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

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.

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 batteries^1,5,6,7, self-healing optical lenses^8,9, mercury and oil sorbents^{5,10,11,12,13,14,15}, 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 polysulfides^19,20. 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 temperatures^1,24. 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.

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 below^3,25.
   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(S_30-90%-DVB_10-70%) 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(S_50%-DVB_50%):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(S_50%-DVB_50%) 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(S_50%-DVB_50%) 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 R_f value of 0.7, and polymers do not move from the baseline, R_f ≈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 (M_n and M_w 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 (T_g) 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 T_g 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.

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 ...

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 S₈^23,26. Lower temperatures can then be used to cause homolytic scission and thiyl radical formation²⁴. For monomers with melting points well below the reaction temperature...

Materials

List of materials used in this article

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