Synthesis of Terpolymers at Mild Temperatures Using Dynamic Sulfur Bonds in Poly(S-Divinylbenzene)

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Summary

The goal of this protocol is to initiate polymerization using dynamic sulfur bonds in poly(S-divinylbenzene) at mild temperatures (90 °C) without using solvents. Terpolymers are characterized by GPC, DSC and 1H NMR, and tested for changes in solubility.

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Westerman, C. R., Walker, P. M., Jenkins, C. L. Synthesis of Terpolymers at Mild Temperatures Using Dynamic Sulfur Bonds in Poly(S-Divinylbenzene). J. Vis. Exp. (147), e59620, doi:10.3791/59620 (2019).

Abstract

Elemental sulfur (S8) is a byproduct of the petroleum industry with millions of tons produced annually. Such abundant production and limited applications lead to sulfur as a cost-efficient reagent for polymer synthesis. Inverse vulcanization combines elemental sulfur with a variety of monomers to form functional polysulfides without the need for solvents. Short reaction times and straight forward synthetic methods have led to rapid expansion of inverse vulcanization. However, high reaction temperatures (>160 °C) limit the types of monomers that can be used. Here, the dynamic sulfur bonds in poly(S-divinylbenzene) are used to initiate polymerization at much lower temperatures. The S-S bonds in the prepolymer are less stable than S-S bonds in S8, allowing radical formation at 90 °C rather than 159 °C. A variety of allyl and vinyl ethers have been incorporated to form terpolymers. The resulting materials were characterized by 1H NMR, gel permeation chromatography, and differential scanning calorimetry, as well as examining changes in solubility. This method expands on the solvent-free, thiyl radical chemistry utilized by inverse vulcanization to create polysulfides at mild temperatures. This development broadens the range of monomers that can be incorporated thus expanding the accessible material properties and possible applications.

Introduction

Conversion of organosulfur compounds to S8 during petroleum refinement has led to the amassing of large stockpiles of sulfur1. Elemental sulfur is primarily used for the production of sulfuric acid and phosphates for fertilizers2. The relative abundance provides a readily available and inexpensive reagent making elemental sulfur an ideal feedstock for materials development.

Inverse vulcanization is a relatively new polymerization technique that repurposes sulfur into functional materials3. The S8 ring converts to a diradical, linear chain upon heating above 159 °C. The thiyl radicals then initiate polymerization with monomers to form polysulfides3. In addition to traditional radical polymerizations, inverse vulcanization has been utilized to initiate polymerization with benzoxazines4. The resulting polymers have been used for a wide range of applications including cathodes in Li-S batteries1,5,6,7, self-healing optical lenses8,9, mercury and oil sorbents5,10,11,12,13,14,15, thermal insulators15, to aid in the slow release of fertilizer16 as well as demonstrating some antimicrobial activity17. One group has provided a thorough systematic analysis of these polysulfides providing more information about the insulating character and mechanical properties with varied S content18. The specific details may aid in further applications development. The dynamic bonds present in these materials have also been utilized to recycle the polysulfides19,20. However, the high temperatures required by inverse vulcanization, typically 185 °C, and lack of miscibility with S8, limit the monomers that can be used3.

Early efforts focused on the polymerization of aromatic hydrocarbons, extended hydrocarbons, and natural monomers with high boiling points5. These methods have been expanded by using poly(S-styrene) as a prepolymer improving miscibility between S8 and more polar monomers including acrylic, allylic, and functionalized styrenic monomers21. Another method utilizes nucleophilic amine activators to enhance reaction rates and lower reaction temperatures22. However, many monomers have boiling points well below 159 °C and thus require an alternate method for polysulfide formation.

In the stable crown form, S-S bonds are the strongest, thus requiring high temperatures for cleavage23. In polysulfides, sulfur is present as linear chains or loops, allowing S-S bonds to be cleaved at much lower temperatures1,24. By using poly(S-DVB) (DVB, divinylbenzene)as a prepolymer, a second monomer with a lower boiling point such as 1,4-cyclohexanedimethanol divinylether (CDE, boiling point of 126 °C), can be introduced24. This work demonstrates further improvement by lowering the reaction temperature to 90 °C with a family of allyl and vinyl ether monomers. Reactions incorporating a second monomer remain solvent-free.

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Protocol

1. Synthesis of poly(S-divinylbenzene)

  1. To prepare poly(S-divinylbenzene), combine elemental sulfur (S8) and divinylbenzene (DVB) at various weight ratios (30:70, 40:60, 50:50, 60:40, 70:30, 80:20, and 90:10 of S8:DVB). Prepare the reaction according to prior methods described below3,25.
    NOTE: All reactions here were conducted on a 1.00 g scale. A typical reaction contains 500 mg of S8 and 500 mg of DVB.
    1. Place the reagents in a 1-dram vial equipped with a magnetic stir bar. Insert the vials in an oil bath at 185 °C for 30 min.
    2. Remove the samples from the oil bath and immediately quench the reaction by placing the vials in liquid nitrogen. Break open the vials to remove the polymer. Liquid nitrogen not only quenches the reaction but also aids in complete removal of the polymer.
      CAUTION: All samples are extremely hot upon removing from bath. Use caution when handling the samples. Use proper PPE when dealing with broken glass.

2. Preparation of terpolymers

  1. Synthesize terpolymers by combining poly(S-DVB) and an additional monomer in a 1-dram vial equipped with a magnetic stir bar.
    NOTE: All samples were prepared on a 600 mg scale. Monomers examined include 1,4-cyclohexanedimethanol divinyl ether (CDE), cyclohexyl vinyl ether (CVE), and allyl ether (AE).
    1. Crush poly(S30-90%-DVB10-70%) with a mortar and pestle for higher surface area interaction with CDE. Alter the composition by varying the ratio of poly(S-DVB):CDE from 1:1 to 1:100 by weight. Additional monomers are only tested at a 1:1 ratio of poly(S50%-DVB50%):monomer.
    2. Place the samples in an oil bath at 90 °C for 24 h and then cool to room temperature. Longer reaction times are required for some monomers.
    3. Some reactions will not result in complete monomer incorporation. For those reactions, dissolve the soluble polymer portions in dichloromethane (DCM) and precipitate in cold methanol. For samples with limited solubility, wash the solid polymer samples with cold methanol to remove any unreacted monomer.
  2. Prepare terpolymers using maleimide
    NOTE: Maleimide does not have a low boiling point. However, it does have just one reactive site towards thiyl radical modification.
    1. Synthesize poly(S-DVB) prepolymer using slightly altered methods. Combine sulfur and DVB at a 30:70 S8:DVB ratio. Combine the reagents in a glass vial equipped with a magnetic stir bar on a 5.00 g scale.
    2. Place the vials in an oil bath at 185 °C for 1 h. Immediately place the samples in liquid nitrogen upon removal from the oil bath.
    3. Combine the prepolymer with maleimide in a 1-dram glass vial with a 3:1 poly(S-DVB):maleimide ratio (w/w). Prepare all samples on a 200 mg scale and dissolve in 10 mg/μL of dimethylformamide (DMF). Place the vials in an oil bath at 100 °C for 24 h.
      CAUTION: Inverse vulcanization reactions produce a small amount of gas. For reactions above a 1.00 g scale, use larger vials or drill a hole in the vial cap to prevent the build-up of pressure.
      NOTE: Prepolymers that are more malleable cannot be crushed into a powder. However, these polymers soften very easily when heated providing miscibility with most monomers.
  3. Monitor percent conversion for the addition of a variety of monomers (CDE, CVE, AE, and maleimide).
    1. Prepare all samples using poly(S-DVB) as described in section 2.2.1-2.2.2. Synthesize poly(S-DVB-CDE), poly(S-DVB-CVE), and poly(S-DVB-AE) in a 1-dram glass vial with a 3:1 poly(S-DVB):monomer ratio by weight. A typical sample should have 150 mg of prepolymer and 50 mg monomer. No solvent is needed for most of these polymerizations. However, in order for maleimide and poly(S-DVB) to interact fully, 20 μL of DMF must be added.
    2. Remove the samples at various time points (t= 0, 15, 30, 60, 180, 360, 720, and 1440 min). Dissolve the polymer in 600 μL of chloroform-d and analyze them by 1H NMR.

3. Control polymerizations

  1. Synthesize poly(S50%-DVB50%) as described in section 1.1. The sample can then be used in three control reactions. Combine the finely crushed polymer with additional DVB. Place only poly(S-DVB) in a separate vial. Heat all samples at 90 °C for 24 h, then cool them to room temperature.
  2. Test elemental sulfur under a variety of conditions in order to confirm that sulfur from the polymer rather than S8 is required for polymerization. Conduct individual reactions by combining S8 with CDE, DVB, and AE, as well as using S8 alone. Combine S8, CDE, and poly(S50%-DVB50%) in another vial. Heat all samples at 90 °C for 24 h, then cool to room temperature.

4. Polymer characterization

  1. Use thin layer chromatography (TLC) for initial detection of S8 in polymers. Spot polymer samples onto silica TLC plates with hexanes eluent. In hexanes, S8 has a Rf value of 0.7, and polymers do not move from the baseline, Rf ≈0.
  2. Analyze all polymers by 1H NMR in chloroform-d. Integrate the resulting 1H-NMR spectra to determine the extent of the reaction. Prepare all samples by dissolving the polymer of interest in chloroform-d. Perform simple filtration to remove undissolved particulates.
  3. Analyze the samples by gel permeation chromatography (GPC) using DCM eluent. Use a GPC with two mesopore columns in sequence and a refractive index detector for analysis.
    1. Due to the relatively low solubility of most terpolymers and broad polydispersity, dissolve each polymer at a seemingly high concentration, 75 mg/mL in DCM. Remove particulates from the soluble portion using a 0.45 μm hydrophobic filter.
    2. Determine the number average and weight average molecular weights (Mn and Mw respectively) based on a calibration curve of polystyrene standards. Use these values to obtain the polydispersity index (PDI).
  4. Study the thermal properties of the polymer samples. Fill the aluminum pans with 30-50 mg of polymer providing enough sample to adequately discern the glass transition temperature (Tg) from the resulting thermograms. Scan the samples from -50 °C to 150 °C at a rate of 10 °C/min, cool back to -50 °C at 20 °C/min and heat them again to 150 °C at 10 °C/min. Obtain all Tg values from the second scan.

5. Solubility studies

NOTE: Terpolymers demonstrated highest solubility in DCM. Solubility of the polymers can be altered by varying the composition.

  1. Measure approximately 150 mg of each polymer into a pre-weighed vial and dissolve in DCM to reach 75 mg/mL. Dissolve the samples for 8 h before removing the soluble portion. Wash the insoluble portion with DCM two times and dry the remaining insoluble sample in an oven for 10 min to remove remaining solvent.
  2. Reweigh the vial after letting the vial cool to room temperature. Calculate the percent solubility by determining the difference in starting and final weights.

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Representative Results

Poly(S-DVB) was synthesized according to published protocols using high temperatures (185 °C) to initiate S8 ring cleavage forming radicals3. These radicals then initiate polymerization with DVB. The molten sulfur and liquid DVB eliminate the need for solvents. Within 30 min, sulfur and DVB react completely for form poly(S-DVB). Upon removal from the vial, the polymer is a hard, brittle material at lower sulfur content (30-40%). Mid-range sulfur content (50-60%) produces a tacky material that is very malleable, and higher sulfur content (70-90%) forms a tough and flexible solid. Furthermore, the polymers become more opaque with increasing sulfur content. This material then serves as a prepolymer for further modification. Dynamic sulfur bonds within poly(S-DVB) can be utilized to initiate polymerization with additional monomers (Figure 1).

A family of vinyl and allyl ethers were combined with poly(S-DVB) and heated at 90 °C to form terpolymers. Both monofunctional and difunctional monomers were tested. These monomers were chosen due to their relatively low boiling points (95 - 147 °C) to examine the effectiveness of reduced reaction temperatures. All monomers tested were successfully polymerized, confirmed by NMR (Figure 2). Maleimide which has a higher boiling point and a higher melting point, ~90 °C, was also tested to determine if this method would still be successful. A small amount of solvent was required to obtain interaction with the prepolymer. The monomer content for all polymerizations were monitored over the course of 48 h (Figure 3). Control reactions were performed to determine the role of poly(S-DVB) versus S8 in the polymerization (Figure 4). The products were examined by 1H NMR and TLC to examine changes to the polymer structure, monomer incorporation, and to determine if S8 was fully incorporated.

A variety of polymerizations were conducted to examine the polymer structure of poly(S-DVB-CDE). The sulfur, DVB, and CDE content were altered and characterized by GPC and differential scanning calorimetry (DSC) (Figure 5). Both increased sulfur content and the addition of CDE led to an overall decrease in the Tg. After an initial decrease in the molecular weight, the addition of CDE led to an overall increase in chain length.

The inclusion of CDE into poly(S-DVB) changed the characteristics of the prepolymer. With a 1:1 poly(S50%-DVB50%):CDE ratio, the polymer exhibited a mocha brown color compared to the reddish black prepolymer color. The material was also able to resist any physical force applied by hand to break apart the polymer. Even light scratching of the polymer showed minimal surface deformation. This is in stark contrast to the poly(S50%-DVB50%) prepolymer’s tacky and malleable properties.

The terpolymer solubility in DCM was also examined by varying S8:DVB ratio and the CDE content (Figure 6). Maximum solubility was achieved for poly(S-DVB) synthesized with 40-50% sulfur. The addition of CDE led to decreased polymer solubility in these samples. However, the opposite occurred for high sulfur content poly(S-DVB). Initially, very low solubility in DCM was observed, but incorporating CDE into the polymers substantially improved solubility in DCM.

Figure 1
Figure 1: Formation of poly(S-DVB) prepolymer via inverse vulcanization followed by mild temperature modification to form poly(S-DVB-CDE). The additional monomers used to modify the poly(S-DVB) are depicted in blue. Please click here to view a larger version of this figure.

Figure 2
Figure 2: 1H NMR spectra of poly(S-DVB) in red, poly(S-DVB-CDE) in purple, and 1,4-cyclohexane dimethanol divinyl ether monomer in blue. The spectrum of poly(S-DVB-CDE) shows the presence of aromatic protons from the prepolymer, alkyl protons from the cyclohexane ring, and the formation of new HC-S bonds. This figure has been reproduced from Macromolecules 51, 7233-7238 (2018)24. Reprinted with permission from Macromolecules Copyright 2018 American Chemical Society. Please click here to view a larger version of this figure.

Figure 3
Figure 3: Percent conversion of 1,4-cyclohexanedimethanol divinyl ether (CDE), cyclohexyl vinyl ether (CVE), allyl ether (AE), and maleimide by poly(S-DVB). Compounds with vinyl groups, CVE and CDE, reacted the most rapidly. The internal double bond of maleimide reacted notably slower than the other monomers tested. Please click here to view a larger version of this figure.

Figure 4
Figure 4: Characterization of control polymerizations. (A) 1H NMR spectrum of control polymerizations: reheated poly(S-DVB), poly(S-DVB) in chloroform-d. (B) Thermograms of S8 and CDE without (red) and with (blue) poly(S-DVB). (C) Detection of elemental sulfur by TLC including elemental sulfur (S8), poly(S50%-DVB50%) [1], S8 + poly(S50%-DVB50%) + CDE [2], S8 + CDE [3] and poly(S-DVB-CDE) [4]. Please click here to view a larger version of this figure.

Figure 5
Figure 5: Polymer molecular weight and glass transition temperature at varied sulfur content. (A) Characterization data of poly(S-DVB-CDE) by GPC and DSC. (B) GPC traces and (C) DSC traces of poly(S-DVB) prepolymer made with 30% S (red), and poly(S-DVB-CDE) produced by concurrent addition of poly(S-DVB) and CDE (black) and produced by heating prepolymer for 5 min before CDE addition (blue). This figure has been modified to include multiple figures from Macromolecules 51, 7233-7238 (2018)24. Reprinted with permission from Macromolecules Copyright 2018 American Chemical Society. Please click here to view a larger version of this figure.

Figure 6
Figure 6: Solubility of poly(S-DVB) and poly(S-CDE). (A) Image and (B) percent solubility of poly(S-DVB) at varied S8:DVB ratios. (C) Image of poly(S-DVB-CDE) made with 50% S at varied prepolymer:CDE ratios. (D) Percent solubility of poly(S-DVB-CDE) with 30-90% S at varied prepolymer:CDE ratios. This figure has been reproduced from Macromolecules 51, 7233-7238 (2018)24. Please click here to view a larger version of this figure.

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Discussion

The primary benefit of this method is the ability to form polysulfides at mild temperatures, 90 °C versus >159 °C for traditional inverse vulcanization. The extended sulfur chains and sulfur loops in poly(S-DVB) are less stable than S-S bonds in S823,26. Lower temperatures can then be used to cause homolytic scission and thiyl radical formation24. For monomers with melting points well below the reaction temperature, this can still be accomplished without the need for solvents. Stable polymers were obtained with 1,4-cyclohexanedimethanol CDE, CVE, and AE (Figure 1). This is especially noteworthy for CVE, a monofunctional compound. There has been successful polymerization of styrenic compounds by inverse vulcanization21,27. However, without cross-linking from the monomer, most other monofunctional compounds do not form stable polysulfides leading to depolymerization and precipitation of S8. Poly(S-DVB-CVE) showed no signs of depolymerization, even after 6 months24.

A slightly different protocol is needed for monomers with high melting points. Maleimide was also incorporated to determine how monomers with melting points near the reaction temperature would affect the reaction. Initially, no interaction between the prepolymer and maleimide was observed. For this monomer, a higher reaction scale was utilized for the prepolymer synthesis. Larger reaction scales tend to decrease the Tg of poly(S-DVB) allowing better interaction with the monomer. Although this improved the interaction some, there was still distinct separation between poly(S-DVB) and the maleimide. A small amount of DMF had to be added at 10 mg reactants/μL DMF for a successful reaction. However, poly(S-DVB) still provides the benefit of stabilization for this monofunctional compound.

Fabrication of all terpolymers was monitored by 1H NMR to determine the percent monomer incorporation. Monomers with vinyl groups, CDE and CVE, reacted the most rapidly (Figure 2). Difunctional CDE demonstrated > 90% conversion and solidification within 1 h. Allyl ether took slightly longer to react likely due to the depressed reaction rates of allyl groups28. Maleimide took the longest to react and only reached ~70% incorporation after 48 h. The slow initial reactivity may be due in part to the limited interaction between the poly(S-DVB) and maleimide. However, maleimide also contains an internal alkene that does not readily undergo radical propagation. This is likely a major contribution to the limited maleimide incorporation overall.

Control polymerizations were conducted by heating poly(S-DVB) in the presence of additional DVB and reheating poly(S-DVB) with no additional monomer. Polymer structures were examined by 1H NMR to compare the formation of HC-S bonds (2-5 ppm) versus HC-C bonds (1-2 ppm) formed by radical propagation (Figure 3A). Heating poly(S-DVB) in the absence of monomers showed no notable changes to the polymer structure. Combining poly(S-DVB) with additional DVB resulted in comparable HC-C:HC-S bond formation relative to the prepolymer. Unlike CDE, alkene peaks were observed by 1H NMR even when poly(S-DVB-DVB) was precipitated into methanol. The presence of partially incorporated monomers with unreacted double bonds have been observed in other polymer systems29.

Additional controls were conducted by combining S8 with different monomers in the absence of poly(S-DVB) prepolymer to determine if the prepolymer alone initiates polymerization. Although S8 must be heated above 120 °C to melt on its own, an incomplete reaction occurred when S8 was combined with CDE at 90 °C for 72 h. However, when examined by TLC and DSC, there was still unreacted elemental sulfur remaining (Figure 3B,C). When the same control was repeated with DVB and AE, no reaction occurred after 72 h. To determine if additional sulfur rather than just additional monomers could be incorporated, a reaction combining a 1:1:1 weight ratio of S8:CDE:poly(S-DVB) was conducted. A substantial portion of the added sulfur was polymerized; however, the reaction was incomplete leaving some unreacted S8 in the sample (Figure 3B).

Prepolymer and terpolymer solubility was investigated in DCM at 75 mg/mL (Figure 5). Poly(S-DVB) with high sulfur content demonstrated limited solubility, which has been observed previously3. However, the incorporation of some CDE improved solubility substantially. Beyond 1:50 poly(S-DVB):CDE ratio, extensive cross-linking led to a less soluble, more gel-like network. Although terpolymer samples appear homogenous (Figure 5A), solubility tests demonstrated samples contain fully soluble and fully insoluble portions. Polymer insolubility may be caused by regions of high sulfur content, extensive cross-linking and/or high molecular weight. Only the soluble fraction was used for characterization by NMR and GPC (Figure 4). Since there are likely competing effects between high sulfur content and cross-linking, it is difficult to discern how the NMR spectra are affected by insolubility. For GPC, the soluble fraction likely underestimates the overall molecular weight and PDI of the terpolymers. Despite these challenges, poly(S-DVB-CDE) demonstrated improved solubility relative to many of the prepolymer samples providing more complete information.

The field of inverse vulcanization has grown rapidly since it was first introduced. Short reaction times, solvent-free synthesis, and the development of functional materials make this method very attractive. However, high reaction temperatures preclude the use of many monomers. Utilizing dynamic sulfur bonds of polysulfides to initiate polymerization has decreased the reaction temperature substantially to 90 °C. Additionally, monofunctional monomers can be combined into stable polymers. This development allows more monomers to be utilized creating a diverse set of materials. Developing polymers with a wider range of physical properties will likely expand the use of recycled waste to create functional polymers.

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Disclosures

The authors have nothing to disclose.

Acknowledgments

Thanks are owed to The American Chemical Society Petroleum Research Fund (PRF # 58416-UNI7) for financial support.

Materials

Name Company Catalog Number Comments
Sulfur, 99.5%, sublimed, ACROS Organics Fisher Scientific AC201250250SDS
divinylbenzene Fisher Scientific AA4280422
1,4-Cyclohexanedimethanol divinyl ether, mixture of isomers Sigma Aldrich 406171
Cyclohexyl vinyl ether Fisher Scientific AC395420500
Allyl ether Sigma Aldrich 259470
maleimide Sigma Aldrich 129585
dichlormethane Fisher Scientific D37
N,N-dimethylformamide Fisher Scientific D119
Auto sampler Aluminum Sample Pans, 50µL, 0.1mm, Sealed Perkin Elmer B0143017
Auto sampler Aluminum Sample Covers Perkin Elmer B0143003
EMD Millipore 13mm Nonsterile Millex Syringe Filters - Hydrophobic PTFE Membrane, 0.45 um Fisher Scientific SLFHX13NL

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References

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