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• Hydrogels have been the subject of considerable interest in both scientific and industrial contexts, due to their status as three-dimensional network structure materials that can be readily manipulated to control their physical and chemical properties [1,2]. The processes involved in the preparation of hydrogels are inherently complex, typically comprising monomers, chemical cross-linking agents, auxililary agents, and functional ingredients [3,4]. In particular, gelation of hydrogels is tedious and relies on an anaerobic environment, as oxygen is an efficient and unwanted free radical scavenger [5]. The freeze–pump–thaw technique and deoxygenation with inert gas are established methods for the removal of oxygen, although they are relatively time-consuming. Therefore, there are still considerable challenges associated with the preparation of hydrogels at room temperature in an aerobic environment.

  In recent years, self-initiated polymerisation systems for hydrogels have been rapidly developed [6–9]. Oxygen-resistant radical polymerisation can be achieved through the utilisation of self-initiating systems that facilitate the generation of excess free radicals or the conversion of oxygen into non-initiating species [10]. Self-initiating systems comprising transition metals and catechol ligands display peroxidase-like activity and can rapidly activate persulfate at room temperature, thereby generating an excess of radicals [8,11]. Glucose oxidase (GOx) is capable of oxidising glucose to produce hydrogen peroxide while reducing oxygen [12]. It is employed in the deoxidisation of free radical polymerisation under ambient conditions [13]. Shen and colleagues employed a two-enzyme cascade reaction comprising GOx and horseradish peroxidase (HRP) to prepare a composite hydrogel [14]. In this reaction, acetylacetonate (ACAC) served as the substrate for horseradish peroxidase, and the composite hydrogel was prepared via the free radical polymerisation of the hydrogel using a ternary system of HRP-ACAC—H₂O₂. Furthermore, ACAC functions as a redox ligand for laccase, which oxidises ACAC and reduces oxygen to water [15,16]. This distinctive process enables acrylamide to polymerise in an oxygen-rich environment without the necessity for external stimuli or persulfate or peroxide initiators. Additionally, chemical investigations of ACAC have demonstrated that it can be metabolised and broken down by microorganisms in the surrounding environment [17,18].

  Natural enzymes display remarkable catalytic efficiency and substrate specificity [19]. However, the complex preparation procedures, unstable catalytic activity and high cost associated with natural enzymes limit their scalability [19]. Artificial enzymes are regarded as a promising alternative to natural biocatalysts, given that they typically demonstrate superior stability than their natural counterparts under extreme conditions, while also being a cost-effective option [19,20]. Metal oxide artificial enzymes with high surface energy to surface volume ratios represent a promising avenue of research. Materials such as Fe₂O₃, Fe₃O₄, Mn₂O₃ and Mn₃O₄, which possess polymerase-like activities, have been identified as potential candidates for this purpose [21]. It is noteworthy that manganese oxide has been demonstrated to possess oxidising capabilities towards a diverse range of organic compounds, including glucose [22], ascorbic acid [23] and 5-hydroxymethylfurfural [24]. In light of these findings, a self-initiating system utilising transition metal oxides for the oxidation of ACAC in the context of aerobic radical polymerisation was developed in the present study.

  The aim of this study is to reveal that transition metal oxides are able to oxidise ACAC to generate carbon radicals to initiate aerobic self-polymerisation of hydrogels without the need for external stimulation. In this self-initiating system (Fig. 1), the transition metal oxides continuously oxidize ACAC to produce reduced carbon radicals, and some of the carbon radicals reduce oxygen to produce oxidized hydroxyl radicals, and the free radical polymerization of various vinyl monomers is carried out under the effect of the carbon radicals and hydroxyl radicals. As a result, the hydrogel can realize self-coagulation without external stimulation in room temperature oxygen environment. The swelling and shrinkable properties of the hydrogel can then be customized by introducing coordination metals and hydrophobic groups. This novel strategy of customizable hydrogel polymerization for the preparation of swelling and shrinking without external stimuli under room temperature oxygen environment is expected to be a mild and convenient alternative to the established chemical procedures of hydrogels.

  Figure 1

  

  Figure 1.  Schematic representation of a self-initiating system mechanism.

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  To evaluate the potential of self-initiated systems, we initially sought to ascertain the capacity of transition metal oxides to oxidize ACAC. For this purpose, Mn₃O₄ was selected as a model material. In aqueous solution, ACAC exhibits two forms, keto and enol. The ratio is influenced by solution acidity and alkalinity. In the cyclic voltammetry experiment (Fig. S1 in Supporting information), substantial oxidation peaks appeared after adding ACAC to an acidic phosphate buffer and alkaline KOH solution, indicating that Mn₃O₄ is capable of oxidizing ACAC in both acidic and alkaline solutions. Density functional theory (DFT) was utilized to examine the electron transfer between two tautomeric structures of ACAC and Mn₃O₄. Based on the existing studies on Mn₃O₄, Mn₃O₄ (110) was selected for further studies [25]. In the Mn₃O₄ (110)-ACAC model, ACAC is adsorbed on the Mn₃O₄ (110) surface (Table S1 in Supporting information), and the main Mn-O interaction occurs between the Mn atom and the carbonyl's O atom and aldehyde groups of ACAC (Figs. 2a(i) and b(i)). The red region represents charge accumulation, while the green region represents charge depletion. Combined with the Mulliken population analysis (Table S2 in Supporting information), the obtained results reveal that the oxygen atom charge of ACAC accumulates, the carbon atom charge is consumed, the Mn atom charge of Mn₃O₄ is consumed, and the surface electrons transfer from both the carbon atom of ACAC and the Mn atom of manganese tetroxide to the carbonyl oxygen atom of ACAC. With reference to the free radical reactions reported by ACAC chemistry [26–28], we propose several hypothetical pathways in the self-initiating system (Fig. 2c). Based on the aforementioned DFT results, the alcohol form of ACAC with the adsorption mechanism of Mn₃O₄ was selected as the initial point for the free energy distribution of processes 1–3, while the ketone form of ACAC with the adsorption mechanism of Mn₃O₄ was selected as the initial point for the free energy distribution of process 4 (Fig. 2d). The energy barriers corresponding to the transition states TS1–4 in the cleavage reactions of processes 1–4 in order are 50.8, 18.9, 28.1, and 27.2 kcal/mol. In addition, the free energies released for the products PRO1–4 are 2.8, 6.4, 13.8, and 6.2 kcal/mol, respectively.

  Figure 2

  

  Figure 2.  Analytical results of redox reactions of ACAC with Mn₃O₄. (a) DFT results of ACAC (keto) adsorption on Mn₃O₄ surfaces: (ⅰ) Three-dimensional structure of Mn₃O₄(110)-ACAC after geometry optimization; (ⅱ) side-view of the three-dimensional plots of the electron density difference; (ⅲ) corresponding top view. (b) DFT results of ACAC (alcohol) adsorption on the surface of Mn₃O₄. Charge accumulation and charge depletion are represented by red and blue, respectively. Carbon: gray; Oxygen: red; Hydrogen: white; Manganese: violet. (c) Proposed radical generation pathway in Mn₃O₄(110)-ACAC. (d) The potential energy surface of Mn₃O₄(110)-ACAC.

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  Following the determination of the feasibility of radical production from the oxidation of ACAC by Mn₃O₄, a subsequent analysis was conducted on the radicals produced by the self-initiated system via DMPO capture electron spin resonance (ESR) experiments (Fig. 3). The results demonstrate (Fig. 3a) that the self-initiated system produces carbon radicals (*C), hydroxyl radicals (*OH) and peroxyhydroxyl radicals (*OOH). The concentration of *C in the nitrogen atmosphere was markedly higher than that in the air and oxygen atmospheres. Furthermore, the yields of *OH and *OOH exhibited a notable increase in the oxygen atmosphere and a pronounced decline in the nitrogen atmosphere. This phenomenon may be attributed to the further reduction of oxygen via the reaction of carbon radicals produced by the oxidation of ACAC [26–28]. Additionally, it has been demonstrated that conventional artificial metalloenzymes can be employed to construct self-initiating systems (Fig. 3b).

  Figure 3

  

  Figure 3.  Characterization results of free radicals. (a) DMPO spin-trapping ESR spectra of Mn₃O₄-ACAC self-initiated system. (b) DMPO spin-trapping ESR spectra of other self-initiated systems. (c) Digital photographs of free radical quenching experiments with exothermic monitoring results.

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  Subsequently, free radical quenching experiments were conducted to substantiate the oxygen resistance characteristics of the self-initiating systems (Fig. 3c). Tertiary butyl alcohol (TBA) and potassium iodide (SSKI oral solution) are typical free radical quenching compounds for *OH and general radicals [9], in that order. In aqueous solution, the polymerization yields of the self-initiating systems in each atmosphere were found to be approximately equivalent to the exothermic amount, which is indicative of oxygen resistance properties. Once all radicals had been quenched by KI, the hydrogel polymerization process was unsuccessful, with a markedly low exothermic response. In contrast, the polymerization of the hydrogel was still successful when the hydroxyl radical was quenched by TBA. Furthermore, the effect of the polymerization weakened significantly with increasing oxygen concentration. This phenomenon may be attributed to the reduction of oxygen by carbon radicals [26,28], whereby an increase in oxygen concentration results in a higher conversion of *C to *OH, and a subsequent weakening of the polymerization effect as *OH is quenched. Therefore, in the self-initiating system, the primary role in polymerization is played by *C, with *OH serving as an assistant. The oxygen in the air is reduced to *OH by *C, which is the main reason for the oxygen resistance of the system.

  On the other hand, we analysed the valence changes of the precipitates after the reaction of the self-initiating system using XPS (Fig. 4a). With the increase in the addition of the reducing agent ACAC, the Mn³⁺ content in the precipitates decreased significantly while the Mn²⁺ and Mn⁴⁺ contents both increased slightly. This phenomenon may be attributed to the H⁺ released during the oxidation of ACAC to carbon radicals [6,29], which can react with Mn₃O₄ to form MnO₂ and Mn²⁺ (Eq. 1) [30,31].

  $ \mathrm{Mn}_3 \mathrm{O}_4+4 \mathrm{H}^{+} \rightarrow \mathrm{MnO}_2+2 \mathrm{Mn}^{2+}+2 \mathrm{H}_2 \mathrm{O} $

  (1)

  Figure 4

  

  Figure 4.  Analytical results of metal valence states in self-initiated systems. (a) The fitted Mn 2P XPS spectra of precipitates after 1 day of reaction in the self-initiating system. (b) Mn AOS of the samples determined using the LBB colorimetric method. (c) Proposed oxidation–reduction process of self-initiating system.

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  Then, we analysed the average valence state of Mn throughout the self-initiated system using the leucobelin blue I (LBB) colorimetric method [32]. The results showed that the average valence state of Mn decreased after the reaction of the self-initiating system (Fig. 4b). Therefore, the redox process of the self-initiated system is proposed to be summarised in Fig. 4c. H⁺ is released during the oxidation of ACAC to carbon radicals by Mn₃O₄, and Mn³⁺ in Mn₃O₄ is reduced to Mn²⁺, and Mn₃O₄ reacts with H⁺ to MnO₂ and produces Mn²⁺.

  Finally, we attempted to customize the swelling and shrinkage of hydrogels using a self-initiating system instead of the commonly used thermal initiation system. Figs. 5a and b, and Fig. S3 (Supporting information) demonstrate the swelling changes of hydrogels continuously immersed in DI H₂O at 25 ℃ and 60 ℃. Polyacrylic acid, a conventional highly-absorbent material, after neutralization of 60% of the carboxyl groups by sodium hydroxide, benefited from the abundance of -COONa⁺ [33] with remarkably enhanced the water-absorbent property (Fig. 5c), especially in hypertonic solutions such as artificial sweat, artificial seawater, and saline, where the swelling performance of sodium polyacrylate was several times higher than that of PAA (Fig. 5d). NSHs produced by copolymerization of highly hydrophilic monomer AA with hydrophobic monomer LMA under high concentration of cationic surfactant CTAB exhibit stable anti-swelling properties at different temperatures, pH, and solvent environments due to the hydrophobicity provided by strong electrostatic interactions between CTAB, LMA, and AA carboxyl groups (Fig. 5) [34,35]. The temperature-sensitive monomer product of the copolymerization of NIPAM and AA has remarkable shrinkage properties in hot water (Figs. 5a and b), and the temperature of the product phase transition could be readily tuned by adjusting the ratio of NIPAM to AA [36,37]. The ATR-FTIR (Fig. S4 in Supporting information), SEM and EDS results (Table S3 in Supporting information) indicated the successful modification of the functional components regulating the hydrogel swelling and shrinking properties. Therefore, this self-initiation strategy is a promising method for hydrogel polymerization as an alternative to thermal initiation systems or other methods that require oxygen avoidance and external stimulation. However, the application of hydrogel products based on self-initiating systems requires attention to possible undesirable effects of residual metal components, such as electrical resistance [30], photothermal effects [38], enzyme-like activities [23] and toxicological effects [39].

  Figure 5

  

  Figure 5.  Swelling behaviors of the hydrogels. (a) Photographs of hydrogels continuously immersed in DI H₂O at 25 ℃ and 60 ℃ for 1.5 h exhibiting various swelling ratios. Swelling ratio of the hydrogels (b) in deionized water at different temperatures, (c) in solutions of different pH values, and (d) in different solvents.

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  In this study, a novel self-initiating system consisting of transition metal oxides and ACAC was demonstrated, and the oxidation mechanism of Mn₃O₄ on ACAC was analyzed by cyclic voltammetry and molecular dynamics simulation, and then ESR and free radical quenching experiments were used to identify the free radical species of the oxidation products of ACAC, and the valence change of Mn during the redox process was analyzed by XPS and LBB colorimetry, and finally the system was used to tailor the hydrogel swelling and shrinking properties. All the results show that this strategy can avoid the factors limiting the design and application of hydrogels in the current hydrogel polymerization strategies, such as oxygen separation, high-temperature heating, ultraviolet radiation and peroxide reagents, which provides new insights and ideas for the future development of hydrogels.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Chaojian Xu: Writing – original draft, Investigation, Data curation, Conceptualization. Juxin Yin: Writing – review & editing, Supervision, Resources, Project administration, Conceptualization. Sihong Wang: Validation, Investigation. Yue Pan: Software, Conceptualization. Qianhe Zhang: Investigation. Ningkang Xie: Validation. Shuo Yang: Supervision, Conceptualization. Shaowu Lv: Writing – review & editing, Project administration, Funding acquisition, Conceptualization.

  Acknowledgments

  This research was funded by the National Key R&D Program of China (No. 2022YFF0904000), Cross-disciplinary Innovation Project of Jilin University (No. JLUXKJC2021ZZ01), the financial support from National Natural Science Foundation of China (No. 62201497). The authors would like to express their gratitude to EditSprings (https://www.editsprings.cn) for providing professional language services and to SCI-GO (www.sci-go.com) for providing computing resources.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111075.

  
  
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• 

  Figure 1  Schematic representation of a self-initiating system mechanism.

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  Figure 2  Analytical results of redox reactions of ACAC with Mn₃O₄. (a) DFT results of ACAC (keto) adsorption on Mn₃O₄ surfaces: (ⅰ) Three-dimensional structure of Mn₃O₄(110)-ACAC after geometry optimization; (ⅱ) side-view of the three-dimensional plots of the electron density difference; (ⅲ) corresponding top view. (b) DFT results of ACAC (alcohol) adsorption on the surface of Mn₃O₄. Charge accumulation and charge depletion are represented by red and blue, respectively. Carbon: gray; Oxygen: red; Hydrogen: white; Manganese: violet. (c) Proposed radical generation pathway in Mn₃O₄(110)-ACAC. (d) The potential energy surface of Mn₃O₄(110)-ACAC.

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  Figure 3  Characterization results of free radicals. (a) DMPO spin-trapping ESR spectra of Mn₃O₄-ACAC self-initiated system. (b) DMPO spin-trapping ESR spectra of other self-initiated systems. (c) Digital photographs of free radical quenching experiments with exothermic monitoring results.

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  Figure 4  Analytical results of metal valence states in self-initiated systems. (a) The fitted Mn 2P XPS spectra of precipitates after 1 day of reaction in the self-initiating system. (b) Mn AOS of the samples determined using the LBB colorimetric method. (c) Proposed oxidation–reduction process of self-initiating system.

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  Figure 5  Swelling behaviors of the hydrogels. (a) Photographs of hydrogels continuously immersed in DI H₂O at 25 ℃ and 60 ℃ for 1.5 h exhibiting various swelling ratios. Swelling ratio of the hydrogels (b) in deionized water at different temperatures, (c) in solutions of different pH values, and (d) in different solvents.

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