Recyclable sulfone-containing polymers via ring-opening polymerization of macroheterocyclic siloxane monomers: synthesis, properties and recyclability

Mengdong Guo, Yue Huang, Jinfeng Cao, Guibao Sun, Xia Zhao, Jie Zhang* and Shengyu Feng*
Key Laboratory of Special Functional Aggregated Materials & Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P.R. China. E-mail: fsy@sdu.edu.cn

Received 10th September 2019 , Accepted 19th October 2019

First published on 21st October 2019


Environmental pollution and the dependence on finite resources are intensifying the search for new chemically recyclable polymers. Herein, we report the synthesis and properties of recyclable sulfone-containing polysiloxanes via the anionic ring-opening polymerization (ROP) of a new macroheterocyclic siloxane monomer (P1OX). The resulted polymer with a molecular weight up to 49 kg mol−1 shows good thermal stability and chemical recyclability. Compared to previous recyclable polymers, the recyclable polysiloxanes have simpler synthetic methods without expensive catalysts, milder conditions of depolymerization (using available and cheap KHSO4 at ambient temperature) and high selectivity of depolymerization (92%) back to P1OX. The sulfone-containing polysiloxanes were a brownish rigid plastic with a storage modulus E′ of 203 MPa at room temperature. From these recycled polymers, smooth polysiloxane nanofibers were prepared successfully via electrospinning. These results conclusively indicated that the new family of sulfone-containing polysiloxanes largely extended the scope of application of silicone and sustainable materials.


Introduction

As widely used polymers, silicone materials possess unusual and valuable properties, such as low glass-transition temperature (Tg), excellent electrical insulation properties, biocompatibility and thermal oxidation durability.1–3 They have attracted significant interest in the fields of construction and medicine, in the aviation industry, and so on.4–10 During the development process of polysiloxanes, traditional products such as silicone rubber, silicone resin, silicone oil and silane coupling agents are used.1 However, due to the lack of appropriate synthetic methods, much less attention has been paid to the new products of polysiloxanes, such as silicone plastics and fibers.

Modification of silicone polymers is an effective method to expand the scope of accessible polysiloxanes. The incorporation of sulfone into polysiloxanes can result in materials with additional unique properties, such as high gas permeability, high transmittance and variable electromechanical properties.11,12 Much progress has been made in the field of sulfone-containing polysiloxanes. Opris et al. fabricated polysiloxane elastomers with sulfone side groups, which exhibited high permittivity and low conductivity.13,14 Swager and his co-worker developed silicone elastomers prepared by “click” reaction between an alkyne functional poly(olefin sulfone) and an azide functional silicone.15 Besides, polysulfone – polysiloxane block copolymers were prepared by polycondensation reaction and these exhibited typical semiconducting characteristics.16 The above-mentioned sulfone-containing polysiloxanes have proved to be useful in actuators, capacitors, liquid or gas membrane separation. Of note, the current synthesis methods of sulfone-containing polymers are mainly condensation polymerization and graft reaction.11 Different from the above-mentioned reaction, ring-opening polymerization (ROP) provides precise control over molecular weight and low molecular weight distribution. However, little research has been done on ROP to obtain the main chain sulfone-containing polysiloxanes. Therefore, the ROP strategy needs to be explored to synthesize novel sulfone-containing silicone materials, and it can be used to explore the potential products of silicon-based polymers.

Many opportunities exist for the design of recyclable polymeric materials to reduce environmental pollution and the dependence of industry on the finite supplies of raw feedstock.17,18 Chemical recycling offers the potential to be an ideal method for depolymerizing macromolecular waste back to monomers or transforming it to new value-added polymers.19–21 Recently, a breakthrough in recyclable polymers, such as polyesters,22,23 polycarbonates24 and polyurethanes,25 has been made by utilizing advanced catalysis or dynamic covalent chemistry.19 However, current studies are mainly focusing on carbonyl-derivative polymers. Obtaining chemically recyclable polymers with a wide variety of structures and properties remains a great challenge in the development of sustainable chemistry.

Herein, chemical recycling and ROP were combined to produce a new family of sulfone-containing polysiloxanes (RSCP). The novel polymer is a product of anionic ring-opening polymerization (ROP) of an emerging macroheterocyclic aromatic siloxane (P1OX). The ROP strategy had high conversion (up to 98%) and produced sulfone-containing polysiloxanes with the number average molecular weight (Mn = 2.4–49.8 kg mol−1) and narrow polydispersity (Đ = 1.28–1.53). The resulted polymer was a brownish rigid plastic with a storage modulus E′ of 203 MPa and had good thermal stability. Besides, smooth and continuous polysiloxane nanofibers were obtained successfully via electrospinning. Notably, RSCP can be selectively recycled back into the original macroheterocyclic monomer with high selectivity (92%) and isolated yield (87%) in the presence of KHSO4 at room temperature. In this paper we developed new recyclable polymers and expanded on the potential application of silicone materials. Given the ever-increasing attention on recyclable polymers, the materials will drastically impact the fields of polymer and sustainable chemistry in the future.

Experimental

Materials

All reagents were purchased as analytical-grade products from Aladdin Co. (China) and Energy chemical. 1,3-Diethenyl-1,1,3,3-tetramethyl-disiloxane (MMvi) was available as a commercial product.

Synthesis of P1

A reaction mixture of MMvi (1.86 g, 10 mmol), 1,4-phenylenedimethanethiol (R1) (1.70 g, 10 mmol) and DMPA (1 wt%, 0.04 g) was irradiated with slow stirring for 15 min under a 100 W UV light (λ = 365 nm) to initiate thiol–ene step-growth polymerization. Then, the mixture was purified by precipitation in methanol to obtain P1 as a viscous yellow liquid with a high yield of 93%. 1H NMR (400 MHz, Acetone-d6, δ, ppm): 7.28 (s, 4H, phenyl ring); 3.72 (d, 4H, Ph-CH2-); 2.46 (m, J = 8.6 Hz, 4H, S-CH2-); 0.87 (t, J = 8.8 Hz, 4H, -CH2-CH2-Si); 0.05 (s, 12H, Si-CH3). 13C NMR (100 MHz, Acetone-d6, δ, ppm): 138.64 (s, 2C, phenyl ring); 130.05 (s, 4C, phenyl ring); 36.51 (s, 2C, Ph-CH2-S); 27.06 (s, 2C, -S-CH2-CH2); 19.61 (s, 2C, CH2-CH2-Si); 1.08 (s, 4C, Si-CH3). 29Si NMR (400 MHz, Acetone-d6, δ, ppm): 6.61 (s).

Synthesis of P1OX

A solution of P1 (1.78 g, 5 mmol) in 20 ml of THF and 7.68 g (25 mmol) of oxone in 20 ml H2O were added in a 3-neck flask with a reflux cooler. The reaction was stirred for 12 h at room temperature under an argon atmosphere. Another portion of oxone (5.0 mmol) in 20 ml of H2O and 20 ml of THF were added in this flask. The reaction mixture was stirred for a further 12 hours. Then, the mixture was filtered and washed with dichloromethane (100 ml) and saturated NaCl solution (2 × 40 ml). The aqueous phase was extracted with CH2Cl2. The organic phases were combined and dried over moderate anhydrous Na2SO4. Afterwards, the solution was concentrated and purified by column chromatography as a white bulk crystal with a yield of 65%. 1H NMR (400 MHz, Acetone-d6, δ, ppm): 7.66 (s, 4H, phenyl ring); 4.44 (s, 4H, Ph-CH2-SO2); 2.65 (m, 4H, -SO2-CH2-CH2); 0.57 (m, 4H, CH2-CH2-Si); 0.08 (s, 12H, Si-CH3). 13C NMR (100 MHz, Acetone-d6, δ, ppm): 132.18 (s, 2C, phenyl ring); 131.94 (s, 4C, phenyl ring); 60.55 (s, 2C, Ph-CH2-SO2); 47.32 (s, 2C, -SO2-CH2-CH2); 10.57 (s, 2C, CH2-CH2-Si); 0.18 (s, 4C, Si-CH3). 29Si NMR (400 MHz, CDCl3, δ, ppm): 8.28 (s). FT-IR (v, cm−1): 1315 (-SO2-); 1173 (-SO2); 844.5 (-Ph-). HR-MS (FAB, m/z): Calculated for C16H28O5S2Si2 (M + Na+): 443.0814; Found, 443.0850. (M + H+): 421.0917; Found: 421.1032. Tm = 267 °C (DSC).

Synthesis of RSCP (the typical ROP of P1OX)

Typically, a solution of P1OX (0.29 g, 0.7 mmol) in anhydrous DMSO (1 ml) was added in a 3-neck flask. Afterwards, KOH (3.9 mg, 0.07 mmol) was added in this flask. The mixture was stirred for a given amount of time at 145 °C under an argon atmosphere to obtain RSCP. Then, benzoic acid (8.5 mg, 0.07 mmol) was added to quench the polymerization reaction. The mixture was heated to 160 °C in a vacuum to remove the solvent. The product was a yellow solid with a yield of 90%. 1H NMR (400 MHz, Acetone-d6, δ, ppm): 7.50 (s, 4H, phenyl ring); 4.42 (s, 4H, Ph-CH2-SO2); 2.96 (m, 4H, -SO2-CH2-CH2); 1.05 (m, 4H, CH2-CH2-Si); 0.14 (s, 12H, Si-CH3). 13C NMR (100 MHz, Acetone-d6, δ, ppm): 132.24 (s, 6C, phenyl ring); 58.08 (s, 2C, Ph-CH2-SO2); 47.98 (s, 2C, -SO2-CH2-CH2); 10.30 (s, 2C, CH2-CH2-Si); 0.43 (s, 4C, Si-CH3). 29Si NMR (400 MHz, CDCl3, δ, ppm): 8.26 (s).

Depolymerization of RSCP (synthesis of R1OX)

Typically, a solution of RSCP (Mn = 5.8 kg mol−1, 52.5 mg) in 15 ml of THF was added in a capped glass flask. Then, the aqueous solution of KHSO4 (160 mM, 15 ml) was added to the solution of RSCP. After 24 h stirring under room temperature, 50 ml of CH2Cl2 was added in the flask. The product was washed with water and saturated NaCl solution. The aqueous phase was extracted with CH2Cl2. The organic layer was dried over anhydrous Na2SO4 and evaporated. A white crystal was obtained in a yield of 87%. 1H NMR (400 MHz, Acetone-d6, δ, ppm): 7.66 (s, 4H, phenyl ring); 4.45 (s, 4H, Ph-CH2-SO2); 2.67 (m, 4H, -SO2-CH2-CH2); 0.57 (m, 4H, CH2-CH2-Si); 0.08 (s, 12H, Si-CH3). 13C NMR (100 MHz, Acetone-d6, δ, ppm): 132.90 (s, 2C, phenyl ring); 132.66 (s, 4C, phenyl ring); 61.19 (s, 2C, Ph-CH2-SO2); 47.96 (s, 2C, -SO2-CH2-CH2); 11.24 (s, 2C, CH2-CH2-Si); 0.90 (s, 4C, Si-CH3). 29Si NMR (400 MHz, CDCl3, δ, ppm): 8.30 (s). HR-MS (FAB, m/z): Calculated for C16H28O5S2Si2 (M + H+): 421.0979; Found: 421.0927.

Characterization

Nuclear magnetic resonance (1H NMR, 13C NMR, and 29Si NMR) spectra were recorded on a Bruker Advance 400 spectrometer at room temperature. The NMR data were analyzed using MestReNova software. Acetone-d6 ((CD3)2CO) or Deuterated chloroform (CDCl3) was used as a solvent. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker Tensor 27 infrared spectrophotometer by using the KBr pellet technique within the 4000 cm−1 to 400 cm−1 region. High-resolution mass spectra were obtained using positive mode on Agilent Technologies 6510 Q-TOF LC-MS. Gel permeation chromatograph (GPC) measurements were performed using THF as the eluent at a flow rate of 1.0 mL min−1 and Polystyrene (PS) as the standard at 313 K with a Waters 515 liquid chromatograph (Milford, MA) equipped with a refractive-index detector 2414 to obtain the molecular weight and molecular-weight distribution. Thermo-gravimetric analysis (TGA) was performed under a N2 atmosphere on TA ADTQ600 apparatus at a temperature range of room temperature up to 800 °C with a heating rate of 10 °C min−1. DSC measurements were determined with a SDTQ 600 of TA Instruments. The samples were loaded in aluminum pans and heated from −50 to 150 °C. The heating and cooling temperature ramping rates were 10 °C min−1. A Mettler Toledo DMA/SDTA861e instrument was used to study the dynamic mechanical properties of the polysiloxane samples. The samples with dimensions of approximately 5.0 × 5.0 × 1.0 mm3 were analyzed from −50 to 100 °C at a heating rate of 3 °C min−1. The measurements were performed in shear mode at a frequency of 1 Hz. The thiol–ene reaction was performed by irradiation using UV light from a Spectroline Model SB-100P/FA lamp (365 nm, 100 W). The UV intensity is 4500 mW cm−2. Scanning electron microscopy (SEM) images were obtained by using a Hitachi S-4800 instrument (7 kv). Samples were cut and coated with a thin layer of gold before investigation. Powder X-ray diffraction (PXRD) was carried out on a Riguku D/MAX 2550 diffractometer with Cu-Kα radiation, 40 kV, 20 mA with the 2θ in the range of 10°–80° (scanning rate of 10° min−1) at room temperature.

Results and discussion

Synthesis of macroheterocyclic siloxane monomers

A macroheterocyclic sulfone-containing siloxane monomer (P1OX) was obtained from cheap, commercially available reagents in two steps.26–28 First, thiol–ene step-growth photopolymerization between equimolar 1,4-phenylenedimethanethiol (R1) and 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (MMvi) was implemented under ultraviolet irradiation (365 nm, 100 W). The photoinitiator was 2,2-dimethoxy-2-phenylacetophenone (DMPA). The reaction was carried out for 15 min to obtain linear aromatic sulfur-bridged siloxane copolymers (P1). Next, the selective oxidation and rearrangement of P1 with oxone, a commercially available salt from Caro's acid (H2SO5),29,30 were carried out in a 50% aqueous THF solution to obtain P1OX in 65% overall yield. The composition of P1OX was verified by NMR (1H, 13C, 29Si), FT-IR and high-resolution mass spectrometry (HR-MS) (Fig. S1–4). Crystallographic analysis revealed that P1OX belonged to a triclinic crystal system and this 13-membered ring was constructed without severe internal strain as shown by the Si–O–Si bond angle of 152.5° (Scheme 1 and Tables S1–2).
image file: c9py01363f-s1.tif
Scheme 1 Synthesis and recycling of RSCP.

Homopolymerization of P1OX

The ROP of P1OX under different polymerization conditions is investigated in Table 1. To reduce the energy input in the polymerization, the ROP of P1OX was performed by solution polymerization due to the high melting point of P1OX (Tm = 267 °C, Fig. S5). At the same time, we hypothesized that anionic ROP can be applied to P1OX due to its similarity with the commercial cyclosiloxane.31,32 Low activity at 110 °C was observed in the system with tetramethylammonium hydroxide ((CH3)4NOH). Deactivation of the catalysts could occur once the temperature exceeded 130 °C.33 Interestingly, ROP of the monomers using commercial lithium hydroxide (LiOH) as the catalyst in dimethylsulfoxide (DMSO) at 145 °C successfully yielded the sulfone-containing polysiloxanes (RSCP) with a high conversion (90%), a number-average molecular weight (Mn) of 2.4 kg mol−1 and a low dispersity (Đ, defined as the ratio of weight-average weight to Mn) of 1.34 (Table 1, entry 2). Based on these promising results, we selected different alkali hydroxides as catalysts to conduct the ROP of P1OX (entries 2–4). Among the screened alkali hydroxides, potassium hydroxides (KOH) achieved the best control over the ROP of P1OX, as demonstrated by the high conversion (up to 98%) and Mn (5.8 kg mol−1) and the low Đ (1.35) of RSCP. Different solvents were used to conduct the ROP (entries 4–6). During polymerization for 8 h, the fine conversion of ROP in DMSO (98%) was higher than the conversion in DMF (68%) and DMAc (63%), which conformed to DMSO as a common accelerator of polymerization in organosiloxane chemistry.34 The Mn values (Table 1, entries 4 and 7–10) up to 49.8 kg mol−1 were close to the expected Mn determined by the feed M/C ratios, which indicated the RSCP with predictable molecular weight. On the basis of the above-mentioned results, ROP is an effective strategy to obtain the main-chain sulfone-containing polysiloxanes with high molecular weight and narrow molecular weight distribution.
Table 1 Results of the ROP of P1OX
Entry Catalyst M/Ca Solvent Time (h) Temp. (°C) Conv.b (%) Mtheoryn[thin space (1/6-em)]c (kDa) Mn[thin space (1/6-em)]d (kDa) Đd
a The catalyst to monomer molar ratio.b As calculated by 1H NMR spectra.c Calculated by (M/C) × Conv. % × (molecular weight of P1OX).d Mn and Đ were determined by GPC at 40 °C in THF relative to polystyrene (PS) standards. (DMSO = dimethylsulfoxide; DMF = dimethylformamide; DMAc = dimethylacetamide).
1 (CH3)4NOH 10/1 DMSO 8 110 30 1.3
2 LiOH 10/1 DMSO 8 145 90 3.8 2.4 1.34
3 NaOH 10/1 DMSO 8 145 95 4.0 4.2 1.32
4 KOH 10/1 DMSO 8 145 98 4.1 5.8 1.35
5 KOH 10/1 DMF 8 145 68 2.8 3.0 1.28
6 KOH 10/1 DMAc 8 145 63 2.6 2.7 1.31
7 KOH 20/1 DMSO 8 145 94 7.9 8.6 1.37
8 KOH 40/1 DMSO 16 145 93 15.6 15.1 1.46
9 KOH 80/1 DMSO 16 145 94 31.6 34.7 1.53
10 KOH 120/1 DMSO 16 145 92 46.4 49.8 1.48


The RSCP structure with the desired main chain was further confirmed by NMR (Fig. 1 and S6–7) and FT-IR (Fig. S4). Two proton peaks at 2.65 ppm and 0.57 ppm corresponding to -SO2-CH2-CH2- and -CH2-CH2-Si- respectively shifted toward the lower field after ROP in 1H NMR spectra, further confirming the successful formation of RSCP. As shown in Fig. S4, the FTIR spectra of RSCP indicated the presence of the characteristic absorption bands of the sulfone group (1311 cm−1). After the ROP, the ν(Si–O) vibration peak at 1050 cm−1 became broader than the peak of the monomer P1OX.35 According to the classic anionic ROP of cyclosiloxanes, the polymerization mechanism of P1OX is proposed in Scheme 2.32 The ring-opening reaction of P1OX occurred through the nucleophilic attack of OH at a silicon atom of P1OX. The ring of P1OX was opened to obtain the linear molecule with a one-terminal nucleophilic reagent of Si–O. Then, the nucleophiles tended to attack the other monomer at a silicon atom, resulting in the occurrence of the ring-opening reaction. This step was a propagating reaction to achieve polymerization. Lastly, in order to quench the polymerization, acid was added to integrate with the nucleophilic reagent of Si–O to form Si-OH. Different from the classic anionic ROP of cyclosiloxanes, the ROP required high reaction temperature due to the low ring tension of P1OX.


image file: c9py01363f-f1.tif
Fig. 1 Chemical recyclability of P1OX. 1H NMR of (a) R1OX obtained by the depolymerization of RSCP using KHSO4; (b) RSCP obtained by the ROP of P1OX; (c) P1OX.

image file: c9py01363f-s2.tif
Scheme 2 Proposed reaction mechanism of the closed-loop recycling of RSCP.

Thermal properties of recyclable polysiloxanes

We then examined the thermal properties of RSCP by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA measurements provided in Fig. 2a and S8 showed good thermal stability of all siloxane compounds up to about 300 °C. The degradation temperatures Td (defined by the temperature at 5% weight loss) and maximum decomposition temperatures Tmax (defined by the peak value in the relative derivative thermogravimetry (DTG)) of RSCP were 328 °C and 382 °C, respectively. The TGA data showed that sulfone-containing siloxanes (P1OX and RSCP) exhibited higher Td and Tmax than the sulfur-bridged polysiloxane P1 with a Td of 303 °C and a Tmax of 327 °C. This phenomenon may be due to the fact that sulfone moiety, the highest oxidation state of sulfur, has better heat resistance than sulfur ether moiety.36 Additionally, a slight enhancement of Tmax after ROP from 374 °C (P1OX) to 382 °C (RSCP) was observed, as shown in Fig. S8.
image file: c9py01363f-f2.tif
Fig. 2 Thermal properties of P1 and P1OX and RSCP (Mn = 49.8 kg mol−1) (a) TGA curves of P1 and P1OX and RSCP; (b) DSC curves of P1 and RSCP; (c) Overlay of storage modulus E′ and loss modulus E′′ for RSCP measured by DMA; (d) PXRD diffraction patterns of P1OX, RSCP.

As seen from the DSC curves in Fig. 2b, the great improvement in the glass transition temperature (Tg) from −48 °C (P1) to 50 °C (RSCP) was a result of thioether oxidation reaction, which showed that the sulfone-containing polymer had a higher Tg with a strong rigidity.11 Besides, as shown in Fig. 2c, polymers only had one Tg and no crystalline peak in the temperature range of −50 °C to 100 °C. The macrocyclic monomer P1OX and the homopolymer RSCP were also characterized by powder X-ray diffraction (PXRD), as shown in Fig. 2d. As expected, P1OX exhibited many sharp diffraction peaks, because P1OX was easily crystallized as white and transparent crystals with the volatilization of the solvent. Besides, RSCP was amorphous and had no obvious crystalline diffraction peaks, which was identical to the result of no crystallization peak in their DSC analysis.

The obtained sulfone-containing polysiloxanes are brownish rigid solids (Fig. 1b). In order to quantify their thermomechanical properties, RSCP was melt processed for the preparation of specimens for dynamic mechanical analyses (DMA). The melting process was carried out in a mold by heating for 30 min at 120 °C and subsequent cooling to room temperature. Fig. 2c shows the DMA thermogram of RSCP. The resulted polymer was highly rigid with a storage modulus E′ of 203 MPa at room temperature (the glassy state). The Tg of RSCP determined from the loss modulus E′′ and tan[thin space (1/6-em)]δ curves (Fig. S9) were 58 °C and 61 °C, respectively. However, the E′ of RSCP decreased by more than three orders of magnitude after the glass transition region from 55 to 61 °C. Then, the state of materials quickly shifted from the glassy state to the viscous flow state. This phenomenon was also in good agreement with the tan[thin space (1/6-em)]δ curves, which sharply increased after the first peak near 60 °C.

Sulfone-containing polysiloxane fibers

Developing polysiloxane fibers without cross-linking remains difficult, due to the low glass transition temperature (Tg) and flexible main-chain structure.37 The ROP of P1OX resulted in the successful incorporation of sulfone and aromatic rings into polysiloxanes. Based on the above-mentioned DSC analysis, the ROP strategy led to a great improvement in Tg, compared with a value of −123 °C for polydimethylsiloxane.3 Besides, the main-chain of RSCP became more rigid due to the presence of sulfone and aromatic rings. As will be described below, the resulted polymer materials showed outstanding spinnability. A transparent and continuous fiber (Fig. 3a) was easily drawn using a pin from viscous polymer liquids at 120 °C. A single fiber was even tied and it bore a slight uniaxial stress (Fig. 3b). Due to the fluctuant rate and intensity of manual drawing, the RSCP fiber was partly rough and flat-like with a regular diameter of 100 μm (Fig. 3c), observed by scanning electron microscopy (SEM). Furthermore, we chose electrospinning to obtain micro/nano fibers. When a solution of RSCP in chloroform at a high concentration (25 wt%) was used for electrospinning, smooth rod-like fibers with a diameter ranging from 1.8 to 3.5 μm were obtained (Fig. 3d). Besides, droplets instead of fibers were obtained (Fig. 3e) by using a solution of RSCP in DMF/CHCl3 (1/10) with a low viscosity (0.053 Pa s). At a solution of RSCP in DMF/CHCl3 (1/10) with a high viscosity (0.114 Pa s), the SEM images (Fig. 3f–h) represented the formation of rod-like nano-fibers. Fig. 3f is an overview of polymer nanofibers. Fig. 3g is of higher magnification and shows the nanofibers with a diameter of about 200 nm. Furthermore, Fig. 3d shows that the fibers are smooth. These results demonstrated the importance of the solvent and viscosity since they affected the binding of the polymer chain by the different rate of evaporation.38 The above-mentioned spinnability of RSCP makes it a potential candidate for application in the textile industry, which promotes the development of novel silicone materials.
image file: c9py01363f-f3.tif
Fig. 3 Images of RSCP fibers. (a) Photograph of fibers drawn using a pin; (b) photograph of a single fiber (the blue circle indicates the knot of the fiber); (c) SEM images of the fibers; (d) SEM images of the electrospun micro-fibers; (e) SEM images of the electrospun droplets; (f–h) SEM images of the electrospun nano-fibers.

Depolymerization of RSCP

Based on the controlled equilibration and rearrangements of siloxanes, we explored the depolymerization of RSCP. The depolymerization process of RSCP was investigated by NMR spectroscopy from the distinct differences observed in the chemical shifts between the P1OX monomer and RSCP. The optimization of the depolymerization conditions of RSCP was performed with KHSO4 in an aqueous THF solution for 24 h without an additional energy input in Fig. 4a. As observed in the optimization experiments, the conversion of depolymerization increased from 13% to 92% (calculated by 1H NMR) with the increase in the concentration of KHSO4 from 40 mM to 160 mM. As shown in Fig. 4a, the depolymerization process was conducted with a high isolated yield (up to 87%) and a high conversion (up to 92%). After some simple work-up procedures, the recycled P1OX (R1OX) was obtained as a white crystal, as shown in Fig. 1a. Analysis by GPC, as shown in Fig. S11, revealed the accomplishment of depolymerisation of RSCP, supported by the disappearance of the polymer peak at a retention time of 25.9 min and the emergence of the monomer peak at a retention time of 30.8 min. Furthermore, Fig. 4b shows that the closed-loop recycling was performed twice. The conversion of ROP from the original P1OX was about 98%, and the conversion of ROP from the recycled P1OX from the first and second recycling was 94% and 95%, respectively. The Mn of RSCP (5.8 kg mol−1) from the original P1OX was close to the Mn of RSCP from the 1st and 2nd recycling (4.4 kg mol−1 and 5.2 kg mol−1, respectively) This result indicated a good retention of closed-loop recycling. These degradation results highlighted the successful design of recyclable polymers based on the controlled equilibration siloxanes via a closed, circular and milder method.
image file: c9py01363f-f4.tif
Fig. 4 (a) The conversion and yield of depolymerization in different concentrations of KHSO4; (b) the conversion of ROP and the Mn of RSCP from the original P1OX, 1st recycle P1OX, 2nd recycle P1OX. The conversion was calculated by 1H NMR spectra. Mn was determined by GPC.

The mechanism of depolymerization to obtain macrocyclic monomers can be interpreted as the cleavage and rearrangement of siloxanes derived from the acidic conditions. Next, a multistep mechanism of the depolymerization of RSCP is proposed in Scheme 2. Under the strong acidic conditions provided by KHSO4, H+ tended to combine with the O atom, resulting in a weakening of the Si–O–Si bond. Next, a complete cleavage of the Si–O–Si bonds and the subsequent formation of the Si-OH and Si-SO4 groups were caused by the attack of the environmental SO42− toward the Si atom. The two-terminal condensation reaction between Si-OH and Si-SO4 groups was carried out by the elimination of HSO4 to obtain the thermodynamically stable cyclic monomer. The driving force for the depolymerization depended on the release of the rigidity in RSCP.28 At the same time, poor depolymerization conversion of RSCP with K2SO4 (5%) or HCl (22%) was detected as comparative studies in Fig. S12, which further confirmed the importance of H+ and SO42− in the mechanism of depolymerization.

In order to better understand the cyclization reaction in the depolymerization process of RSCP, a computational study was carried out to obtain the free energy by density functional theory (DFT). First, the structures of P1OX and uncyclization P1OX (UCP1) were optimized (Fig. S13). Due to the existence of the intermolecular hydrogen bond, the entire molecule was curved and the Si-OH and Si-SO4 groups in the two-terminal molecules of UCP1 were much closer. The results were beneficial for the occurrence of the two-terminal condensation reaction, and UCP1 tended to cyclization. Detailed DFT calculation parameters are given in the ESI (Tables S3 and 4). Next, the elimination of HSO4 to yield P1OX had a relative Gibbs free energy of −12.0 kcal mol−1, which meant that the formation of P1OX was a spontaneous process and therefore beneficial in thermodynamics. The calculation results were in keeping with the fact that the depolymerization of RSCP was performed under room temperature without extra energy importation.

Conclusions

In conclusion, we have reported the first recyclable sulfone-containing polysiloxanes derived from an anionic ring-opening polymerization of P1OX. The polymers possess good thermal stability and recyclability for depolymerizing at room temperature in the presence of KHSO4 into the starting siloxane monomer in high selectivity and yield. Smooth and continuous polysiloxane nanofibers were obtained successfully via electrospinning. Future work will focus on whether the reported monomer P1OX has the potential of copolymerization with different cyclosiloxane monomers, which will largely expand the scope and range of polysiloxanes. Besides, the siloxane segments are inserted into the main chains of the unsustainable macromolecules for the creation of new functional and recyclable materials to further reduce solid polymer waste. Therefore, this methodology of the controlled equilibration and rearrangements of siloxanes opens the field of recyclable chemistry in a brand new class of polymer and advances the development of novel polysiloxane materials in academic research and practical applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21774070) and Shandong province major scientific and technological innovation projects (2017CXGC1112).

Notes and references

  1. M. P. Wolf, G. B. Salieb-Beugelaar and P. Hunziker, Prog. Polym. Sci., 2018, 83, 97–134 CrossRef CAS.
  2. E. Yilgör and I. Yilgör, Prog. Polym. Sci., 2014, 39, 1165–1195 CrossRef.
  3. E. Pouget, J. Tonnar, P. Lucas, P. Lacroix-Desmazes, F. O. Ganachaud and B. Boutevin, Chem. Rev., 2009, 110, 1233–1277 CrossRef PubMed.
  4. Y. Spiesschaert, M. Guerre, L. Imbernon, J. M. Winne and F. Du Prez, Polymer, 2019, 172, 239–246 CrossRef CAS.
  5. O. van den Berg, L.-T. T. Nguyen, R. F. A. Teixeira, F. Goethals, C. Özdilek, S. Berghmans and F. E. Du Prez, Macromolecules, 2014, 47, 1292–1300 CrossRef CAS.
  6. Y. Zuo, J. Cao and S. Feng, Adv. Funct. Mater., 2015, 25, 2754–2762 CrossRef CAS.
  7. J. Cao, D. Wang, P. An, J. Zhang and S. Feng, J. Mater. Chem. A, 2018, 6, 18025–18030 RSC.
  8. J. Croissant, D. Salles, M. Maynadier, O. Mongin, V. Hugues, M. Blanchard-Desce, X. Cattoën, M. Wong Chi Man, A. Gallud, M. Garcia, M. Gary-Bobo, L. Raehm and J.-O. Durand, Chem. Mater., 2014, 26, 7214–7220 CrossRef CAS.
  9. J. Graffion, X. Cattoën, M. Wong Chi Man, V. R. Fernandes, P. S. André, R. A. S. Ferreira and L. D. Carlos, Chem. Mater., 2011, 23, 4773–4782 CrossRef CAS.
  10. X. Zhu, C. Melian, Q. Dou, K. Peter, D. E. Demco, M. Möller, D. V. Anokhin, J.-M. Le Meins and D. A. Ivanov, Macromolecules, 2010, 43, 6067–6074 CrossRef CAS.
  11. C. Dizman, M. A. Tasdelen and Y. Yagci, Polym. Int., 2013, 62, 991–1007 CAS.
  12. A. K. Verma, Prog. Polym. Sci., 1986, 12, 219–228 CrossRef CAS.
  13. D. M. Opris, Adv. Mater., 2018, 30, 1703678 CrossRef PubMed.
  14. S. J. Dünki, E. Cuervo-Reyes and D. M. Opris, Polym. Chem., 2017, 8, 715–724 RSC.
  15. J. M. Lobez and T. M. Swager, Macromolecules, 2010, 43, 10422–10426 CrossRef CAS.
  16. G. Rusu, A. Airinei, V. Hamciuc, G. Rusu, P. Râmbu, M. Diciu, P. Garoi and M. Rusu, J. Macromol. Sci., Part B: Phys., 2009, 48, 238–253 CrossRef CAS.
  17. X.-B. Lu, Y. Liu and H. Zhou, Chem. – Eur. J., 2018, 24, 11255–11266 CrossRef CAS PubMed.
  18. X. Zhang, M. Fevre, G. O. Jones and R. M. Waymouth, Chem. Rev., 2018, 118, 839–885 CrossRef CAS PubMed.
  19. P. R. Christensen, A. M. Scheuermann, K. E. Loeffler and B. A. Helms, Nat. Chem., 2019, 11, 442–448 CrossRef CAS PubMed.
  20. A. Rahimi and J. M. García, Nat. Rev. Chem., 2017, 1, 0046 CrossRef.
  21. J.-B. Zhu, E. M. Watson, J. Tang and E. Y.-X. Chen, Science, 2018, 360, 398–403 CrossRef CAS PubMed.
  22. J.-B. Zhu and E. Y.-X. Chen, Angew. Chem., Int. Ed., 2018, 57, 12558–12562 CrossRef CAS PubMed.
  23. M. Hong and E. Y. Chen, Nat. Chem., 2016, 8, 42–49 CrossRef CAS PubMed.
  24. Y. Liu, H. Zhou, J.-Z. Guo, W.-M. Ren and X.-B. Lu, Angew. Chem., Int. Ed., 2017, 56, 4862–4866 CrossRef CAS PubMed.
  25. Y. Yanagishita, M. Kato, K. Toshima and S. Matsumura, ChemSusChem, 2008, 1, 133–142 CrossRef CAS PubMed.
  26. F. Goethals, D. Frank and F. Du Prez, Prog. Polym. Sci., 2017, 64, 76–113 CrossRef CAS.
  27. J. M. Sarapas and G. N. Tew, Angew. Chem., Int. Ed., 2016, 55, 15860–15863 CrossRef CAS PubMed.
  28. Y. Zuo, Z. Gou, J. Cao, Z. Yang, H. Lu and S. Feng, Chem. – Eur. J., 2015, 21, 10972–10977 CrossRef CAS PubMed.
  29. B. Yu, A.-H. Liu, L.-N. He, B. Li, Z.-F. Diao and Y.-N. Li, Green Chem., 2012, 14, 957 RSC.
  30. Z. Gou, Y. Zuo, J. Qi, Z. Li and S. Feng, Polym. Chem., 2016, 7, 5496–5500 RSC.
  31. W. Grubb and R. C. Osthoff, J. Am. Chem. Soc., 1955, 77, 1405–1411 CrossRef CAS.
  32. S. W. Kantor, W. T. Grubb and R. C. Osthoff, J. Am. Chem. Soc., 1954, 76, 5190–5197 CrossRef CAS.
  33. P. Zheng and T. J. McCarthy, J. Am. Chem. Soc., 2012, 134, 2024–2027 CrossRef CAS PubMed.
  34. S. Li and S. Feng, RSC Adv., 2016, 6, 99414–99421 RSC.
  35. B. Aydogan, D. Sureka, B. Kiskan and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 5156–5162 CrossRef CAS.
  36. O. van den Berg, T. Dispinar, B. Hommez and F. E. Du Prez, Eur. Polym. J., 2013, 49, 804–812 CrossRef CAS.
  37. M. Ma, R. M. Hill, J. L. Lowery, S. V. Fridrikh and G. C. Rutledge, Langmuir, 2005, 21, 5549–5554 CrossRef CAS PubMed.
  38. A. Celebioglu and T. Uyar, Chem. Commun., 2010, 46, 6903–6905 RSC.

Footnote

Electronic supplementary information (ESI) available. CCDC 1888686. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9py01363f

This journal is © The Royal Society of Chemistry 2019