Degradable hypercrosslinked porous polymer generated from aromatic polyketone

With the rapid development of industrialization and agriculture, environment pollution caused by toxic organic chemicals and heavy metal ions has attracted global attention^1,2. For the sustainable development of the environment for human, it is very urgent to develop new efficient methods to remove pollutants. Adsorption technique has been widely applied in the removal of pollutants due to its low cost, easy operation, and practical application on a large scale^3,4. Adsorbent is the core component of the adsorption technique, therefore numerous porous materials such as activated carbon^5,6, zeolites⁷, and inorganic adsorbents^8,9 have been developed as adsorbents for removal of pollutants^10,11.

As a new generation porous material, hypercrosslinked polymers (HCPs) have exhibited practical and potential applications in the field of the pollutant removal due to their unique advantages such as high specific surface area, low mass density, favorable pore structures, and high stability^{10,11,12,13,14,15,16}. Currently, the common HCPs are hypercrosslinked polystyrene-based polymers, which are prepared from polystyrene (PS) by Friedel-Crafts reaction^17,18,19. However, hypercrosslinked polystyrenes often exhibit relatively low adsorption performance toward pollutants due to the absence of polar functional groups. To improve the adsorption property toward removal of pollutants, functional groups such as amino, amide, hydroxyl, and carbonyl groups have been introduced on hypercrosslinked polystyrene (HCPS) by internal crosslinking^{20,21,22,23,24}. However, pre- and post-functionalization (Scheme 1A, B) are originated from a small proportion of ‒CH₂Cl groups, thereby usually leading to quite low installing amounts of functional groups. In addition, the “external crosslinking” is an alternative technique to synthesize functional HCPS (Scheme 1C) with a large number of functional groups^25,26,27,28. However, the “external crosslinking” technique has rarely been used in functional PS to prepare functional HCPS because functional groups often suppress the external crosslinking reaction. Direct synthesis of functional HCPS originated from functional PS by an “external crosslinking” technique remains highly challenging²⁹.

Functionalized HCPS synthesized by internal crosslinking followed by post-functionalization (A), pre-functionalization (B), and direct external crosslinking method (C).

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As a class of highly efficient adsorbents for the removal of pollutants, HCPs themselves are also a source of white plastic pollution. The waste HCPs are often non-degradable in the natural environment³⁰. More seriously, the waste HCPs are not even treated by decomposition combustion due to their hypercrosslinked structure, which is an efficient method for plastic resin. The development of degradable plastics is a promising solution to solve the global plastic pollution, but degradable HCPs have never been reported to the best of our knowledge³¹. Only one sample of 2-hydroxyterephthalic acid (2-HTA) modified hypercrosslinked PS (2-HTA-HCP) introduces degradation. A ~12.4 wt% of degradability is ascribed to the 2-HTA modified fraction, but the hypercrosslinked PS backbone remains³². Therefore, the development of degradable HCPs is highly desirable based on the full environmental protection concept.

The objective of this study is to prepare degradable HCPs originating from functional polymers by the external crosslinking method²⁹. Aromatic polyketones with the alternating structure were selected as pre-polymers because of their degradability caused by in-chain carbonyl groups³³. Our group previously reported the synthesis of atactic aromatic polyketone by palladium-catalyzed coordination polymerization of CO and vinyl arene^{34,35,36,37,38,39}. In this paper, functional aromatic polyketones were directly used to prepare hypercrosslinked polyketone (HCPK) by an “external crosslinking” method (Scheme 1C). The prepared HCPK with a high density of tert-butoxy functional group had mesopores structure and large surface area, thereby showing a good adsorption capacity for the removal of aniline. More importantly, the HCPK could be decomposed by UV light, which represented the first sample of photodegradable HCP.

Materials

Polyketones (PKs) were prepared by the coordination polymerization of CO and vinyl arene using a α-diimine palladium catalyst according to our previous work³⁷. Polyketone (PK-H) generated from CO and styrene has M_n = 47,200 with M_w/M_n = 1.10, while polyketone (PK-O^tBu) generated from CO and 4-tert-butoxystyrene has M_n = 39,100 with M_w/M_n = 1.25 (Supplementary Table S1). Formaldehyde dimethyl acetal (FDA), 1,2-dichloroethane (DCE), methanol, anhydrous ferric chloride, and aniline were obtained from Energy Chemical. Other commercially available reagents were purchased and used without purification.

Synthesis of hypercrosslinked polymers

Hypercrosslinked polyketones (HCPK-H and HCPK-O^tBu) were synthesized by an “external crosslinking” method²⁹. A typical synthesis is described as follows. 0.5 g of polyketone (PK-H or PK-O^tBu) was firstly dissolved in 400 mL of 1,2-dichloroethane (DCE), and then 0.60 g of anhydrous ferric chloride was added at 40 °C. When the reaction temperature was heated to 80 °C, FDA solutions (0.5 mL) in 50 mL of DCE were slowly added into the mixtures using a syringe. After 24 h, the reaction solution was concentrated to ~10 mL, and the polymer solid was isolated by precipitation into methanol and filtration to remove unreacted reactants. The obtained hypercrosslinked polyketone (HCPK) was further purified by extraction with methanol in a Soxhlet apparatus for 24 h, followed by drying in a vacuum oven at 50 °C to a constant weight.

Characterizations

Fourier transform infrared (FTIR) spectra were recorded using a Bruker TENSOR 27 FTIR spectrometer. Photoluminescence (PL) spectra were recorded using an Edinburgh Instruments FLS980 spectrofluorometer with a scanning speed of 240 nm/min. The UV-vis spectra of aniline aqueous solution were acquired using a Shimadzu UV-3600 spectrometer. The NMR spectra were recorded on a Bruker Advance III HD 400 MHz. Differential scanning calorimetry (DSC) analyses were carried out with a PerkinElmer DSC-4000 system. The DSC curves were recorded as second heating curves at a heating rate of 10 °C/min from 60 to 200 °C under nitrogen flow. Thermogravimetric analysis (TGA) was carried out with a PerkinElmer Pyris1 thermal gravimetric analyzer at a heating rate of 10 °C/min from 0 to 800 °C under nitrogen flow. GPC analyses of the molecular weights and molecular weight distributions of the polyketones and the degradation products were acquired using a Waters-1515 GPC chromatography equipped with a differential refractive-index (RI) detector. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL/min. Field-emission scanning electron microscopy (SEM) images were recorded using a FEI Sirion 200 field-emission scanning electron microscope operating at 10 kV. Transmission electron microscopy (TEM) was conducted using a FEI Tecnai G2 Spirit TEM operating at 120 kV. Brunauer–Emmett–Teller (BET) analysis was conducted using a Micromeritics ASAP-2460 multiport surface area and porosity system, while pore size distributions were calculated using NL-DFT methods. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer.

Adsorption experiments of aniline

Equilibrium adsorption

At room temperature (298 K), 10 mg of hypercrosslinked polyketone (HCPK-H or HCPK-O^tBu) was added into separate conical tubes containing 10 mL aniline aqueous solution with concentrations of 100, 200, 300, 400, 500, 1000 ppm. The slurry was stored in a shaker for 12 h at room temperature so that the adsorption reached the equilibrium. The HCPK was filtered out, and the filtrates were analyzed using UV-vis to determine the concentration of remaining aniline based on the standard curve of aniline aqueous solution (Supplementary Fig. S1).

The equilibrium capacity (Q_e) was calculated by Eq. (1)

Q_e is the equilibrium capacity (mg/g), C₀ and C_e are the concentrations of aniline in the aqueous solution before and after adsorption (mg/L), respectively. V is the solution volume (L), W is the mass of adsorbent (g).

Adsorption kinetics

At room temperature (298 K), 100 mg of hypercrosslinked polyketone (HCPK-H or HCPK-O^tBu) was added to conical tubes containing 100 mL of aniline aqueous solution with a concentration of 500 ppm. A sample was obtained at different times, and the concentration of the remaining aniline was determined using UV-vis spectroscopy based on the standard curve of the aniline aqueous solution.

Photodegradation of hypercrosslinked polyketones

20 mg of hypercrosslinked polyketone (HCPK-H or HCPK-O^tBu) was added to a 50 mL flask, followed by the addition of 20 mL of THF. The mixture was sonicated for 10 min to ensure uniform dispersion of the solid. The flask was then placed on a stirrer, with the temperature maintained at a constant 25 °C. The reaction system was irradiated with 312 nm UV light (150 W), and the irradiation time was recorded. After the predetermined reaction time, the solvent in the reaction mixture was removed by rotary evaporation, and the resultant product was isolated for analysis.

Synthesis and characterization of hypercrosslinked polyketones

Direct hypercrosslinking of polystyrene (PS) was pioneered by Davankov using the “internal crosslinking” method³⁹, but this approach is infeasible for polyketones. Herein, hypercrosslinked polyketone (HCPK) was prepared by an “external crosslinking” method using FDA as an external crosslinking agent (Scheme 2)^26,29. Although the electron-withdrawing carbonyl group was introduced on the main chain, the hypercrosslinking of polyketones still smoothly proceeded with a quantitative yield. To the best of our knowledge, this is one of the extremely rare reports on the direct synthesis of functional PS-based hypercrosslinked polymers by the “external crosslinking” technique²⁹. Herein, we did not optimize the reaction conditions to prepare hypercrosslinked polyketones with controlled porosities. We focused on the synthesis feasibility and the degradability of hypercrosslinked polyketones. Polyketones are white solids and easily dissolve in common organic solvents such as dichloromethane (DCM), chloroform, toluene, and tetrahydrofuran (THF), whereas hypercrosslinked polyketones (HCPKs) become dark brown and are insoluble in any solvents after the hypercrosslinking reaction (Supplementary Fig. S2). These huge differences in color and solubility suggest the occurrence of the crosslinking reaction.

Synthetic route of HCPK-H or HCPK-O^tBu.

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The occurrence of the crosslinking reaction was spectroscopically proven. As shown in Fig. 1A, Fourier transform infrared (FTIR) spectroscopy of pre-crosslinked PK-H shows the characteristic absorption band of the carbonyl group at 1707 cm⁻¹. However, this characteristic absorption band clearly weakens or disappears for hypercrosslinked polymers (HCPK-H and HCPK-O^tBu) because crosslinking restricts the vibration of the carbonyl group. Besides, two new absorption bands are present at 1608 and 1512 cm⁻¹ for hypercrosslinked polymers, which are a result of multisubstituted benzene rings³⁹. In addition, the water peak appears at 3400 cm⁻¹ for hypercrosslinked polymers but is not observed for the polyketone sample PK-H. This is mainly attributed to the strong adsorption of porous hypercrosslinked polymers toward water. Meanwhile, the photoluminescence (PL) spectra also support the occurrence of the crosslinking reaction. We previously reported that aromatic polyketone is a kind of simple and efficient non-conjugated luminescent macromolecule, which can emit intrinsic blue light (~440 nm) by the clusterization-triggered emission mechanism^35,37. As shown in Fig. 1B, hypercrosslinked polymers HCPK-H and HCPK-O^tBu do not exhibit fluorescence emissions because the hypercrosslinking structure restricts formation of aggregate.

FTIR spectra (A), PL spectra (B) (blue and green curves are overlapped), TGA curves (C) and DSC curves (D) of PK-H, HCPK-H, and HCPK-O^tBu.

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Hypercrosslinked polymers were further analyzed by thermal analysis. Thermal gravimetric analysis (TGA) (Fig. 1C) shows that pre-crosslinked PK-H sample has a decomposition temperature of 374 °C, and is fully decomposed at >450 °C. However, hypercrosslinked polymers show an increased decomposition temperature and higher residue weight (Fig. 1C) due to carbonization. At 800 °C, HCPK-H has higher relative residue weight (45.2 wt%) than HCPK-O^tBu (18.2 wt%), suggesting that HCPK-H has more abundant hypercrosslinking. Differential scanning calorimetry (DSC) analysis shows that polyketone PK-H has a glass transition temperature (T_g) of 110 °C while hypercrosslinked polymers do not have a T_g (Fig. 1D) because the hypercrosslinking restricts the movement of polymer chain segments^40,41.

Hypercrosslinked polymers often have porous structure, and their porosities were characterized by nitrogen adsorption measurements. As shown in Fig. 2, HCPK-O^tBu shows obvious adsorption isotherm whereas HCPK-H hardly exhibits adsorption isotherm. Totally, HCPK-O^tBu shows larger pore volume (V_total) and surface area (S_BET) than HCPK-H (Table 1). Clearly, this big difference should be the result of tert-butoxy groups²⁹. On the one hand, the strong electron-donating ability of tert-butoxy group facilitates Friedel-Crafts hypercrosslinked reaction. Consequently, the crosslinking density of the polymer increased, thereby facilitating the pore formation. On the other hand, bulky tert-butoxy group occupies the interchain spaces, thereby benefiting to the pore formation. According to the IUPAC classification, the adsorption isotherm of HCPK-O^tBu is similar to type IV, indicating abundant mesopores are dominant in the materials. This observation suggests that hypercrosslinking mainly takes place by interchain Friedel-Crafts reactions because the carbonyl suppresses the intrachain Friedel-Crafts reactions. The pore size distribution curves (Fig. 2B) clearly show that HCPK-O^tBu has a relatively broad pore size distribution and exhibits hierarchical porosity, including most mesopores (10-20 nm) and a small number of micropores (>50 nm) (Fig. 2B). The field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to directly observe the morphology and the pore size of two hypercrosslinked polymers. As shown SEM images in Fig. 3, two hypercrosslinked polymers are intrinsically amorphous in shape. HCPK-O^tBu displays abundant pores throughout the hypercrosslinked materials, while HCPK-H exhibits more compact morphology with almost no visible pores. TEM observations further reveal that the mesopore sizes are ~20 nm for HCPK-O^tBu, which is consistent with pore size distribution analysis.

N₂ adsorption isotherms (A) and pore size distributions (B) of HCPK-H and HCPK-O^tBu.

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SEM (up) and TEM (down) images of HCPK-H (left) and HCPK-O^tBu (right).

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Adsorption of aniline from aqueous solution

HCPK-O^tBu has mesopore structure, large surface area, and abundant tert-butoxy and carbonyl groups, which is expected to possess outstanding adsorption performance. Herein, aniline was chosen as an organic pollutant to study adsorption of HCPK-O^tBu adsorbent because it is highly toxic and widely existed in industrial wastewater.

Adsorption isotherms

The equilibrium adsorption of HCPK-O^tBu was evaluated at 298 K using aniline as an adsorbate. Figure 4 shows isothermal adsorption curve of aniline on HCPK-O^tBu. Langmuir and Freundlich models were separately applied to fit the isotherm data⁴². The Langmuir isotherm equation and the linearized form of the Freundlich isotherm equation are shown in the formula (2) and (3), respectively.

where Q_e is the equilibrium capacity (mg/g), and C_e is the concentration of aniline in the aqueous solution after adsorption (mg/L). Q_m and b are the Langmuir constants corresponding to maximum adsorption capacity and adsorption energy, respectively. K_F and 1/n are the Freundlich constants related to sorption capacity and sorption intensity, respectively.

Adsorption isotherms (left) and kinetics (right) of aniline on HCPK-O^tBu in aqueous solution.

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The isotherm parameters of these two adsorption models were calculated according to the two-parameter isotherms and summarized in Supplementary Table S2. According to the correlation coefficients R² (Supplementary Fig. S3 and Supplementary Table S2), the Langmuir model better fits the experimental data in the range of concentrations studied, indicating that the aniline adsorption obeys a monolayer model. According to the Langmuir model, the maximum adsorption capacity (Q_max) of HCPK-O^tBu is calculated to be 158.73 mg/g. This maximum adsorption capacity (Q_max) for aniline is superior to most of the reported values of inorganic adsorbents and is comparable to other HCP adsorbents (Supplementary Table S4).

Adsorption kinetics

The adsorption kinetics were further studied by measuring the amounts of aniline adsorbed at different times at 298 K and an aniline aqueous solution with concentration of 500 ppm. As shown in Fig. 4, the adsorption capacity of HCPK-O^tBu increases sharply at 100 min and then reaches equilibrium. Lagergren pseudo-first-order (formula (4)) and pseudo-second-order (formula (5)) kinetic models were used to fit the adsorption kinetics⁴³.

where Q_e and Q_t are the amount of aniline adsorbed per unit weight of HCPK-O^tBu (mg/g) at equilibrium and at time t, respectively, k₁ is the rate constant (min⁻¹) of pseudo-first-order adsorption and k₂ is the rate constant (g/min/mg) of pseudo-second-order adsorption.

The fitting parameters of the pseudo-first-order and pseudo-second-order models are shown in Supplementary Table S3 and Supplementary Fig. S4. According to the correlation coefficients (R²), the adsorption kinetic of aniline on the HCPK-O^tBu is more consistent with the pseudo-second-order model. This result indicates that the rate-determining step of aniline adsorption is kinetically controlled⁴⁴.

Mechanistic studies

In addition to simple physical adsorption, the interactions between functional carbonyl and tert-butoxy groups with aniline are also expected to exist as chemical adsorption. X-ray photoelectron spectroscopy (XPS) analysis of HCPK-O^tBu before and after adsorption was conducted to identify complex interactions.

As shown in Fig. 5, a new peak at 400.08 eV appears for HCPK-O^tBu after adsorption, which is safely assigned to the N element originating from the adsorbed aniline. The curve-fitted C 1s core-level spectrum of HCPK-O^tBu shows a shift before and after adsorption. The curve-fitted O 1s core-level spectrum of HCPK-O^tBu before adsorption clearly shows the carbonyl group at 532.59 eV and the tert-butoxy group at 533.08 eV⁴⁵. After adsorption, a new peak at 534.56 eV appears (Fig. 6), which is attributed to PhNH₂ ← O hydrogen-bonding interactions¹⁸. Therefore, it is concluded that the excellent adsorption capacity of HCPK-O^tBu toward aniline is a result of cooperative operation of abundant mesopore structures and large surface area of HCP as well as the interactions between aniline and functional carbonyl and tert-butoxy groups^29,31.

XPS spectra of HCPK-O^tBu before and after adsorptions of aniline.

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C1s (A) and O1s (B) core-level curve-fitted spectra of HCPK-O^tBu before and after adsorptions of aniline.

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Photodegradation of hypercrosslinked polyketones

Traditional hypercrosslinked polymers are often insoluble, non-meltable, and non-degradable. As a source of white plastic pollution, waste hypercrosslinked polymers will be more difficult to handle than pre-hypercrosslinked polymer resins. In our work, polyketones as pre-hypercrosslinked polymers are photodegradable due to photonic in-chain carbonyl groups⁴⁶. Therefore, the photodegradation of hypercrosslinked polyketones was also studied.

Photodegradation of the suspensions of HCPK-H and HCPK-O^tBu in THF was conducted under 312 nm UV light irradiation. After 20 min, the color of the suspension obviously changed from colorless to red-brown, indicating the occurrence of photodegradation. After 1 h, suspended solids nearly disappeared, and the suspension transformed into a homogeneous solution in THF (Fig. 7). The solutions were evaporated and the degradation products were isolated for analysis. As shown in Table 2, the degradation product of the HCPK-H has a molecular weight of 3.29 kg/mol, representing a ~14-fold decrease compared to the pre-crosslinked PK-H (M_n = 47.2 kg/mol). The degradation product of the HCPK-O^tBu possessing a more mesoporous structure has a molecular weight of 2.27 kg/mol, corresponding to a ~17-fold decrease compared to the pre-crosslinked PK-O^tBu (M_n = 39.1 kg/mol). This observation indicates that HCPK-O^tBu degrades more intensively under light irradiation. With prolonged irradiation time, the molecular weight of the degradation products continuously decreases. In addition, the same photodegradation experiment was also conducted after the equilibrium adsorption of aniline using HCPK-O^tBu. It is found that the adsorption of aniline had no influence on the photodegradation process of the hypercrosslinked polyketone.

Images of HCPK-X (X = H, O^tBu) in THF before and after photodegradation and photodegradation pathways.

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It is known that the photodegradation of polyketones primarily occurs through the Norrish reaction (I and II types)⁴⁷. Aromatic polyketones specifically undergo the Norrish type I reaction because of the absence of γ-H⁴⁸. Upon UV light irradiation, the homolytic cleavage of the in-chain carbonyl groups takes place to produce free radicals. These free radicals may subsequently undergo disproportionation to form various fragments. In principle, the photodegradation of hypercrosslinked polyketones also obeys the Norrish I mechanism. To confirm the photodegradation mechanism, the chain-end analysis of the degradation products was carried out by ¹H NMR technique. As shown in Supplementary Fig. S6, the ¹H NMR spectrum of the degradation products of HCPK-H is complex due to a complex mixture. Although the clear assignments of all resonances are very difficult, several characteristic peaks can be clearly assigned⁴⁹. The signal at 8.1 ppm is undoubtedly attributed to the aldehyde group (-CHO), and the peaks within the 4.0–6.0 ppm are assigned to the protons of double bonds (C=C). The signals at the 1.2–2.5 ppm are assigned to the terminal methylene (CH₂) and methine (CH) groups. Clearly, the presence of aldehyde, double bonds, and terminal methylene (CH₂) and methine (CH) groups proves the Norrish type I degradation pathways of hypercrosslinked polyketones (Fig. 7).

In summary, we have successfully synthesized hypercrosslinked porous polymers derived from aromatic polyketones with abundant functional groups using an external crosslinking technique. Introduction of the tert-butoxy group promotes the Friedel-Crafts alkylation-derived hypercrosslinked reaction, and the obtained HCPK-O^tBu is a typical mesoporous material (average pore size = 11.1 nm). HCPK-O^tBu possesses good adsorption capability for aniline removal from aqueous solution, and the adsorption equilibrium and kinetic process obey the Langmuir isotherm model and the pseudo-second-order kinetic model, respectively. Enhanced adsorption capability of HCPK-O^tBu is ascribed to interactions between aniline and carbonyl and tert-butoxy groups. More importantly, hypercrosslinked aromatic polyketones exhibit rapid and efficient photodegradation under UV light irradiation, which represents the first degradable hypercrosslinked polymers up to date. This study opens a door for developing environmentally friendly and degradable hypercrosslinked polymers.