New sulfur-containing polymeric sorbents based on 2,2′-thiobisethanol dimethacrylate

Materials

The following chemicals were obtained from commercial sources: methacryloyl chloride, triethylamine, 2,2′-thiobisethanol, benzoyl peroxide (BPO), polyvinyl alcohol, methylene chloride from Sigma-Aldrich; sodium chloride, calcium chloride, sodium hydroxide, cyclohexanol, cyclohexanone, ethyl acetate, toluene, n-heptane from POCH, Poland. Deionized water (Millipore) was used for making the aqueous phase and solutions during sorption studies.

Preparation of monomer

Synthesis of 2,2′-thiobisethanol dimethacrylate (TEDM) involves the reaction of 2,2′-thiobisethanol with methacryloyl chloride in the presence of triethylamine in methylene chloride [23]. The reaction was carried out in a three-neck flask equipped with a mechanical stirrer, a thermometer and a dropping funnel. 24.42 g of 2,2′-thiobisethanol, 40.4 g of triethylamine, 396 g of methylene chloride are placed into the flask and then cooled in a water-ice bath to a temperature below 0°C. Forty two grams of methacryloyl chloride was placed in a dropping funnel, added dropwise to a flask with thiobisethanol for over an hour, the temperature was kept below 10°C. After the addition was completed, the solution was stirred for 5 h at room temperature. The resulting precipitate was filtered off under reduced pressure. The filtrate containing monomer was washed several times with 5% sodium hydroxide solution in order to wash out the monoester, and then with 5% sodium chloride solution to obtain a neutral pH of the solution.

Monomer characterization

Determination of monomer solubility parameter

A Small’s theory [24] was used to determine the solubility parameter (δ) of synthesized TEDM. Table 1 contains molar-attraction constants (F_j) for individual groups present in the monomer. The solubility parameter (δ) was determined from the formula (eq. 1):

Table 1:

Molar-attraction constants (F_j) for individual groups present in the monomer [24].

Group	F j [MPa1/2 cm3 mol−1]
–CH3	437
=CH2	388
	39
–COO–	634
–CH2–	272
–S–	460

(1) δ i = ∑ j F j V i = ρ i ∑ j F j M i

ρ_i – monomer density, for TEDM ρ_TEDM=1.127 [g cm⁻³],

M_i – molar mass of the monomer, for TEDM M_TEDM=258.37 [g mol⁻¹],

F_j – molar-attraction constant [MPa^1/2 cm³ mol⁻¹].

Preparation of polymeric sorbents

Specific polymeric sorbents for metal ions recovery were obtained in radical suspension polymerization. The continuous water phase comprised 5% w/w calcium chloride and 1% w/w poly(vinyl alcohol) (PVA) (calculated for organic phase). The dispersed organic phase contained monomer (TEDM), initiator – BPO (0.5% w/w calculated for monomer) and solvents: ethyl acetate, cyclohexanol, cyclohexanone, n-heptane or toluene. The ratio of solvents to monomers in the polymerization mixture was 1:1 w/w. The homopolymer poly(2,2′-thiobisethanol dimethacrylate) (polyTEDM) (P1), the copolymer of 2,2′-thiobisethanol dimethacrylate/ethylene glycol dimethacrylate (TEDM/EGDMA) (P2) and copolymers of 2,2′-thiobisethanol dimethacrylate/pentaerythritol tetraacrylate (TEDM/PETEA) (P3–P5) were synthesized.

Polymer characterization

Sorption experiments

Sorption experiments have been carried out using newly obtaining polymeric materials and solutions of metal ions (20 mg dm⁻³) having 0.1 M concentration of hydrochloric acid (except cobalt aqueous solution). A batch method was used in which metal ions solution was contacted in 50 mL Erlenmeyer flask with an appropriate amount of polymer sorbent (15 mg). After shaking at room temperature for 48 h, the sorbent was separated by filtration and the concentrations of metal ions were measured using atomic absorption spectroscopy method (AAS) on a GBC Avanta instrument. AAS analysis was performed for the ions of Ag(I), Cu(II), Ni(II), Co(II), Au(III). The retention rate of the metal ions, R (%) was calculated using eq. 3:

where C₀ and C_eq (mg dm⁻³) are the liquid-phase concentrations of metal ions at initial and at equilibrium.

Monomer synthesis

Synthesis of TEDM consists of the reaction of 2,2′-thiobisethanol with methacryloyl chloride in the presence of triethylamine in methylene chloride. Monomer synthesis reaction is shown in Fig. 1. The purified synthesis product was obtained as a honey color liquid substance. Because of the low efficiency of the process (see Table 2), and thus small quantities of products, the synthesis was carried out five times. The obtained synthesis yields of monomer were about 35%. Substantial reduction in the reaction yields was due to the multistep purification process. During liquid-liquid extraction, a portion of obtained diester was lost because it is slightly soluble in water and can migrate to the inorganic phase. To confirm that the desired compounds were obtained all products of monomer syntheses were analyzed using ¹H NMR and ¹³C NMR spectroscopy. For example, Figs. 2 and 3 present the results of the analysis carried out for the synthesis of product M4. In Fig. 2 the signal labeled A is derived from the protons of the methyl group CH₃. Peak B is the triplet corresponding to protons of the methylene group directly linked to the sulfur atom. Adjacent to it, the second methylene group CH₂ is revealed in the spectrum as a signal C. The protons in the vinyl group =CH₂ are detected as two coupled peaks D and E. The nuclear magnetic resonance shows that the synthesis proceeded well to give the desired product. Other small peaks are impurities. ¹³C NMR spectrum presented in Fig. 3 also confirms the structure of 2,2′-thiobisethanol dimethacrylate. The peak labeled 1 is derived from the carbon atom of the methyl group. The carbon atom in the methylene group linked directly to the sulfur corresponds to the peak 2. However, the signal 3 is characteristic for carbon atoms in the methylene group linked to an oxygen atom. Peak 4 originates from the carbon atom in the carboxyl group. The tertiary carbon adjacent to the methyl group is revealed in the spectrum as a peak 5. The carbon atom of the =CH₂ group can be assigned to the signal 6. The not labeled peak is derived from the solvent. Additionally, the purity of the obtained monomers was confirmed by GC/MS method. In the Fig. 4, there is an example of the GC chromatograms for M1, M2 and M3 products of monomer syntheses. On each of them is a peak with similar retention times – about 30.20 min. which is identified as a peak derived from the TEDM. Other peaks with very low intensities are the small residues after the purification process. Additionally, MS spectrum of the product M1 was compared with a reference spectrum of 2,2′-thiobisethanol dimethacrylate (Fig. 5). It can be seen that all signals in the spectrum of the obtained product, correspond to the signals of the spectrum of the TEDM standard.

Fig. 1:

Synthesis of 2,2′-thiobisethanol dimethacrylate monomer.

Fig. 2:

¹H NMR spectrum of the purified product M4.

Fig. 3:

¹³C NMR spectrum of the purified product M4.

Fig. 4:

GC chromatograms of products of syntheses of monomers M1, M2, M3.

Fig. 5:

The mass spectrum of 2,2′-thiobisethanol dimethacrylate standard and the product of the synthesis of monomer M1.

Polymers syntheses

In order to obtain polymer beads, poly(2,2′-thiobisethanol dimethacrylate) and TEDM copolymers were synthesized in radical suspension polymerization. Three groups of sulfur-containing materials were polymerized. One is a homopolymer of TEDM, the second one is TEDM copolymer with ethylene glycol dimethacrylate and the last group are copolymers of TEDM with pentaerythritol tetraacrylate. The synthesized monomer and selected comonomers are more than bifunctional and therefore during polymerization a three-dimensional polymer network has been created. In Fig. 6 a fragment of the poly(2,2′-thiobisethanol dimethacrylate) structure is presented. Five polymers labeled P1 to P5 were prepared. Their chemical compositions are presented in Table 3, where we can find the kind of comonomers which are used in the syntheses and their weight ratio. The polymerization of TEDM and its copolymerization with other acrylic monomers lead to the polymer particles with various polymerization yields (see Table 3). It was found that the final monomer conversion is 83% for the pure TEDM and its copolymer with EGDMA. For TEDM/PETEA copolymers the polymerization yields depend on the PETEA content. With its increase the copolymerization yields also increase – for P3 13%, for P4 24% and for P5 93%. The same relationship was observed in the copolymerization of PETEA with styrene [25], where the authors suggested that the polymerization yield is closely related to polymer particle formation, during which the soluble oligomer radicals were captured from the solution by the vinyl groups on the particle surface throughout the polymerization. Additionally, in radical polymerization of multifunctional monomers, the gel stage in the polymerization of more than 4-functional (f>4) monomers is observed to be appreciably earlier than with conventional 4-functional (f=4) cross-linkers [26]. Autoacceleration results from a decreasing rate of termination compared to initiation and propagation as radicals on chains and networks move less and less easily, while monomer and oligomer mobilities remain high. Consequently, the radical concentration accumulates and so propagation accelerates. At some point the apparent mobility of free radicals is controlled mainly by reaction diffusion, i.e. through propagation reactions along kinetic chains [27], [28]. It causes that in the polymerization of multimethacrylates and multiacrylates reaction diffusion become the dominant termination mechanism early in the reaction (as early as 5% conversion) [28], while for linear polymerizations in which reaction diffusion controlled termination occurs only after 40–60% conversion. Also, for this reason, we can observe an increase in copolymerization efficiency with the increase in the content of multifunctional monomer (PETEA, f=8) in the studied systems.

Fig. 6:

Fragment of the poly(2,2′-thiobisethanol dimethacrylate) structure.

Table 3 also contains the type of solvents mixtures used during polymerization. The solvents are responsible for a creation of porous structure in obtained materials. The basis for selecting the appropriate solvents used in the polymerization process was the value called the solubility parameter (δ). Solvents having δ similar to δ of monomer gave smaller pores and a higher surface area (sol solvents). The use of non-sol solvents (the difference in solubility parameters is greater than 1 Hildebrand unit) resulted in worse solvation of monomer and consequently, the pore size became bigger, which in turn caused the decrease of the surface area. The values of solubility parameters for cyclohexanol, cyclohexanone, ethyl acetate, toluene and n-heptane are 23.3 MPa^1/2, 20.3 MPa^1/2, 18.6 MPa^1/2, 18.2 MPa^1/2 and 15.1 MPa^1/2, respectively. Whereas δ for 2,2′-thiobisethanol dimethacrylate was determined using Small’s theory according to the eq. 1; calculated value is 19.8 MPa^1/2. The differences in the pore structure of the synthesized sorbents are noticeable on scanning electron microscopy images, which are presented in Fig. 7. It is clearly visible that the polymerization of pure TEDM does not lead to polymer material in a spherical shape. The monomer type affects the surface energy of the polymerizing mixture droplets. This energy is a function of the composition of this mixture which changes with the progress of the reaction. It can be varied also by selection of the type and concentration of the stabilizer. It is also a function of temperature, so it’s become even more complicated. Therefore, in order to improve the shape of polymeric grains, it was decided to add a different comonomer (EGDMA or PETEA), which changed the surface energy of droplets of the polymerization mixture. The results showed that only the addition of PETEA to TEDM has provided the satisfactory shape and rigid structure of the synthesized copolymers beads. However, the lack of spherical shape of polymeric grains will affect the sorption kinetics of the tested metal ions, but not their sorption efficiency. The fragmentation of polymeric grains shortens the diffusion path, which improves the sorption kinetics, but due to the donor-acceptor mechanism of sorption, the sorption efficiency, at equilibrium, should remain constant and depends only on the sulfur content.

Fig. 7:

SEM images of the synthesized sulfur-containing polymeric sorbents.

For the characterization of the porous structure in the prepared sorbents the surface area, pore volume and pore size were measured. Results are presented in Table 4. The obtained polymers have a mesoporous structure. The pore size is about 3.8–12.6 nm. The specific surface area values vary in the interval between 12 and 66 m² g⁻¹. The highest surface area has P2 – copolymer TEDM with EGDMA. Generally, obtained materials have poorly developed porous structure. Despite a significant increase in the proportion of sol solvent in the polymerization mixture of polymer P4 and P5, a satisfactory increase in surface area development has not been achieved. This behavior may result from the branched structure of PETEA. The increased proportion of PETEA in investigated copolymers probably increases the proportion of intramolecular cyclization in polymer chains, which in turn can reduce the real degree of cross-linking. The insufficient degree of cross-linking leads to the collapse of the polymer chains during the drying process (after removal of the diluent) and hence generated porous structure is lost.

The chemical structure of the obtained polymeric sorbents was examined using elemental analysis. The S, C, H and O contents were determined. The results are presented in Table 5. In all cases, the experimental data are comparable with theoretical atoms content. The sulfur content is in the range from 3 to 12%, depending on the amount of introduced sulfur-containing monomer to polymer structure. The presence of sulfur atoms in the polymer structure can increase its specific sorption capacity by creating additional interactions between the sorbent and sorbate. P2 has the highest sulfur content and also the highest water regain – 2.43 g of water per gram of polymer. P1 and P3 have high sulfur contents and good swelling properties too (see Tables 4 and 5). The higher polymer swelling increases the availability of atoms responsible for specific interactions between sorbate and sorbent. For this reason, these three polymers should exhibit the best sorption properties.

In order to determine the sorption capacity of the obtained poly(dimethacrylate)s materials containing sulfur atoms, the metal ions sorption was carried out. Based on the batch studies the retention rate (R) for metal ions like Ag(I), Cu(II), Ni(II), Co(II), Au(III) are presented in Table 6. Analysis of the results leads to the conclusion that for the investigated sorbent-sorbate systems, the Pearson HSAB theory can be used, according to which gold and silver ions are soft acids that have a high affinity for soft bases, which include sulfur and its compounds (thiol, thiourea or thiocyanate groups) [29]. The retention rate of metal ions from 0.1 M hydrochloric acid solutions (except cobalt aqueous solution) was examined. The tested materials showed no affinity for copper, cobalt and nickel ions, which belong to hard and transition acids according to HSAB. The precious metal ions were removed in the range from 13 to 98% for gold(III) ions, and from 3 to 27% for silver(I) ions. The obtained results can be compared to the percentage of sulfur in investigated polymers. Because the donor sulfur atoms can form bonds with soft acids, they are responsible for the sorption of these metal ions. The studied metal ions are most intensively sorbed by P2, P1 and P3 polymers. The best binding polymers of both Au(III) and Ag(I) are characterized by the highest percentage of sulfur. However, among this group, the P1 polymer has the most sulfur content, but the most intense sorption takes place on the P2 polymer. Most probably, this is due to easier access to the sulfur atoms, because P2 is having better swelling in sorbate solution. Its water regain is almost twice higher than water regain of P1 (see Table 4). The relation between metal ions sorption and sulfur content is also not observed for polymer P4. It is a material having a relatively large amount of sulfur, but its retention rate is not satisfactory (R=13%). However, it should be noted, that this material has a small water regain, so active sorption centers are available only on the surface of the polymer particles.