Introduction

In modern society, fossil resources, such as petroleum and coal, are widely used as primary energy sources. However, these resources are nonrenewable, face the risk of depletion, and cause significant environmental pollution. Consequently, the development of renewable energy and sustainable materials has become a global priority [1,2,3]. Among the various renewable resources, biomass has attracted considerable attention because it is abundant, renewable, and often biodegradable. The utilization of biomass-derived materials is thus expected to contribute to the reduction of carbon dioxide emissions and to the establishment of a more sustainable society [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18].

Lignin is a promising biomass resource because it is not widely utilized as a fine chemical in industry [19,20,21] and is a major component of wood, accounting for approximately 15–30% of its dry weight. As a result, it has been the focus of considerable research. Despite its abundance and unique aromatic structures, lignin has traditionally been underutilized and primarily burned as a low-value energy source in industrial processes. Therefore, its application in the production of value-added materials, such as plastics and functional polymers, has been limited [22, 23]. In the biodegradation pathway of lignin, several intermediates are formed following the cleavage of aromatic rings. Among them, 2-pyrone-4,6-dicarboxylic acid (PDC) plays a central role in this study. PDC is a key intermediate generated through the microbial transformation of lignin-derived aromatic compounds and is considered an important platform in the valorization of lignin. Prior studies have demonstrated success in selectively and quantitatively producing PDC on a large scale [24]. The physical properties of PDC have been comprehensively investigated, and one important feature of PDC is its high acidity, with a pKa of 1.1 [25, 26]. Furthermore, previous studies have demonstrated that PDC can be polymerized to afford its polyesters despite its acidic nature, and the resulting polyesters are biodegradable in pond water [27,28,29]. These results highlight the environmental compatibility and potential of these materials as sustainable building blocks for functional polymeric materials.

Although more PDC-based linear polymers, such as polyurethanes and polyamides, have been synthesized [28, 30,31,32], to the best of our knowledge, no side-chain-type polymers have been reported [33]. We selected poly(vinyl alcohol) (PVA) as a commercially available reactive polymer backbone, and one of the carboxylic acids in PDC was covalently bonded to the hydroxyl groups of PVA. The other carboxylic acid group remained unreacted as a side chain and is expected to dissociate, providing mobile negative charges. The resulting polymer is thus anticipated to function as an anionic polymer electrolyte.

Results and discussion

Synthesis and characterization

Both PVA and PDC are water soluble. They were dissolved in water and reacted at 110 °C for 24 or 48 h using hydrochloric acid as a catalyst (Scheme 1). To avoid excessive crosslinking, the molar ratio of hydroxyl groups (PVA) to carboxylic acid groups (PDC) was controlled at 1:5. After the reaction was complete, the excess PDC was removed by washing with ethyl acetate, yielding the corresponding PVA-bearing PDC side chains (PVA-PDC). The aqueous solution of the resulting PVA-PDC was cast onto a Teflon plate and dried at room temperature to yield a yellow free-standing film. On the basis of the weight yield, the reaction efficiency was determined to be 7.3% after 24 h and 14.9% after 48 h. This finding is consistent with the observation that the solution color became darker with increasing reaction time. The relatively low yields of the polymer reactions are likely due to the partial dissolution of the target polymer in ethyl acetate. The 1H NMR spectrum of PVA-PDC suggested the absence of PDC protons because of the low PDC content and significant peak broadening (Fig. S1, Supporting Information). Although PDC is an asymmetric molecule, the reactivity ratio of its two carboxylic acid groups could not be clarified from the analysis of the resulting polymer.

Scheme 1
Scheme 1The alternative text for this image may have been generated using AI.
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Synthesis of PVA bearing PDC side chains

The functional groups of the polymers were analyzed by Fourier transform-infrared (FT-IR) spectroscopy. The most notable difference between PVA and PVA-PDC is the presence of carboxylic acid groups. PVA does not exhibit any carbonyl (C = O) peaks, whereas for PVA-PDC, the characteristic stretching vibration of the C = O bond in carboxylic acids appears at approximately 1700 cm–1 (Fig. 1). Therefore, the strong band observed near 1700 cm–1 in PVA-PDC indicates the presence of carboxylic acid moieties from PDC. Furthermore, for PVA-PDC, the intensity of this peak increased with increasing reaction time from 24  to 48 h, suggesting that a greater amount of PDC was incorporated into the polymer chain as the reaction proceeded.

Fig. 1
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FT-IR spectra of PVA and PVA-PDC in the range of a 4000–600 cm–1 and b 1800–1600 cm–1

The thermal properties of PVA and PVA-PDC were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). PVA-PDC (24 h) was used in the following experiments because of the limited sample amount of PVA-PDC (48 h). To determine the effect of moisture, each polymer was dried in a vacuum at 60 °C for at least 6 h. According to the TGA curves, the 5% weight loss temperature (Td5%) of PVA was 241.5 °C, whereas that of PVA-PDC was 169.4 °C (Fig. S2, Supporting Information). This result is attributed to the substitution of a small number of PDC side chains, which likely disturbed the packing of the polymer chains and generated initiation sites for thermal decomposition. In contrast, the DSC thermograms of PVA and PVA-PDC suggested the opposite trend. The glass transition temperature (Tg) of pristine PVA was determined to be 64.5 °C, whereas that of PVA-PDC (24 h) increased to 90.7 °C (Fig. 2). Carboxylic acids are known to form stable cyclic dimers through two cooperative hydrogen bonds, resulting in significantly stronger intermolecular interactions than those formed by hydroxyl groups [34]. Therefore, the intermolecular interactions between the PDC side chains are considered to increase the Tg of the polymer.

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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DSC curves (2nd heating scan) of PVA and PVA-PDC (24 h)

pH titration

Owing to the presence of side-chain carboxylic acid groups, PVA-PDC is expected to become a biomass-based polyelectrolyte [35]. Therefore, acid‒base titration was carried out to quantify the carboxylic acid groups in PVA-PDC (24 h), and the results were compared to those of PVA. The polymers were dissolved in water and titrated with 0.1 M NaOH. From the titration curve, the equivalence point was determined to be approximately pH 8 (Fig. 3a). On the basis of the volume of NaOH consumed at the equivalence point, the number of carboxylic acid groups was calculated. The degree of incorporation of PDC into the side chains of PVA-PDC (24 h) was thus estimated to be 5.24%. This value is consistent with that estimated from the weight yield (7.3%) (vide supra).

Fig. 3
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a pH titration curve of PVA-PDC (24 h) with 0.1 M NaOH and b the corresponding Henderson–Hasselbach plots

Furthermore, the degree of dissociation (α) was plotted as a function of pH, yielding Henderson–Hasselbach plots with a linear relationship (Fig. 3b). From this linear plot, the y-intercept was determined. Because the y-intercept corresponds to the acid dissociation constant (pKa), the pKa of PVA-PDC (24 h) was determined to be 4.41. Notably, the intrinsic pKa of PDC is approximately 1.1 [25]. The significant increase in pKa upon introduction into the polymer side chain indicates that the acidity of PDC is markedly reduced when it is covalently bound to the polymer backbone. However, the value is still reflected by the strong acidity of PDC. For example, compared with poly(acrylic acid), whose pKa is approximately 4.75 [36], PVA-PDC (24 h) is more strongly acidic. These results suggest that the intrinsically strong acidity of PDC contributes to the relatively low pKa of the modified polymer, even at a low degree of incorporation.

Mechanical strength

Free-standing films of PVA and PVA-PDC (24 h) were fabricated by solution casting onto Teflon substrates, followed by drying under ambient conditions. The resulting films were carefully peeled off and subjected to tensile testing. The specimens were prepared with dimensions of 0.2 mm in thickness, 2 mm in width, and 12 mm in gauge length. The tensile-tested samples after fracture are shown in Figure S3 (Supporting Information). A clear difference in optical images was observed between the two polymers. PVA-PDC exhibited a rigid and brittle mechanical response, which was characterized by limited elongation prior to failure and abrupt fracture. In contrast, pristine PVA demonstrated a soft and highly ductile nature, showing significant elongation before breaking. This property was quantitatively evaluated by tensile testing. The stress‒strain curves revealed a pronounced difference in mechanical stiffness between the two polymers. Pristine PVA exhibited a Young’s modulus of 203 MPa, which is consistent with its soft and ductile nature (Fig. 4). In sharp contrast, the Young’s modulus of PVA-PDC dramatically increased to 2240 MPa, representing more than a tenfold increase in stiffness. The incorporation of PDC units into PVA significantly altered the mechanical characteristics of the polymer films. The increased stiffness and brittleness of PVA-PDC suggest restricted polymer chain mobility, likely originating from intermolecular interactions and ionic associations introduced by the PDC moieties. Conversely, the flexibility and extensibility of pristine PVA can be attributed to its relatively high chain mobility and hydrogen-bonding-mediated network structure. These results indicate that the introduction of PDC effectively modulates the mechanical properties of PVA, which transforms the material from a ductile polymer into a relatively rigid and brittle material.

Fig. 4
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Strain‒stress curves of PVA and PVA-PDC

To obtain direct microstructural evidence for the aforementioned changes in thermal and mechanical properties, the surface morphologies of the films were investigated using atomic force microscopy (AFM) (Fig. S4). Pristine PVA has a continuous, undulating surface morphology with a height variation of ~80 nm, reflecting its regular chain packing and flexible nature. After the introduction of PDC side chains, the PVA-PDC film displayed a significantly flatter matrix (the height variation is reduced to ~22 nm), accompanied by numerous discrete, island-like aggregates. These localized features indicate strong intermolecular hydrogen bonding between the pendant PDC groups. Such aggregated domains act as physical cross-linking sites within the polymer network, significantly restricting polymer chain mobility and disrupting the uniform packing of PVA chains. This microstructural transformation provides clear structural evidence for the pronounced increase in Tg and mechanical stiffness discussed above.

Biodegradation test

The biodegradability of PVA and PVA-PDC was evaluated for 30 days under controlled composting conditions using a Microbial Oxidative Degradation Analyzer (MODA-CS; Yahata Bussan Co., Ltd., Japan) according to ISO 14855-2. Reference samples, e.g., cellulose and PDC, were also evaluated under the same conditions. PVA is a petroleum-derived polymer, but it is widely recognized as a biodegradable polymer [37,38,39]. PDC is a lignin-derived metabolic intermediate, and its biodegradability was recently demonstrated by a biochemical oxygen demand (BOD) test [28]. Therefore, on the basis of its chemical structure, PVA-PDC is expected to be biodegradable. The degree of biodegradation was calculated from the cumulative amount of CO2 evolved during the test, gravimetrically determined from the weight increase of the CO2 absorption columns. After the blank value was subtracted, the net CO2 generated from the sample was compared with the theoretical amount of CO2 on the basis of its carbon content to obtain the biodegradation percentage. As shown in Fig. 5, the cellulose began to decompose efficiently from the start of the experiment and exhibited a biodegradation rate exceeding 80% after 30 days. In contrast, PDC exhibited an induction period of approximately one week, after which rapid degradation occurred over several days, eventually reaching a plateau at approximately 40% biodegradation. This behavior differs from that observed in the BOD test conducted in pond water, which showed a monotonic increase and quantitative degradation, suggesting that the degradation behavior depends on the environmental conditions. Both PVA and PVA-PDC displayed monotonic increases in biodegradation similar to that of cellulose. However, the degradation plateaued at approximately 30%. Although additional cleavage of the ester linkage between the PVA main chain and the PDC side chains is needed, little difference was observed between the biodegradation behaviors of PVA-PDC and those of PVA.

Fig. 5
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Biodegradation profiles of PVA and PVA-PDC in powder form compared with those of cellulose and PDC for 30 days under controlled compost conditions. The experiments were conducted in accordance with ISO 14855-2. Average plots of two test samples

Conclusions

In conclusion, we have successfully developed a novel side-chain-type polymer bearing PDC units through selective mono-esterification of PDC onto PVA. The introduction of pendant PDC moieties significantly influenced the thermal and mechanical properties, highlighting the strong impact of side-chain functionalization on intermolecular interactions and chain rigidity. Acid‒base titration confirmed that the resulting PVA-PDC behaves as a polyelectrolyte, exhibiting higher acidity than poly(acrylic acid) does, while the results of the composting tests demonstrated biodegradability comparable to that of pristine PVA.

This study presents a new synthetic strategy for biomass-derived functional polymers using PDC and demonstrates that PDC is a promising precursor for biomass-derived polyelectrolytes. Future work will focus on increasing the PDC incorporation ratio and exploring the structure‒property relationships in various polymer backbones, as well as investigating potential applications in sustainable ionic materials and environmentally benign functional devices. Increasing the PDC amount increases the biomass content and contributes to reducing the use of fossil resources. We believe that realizing functionalities derived from PDC is key to practical applications.