sulfonated, and then the sulfonated dicyclopentadienone monomer and diynyl benzene and derivatives thereof are utilized to synthesize sulfonated phenyl polyphenylene,The method can accurately control the distribution of sulfonic acid groups, is beneficial to obtaining sulfonated phenyl polyphenylene with a clear structure, and is beneficial to constructing an ordered ion transmission channel in the polymer electrolyte membrane. In 2015, Holdcroft et al. Reported the preparation method of sulfonated phenyl polyphenylene with definite structure, and biscyclopentadienone monomer precisely modified by sulfonic acid group was used in the reaction, as shown in Fig. 8^[65]. The sPPP-H⁺ polymer proton exchange membrane prepared by sulfonated bicyclopentadienone monomer has excellent proton conductivity, which can reach 106 mS·cm^-1 at 30 ℃/95% RH. In addition, the authors have clarified that the ratio of intermediate and para-aryl bonds in the main chain of the polymer prepared by the [4 + 2] Diels-Alder cycloaddition method is about 1:1. The as-prepared polymer contain four sulfonic acid groups per repeat unit and has a 3.47 mmol·g^-1 IEC as measure by titration, but that high IEC results in a polymer film with a high water swelling ratio in wat at 80. degree. C.

Over the past decade, sulfonated phenylpolystyrene polymer proton exchange membranes have been developed rapidly. Many studies have shown that these polymers have excellent chemical stability, mechanical properties, thermal stability, proton conductivity and low fuel permeability. Some properties are shown in Table 2. They are potential alternative proton exchange membrane materials. However, the high water absorption and swelling rate still restrict the development of these materials. When the sulfonic acid group content of each repeating unit in the polymer is more than 2, the polymer will swell excessively in water because it is too hydrophilic, such as sPPP-H⁺ membrane (IEC=3.47 mmol·g^-1), whose water absorption and volume swelling rate are 319% and 363%, respectively. Researchers have used many strategies to solve this problem, such as introducing bulky hydrophobic spacer units into the polymer backbone to increase the hydrophobicity of the polymer membrane, reduce the amount of water absorption, and thus reduce the swelling rate. In 2017, Holdcroft et al. Introduced bulky biphenyl and naphthalene into the main chain of phenyl polyphenylene, and prepared two new sulfonated phenyl polyphenylene polymer electrolyte membranes sPPB-H⁺ and sPPN-H⁺, as shown in Fig. 9 a and B^[66]. Compare to a sPPP-H⁺ membrane (IEC=3.47 mmol·g^-1),The water uptake and swelling ratio of the prepared sPPB-H⁺ membrane (IEC=3.19 mmol·g^-1) and sPPN-H⁺ membrane (IEC=3.28 mmol·g^-1) decreased. However, the proton conductivity is increased, and it can reach 268 mS·cm^-1 at 80 ℃/95% RH. Subsequently, a series of polymer electrolyte membranes with different degrees of sulfonation were prepared by copolymerization of sulfonated dicyclopentenone monomer and unsulfonated dicyclopentenone monomer with diynylbenzene^[67]. The content of the hydrophilic unit can be controlled by adjusting the feeding ratio, thereby controlling the water absorption rate and the swelling rate of the polymer film. With the decrease of the content of hydrophilic unit, the water absorption and swelling ratio of the polymer membrane decreased correspondingly, and the water absorption and volume swelling ratio of the sPPP(0.5)(H⁺) membrane were the lowest, which were 65. 1% and 67. 7%, respectively, but the proton conductivity of the polymer membrane decreased to 0.5 mS·cm^-1 at 30 ℃/95% RH. Although reducing the content of hydrophilic units can improve the dimensional stability of the membrane, low IEC will lead to a sharp decline in proton conductivity.

The swelling rate of sulfonated polystyrene polymer is high, on the one hand, because the degree of sulfonation of this kind of polymer is generally high, on the other hand, because the rigidity of the polymer backbone is strong, it is not easy to form strong entanglement. Therefore, reducing the linearity of the polymer backbone and increasing the entanglement between the polymer molecular chains can effectively improve the dimensional stability of these materials^[68,69]. Holdcroft et al. introduced bulky branching sites into the polymer backbone to prepare a series of sulfonated phenylpolystyrene polymers with different degrees of branching, as shown in Figure 9c^[70]. The prepared proton exchange membrane has good dimensional stability, and compared with the unbranched proton exchange membrane, the water absorption rate and the swelling rate of the prepared proton exchange membrane are respectively reduced by 40.2 to 62.3% and 19.8 to 57.7% at 20.5 to 0.5 deg C, and the prepared proton transfer membrane still has high proton conductivity, and the proton conductivity can reach 212 mS·cm^-1 at 80 deg C/95% RH. In addition, the authors also introduced sterically hindered pyridine groups into the main chain structure of sulfonated phenyl polyphenylene to prepare a new type of polymer electrolyte membrane (sTPPyPP-H⁺), as shown in Fig. 9d^[71,72]. The acid-base interaction between the pyridine group and the sulfonic acid group is beneficial to the dimensional stability of the polymer film, and the water absorption and swelling ratio of the polymer film are 54.4% and 62.4%, respectively. Compared with the proton exchange membrane sTPPPP-H⁺ without pyridine group, the sTPPyPP-H⁺ membrane has higher thermal stability and mechanical properties, its elongation at break increases from 37% to 55%, and its thermal decomposition temperature increases from 256 ° C to 326 ° C, as shown in Table 2.

High temperature polymer proton exchange membrane fuel cells (HT-PEMFCs) usually operate in the temperature range of 120 ~ 200 ℃, so membrane materials are required to have excellent chemical stability and mechanical properties at high temperatures, while exhibiting high low-humidity or anhydrous proton conductivity^[19,73,74]. Phenylpolyphenylene with all-phenyl structure has excellent chemical stability and thermal stability, and is a potential high-temperature proton exchange membrane material. Kim et al. Modified phenylpolyphenylene with quaternary ammonium nitrogen groups to prepare phosphoric acid-doped phenylpolyphenylene proton exchange membrane PA-doped QAPOH, as shown in fig. 10^[75]. Due to the strong ion-pair interaction between quaternary ammonium nitrogen and dihydrogen phosphate, the PA-doped QAPOH membrane still has strong phosphoric acid retention ability even at high humidity, so it can be used in a wide temperature range (80 ~ 180 ℃), which makes up for the shortcomings of PFSA membrane and phosphoric acid doped PBI membrane. The durability test showed that the voltage of the membrane electrode assembled with PA-doped QAPOH membrane hardly decayed after 500 H operation at 120 ℃. On the one hand, the prepared polymer electrolyte membrane has high phosphoric acid retention capacity; On the other hand, it has good anti-oxidation stability, mechanical properties and low fuel permeability. This study shows that phenylpolystyrene polymer has great application potential in high temperature polymer electrolyte membrane, but its molecular structure has limited adjustable space, which hinders the development of this kind of polymer.

Over the past decade, polyarylalkanes have developed rapidly and been widely used in fuel cells, electrolyzers, acid recovery and other fields^{[33,76,77][34,78][79⇓~81]}. For example, Zhe Wang et al. Prepared polyarylpiperidine anion exchange membranes for acid recovery by superacid-catalyzed reaction^[79]. The prepared anion-exchange membrane exhibited excellent diffusion dialysis performance with a diffusion dialysis coefficient H⁺ and a separation factor of 0.045~0.053 m·h^-1 and 32 – 56, respectively. The main chain of polyarane only contains phenyl-phenyl bond and phenyl-alkyl bond with high bond energy, and its chemical properties are much more stable than those of polymers containing heteroatom bonds, so it has stronger ability to resist the attack of free radicals such as · OH and · OOH in the actual working environment of fuel cells, and has excellent oxidation stability^{[32,82][27⇓~29]}. Generally speaking, the backbone structure of all-aryl has strong rigidity, which leads to poor toughness of polymer films, and the strong π-π interaction between aromatic ring groups will reduce the solubility of polymers. In contrast, polyarylalkanes synthesized by superacid-catalyzed reaction have aryl-alkyl bonds with large rotational freedom in addition to the phenyl structure, which increases the flexibility of the polymer chain. Therefore, this kind of polymer not only has strong antioxidant stability, but also has good solubility and mechanical properties, which is one of the most potential alternative membrane materials.

The polyarane was synthesized by a typical Friedel-Crafts reaction, as shown in Fig. 11^{[83⇓⇓⇓~87]}. Aromatic monomers such as biphenyl, p-terphenyl and m-terphenyl are polymerized with ketone monomers such as perfluoroacetophenone, pentafluorobenzaldehyde and N-methylpiperidone in the presence of BrØnsted superacid at room temperature or in an ice bath. No metal catalyst is needed in the reaction process. In addition to being a solvent, the superacid also has a catalytic effect, protonating the ketone to produce an alcohol intermediate, and then dehydrating to produce a carbocation that reacts with the aromatic monomer, so the reaction is also called a superacid catalytic reaction. The amount of superacid directly affects the molecular weight of the polymer, which is generally 5 to 12 times the amount of the non-reactive aromatic monomer. If the dosage is too small, the molecular weight of the polymer will be too small, and its mechanical properties will not be enough to meet the actual application requirements, but if the dosage is too large, it will lead to crosslinking or no reaction, so the appropriate amount of acid is the key to the reaction. Zolotukhin et al. Prepared a series of polyarylalkane polymers, and the weight-average molecular weight M_w of the polymers increased from 5.7×10⁵g·mol^-1 to 1.1×10⁶g·mol^-1 when the amount of acid increased from 4 to 8.8 times, but the polymers began to crosslink when the amount of acid increased to 10 times^[85]. The aromatic monome and that ketone monomer can also be fed in a non-stoichiometric ratio. If the ketone is use in an amount greater than the aromatic monomer, the excess ketone will significantly reduce the reaction time and increase the molecular weight of the polymer while maintaining a narrow molecular weight distribution. Zolotukhin's group prepared polyarylalkanes from 1,1,1-trifluoroacetone and biphenyl. The weight-average molecular weight M_w of the polymer prepared with stoichiometric feeding is 2.03 1.1×10⁶g·mol^-1,PDI, while the weight-average molecular weight M_w of the polymer prepared with non-stoichiometric feeding is increased to 1.6 1.3×10⁶g·mol^-1,PDI^[88].

Sulfonic acid groups are excellent proton donors and are widely used in proton exchange membranes^[91⇓~93]. However, there are few studies on sulfonated polyarylalkanes proton exchange membranes, on the one hand, because polyarylalkanes were studied later than other polymers, and on the other hand, because it is difficult to introduce sulfonic acid groups into polyarylalkylenes. In 2015, Kim et al. Synthesized polyarylisatin by superacid-catalyzed reaction, and then used the substitution reaction of 3-bromo-1-propanesulfonic acid to graft propanesulfonic acid onto the amide nitrogen of isatin to prepare a series of sulfonated polyarylalkane proton exchange membranes (PPSIBs), as shown in Fig. 12A^[94]. The sulfonation degree of the polymer can be adjusted by controlling the amount of 3-bromo-1-propanesulfonic acid, so that the proton exchange membrane with different IEC can be prepared. The prepared PPSIBs membrane can show better dimensional stability under higher IEC conditions. For example, the IEC of PPSIB50 membrane is 1.29 mmol·g^-1(Nafion211 is 0.90 mmol·g^-1), and its area swelling ratio and thickness swelling ratio are 12.9% and 11.7%, respectively. The authors believe that this is because the polymer membrane has a good microphase separation structure, and most of the water enters the hydrophilic phase region responsible for ion conduction. The high IEC endows the PPSIBs membrane with high proton conductivity, which can reach 121.7 mS·cm^-1 at 80 ° C/90% RH. Ding Jianning et al. prepared a series of polymer membranes (MFxDPy-PS and HFxDPy-PS) containing fluorene ring and isatin in the main chain by the ring-opening reaction of 1,3-propanesultone, as shown in Fig. 12b^[95]. The existence of fluorene ring and isatin group makes the main chain structure of the polymer membrane very rigid. The area swelling ratio of these polymer films was less than 20% at 80 ° C. Kim et al. Prepared a sulfonated polyarylisatin proton exchange membrane (SPP) by introducing sulfonic acid groups into the backbone of polyarylisatin using the sulfonation reaction of chlorosulfonic acid, as shown in Figure 12c^[96]. The degree of sulfonation of SPP membrane can be controlled by adjusting the amount of chlorosulfonic acid. The thermal decomposition temperature of SPP membrane can reach 250 ℃. In addition, the water uptake of SPP films ranged from 31.8% to 62.5%, and the area and thickness swelling ratios ranged from 13.7% to 26.9% and 7.9% to 14.4%, respectively. The polymer membrane has a good microphase separation structure, and the water content in the hydrophobic phase region which plays a mechanical supporting role is very small, so the polymer membrane has good dimensional stability. The proton conductivity of SPP membrane can reach 88.6 mS·cm^-1 at 80 ℃/90% RH. Xu Tongwen et al. Synthesized polyoxyxanthene through the superacid-catalyzed reaction between diphenolic hydroxyl monomer and 2,2,2-trifluoroacetophenone, and then sulfonated it with chlorosulfonic acid to prepare sulfonated polyoxyxanthene proton exchange membranes (SPX-HFP and SPX-BP)^[97]. These polymer films have excellent dimensional stability, and their volume swelling ratios are all less than 10% at 80 ° C. Bae et al. Synthesized polyarylalkanes containing halogenated hydrocarbons in the side chain through a superacid-catalyzed reaction, replaced the halogen atoms with potassium thioacetate, and then converted the thioacetate group into a sulfonic acid group with 3-chloroperbenzoic acid (mCPBA) to prepare a new sulfonated polymer membrane material, as shown in Figure 12d^[98]. Because the hydrophilic sulfonic acid group is modified to the end of the alkyl side chain, the hydrophilic sulfonic acid group can be separated from the hydrophobic main chain to form a hydrophobic phase region for providing mechanical support and a hydrophilic phase region for conducting protons, the obtained polymer membrane has good oxidation resistance stability, mechanical properties and dimensional stability. In 2023, Yang Hui et al. Synthesized Poly (arylalkane) (FLx-BPy) containing rigid fluorene ring in the main chain by superacid catalytic reaction, and similarly, a series of sulfonated poly (arylalkane) polymer membrane Poly(FLx-BPy)-SO₃H were prepared by the reaction of potassium thioacetate and 3-chloroperbenzoic acid (mCPBA) with alkyl halide^[99]. The molecular characteristics of the combination of the rigid main chain and the flexible disulfonic acid terminated side chain endow the polymer membrane with a typical microphase separation structure, so that the polymer membrane has excellent chemical stability and proton conductivity. The Poly(FL60-BP40)-SO₃H membrane has a high proton conductivity of 202 mS·cm^-1 at 80 ° C, and the power density of its assembled fuel cell exceeds that of 2.46 W·cm^-2. The sulfonation method is relatively simple, and can directly synthesize the target polymer by using the precursor of the anion exchange membrane, thereby providing a new way for the development of the sulfonated polymer proton exchange membrane. Jannasch et al. Used p-terphenyl, perfluoroacetophenone and acetophenone to prepare Px membrane materials through superacid catalytic reaction, and then carried out selective and quantitative thio-oxidation process on pentafluorophenyl to prepare sulfonated polyarane SPx^[100]. Polymers with different pentafluorophenyl contents can be obtained by adjusting the molar ratio of perfluoroacetophenone and acetophenone in the polymerization process, and then thio-oxidation is performed at the para-position of the pentafluorophenyl group to obtain polymer membrane materials with different degrees of sulfonation, as shown in fig. 12e. When the SPx film was immersed in Fenton's reagent at 80 ℃ (3%H₂O₂, 2 ppm Fe²⁺),1 h), there was no mass loss of the polymer film, and there was no change in the NMR spectrum, and the polymer film was damaged after 24 H. The authors believe that the absence of heteroatom bonds in the main chain structure of SPx is the reason for its high antioxidant stability. Moreover, the high IEC of these polymers contributes to the high proton conductivity, and the proton conductivity of SP100 membrane (IEC=1.83 mmol·g^-1) can reach 260 mS·cm^-1 at 120 ℃ under fully hydrated conditions. However, polyarylalkanes are not easily brominated or chloromethylated because they often contain electron-withdrawing groups in the main chain structure. Therefore, it is difficult to design such molecules into comb-shaped molecules with microphase separation behavior. In 2022, Jannasch et al. Prepared polyarylalkane polymers containing bromoethyl groups by superacid-catalyzed reaction, then grafted polypentafluorostyrene side chains with bromoethyl groups as the initiation site of atom transfer radical polymerization (ATRP), and finally prepared sulfonated polyarylalkanes with comb structure by thio-oxidation reaction at the para-position of pentafluorophenyl group^[101]. The degree of sulfonation of the polymer can be controlled by adjusting the grafting density or length of the polypentafluorostyrene side chain, so that proton exchange membranes with different IECs can be obtained. To evaluate the antioxidant stability of the prepared membrane, the S64-10-1.80 membrane was soaked in Fenton's reagent (3%H₂O₂, 2 ppm Fe²⁺) at 80 ° C, and then the NMR spectra were examined after different time. It was found that the intensity of the fluorine signal peak in the pentafluorophenyl group of the side chain decreased by 70% after 1 H, and the signal peak disappeared after 5 H, while the signal peak of the hydrogen in the benzene ring of the main chain did not change. This indicates that only the side chain has been degraded, while the all-carbon backbone containing the aryl group remains structurally intact. In 2022, Liu Peinian et al. Prepared three sulfonated polyarylalkanes (SP1-3) with different structures by the reaction of sodium phenolate and pentafluorophenyl para-fluorine, as shown in Figure 12f^[90]. The prepared polymer film has excellent dimensional stability, and its swelling ratio is up to 12. 9% ± 3.1% at 80 ℃/100% RH, which is significantly lower than that of Nafion 212 (35. 5% ± 4.3%).

Compared with sulfonated proton exchange membranes, the corresponding phosphonated proton exchange membranes have the advantages of good thermal stability, high oxidation stability, low fuel permeability and good dimensional stability, which have attracted more and more attention from researchers^{[102⇓⇓⇓~106]}. In general, the phosphonic acid group in most phosphonated aryl polymers is introduced by the classical Michaelis-Arbuzov reaction involving nucleophilic substitution of a trialkylphosphite and a benzyl or halogen atom. However, the reactivity of aromatic halides is low, and it is not easy to introduce the phosphonic acid group through the classical Michaelis-Arbuzov reaction, so it is necessary to add palladium catalyst to the reaction system to phosphonate the benzene ring, but the reaction is usually not complete, leaving a lot of aryl halides which are not phosphonated^[102]. The fluorine at the para position of the pentafluorophenyl group is activated by the electron-withdrawing fluorine atoms at the ortho and meta positions to be highly reactive, so that the phosphonic acid group can be selectively and quantitatively introduced by Michaelis-Arbuzov reaction with Tris (trimethylsilyl) phosphite^[107⇓~109]. Jannasch et al. Prepared a series of phosphonated polyarylalkane polymers PPx by Michaelis-Arbuzov reaction using the precursor Px for the preparation of sulfonated proton exchange membranes^[110]. The thermal decomposition temperature of PPx is about 400 ℃, which is significantly higher than that of the corresponding sulfonated polymer SPx (277 ~ 284 ℃), which may be due to the high stability of C — P bond. The structure and quality of the PPx membrane did not change after soaking in Fenton's reagent at 80 ℃ for 5 H. With the increase of phosphonation degree, the oxidation stability of the polymer film was gradually enhanced, and the PP100 film remained transparent and tough after 11 days, as shown in Figure 13. In contrast, the corresponding sulfonated polymer membrane was broken after 24 H. Therefore, phosphonated polyarylalkane polymers are a class of highly stable proton exchange membrane alternative materials with great potential.

Polyarylalkanes have excellent thermal stability, oxidation resistance, mechanical properties and acid resistance, and are the most potential alternative materials for high temperature proton exchange membranes. In recent years, phosphoric acid doped polyarylalkane proton exchange membranes have gradually appeared in people's vision and attracted the attention of many researchers. In 2019, Lu Shanfu et al. Synthesized polyarylpiperidine (PTP) by superacid catalytic reaction, and then used the tertiary amine on the piperidine ring to adsorb phosphoric acid to prepare a new type of high temperature polymeric material exchange membrane material^[111]. Although the prepared PTP/PA membrane has a slightly lower phosphoric acid doping level (5.80) than the OPBI/PA membrane (7.01), its proton conductivity and fuel cell performance are significantly higher than those of the OPBI/PA membrane, which are 96.0 mS·cm^-1 and 1220.2 mW·cm^-2, respectively, at 180 ° C. The authors believe that this difference is due to the fact that the prepared PTP/PA membrane has a microscopic phase separation structure, while the OPBI/PA membrane does not. Moreover, the prepared PTP/PA membrane has good mechanical properties, and the breaking strength is 12. 0 ± 0.3 MPa, which is three times that of the OPBI/PA membrane. In addition, the PTP membrane also has excellent anti-oxidation stability, and it only breaks after being immersed in Fenton's reagent at 68 ℃ for 200 H, as shown in Table 3, which ensures the long service life of the fuel cell assembled with it. Although PTP polymer has good mechanical properties and oxidation stability, its solubility is not good. Unprotonated PTP polymers are poorly soluble in solvents such as DMF and N-methylpyrrolidone (NMP), which limits the development of PTP polymers. In order to improve the solubility of PTP polymer, Yang Jingshuai et al. modified its chemical structure by grafting bulky pyridine, benzimidazole and other groups at the piperidine ring, as shown in Fig. 14^[112]. The grafted basic group improved the phosphoric acid adsorption capacity of the polymer membrane to some extent, and the phosphoric acid doping amount of the ungrafted PTP-TFA membrane was 105% ± 5%, while that of PTP-33% Py, PTP-26% BeIm, and PTP-41% BeIm was 136% ± 6%, 166% ± 5%, and 215% ± 6%, respectively, and its proton conductivity could reach 88 mS·cm^-1, which was much higher than that of the phosphoric acid-doped PTP-TFA membrane. The prepared polymer films were soaked in Fenton's reagent (3.0%H₂O₂,4.0 ppm Fe²⁺), and the temperature was controlled at 68 ° C, all of which were broken after 80 H, as shown in Table 13. The decomposition of quaternary ammonium and benzimidazole groups in Fenton's reagent due to the attack of free radicals is the main reason for the degradation of polymer films, and the introduction of electron-rich groups will accelerate their degradation.

The tertiary amine in the piperidine ring is easily decomposed by the attack of free radicals when it becomes quaternary ammonium. In contrast, the pyridine group has more antioxidant stability. Yang Jingshuai et al. Prepared a new type of polymer membrane containing pyridine groups by superacid-catalyzed reaction^[113]. To evaluate the antioxidant stability of the prepared membranes, PBAP membrane, PTAP membrane and PBI membrane were soaked in Fenton's reagent (3.0%H₂O₂,4.0 ppm Fe²⁺) at 68 ° C, and the temperature was kept constant. It was found that the PBAP membrane and the PBI membrane were broken after 75 and 250 H, respectively, while the PTAP membrane remained intact after 450 H and had a mass of 86.3% of the initial mass, as shown in Table 3. After soaking in Fenton reagent for 450 H, the PTAP membrane can still absorb 200% of phosphoric acid, and its proton conductivity can reach 86 mS·cm^-1 at 180 ℃. Phosphoric acid doped PTAP membrane was assembled into a fuel cell, and its voltage was almost unchanged when the current density was 200 mA·cm^-2 for 100 H, and then the voltage remained stable when the current density was increased to 400 mA·cm^-2,60 h.After several shutdown and restart tests, the open circuit voltage is still maintained at about 1. 0 V. These tests show that the polymer membrane material has high oxidation stability and low fuel permeability, thus ensuring its durability in fuel cells.

The loss of phosphoric acid is a major problem in the development of high temperature polymer fuel cells, which seriously affects the service life of the cell. Reducing the loss of phosphoric acid without reducing the proton conductivity is of great significance to the development of high temperature polymer membrane materials^[18,114,115]. In 2021, Zhang Suobo et al. Prepared 2-IMPIM and 4-IMPIM membrane materials with microporous structure by using PIM molecules with distorted structure as comonomers and 2-imidazolecarboxaldehyde (4-imidazolecarboxaldehyde) through superacid catalytic reaction^[116]. The continuous microporous structure can not only increase the phosphoric acid doping level of the polymer membrane, but also provide a fast channel for proton transport. At 180 ℃, the proton conductivity of the polymer membrane can reach 330.3 mS·cm^-1, which is much higher than that of the phosphoric acid doped polybenzimidazole membrane. The theoretical study shows that the interaction between the imidazole group in the IMPIM molecule and phosphoric acid is 2. 5 times stronger than that of polybenzimidazole, and the capillary force provided by the micropores further strengthens the phosphoric acid loading strength, so the polymer membrane has good phosphoric acid retention ability, and at 80 ℃/40% RH, the phosphoric acid retention of 2-CIMPIM membrane and 4-IMPIM membrane is 86% and 83%, respectively. In 2022, Li Nanwen et al. Prepared polyarylalkanes with imidazole groups in the side chain by superacid-catalyzed reaction^[117]. The phosphoric acid retention of the prepared PIBI-Q70/PA membrane was 20% higher than that of the mPBI/PA membrane after 100 H retention at 80 ℃/40% RH. The molecular structure of polyarylalkanes is rich and diverse, and the adjustable space is broad, which is helpful for researchers to design high-temperature proton exchange membrane materials with excellent comprehensive performance from the molecular structure.