Synthesis of Poly(p-Terphenyl N,N-Dimethylpiperidinium)s Using Asymmetric Ketone-Based Branching Agent

Macromolecular Materials and Engineering www.mame-journal.de RESEARCH ARTICLE Synthesis of Poly(p-Terphenyl N,N-Dimethylpiperidinium)s Using Asymmetric Ketone-Based Branching Agent Ihor Tkachenko1Tetiana Samoilenko1Mariana Gumenna1David Kalabis2Ruslan Kravchuk3 Vassili Nazarenko3Iva Matolínová2Valery Shevchenko1 1Institute of Macromolecular Chemistry of the NAS of Ukraine, Kyiv, Ukraine 2Charles University, Prague, Czech Republic 3Institute of Physics of the NAS of Ukraine, Kyiv, Ukraine Correspondence: Ihor Tkachenko ([email protected].ua) Received: 29 May 2025 Revised: 22 July 2025 Accepted: 4 August 2025 Funding: This work was supported by the CERIC-ERIC Consortium for the access to experimental facilities and financial support (Proposal Id 20242137), the Grant of the National Academy of Sciences of Ukraine to research laboratories/groups of young scientists (No. 28/02-2024(5)), “The Energy Conversion and Storage,” funded as project no. CZ.02.01.01/00/22_008/0004617 by Programme Johannes Amos Comenius, call Excellent Research. The authors acknowledge funding from the NATO SPS project G6030. Keywords: AB2-type monomer | anion-exchange membranes | asymmetric branching agent | branched ether-free aromatic polymer backbone | poly(aryl piperidinium)s | quaternization | superacid-catalyzed Friedel-Crafts polyhydroxyalkylation reaction ABSTRACT Poly(aryl piperidinium)s containing both alkaline stable ether-free aromatic polymer backbone and heterocyclic quaternary ammonium groups are currently considered as one of the best candidates for the development of anion-exchange membranes. The branching modification strategy allows accelerating the polymerization process and receiving the polymers with high molecular weight and enhanced characteristics. So far, the application of asymmetric branching agents is very limited. In this study, 4-biphenylyl trifluoromethyl ketone (BTK) was used as AB2-type structuring monomer in superacid-catalyzed FriedelCrafts polyhydroxyalkylation together with p-terphenyl which is B2-type monomer, and N-methyl-4-piperidone which is A2-type monomer. Polymers with different degrees of branching (1.5 and 3 equivalents of BTK) are synthesized. The presence of unreacted terphenyl, revealed by 1H NMR and wide-angle X-ray diffraction, is highlighted, and measures are proposed to prevent its occurrence. On the basis of neutral polymers (NB-PTP-1.5 and NB-PTP-3), their quaternized counterparts (QB-PTP-1.5 and QBPTP-3) with excellent film-forming properties are obtained. Static light scattering measurements show that the values of molecular weight of different polymers are close, whereas particle size is bigger for a more branched polymer (according to dynamic light scattering analysis). Thermooxidative resistance of quaternized branched polymers is higher than that of linear polymers. Alkaline stability of polymers is confirmed by 1HNMR. 1 Introduction The application of anion exchange membranes (AEMs) for fuel cells and water electrolyzers is believed to start a real revolution in practical electrochemistry [1]. Many authors [2–4] highlight their overwhelming superiority in comparison with proton exchange membranes, particularly faster electrochemical kinetics, weaker corrosion, and the possibility to use non-precious-metal catalysts. Such a set of properties can decrease expenses, facilitate commercialization, and make hydrogen an economically competitive alternative energy source [2, 5, 6]. While water electrolyzers provide obtaining of green hydrogen without CO2and other greenhouse gas emissions at relatively low temperatures, fuel cells utilize it as a sustainable, energy-efficient fuel to produce This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2025 The Author(s). Macromolecular Materials and Engineering published by Wiley-VCH GmbH Macromolecular Materials and Engineering, 2025; 310:e00225 https://doi.org/10.1002/mame.202500225 1of13 electricity with only water as a by-product. Facing the worldwide issues of climate change and depletion of fossil fuels, scientists naturally could not leave the development of AEMs out of consideration. Although much effort was made and huge success was achieved in this sphere [1, 3, 7]uptonow,onlyafewAEM products are commercially available [8]. Moreover, even these membranes are claimed to show quite a low level of technological readiness [5]. The most challenging is to maintain their high durability and ionic conductivity together with alkaline and dimensional stability [3, 5, 9]. With a view to overcome the aforementioned problems, current scientific research is focused on the designing of new polymer chemistries of AEMs, including the nature of the backbone, cation, quaternizing agent, and the combination thereof. Whereas polymer chain ensures the mechanical and thermal stability of a membrane, quaternary groups endow it with ionic conductivity and polarity. The reasonable amount of cationic functional groups should be introduced, so as to keep balance between high conductivity and excessive water absorption, leading to the deterioration of membrane’s mechanical strength [4, 10]. Another feature of anion exchanging groups have to be taken into account is their susceptibility to degradation in alkaline media that obviously causes a decrease in ion conductivity [5]. It is characteristic to the majority of investigated ammonium, phosphonium, sulfonium, and organic-metal cations [10]. Quaternary ammonium groups are the most common cations in AEMs, but they often degrade in hydroxide (OHˉ) environments via nucleophilic substitution or Hofmann elimination [5]. Since Hofmann elimination needs a β-hydrogen atom, researchers often use benzyltrimethylammonium groups, which lack this structural feature [11–13]. However, it has been turned out that the negative inductive effect of phenyl substituents makes the benzyltrimethylammonium group more sensitive to SN2 degradation instead [5, 10]. Accordingly, some alkyl ammonium groups are shown to be more alkali resistant [14]. Particularly, AEMs with long cationic alkyl side chains exhibiting improved alkaline stability have gained much scientific attention [15–17]. At the same time, other researchers [18, 19] argue that the membranes bearing ion exchange groups in the structure of a polymer backbone possess higher ion conductivity, mechanical strength, and alkaline durability. In this respect, ionene-type heterocycle-containing polymers like cationic copolyimides [20] or polyimidazolium ionenes [21] seem to be very attractive. Other cyclic quaternary ammonium groups, namely N,N-dimethylpiperidinium and piperidine-based 6-azonia-spiro[5.5]undecane, today are generally recognized to have the highest resistance against both elimination and substitution reactions, which is attributed to their ring structure [14, 22]. Apart from that, not only ionic groups, but also many types of polymer backbones, such as poly(ether ether ketone), polysulfone, and poly(phenylene oxide), are prone to alkaline degradation. It involves polymer chain cleavage, which threatens the mechanical integrity of the sample [5, 10, 23]. Hence, it is preferably to avoid unstable aryl ether bonds within macromolecule structures [23–25]. The pioneering work in this sphere was presented by Olsson, Pham, and Jannasch [26], who synthesized poly(aryl piperidinium) (PAP) AEMs, particularly poly(biphenyl N,N-dimethylpiperidinium) and poly(p-terphenyl N,N-dimethylpiperidinium), containing quaternary piperidinium rings directly in the aromatic ether-free polymer backbones. They are characterized by high hydroxide conductivity and alkaline stability, with the decrease of the last one upon increasing the length of the alkyl chain in a quaternizing agent. Still, water uptake of the membranes, which has the opposite dependence on the number of Carbon atoms in a quaternizing agent, is excessive. To diminish it, the authors proposed to carry out partial quaternization, copolymerization with trifluoroacetone or trifluoroacetophenone, and in situ cross-linking through 1,8dibromooctane, combining Nitrogen atoms [27]. Different strategies have been implemented to further enhance the properties of PAP membranes. Thus, the application of m-terphenyl [28], its combination with p-terphenyl [29, 30] or quaterphenyl units [31–33] is proposed. The synthesis of the main chain with two piperidinium groups in a repeating unit, namely poly(bis-alkylpiperidinium), is also declared [34]. Alternatively, Wang et al. used trifluoroacetophenone to partially substitute piperidone [24]. Altogether, both hydrophilic and hydrophobic co-monomers, such as dibenzofuran [35, 36], dibenzothiophene [36], crown ethers [37], and dibenzyl [38], 9,10-diphenylanthracene [39], 9,9′-dimethylfluorene [40], respectively, are known to be incorporated into a polymer backbone. The representatives of the first group promote phase separation at the microscopic level, while the ones of the second group increase polymer chain rigidity. Copolymerization with 3-bromo1,1,1-trifluoroacetone enables grafting of long multi-piperidine cationic pendants [41]. However, N-alkyl side chains like octadecyl [42], oligo(ethylene glycol) [43], and cation-containing [23, 44–46] fragments are more common. Similarly, multi-cation structures are successfully used as crosslinkers, providing the creation of ion channels [47–50]. Synthesis of branched polymers is believed to be another straightforward, efficient approach in the development of AEMs [51, 52]. It implies incorporation of a monomer with functionality equal to or greater than three, which facilitates the polymerization process and results in the formation of polymers with high molar mass [51]. Being a key property of polymers, molecular weight determines mechanical performance, dimensional, and alkaline stability [53, 54]. In addition, a looser arrangement of macrochains may induce microphase separation and ion clusters assembling [55, 56]. Positive impact of various branching agents on PAP membrane characteristics is described; [52, 54, 56, 57]yet, to the best of our knowledge, application of asymmetric A2B-type monomer is limited to 9-dodecylidene-9H-fluorene [58]. Therefore, the aim of the present study is to explore 4-biphenylyl trifluoromethyl ketone (BTK), a trifunctional monomer, as an asymmetric branching agent for the synthesis of neutral poly(p-terphenyl-N-methylpiperidine)s via a superacid-catalyzed Friedel-Crafts polyhydroxyalkylation reaction, followed by quaternization to produce cationic poly(p-terphenyl-N,Ndimethylpiperidinium)s. These branched polymers are considered promising candidates for future application in anion exchange membranes. The polymers are thoroughly characterized in terms of chemical structure, absolute molecular weight determined by static light scattering, and thermooxidative stability. Special attention is given to the potential presence and 2of13 Macromolecular Materials and Engineering,2025 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License TABLE 1 Polymerization conditions for NB-PTP-x. Entry Reagent equivalents Reaction time (h) Yield (%) mPip BTK TP TFA TFSA NB-PTP-1.5 99 1.5 99.5 99.5 995 7 90 NB-PTP-3 98 3 99 99 990 5 92 removal of residual monomeric impurities in the neutral polymers, occasionally evident in earlier studies, though not explicitly addressed. 2 Experimental Section 2.1 Materials The branching agent 4-biphenylyl trifluoromethyl ketone (BTK) was prepared according to the earlier described procedure [59]. Para-terphenyl (TP, 99%, Sigma–Aldrich), N-methyl4-piperidone (mPip, 98%, Uoslab), 1,1,1-trifluoroacetic acid (TFA, 99%, Uoslab), trifluoromethanesulfonic acid (TFSA, 98%, Uoslab), and iodomethane (98%, Sigma–Aldrich) were all used as received. All organic solvents, including dichloromethane (DCM), dimethyl sulfoxide (DMSO), and diethyl ether, were distilled before use. 2.2 Synthesis of Polymers 2.2.1 Synthesis of Neutral Branched Poly(Terphenyl Piperidine)s (NB-PTP-x) Polymers NB-PTP-x (where x represents the equivalent of BTK used in the monomer ratio as shown in Table 1)weresynthesized by superacid-catalyzed Friedel-Crafts polyhydroxyalkylation reactions. The synthesis of NB-PTP-1.5 is provided as an example. Ketone mPip (0.489 g, 4.320 mmol), aromatic monomer TP (1.000 g, 4.342 mmol), and branching agent BTK (0.016 g, 0.065 mmol) were dissolved in 4.3 mL of DCM (35 % w/v). The mixture was cooled to 0◦C using an ice bath while stirring with a magnetic stir bar, then TFA (0.33 mL, 4.342 mmol) and TFSA (3.9 mL, 43.422 mmol) were gradually added to the solution of monomers. During the reaction, the color of the solution changed from light purple to dark blue. After 7 h, the highly viscous solution was precipitated into a 2 M NaOH solution to terminate the reaction and neutralize the acids with the formation of a white to pale yellow fibrous solid. The polymer was collected by filtration, washed with water, and immersed in 1M K2CO3solution for 12 h at room temperature. Finally, the solid product was filtered again, washed with deionized (DI) water, and vacuum dried at 80◦C. Yield of NB-PTP-1.5 was approximately 90%. 2.2.2 Synthesis of Quaternized Branched Poly(Terphenyl Piperidinium)s (QB-PTP-x) A typical synthesis procedure of QB-PTP-1.5 is as follows: to a 50 mL one-necked flask equipped with a magnetic bar, NB-PTP1.5 (1.0 g, 1 equiv.) and potassium carbonate (0.42 g, 1 equiv.) were suspended in DMSO (10 mL). Methyl iodide (0.6 mL, ∼3 equiv.) was added quickly. The solution was stirred for 24 h at room temperature in the dark. The resulting solution was added dropwise into excess ethyl acetate, yielding the crude product. The precipitate was collected, dried, and dissolved in DMSO, filtered to remove any insoluble solid, and reprecipitated by adding the solution dropwise into ethyl acetate. The separated polymer was dried in vacuo at 60◦C overnight. Yield after precipitation was about 82%. 2.2.3 Synthesis of Quaternized Linear Poly(Terphenyl Piperidinium) (QL-PTP) The linear reference polymer based solely on TP and mPip monomers was synthesized according to ref. [26]. While the original protocol for the neutral polymer precursor specified a reaction time of 5–7 h, in our case, the mixture remained capable of being stirred beyond this time. Therefore, the reaction was allowed to proceed without stirring overnight at 0◦C, followed by precipitation of the highly viscous product in aqueous NaOH. 2.3 Membrane Fabrication and Anion Exchange QB-PTP-x polymer was dissolved in DMSO to form a 3.33% w/v solution. The solution was filtered through a 0.45 µmpolytetrafluoroethylenemembranefilterandevenlypouredontoaclean glass plate. It was then left to evaporate at 90◦Cfor24hto ensure gradual solvent removal. The resulting membrane in the I−form was gently peeled off the substrate after hydration. The polymer solution volume and substrate dimensions were adjusted to obtain membranes with a thickness of approximately 15–25 µm. The corresponding membrane in the hydroxide (OH−)formwas obtained through ion exchange reaction in a 1 M aqueous KOH solution at room temperature for 48 h, followed by washing and immersion in DI water. 2.4 Characterization 2.4.1 1H Nuclear Magnetic Resonance (NMR) Spectroscopy 1H NMR spectra were recorded using a Bruker Avance instrument at room temperature in DMSO-d6 with approximately 5 vol% TFA added to shift the water peak, which facilitates spectrum interpretation. Chemical shifts are given relative to dimethyl sulfoxide (δ=2.50 ppm). Macromolecular Materials and Engineering,2025 3of13 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2.4.2 Fourier Transform Infrared (FTIR) Spectroscopy FTIR spectra of dried polymers before and after quaternization were recorded on a spectrophotometer “Tenzor 37” (Brucker Optics). The scanned wavenumber range was 4000–600 cm−1. 2.4.3 Field-Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-Ray (EDX) Analysis High-resolution FE-SEM analysis of the membranes was performed on a Tescan Mira3 microscope equipped with EDX detector, allowing elemental analysis and mapping for chemical characterization of the materials. 2.4.4 Static Light Scattering (SLS) The molecular weights of QB-PTP-x were determined using an Anton Paar Litesizer 700 instrument by the SLS method. The sample was analyzed at different concentrations in DMSO at 25◦C, applying the Rayleigh equation [60]. The refractive index increment (dn/dc) value at a wavelength of 640 nm was calculated during the sample measurements using water, toluene, and tetrahydrofurane standards as references. The data were then used to construct a Debye plot and calculate the absolute weightaverage molecular weight (MwSLS) using Anton Paar’s Kalliope software. 2.4.5 Dynamic Light Scattering (DLS) DLS was used to find the hydrodynamic diameter and size distribution of synthesized polymer particles. 5 mg/mL solutions in DMSO were tested on the same instrument as SLS. The conversion to the volume distribution was done through the algorithm built into the software. 2.4.6 Inherent Viscosity The inherent viscosities of 0.4 wt.% polymer solutions were determined using a Ubbelohde viscometer, either in DMSO at 25◦C or in 0.1 m LiBr/DMSO at 30◦C[26, 54]. 2.4.7 Thermogravimetric Analysis (TGA) TGA was carried out using a TA Instruments Q-50 thermal analyzer. Approximately 5–10 mg samples were heated from 25◦C to 700◦C at a heating rate of 20◦Cmin −1under an air atmosphere. 2.4.8 Wide Angle X-Ray Diffraction (WAXD) WAXD patterns were obtained by means of a Proto instrument equipped with an X-ray tube containing a copper anode and nickel filter that operates at 30 kV and 20 mA. The 2θrange of 5◦to 50◦was used for analysis. 2.4.9 Ion Exchange Capacity (IEC) The actual IEC value of the membranes was determined using a conventional acid–base back titration method [28]. 2.4.10 Alkaline Stability The alkaline stability of the QB-PTP-x membranes was assessed by comparing their molecular structure using 1H NMR spectroscopy before and after 14 days of exposure to 2 m KOH at 80◦C. 3 Results and Discussion 3.1 Rationale for the Choice of Branching Agent In the majority of studies, symmetric aromatic monomers with 3(A 3-type) or 4 (A4-type) functional groups were incorporated (Figure 1). The most commonly used C3-symmetrical branching agent is 1,3,5-triphenylbenzene [51, 54, 61–64]. Other examples include triphenylmethane [65], triphenylamine [57, 66], triptycene [50, 67], and alkyl-substituted derivatives of triazatruxene [68]. For C4-symmetry tetraphenylmethane [56, 69] is the main monomer, although branched polymers were also synthesized using spirobifluorene [52] and bis(carbazolyl)-substituted aromatics [55, 70]. In addition to differences in chemical composition, these agents also vary in their 2and 3D architectures. Very recently, octaphenylcyclotetrasiloxane was also applied as a branching agent for the construction of multidirectionally branched PAP structures [71]. The use of AB2(or A2B) monomers is predominant in synthetic strategies due to offering a wide structural diversity in branched and hyperbranched polymers [72]. In ref. [58] an asymmetric A2B monomer was employed to synthesize branched polymers via superacid-catalyzed polycondensation. 9-alkylidene-fluorene derivatives were chosen as A2B-type branching agents, where the “A” component consists of two reactive aromatic sites, and the “B” component is a carbon-carbon double bond that readily reacts under acidic conditions (Figure 1). Specifically, using 9dodecylidene-9H-fluorene as a branching agent, a quaternized poly(piperidinium-triphenyl-dodecylidenefluorene) having low water swelling and high mechanical strength was obtained. Moreover, ultra-thin anion exchange membranes (∼8µm) were fabricated from this polymer. Inspired by these achievements, we designed branched polymers using an asymmetric AB2type agent (BTK) that incorporates both a reactive aromatic group (A) and a bifunctional carbonyl group (B2)(Figure1). This synthetic pathway may promote the self-assembly of ionic moieties into well-defined ionic clusters, potentially improving hydroxide conductivity. The choice of an AB2-type branching agent such as BTK may offer several conceptual and practical advantages in the context of polymer design. Its two chemically distinct reactive centers may promote selective interaction with both co-monomers (e.g., TP and mPip), enabling more uniform incorporation of branching points into the polymer backbone. Additionally, the AB2 architecture allows for possible self-condensation of BTK units 4of13 Macromolecular Materials and Engineering,2025 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 1 Chemical structure of branching agents used for PAP membranes. at later stages, further increasing the degree of branching. In contrast, conventional A3-andA 4-type branching agents possess multiple identical functional centers and typically react with only one type of monomer. Since they do not react with each other, their incorporation can be limited, especially under highviscosity conditions; whereas their accumulation at chain ends may hinder further reaction progress. While low incorporation levels of branching agents suffice in most cases, higher loadings (sometimes even up to 15% mol) are often reported without crosslinking [55, 61]. This behavior likely reflects their lower incorporation efficiency, resulting in partially substituted species (e.g., di-substituted products in the case of A3-type agents, or a mixture of diand tri-substituted species for A4-type agents), which may contribute to a lower overall branching efficiency within the polymer architecture. 3.2 Synthesis of Novel Branched Poly(terphenyl piperidinium) Polymers In this study, the synthetic approach for new quaternized branched poly(terphenyl piperidinium) polymers with a different content of branching agent (QB-PTP-1.5 and QB-PTP-3) has been developed. Initially, neutral branched poly(terphenyl piperidine) polymers (NB-PTP-1.5 and NB-PTP-3) are obtained through the reaction of mPip as an A2-type monomer, TP as aB 2-type monomer, and 1.5 or 3 equivalents of BTK as an A2B-type monomer that controls the degree of branching in the macromolecular chains (Figure 2a). The reaction proceeds under superacid-catalyzed Friedel-Crafts polyhydroxyalkylation conditions in dichloromethane using TFSA and TFA acids. The optimized equivalent ratio of monomers to TFSA and TFA in the polymer synthesis is presented in Table 1. The addition of a higher amount of BTK (5 equivalents or more) leads to an increased fraction of cross-linked polymer (exceeding 20%) or even gelation of the reaction mixture. In our view, such a degree of cross-linking is not suitable for further evaluation or membrane fabrication. This outcome, however, highlights the efficiency of BTK as a branching agent, as only a small amount of it is sufficient to induce branching in the resulting polymers. It corroborates with the literature data stating that the molar concentration of a branching agent above 5 % is unfavorable both for membrane casting and its further performance [51, 54]. Moreover, BTK can be synthesized in a straightforward and efficient way [59], which, combined with its low required loading, minimizes BTK impact on the overall cost of the resulting polymers. It was previously found that a slight excess of a carbonyl compound dramatically accelerates superacid-catalyzed polyhydroxyalkylation and leads to a significant increase in polymer molecular weight [73, 74]. In most studies on the synthesis of branched systems using functionalized aromatic branching agents (see Figure 1), a minor excess of the carbonyl monomer (typically mPip) is still employed, although to a lesser extent than in the synthesis of linear analogs [52]. In our case, the branching agent BTK itself contains a functional trifluoroacetyl group. Therefore, to ensure its incorporation into the polymer structure, we deliberately avoid using an additional excess of the carbonyl compound. Nevertheless, neutral polymers are successfully synthesized within a relatively short reaction time (up to 7 h), similar to those reported in other works on branched polymers [52, 54]. In the next step, the corresponding quaternized branched polymers (QB-PTP-1.5 and QB-PTP-3) are obtained by N-alkylation reaction of NB-PTP-1.5 and NB-PTP-3, respectively, with an excess of iodomethane in DMSO solution (Figure 2a) to achieve complete quaternization. Importantly, QB-PTP-x polymers exhibit excellent film-forming properties (Figure 2b). The neutral linear poly(para-terphenyl piperidinium) polymer (NL-PTP) without BTK moieties and its quaternized derivative (QL-PTP) are also synthesized for comparison as described in Refs. [26, 75](FigureS1a). The chemical structures of the synthesized polymers are confirmed by 1H NMR and FTIR spectroscopy. The 1H NMR spectra of NL-PTP and QL-PTP (Figure S1b)arein good agreement with the proposed structures from the previously reported data [26]. Briefly, characteristic signals from terphenyl Macromolecular Materials and Engineering,2025 5of13 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 2 a) Synthetic routes for the branched NB-PTP-x and QB-PTP-x; b) Photo image of QB-PTP-1.5 membrane (5 cm ×15 cm). FIGURE 3 1H NMR spectra of NB-PTP-1.5 a), QB-PTP-1.5 b), NB-PTP-3 c), and QB-PTP-3 d). protons are observed at 7–8 ppm, and the region narrows as the polymer transits from neutral to cationic form. Shifts from the Nprotonated piperidine ring in NL-PTP or the piperidinium ring in QL-PTP, as well as methyl protons, appear between 2.2 and 3.5 ppm. Notably, the splitting of the methylene peaks in the NLPTP spectrum is attributed to protonation of the piperidine ring. The 1H NMR spectra of NB-PTP-x and QB-PTP-x (Figure 3)show signal distribution patterns that closely resemble those of their linear analogs (NL-PTP and QL-PTP). However, an additional low-intensity signal at ∼7.2 ppm appears in the aromatic region of all branched neutral polymer spectra, likely originating from the BTK moiety responsible for branching. To clarify the origin of this signal, we additionally synthesized NL-PTP using the same procedure as for NB-PTP-3 (maintaining identical reaction time and reagent ratios), but omitting the BTK component. A detailed comparison of the 1H NMR spectra of both polymers (Figure S2) clearly showed the absence of the ∼7.2 ppm signal in the NL-PTP spectrum. This observation supports the assignment of the signal to the BTK unit. Notably, this signal most likely corresponds to the protons in the 2,6positions of the disubstituted biphenyl fragments (Figure S2), while the remaining BTK protons are overlapped with the signals of the substituted terphenyl units. FTIR spectra of neutral and quaternized polymers are shown in Figure 4a. Firstly, the spectra contain absorption bands inherent to phenyl rings. Particularly, C─H stretching of benzene appears at 3028 cm−1, the characteristic peaks of C─C aromatic stretching vibrations are located near 1607, 1491, and 1450 cm−1[44], while ring skeleton vibrations can be found at 1003 cm−1[76]. The band centered at 1677 cm−1also corresponds to the benzene structure [47]. Second, there are IR absorption signals of Nmethylpiperidine. Thus, the stretching of methylene and methyl 6of13 Macromolecular Materials and Engineering,2025 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 4 a) FTIR spectra of neutral and quaternized branched polymers; b) Morphology of the surface on FE-SEM micrograph, and c) Fluorine distribution on FE-SEM/EDX image for QB-PTP-3. groups is observed at 2600–3000 cm−1[77]. Quaternization leads to narrowing of this region and registration of only two distinct peaks at 3028 and 2957 cm−1due to formation of a more symmetric chemical structure. Another reason of such behavior may be related to the quaternization-induced disappearance of the group of small bands of lower frequency associated with the lone-pair electrons of the nitrogen atom. Such an effect was explained in ref. [78] and also noticed in ref. [79]. Another evidence of Nmethylation was reported in ref. [46], where the authors claim about the appearance of ─CH3asymmetric bending vibration band with the maximum near 1470 cm−1in the spectrum of piperidinium salt. In the research given it is assumed that this peak is overlapped with the one of benzene fragment at 1450 cm−1in a spectrum of a neutral compound and becomes more pronounced after the addition of the second methyl group. In the meantime, the symmetric bending of C─Hin─CH2─ and ─CH3appears at 1352 and 1381 cm−1, respectively. In the quaternary ammonium group the last peak shifts to 1395 cm−1. Interesting enough, the peak centered at 1285 cm−1in the spectra of uncharged polymers shifts to 1296 cm−1for QB-PTP-3 while splits to three bands at 1258, 1275, and 1294 cm−1for QB-PTP1.5. In ref. [77] all of them are associated with wagging vibrations of different types of ─CH2─groups in piperidine ring. However, in refs. [34]and[80] a peak near 1280 cm−1is believed to be characteristic to C─N bond. More likely that C─N stretching vibrations of tertiary amine appear in the region from 1180 to 1200 cm−1as a complex band that consists of three peaks and changes its shape after quaternization. An absorption band at 1136 cm−1is defined as ─CH2─twisting [77]. In the region of 900– 1000 cm−1, except for the peaks near 920 and 950 cm−1the one centered at 978 cm−1evokes in a charged polymer. In accordance with the literature [46, 47, 81], it corresponds to (CN+) stretching vibrations in the quaternary ammonium group. The additional evidence of its formation is the increase in the intensity of a broad band of hydroxyl stretching centered near 3420–3450 cm−1. It is attributed to higher moisture absorption by a more polar quaternized polymer [24, 47]. FE-SEM testing elucidates the morphology of the synthesized membranes, while EDX mapping provides their elemental analysis. FE-SEM micrograph of the membrane surface (Figure 4b) reveals a smooth and homogeneous structure of the polymer (by the example of QB-PTP-3). In the meantime, Figure 4c displays the distribution of fluorine over the sample, thus confirming the presence and successful embedding of the fluorine-containing branching agent. The measured fluorine content in QB-PTP-3 (0.545%) is about two times higher than in QB-PTP-1.5 (0.255%) and slightly exceeds the theoretically calculated values in both cases. However, due to the possible uniformity of the atomic distribution over the depth of the sample, this analysis may only serve as qualitative, not quantitative one. 3.3 A Closer Look at the Purification of Synthesized Polymers The polymer NL-PTP was first synthesized by Olsson et al. using DCM as the solvent, with a monomer concentration (TP and mPip) of ∼80 wt./v% and a ninefold excess of TFSA, under a reaction time of 5–7 h [26]. We adopted this approach to prepare NL-PTP and its quaternized derivative (QL-PTP). Several studies have since reported modified procedures for NLPTP synthesis. These variations involve changes in monomer stoichiometry, concentration in DCM, or acids (TFSA and TFA) amount. For instance, Tang et al. have used a more concentrated monomer solution (100 wt./v%) with 10 equivalents of TFSA [75]. A key advantage of this method is the shorter reaction time (only 2 h), after which the solution becomes too viscous to stir. Our results confirm that viscosity increases significantly after 2–3 h under similar conditions. However, the 1H NMR spectrum of the resulting polymer (NL-PTP*, where the asterisk (*) here and elsewhere denotes a polymer that either contains impurities or is derived from such polymer) showed signals that appear to originate from unreacted terphenyl, as suggested by comparison with the reference spectrum of pure TP (see spectra 1 and 2 on Figure 5a). Next, we observed similar TP impurities in NB-PTP-x samples synthesized from relatively concentrated monomer solutions (e.g., 80 wt./v% in DCM), as seen in the representative spectrum of Macromolecular Materials and Engineering,2025 7of13 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 5 a) 1H NMR spectra of TP monomer (1), NL-PTP* (2), and NB-PTP-3* (3); b) 1H NMR spectra of NL-PTP* (1), NL-PTP* after reprecipitation (2), and NL-PTP* after quaternization of the non-reprecipitated sample (3); c) WAXD patterns of TP monomer, NB-PTP-3*, and NBPTP-3* after reprecipitation. The asterisk (*) marks polymers that are either impure or derived from impure polymers. “ReP” and “Qtz” in panel (b) indicate the reprecipitation process and the quaternization reaction, respectively. NB-PTP-3* (Figure 5a, spectrum 3). Interestingly, a careful review of NMR data in some previous studies suggests similar impurities [75, 82–85], although they were not explicitly discussed. Reprecipitation is widely applied in polymer purification to separate out residual monomers and low-molecular-weight impurities.Inourcase,reprecipitatingNL-PTP*from DMSO (containing the necessary amount of TFA) into ethyl acetate, followed by the addition of potassium bicarbonate, effectively removes impurities (see spectra 1 and 2 in Figure 5b). Alternatively, as seen in the comparison of 1H NMR spectra 1 and 3 in Figure 5b, direct quaternization of NL-PTP* with methyl iodide followed by precipitation also eliminates most impurities. Complete quaternization typically requires an excess of methyl iodide. However, for applications involving partial quaternization [27], impurity-free starting materials are essential. This highlights the importance of proper purification prior to further modification. WAXD profiles of the initial TP monomer and NB-PTP-3* before reprecipitation confirm the presence of unreacted terphenyl in the polymer (Figure 5c). In addition to the broad diffuse peaks characteristic of the polymer, sharp reflections corresponding to pure TP are observed, particularly in the region around 2θ= 12◦–24◦, where TP exhibits several of its most intense diffraction peaks. After following reprecipitation, carried out in a similar manner to NL-PTP*, NB-PTP-3* displays only a broad diffuse peak around 2θ≈18◦(Figure 5c), indicative of its amorphous nature. Notably, no TP impurities were observed in our synthesis of NB-PTP-x using a 35% w/v monomer concentration and 10 equivalents of TFSA acid relative to 1 equivalent of the TP monomer. Nevertheless, we included a reprecipitation step for all QB-PTP-x polymers, involving precipitation from DMSO into ethyl acetate – an additional purification measure not reported in previous studies. Although this step led to a slight reduction in overall yield (∼5%), it likely enhanced the structural purity of the final polymers. 3.4 Static and Dynamic Light Scattering Studies Most PAPs prepared via polyhydroxyalkylation followed by quaternization, whether of linear or branched structures, are rarely characterized by conventional gel permeation chromatography / size-exclusion chromatography (GPC/SEC) analysis. This is primarily due to the lack of suitable calibration standards, poor solubility of neutral PAP precursors in common solvents, and strong adsorption of the resulting cationic polymers on the column material. As a result, intrinsic viscosity is often used as an indirect indicator of molecular weight. For instance, Wu et al. reported an intrinsic viscosity of 2.24 dL g−1for QL-PTP [54], while Olsson et al. described the same polymer with a much lower value of 0.36 dL g−1[26], which was attributed to low molecular weight. However, this discrepancy is likely to stem from different measurement conditions, specifically, the use of LiBr in Olsson’s study to suppress the polyelectrolyte effect [86] of the cationic polymer, making direct comparison unreliable. Notably, most reports on PAP-type polymers do not mention the use of salts to mitigate electrostatic interactions. In our study, both the reduced and inherent viscosities of QB-PTP-x (measured without LiBr) increased with dilution, clearly demonstrating a pronounced 8of13 Macromolecular Materials and Engineering,2025 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 6 Debye plots from SLS data for QB-PTP-1.5 a) and QB-PTP-3 b) dissolved in DMSO at 25◦C, where line is a linear fit to the data that leads to MwSLS values; c) DLS size distribution of the samples. polyelectrolyte effect (Figure S3). This kind of dependence does not allow proper extrapolation for finding the value of intrinsic viscosity. Moreover, in such systems, viscosity reflects not only molecular weight but also charge density, ionic strength, and interchain interactions [87]. Thus, we additionally measured the inherent viscosity in the presence of LiBr following Olsson’s protocol for better comparison. Both QB-PTP-1.5 and QB-PTP3 exhibited viscosity behavior typical of conventional non-ionic polymers (Figure S3, shown for QB-PTP-3). The inherent viscosity values obtained in the presence of LiBr were 0.56 dL g−1for QB-PTP-1.5 and 0.62 dL g−1for QB-PTP-3. In contrast to the above-mentioned limitations associated with GPC/SEC and inherent viscosity measurements, SLS method [60] offers a more reliable, calibration-free method for absolute molecular weight determination of polymers. In this study, we appliedthis technique to determine the weight-averagemolecular weight (MwSLS) of the QB-PTP-x polymers. To the best of our knowledge, this is the first report to use SLS for molecular weight determination of PAP-type polymers. The MwSLS values of the polymers are obtained from the slope of the Debye plots generated from SLS measurements (Figure 6a,b). As it turned out, both polymers have similar values of molecular weight. Still, the one of QB-PTP-3 having the bigger amount of BTK branching agent is slightly higher. The second virial coefficients, calculated from the Debye plot slopes, are positive for all the samples, indicating that particle-solvent interactions dominate over particle-particle interactions, which means good solution stability. The limited GPC data available for PAPs show that the MwSLS values for QB-PTP-1.5 and QB-PTP-3 are consistent with those reported for other branched systems. For instance, branched polymers synthesized using tetraphenylmethane as a branching agent are characterized by Mwvalues in the range of 28.44– 51.24 kDa [56]. In contrast, hyperbranched PAPs synthesized by grafting onto triazatruxene derivatives have lower molecular weights in the range of 10–16 kDa (measured in DMF with LiBr as the eluent and monodisperse poly(methyl methacrylate) as the standard) [68]. Mwvalue of 66 kDa is also reported for branched poly(aryl-quinuclidinium)s, while quaternized poly(p-terphenyl isatin)s with branched architectures exhibited Mwvalues ranging from 57 to 67 kDa under similar conditions (measured in DMF using polystyrene as the standard) [61, 64]. Next, based on the idea that the intensity of light scattered is proportional to particles’ diameter, DLS is suitable for investigation of colloidal solutions including macromolecular ones. Therefore, it was used to estimate size and size distribution of synthesized polymers. Obtained data is represented in Figure 6c, showing that the higher concentration of the branching agent leads to the formation of bigger particles. The authors [54], who used DLS method for analysis of PAPs, also observed enlargement of particles with increasing of branching degree and connected it with the increasing of molecular weight. However, in our work, where the values of Mware very close, it is supposed that the bigger particle size for a more branched polymer is ascribed to the higher fraction of free volume and looser structure known to be characteristic to branched architectures. Except for the appearance of the small peak in the NMR spectra of branched polymers, the presence of fluorine (confirmed by EDX) and trends in DLS data indirectly support the successful incorporation of BTK. 3.5 Thermal Properties Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the synthesized polymers recorded during their TGA testing are displayed in Figure 7. It is worth noting that the weight loss before 250◦C is caused by the release of bound water and solvent residues in the sample. From the general view of the profiles, it may be concluded that the fact of quaternization rather than amount of branching agent is the key factor determining the decomposition behavior of the polymers. The most significant difference between neutral and charged polymers lies in the presence of an additional step with a quite non-intensive maximum of degradation rate (Тmax) near 370–380◦C for quaternized specimens (Figure 7b). It is attributed to facilitated disintegration of thermally sensitive N,Ndymethylpiperidinium functionality [26, 27, 31], also claimed to proceed in a given temperature range by other scientists [55, 56, 69]. In the meantime, many authors observed decomposition of cationic groups at lower temperatures [46, 88, 89]. At the main stage (Тmax =530–630◦C) the destruction of an aromatic polymer backbone occurs. In general, the polymers in a neutral form are more thermally stable at the initial stage, while the ones in the form of salt degrade more slowly at the end of the process. This fact may be clearly evidenced by the temperatures Macromolecular Materials and Engineering,2025 9of13 14392054, 2025, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202500225 by University Of Chemistry And, Wiley Online Library on [16/12/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License