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Abstract

Inverse vulcanization is a cost-effective method for producing high sulfur-content copolymers by combining elemental sulfur with organic monomers, which has rapidly gained popularity due to its simplicity of synthesis and wide range of applications. Although numerous examples of sulfur-rich copolymers have been synthesized at different reaction rates and temperatures using various monomers, the precise reaction mechanism remains unclear. In this paper, we used tung oil containing conjugated triene as a monomer to synthesize sulfur-rich copolymers under six different reaction conditions and investigate the effects of oxygen and moisture. Our study, which employed DSC, XRD, ¹H NMR, and XPS characterization methods, revealed that oxygen accelerated the reaction rate and decreased the free sulfur content of the products, while moisture shortened the gel times but increased the free sulfur content. These findings confirm that reverse vulcanization involves two simultaneous mechanisms: the free radical mechanism and the anion mechanism. With regard to the radical mechanism, we discuss the source, ease, and reactivity of radicals and show that the creation of radicals depends on the second monomer rather than sulfur. Tung oil not only acts as a comonomer in the reaction but also plays an initiating role in promoting the dissociation of sulfur chains to generate free radicals for addition of non-conjugated double bonds. The effect of the anion mechanism exceeds that of the radical mechanism once certain factors are stimulated, such as the presence of metal ions and sulfide ions. Understanding the detailed mechanisms involved in inverse vulcanization is essential for selecting optimal monomers, which can enhance not only the synthesis process but also the properties of sulfur-rich materials.

Keywords

Inverse vulcanization elemental sulfur reaction mechanism radical mechanism anion mechanism conjugated double bonds

Introduction

From the end of last century, to protect the environment and mitigate acid rain, the petrochemical industry has been purifying sulfur-containing petro-products by hydrodesulfurization and the Clause process, thus generating a large amount of elemental sulfur as a by-product (over 60 million tons per year).^1–3 Although a small percentage of sulfur is used to make sulfuric acid,⁴ tire rubber,⁵ fungicides⁶ and fertilizers,⁷ the supply of sulfur far exceeds the demand, resulting in millions of tons of sulfur being left unused and thus wasting resources. With the increasing scarcity of oil resources, cheaper crude oil, usually containing more sulfur, is further utilized, leading to an increase in sulfur production. Thus, elemental sulfur is a cheap, abundant, and underutilized resource that can be widely used to synthesize new materials.

Since the concept -- ‘inverse vulcanization’ was proposed by Pyun et al.,⁸ in 2013, many researchers have devoted themselves to this field with great enthusiasm. An enormous number of potential applications^9–11 of sulfur-rich materials have been reported, including cathode material for rechargeable batteries,^12–14 materials with high refractive indexes,^15,16 IR optical materials, repairable materials,^17–19 adhesives,^19,20 fertilizers,^21–25 and composite materials.^26,27 An extensive range of substances was successfully used to synthesize sulfur-rich polymeric materials^{12–14,28–30} but selecting the second monomer seemed to be a random process. Their similarity in molecular structure is at least one of the following: double bonds (conjugated or non-conjugated),³¹ aromatic^32,33 or benzene ring, and nucleophilic system.^34,35 For reaction conditions, a wide range of temperatures were applied, from under the melting point of sulfur to near 200°C. Most reactions were carried on in the absence of a catalyst while a few catalysts were introduced to promote them.^36,37 The reason for this situation is that very little understanding of the mechanism of inverse vulcanization was known and corresponding work was scarce. Zhang et al.³⁴ demonstrated the accelerator effect of N-methylimidazole and 4-vinylaniline on inverse vulcanization, which was that nucleophilic attack caused ring-opening of S₈ under lower temperatures. This is undoubtedly true. It has been known for a very long time that S₈ is reduced by sulfide or hydrogen sulfide ions to polysulfide ions in water, where the nucleophilic attack of S^2- ions on the electrophilic S₈ ring is the crucial step.³⁸ Liam James Dodd et al.³⁷ investigated the role of metal catalysts in inverse vulcanization and made several deductions about the mechanism. They are, first, the diethyldithiocarbamate (DEDC) ligand as a nucleophile assisted cleavage of sulfur–sulfur bonds and carbon–carbon double bonds. The second deduction is that an anionic DEDC or an anionic polysulfide assisted to break the C = C bond, which is described as the anionic mechanism. Last, they suggested that the metal DEDCs act as phase transfer agents between the immiscible sulfur and organic phases. Catalytic inverse vulcanization by DEDC has advantages including lower reaction temperatures, reducing the formation of the toxic byproduct hydrogen sulfide, increasing yields of reaction, and the polymerization of unreactive monomers. But the significant difference between the properties of polymeric materials from vegetable oils in the presence or absence of DEDC hinted that the inner structures may be massively different and thus their reaction mechanism too.^29,39 From this, it can be speculated that the mechanism of inverse vulcanization is a mixture of a radical mechanism and an anionic one, and the dominant route might be one of them under different conditions such as temperature lower or higher than 159°C and the usage of catalysts.

Needless to say, more knowledge and understanding about many aspects of elemental sulfur and its nature in inverse vulcanization would enable us to influence the reaction behavior and thus enlarge the practicable applications. Finding out the detailed mechanism of inverse vulcanization is a challenging problem not only because of ambiguous and controversial knowledge about elemental sulfur but complex characteristics of products such as insolubility and nonuniformity. Thus we try to explore the mechanism by controlling the oxygen and moisture during the reaction process. The inverse vulcanization system is tung oil, for which it was found that its unique conjugated triene tremendously decreased reaction temperature compared with the non-conjugated double bonds in normal lipids like sunflower oil.²⁹ The experimental results obtained from particular reaction conditions would reflect whether trace impurities are beneficial or detrimental to the copolymerization and provide evidence to confirm or disprove the mechanisms proposed by other researchers.

Experimental

Materials

Elemental sulfur (reagent grade, 99.9%) was used as received from Alfa Aesar. Tung oil (TO) was purchased from Shandong Lucheng chemicals LTD. Sunflower oil (SFO) was purchased from Shanghai Rongs. Iodine standard titration solution was provided by Quanzhou YiDa Technology. Anhydrous sodium sulfite, acetic acid, starch solution, and all solvents were obtained from Aladdin Biochemical Technology and used as received. All these compounds are of reagent grade except for natural vegetable oils.

Table 1 shows the types and relative contents of each fatty acid in vegetable oils (tung oil and sunflower oil). The content of unsaturated fatty acids in tung oil was 91.1w%, among which the content of tung oil acid (9,11,13-octadecatrienoic acid) was 67.8w%. The content of unsaturated fatty acids such as linoleic acid and oleic acid in sunflower oil was 97.1w%.

Table 1.

Relative content of fatty acids in vegetable oils.

Fatty acids	The contents of fatty acids (WT%)
Fatty acids	Tung oil	Sunflower oil
c16:1	—	14.0
c18:1	12.2	33.6
c18:2	9.0	49.5
c18:3 (ELEOSTEARIC acid)	67.8	—
c20:1	2.10	—

General procedure for the mechanism exploration: preparation of sulfur-rich copolymers based on vegetable oils

All reactions were carried out in 50 mL round-bottom flasks equipped with magnetic stirrers and heated in an oil bath at 130°C. Under vigorous stirring liquid sulfur (2 g) and vegetable oil (2 g) became an emulsion. When the emulsion was darkened and so viscous that it stopped the stir bar, the reaction ended, and the time was recorded as gel time. To determine how moisture and oxygen affected the rate of reaction and gel time, the synthesis processes were conducted under six conditions. In condition A, the flask was open to the air, meaning that oxygen and moisture were present during the reaction process and the byproduct H₂S might emit out. The condition AD, N, and ND means dehydrated air, nitrogen, and dehydrated nitrogen, respectively, flowing through the flask. In these three conditions, H₂S was blown out with the other volatile species generated in the reaction process. In the condition AT, a glass stopper was used to seal the mouth of the round-bottom flask, so the reaction took place in a sealed environment that contained both oxygen and moisture. When the condition NT was exerted, before reaction, the two-neck flask was connected to a double-row tube device to evacuate the reactor first and then pass dry nitrogen gas. Repeat the above operation three times to make the reaction under water-free and oxygen-free confined nitrogen gas and no product left the flask before the end of the reaction. The repetitive experiments were carried on to verify the reproducibility. It was found that the gel times of the reactions had 3%–5% errors, caused by the instrument’s heating rate and stirring intensity.

The nomenclature adopted in this article for the samples is as follows: S, TO, and SFO represent sulfur, tung oil, and sunflower oil. For example, poly (S50-TO-50-SFO-A) corresponds to a copolymer sample prepared from a mixture of tung oil (25w%), sunflower oil (25w%), and sulfur (50w%) under the condition of open air.

Characterization methods

Utilizing an AVANCE III 400 (400 MHz) spectrometer,¹H NMR spectroscopy was carried out on soluble polymer in deuterated chloroform. A TA instrument differential scanning calorimeter (DSC) Model 2910 was used with a heating rate of 10°C/min and a nitrogen flow rate of 60 mL/min. The copolymer samples were also characterized by X-ray diffraction (XRD, Bruker D8-Advance X-ray polycrystalline diffractometer) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi).

This paper also uses the standard method ISO 7269:1995 -- Rubber-Determination of free sulfur to calculate the free sulfur content in samples. In this method, free sulfur in the rubber sample is extracted by sodium sulfite solution to generate sodium thiosulfate, which reacts with the iodine standard solution. The content of free sulfur is calculated by the volume of consumed iodine standard solution.

Results and discussion

Synthesis of sulfur-rich copolymers

S₈ is a powder at room temperature and begins to melt when it is heated to 120°C. When S₈ is completely melted, it forms an immiscible two-phase with tung oil (Figure 1(b)), so homogenization of the reaction mixture is essential to obtain the target reaction product, which can be achieved by heating the reactants to 130°C and vigorous stirring. After a while, an orange emulsion appears (Figure 1(c)), indicating that sulfur begins to generate free radicals and crosslinks with tung oil.⁴⁰ Finally, the increasing viscosity of the liquid implies an increased degree of cross-linking of sulfur with tung oil and an increased molecular weight of the copolymer. When the stir stops (Figure 1(d)), record the gel time, as shown in Table 2.

Figure 1.

Reaction phenomenon of TO reacting with S₈.

Table 2.

Free sulfur contents, gel times of Poly (S50-TO) synthesized under different conditions.

	Gel time, min	Free sulfur content, w%			Relative generation of C-S bonds
	Gel time, min	DSC	Titration	XRD	Relative generation of C-S bonds
Poly (S50-TO-AD)	160	0.07	5.78	2.27	6.96
Poly (S50-TO-A)	70	0.69	8.47	3.13	5.73
Poly (S50-TO-ND)	195	1.95	10.88	6.24	5.23
Poly (S50-TO-NT)	113	2.32	11.08	9.94	4.23
Poly (S50-TO-AT)	93	4.71	12.64	11.60	3.57
Poly (S50-TO-N)	127	5.81	13.75	15.76	1.83

Comparing gel times under six conditions, the shortest reaction time was required when the reaction reagents were open to the atmosphere. That is to say, when they contacted oxygen, moisture, and gaseous substances dissipated, the fastest reaction rate was achieved. When the above system is isolated from the environment, the gel time becomes longer because H₂S generated increases the system pressure and thus decreases the reaction rate. The longest gel time occurred under the condition of dry nitrogen -- no oxygen and moisture. In summary oxygen and moisture shorten the gel time, meaning enhance the rate of reaction. Gel time, named vitrification time in some pieces of literature, is confirmed to be a good measure of the rate of the reaction by an NMR kinetics experiment.³⁷

Analysis of free sulfur in copolymers by DSC, XRD, and titration

The differential scanning calorimetry (DSC) curves of the tung oil-sulfur copolymers synthesized under six conditions are performed in Figure 2(a). In the figures, each curve contains a peak at 118°C which is attributed to the melting of S_β.⁴¹ The peak areas are related to free S_β contents in the copolymers. The free sulfur contents of the samples can be calculated by integrating the melting peaks according to the method provided by M.J.H. Worthington et al.³⁹ The data in Table 2 show that the free sulfur content in poly (S50-TO-N) was the highest and poly (S50-TO-AD) the lowest among the six samples. This finding suggests that the moisture and the inert gas are unfavorable environments for the inverse sulfurization reaction. The fact that poly (S50-TO-AD) has the lowest free sulfur content demonstrates that the dry-air is the best condition for sulfur to participate in the polymerization process.

Figure 2.

(a) DSC curves of Poly (S50-TO), (b) XRD patterns of Poly (S50-TO).

The curves of poly (S50-TO-N) and poly (S50-TO-AT) have a small exothermic peak at 113°C which are the melting peaks of S_α. It is known that the stable STP form of sulfur is orthorhombic α-sulfur but single crystals of α-sulfur do not easily convert to monoclinic sulfur S_β. Instead, they melt at 112°C under ideal conditions.⁴² In our samples, free sulfur mainly exists in the form of S_β. When the copolymers contain a more considerable amount of free sulfur, a small amount of free sulfur is in the state of S_α, indicating that scattered sulfur microcrystals are prone to generate S_β during the cooling process, probably because S_β has smaller melting enthalpy and smaller crystal size.

The XRD patterns of the sulfur-rich copolymers obtained by the six reaction conditions were compared with the S₈ standard pattern⁷ (bottom) in Figure 2(b). The peaks detected at 2θ = 23° for all samples coincide with those of sulfur, proving the presence of free sulfur in the samples. It can also be seen that there is a low and broad band between 2θ = 15–30° in each pattern, which means that there is a non-crystalline amorphous structure in the samples. Both poly (S50-TO-N) and poly (S50-TO-AT) have a sharp peak at 2θ = 16°. Combined with the DSC data, it is presumed that the peak at 2θ = 16° is attributed to the crystalline of S_α. The XRD pattern of poly (S50-TO-A) shows there is a raised and more pronounced broad band between 2θ = 15–30° and no sharp peaks meanwhile, demonstrating that its free sulfur content is low. The data from XRD agree with the finding that most of the sulfur was present in the polymer form when the reaction system was open to the air.^43–45 The crystallinity of the samples was calculated to obtain the free sulfur contents,⁴⁶ which is shown in Table 2. The free sulfur content calculated by this method greatly exceeds the DSC results, but the order of samples remains unchanged.

We also applied the standard method ‘ISO 7265-1995 Rubber - Determination of Free Sulfur’ to measure the contents of free sulfur in copolymers. The results are listed in Table 2, too. The titration method got the largest value among the three methods, followed by the XRD method, and the DSC method the smallest. The data from the three testing methods all exhibited consistent trends, in which the free sulfur contents of poly (S50-TO-AD) are the lowest values. In the titration method, 2 g sample in the state of 1 mm³ particles was mixed with Na₂SO₃ solution. This can assure sample homogeneity and experiment precision, as duplicates in each sample measure within 0.1% of free sulfur. The high results of the titration method compared to DSC are probably given rise from polysulfides in the copolymers, which liberate the free sulfur during the heating with sodium sulfite,⁴⁷ and the thiosulfate thus formed influences the accuracy of the free sulfur determination. Also, this method usually requires 4 h for the reaction of sulfur and sodium sulfite solution. Considering the XRD test again, the polymer must be heat pressed at 140°C to make a sheet before testing, causing some free sulfur was cross-linked with the polymer, lowering the free sulfur content. The DSC method was characterized by taking a sample of about 5 mg and is much faster than the other two methods. In summary, each of them can be used for the comparison of free sulfur contents in the same series of sulfur-rich copolymers even though the three methods have advantages and disadvantages.

It is noted that the order of free sulfur contents is different from that of gel times. Poly (S50-TO-AD) has the lowest sulfur content while poly (S50-TO-N) has the highest. By comparing with conditions ND and AD, we can see that oxygen has a positive effect on decreasing the sulfur content. From conditions N and ND, we can come to the conclusion that moisture is harmful to the reaction degree, causing the increase of free sulfur. The effect of isolation from the environment is the same as that on the gel time, i.e. the polymer obtained by the reaction in the closed system has a higher free sulfur content. The failure to emit H₂S not only slows down the reaction rate but also affects the reaction degree.

¹H NMR analysis of double bonds in copolymers

Figure 3(a) shows the ¹H NMR spectrum of tung oil in which peaks between 5.58-6.41 ppm are the characteristic protons of conjugated trienes and 5.20–5.44 ppm are protons of non-conjugated double bonds. The ¹H NMR spectra of the copolymer samples can be observed in Figure 3(b). All peaks are weak because the materials are all poorly soluble in deuterated chloroform. Zooming on the range of 5.20–6.41 ppm (Figure 3(c)), it can be observed that the peaks assigned to conjugated triens (5.58–6.41 ppm)⁴⁸ and non-conjugated double bonds (5.20–5.44 ppm) disappeared, indicating that not only conjugated trienes but non-conjugated double bonds (unexpectedly) reacted with sulfur at low temperature. The disappeared peak (2.0–2.2 ppm) of the -CH₃ protons near the double bond also confirmed the reaction of the double bond. It is speculated that the conjugated trienes played an essential role in the reaction of non-conjugated double bonds.

Figure 3.

¹H NMR spectra of (a) TO, (b) poly (S50-TO) generated under different conditions, (c) 5.0 ppm–6.7 ppm, (d) 1.2 ppm–1.7 ppm.

As the chemical shift of 1.50–1.60 is given by protons in -CHS groups, its peak area is related to the amount of -CHS group (Figure 3(d)). We calculated the ratio of the peak area of -CHS to the peak area of -CH₃ (0.9–1.0 ppm) in the ¹H NMR spectra and consider it the relative formation of C-S bonds in the copolymers, as shown in Table 1. The trend of decreasing relative C-S production for each reaction condition was the same as the trend of increasing free sulfur content.

Free sulfur in samples is the sulfur that did not participate in the copolymerization and remains original while the relative generation of C-S bonds reflects the amounts of lipid molecules that reacted with sulfur. Thus both of the contents of free sulfur and C-S bonds reflect the reaction degree of vegetable oil and sulfur. The orders of reaction degree and gel time of six conditions are different. The gel time of poly (S50-TO-ND) is the longest one while the reaction degree of poly (S50-TO-N) is the lowest one. These facts and relationships will help analyze the mechanism of sulfur-vegetable-oil copolymerization, which we will discuss later.

Analysis of the valence state of the elements in the copolymer

X-Ray photoelectron spectroscopy (XPS) was employed to understand the chemical composition and chemical bonds of the as-prepared sulfur-rich materials and explore the influence of water and oxygen on the reaction. The dominant peaks of C 1s (∼285 eV), O 1s (∼532 eV), S 2p (∼165 eV), and S 2s (∼229 eV) are observed in the full XPS spectra of Figure 4(a) of poly (S50-TO-AD), Figure 4(e) of poly (S50-TO-A), Figure 4(i) of poly (S50-TO-ND) and Figure 4(m) of poly (S50-TO-N). From the high-resolution C 1s XPS spectra of poly (S50-TO-AD), poly (S50-TO-A), poly (S50-TO-ND), and poly (S50-TO-N) (Figure 4(b), (f), (j), and (n)), four peaks can be identified. The peaks at about 284.8 eV belong to C-C or C = C. According to the ¹H NMR results, almost all the C = C bonds in the samples were reacted, so it can be inferred that the peaks here represent the C-C bond. The peaks around 285.6 and 287.1 eV are attributed to C-S or C-O, and the peaks around 289.1 eV are assigned to the C = O bonds.

Figure 4.

C1s, S2p, and O1s XPS spectra of poly (S50-TO-AD) (a–d), poly (S50-TO-A) (e–h),poly (S50-TO-ND) (i-l) and poly (S50-TO-N) (m-p),respectively.

Figure 4(c), (g), (k) and (o) show the binding energy of sulfur in poly (S50-TO-AD), poly (S50-TO-A), poly (S50-TO-ND), and poly (S50-TO-N), respectively. Due to the spin orbit coupling, the S 2p spectra are made up of S 2p3/2 and S 2p1/2 components having 2:l relative intensity.^49,50 In the S2p spectra, four doublets were observed at 163.8/165.0 eV and 164.2/165.4 eV, which could be attributed to the C–S bond and S–S bond. These two couples of peaks illustrate that most of the sulfur atoms are present as C-S and S-S bonds under four reaction conditions. This matches the calculation results of free sulfur from DSC, XRD, and the relative generation of C-S bonds from ¹H NMR. The peaks at 162.0/163.2 eV in Figure 4(g) are typical of S²⁻, and the component at 168.5/169.7 eV can be associated with sulfur cation species.^51–53 The emergence of sulfur anions and cations in the sulfur-rich polymers obviously enlightens the exploration of reaction mechanisms. The existence of anions would support the anion mechanism and oppose the radical mechanism, which is covered in the next section. The peaks of cations, which also appeared in the XPS spectra in many studies involving sulfur, are often interpreted as sulfates formed by surface oxidation or contamination.^49,53–55 It is possible to imagine that sulfur cations exist in the sample bulk rather than on the surface. They are not necessarily combined with oxygen but exist in the charge-separated form.

Discussion of the inverse vulcanization mechanism of tung oil with sulfur

In recent years, promising sulfur-rich polymers and their applications are constantly emerging moreover the investigation of inverse vulcanization mechanism was attracted much attention but the mechanism still remains unclear. Here we proposed two probable mechanisms of inverse vulcanization between tung oil and sulfur in the absence of catalysts by summarizing several striking publications. First, it could be a radical mechanism. This seems to be a doubtless view because, since the concept of vulcanization has been used in the synthesis of polymers, it has been believed that 159°C is the starting temperature that cycloocta sulfur can homo-cleavage to generate sulfur diradicals. However, the synthesis of a large number of polymers took place below 159°C, and the same as the synthesis of tung oil-sulfur polymers. Nevertheless the fact that synthesis temperatures below 159°C are not enough to deny the free radical mechanism. At 120°C, the just mobile melt already contains many complicated species besides cycloocta sulfur, such as diradical sulfur chains present at appreciable concentrations, very reactive S₃ and S₄ molecules, and rings with an average of 17.6 atoms per ring.^38,56 On the other hand, there is a significant amount of unbroken cycloocta sulfur still present above 159°C.⁴² Also, the critical temperature can be decreased by adding S6 or elevated by adding thermally stable, cyclic sulfur compounds. Thus there would be a big opportunity that the second raw material in the inverse vulcanization would alter the critical temperature and create a significant amount of sulfur diradicals to process the reactions. Two publications from Bartlett et al.⁵⁷ about the inhibition mechanisms for the polymerization of styrene at 80.89°C and vinyl acetate at 45°C⁵⁸ indicated that sulfur was involved in polymerization at temperatures far below the sulfur’s melting point and critical temperature. The free radicals from styrene and vinyl acetate react with cycloocta sulfur to form RS₈· free radicals, resulting in the formation of polysulfides RS_xR. These facts could shed interesting light on the generation of radicals in inverse vulcanization in which the generation of RS₈· free radicals would be dependent on the reactivity of free radicals instead of temperature. The mechanism is shown in Scheme 1.

Scheme 1.

Free radical mechanism of reverse vulcanization reaction.

Second, it could be an anion (nucleophilic) mechanism where the polysulfide anion act as a nucleophile. The anion mechanism has successfully explained the benefits to inverse vulcanization by the addition of a metal diethyldithiocarbamate catalyst (DEDC). The DEDC ligand acting as a nucleophile may assist in the cleavage of S-S bonds and C-C double bonds, creating polysulfide anions which is a potential reactive intermediate.⁵⁹ Polysulfide anions could easily be generated by the reaction of sulfur and Na₂S.^60,61 Dodd et al.³⁷ detailed discussed the role of polysulfide anion in catalytic inverse vulcanization. Here, we add some more details about polysulfides. The reaction of sulfur and Na₂S generates nonasulfide ions $S_{9}^{-}$ , which are rapidly degraded to shorter chains, such as $S_{8}^{-}$ , $S_{4}^{-}$ , and $S_{2}^{-}$ . Krein et al.’s⁶² work showed that polysulfide anions were amenable to phase transfer catalysis which is another benefit to vulcanization because sulfur melts and organic phase are immiscible. Would inverse vulcanization without catalysts not contain polysulfide anions and not be an anion mechanism? We don’t think so. The charged sulfur chain may come from hetero-cleavage of cycloocta sulfur in a special circumstance (Scheme 1).^63,64 Earlier research work suggested the possibility of the formation of linear dipoles in sulfur melts.⁶⁵ The XPS spectrum of poly (S50-TO-A) provided evidence of the anion mechanism that there is a pair of peaks at 162.0/163.2 eV represented S^2- (Figure 4(g)). The XPS spectra also show the peaks which are attributed to sulfur cations. The sulfur ion might be related to the hetero-cleavage of sulfur, where the anion part might react with the double bonds, and the remaining cation could combine with oxygen from inter-molecules or extra-molecules. Lian et al.’s⁶³ work suggested sulfur radicals abstract allylic H atoms to generate the thiol groups, which are then converted to polysulfide anion. This process occurs above 150°C without a catalyst or at 80°C with a catalyst. The above are possible sources of sulfur anions.

After analyzing the two possible mechanisms of inverse vulcanization in detail, let’s consider the effects of oxygen in the reactions and whether these effects could fit the above two mechanisms. Comparing poly (S50-TO-ND) and poly (S50-TO-AD) in Table 2, it can be shown that when the system contains oxygen, the gel time is shortened by about half, the free sulfur content in the sample is reduced, and the C-S bond generated is increased, indicating that oxygen enhanced the reaction rate and reaction degree. When oxygen is mentioned in polymer science, it is known that it is an inhibitor. The inhibition occurs when the reactivity of peroxyl radical, which is formed by oxygen and alkyl radical, is lower than that of the alkyl radical in radical polymerization. For lipid oxidation which also takes place by radical mechanism, however, the oxygenation of L· to peroxyl radical LOO·, is an essential step. The catalysis of metals, light, heat, lipoxygenase, etc. produces L· free radicals. Although only trace amounts of initiators are needed to drive the radical chain reaction initiation, L· participates in oxidation by conversion to LOO·. LOO· is involved in hydrogen abstraction, cyclization, and addition to generate oxidation products such as aldehydes, ketones, epoxides, dimers, and polymers.⁶⁶ The hydrogen abstraction itself and consequent conversion of the double bonds to conjugated increase the addition reactions of LOO· and conjugated groups and produce the characteristic polymers and increased viscosity. In the same way, LOO· excited by oxygen is reasonably assumed important to reactions proceeded as the radical mechanism in vulcanization.

The vulcanization of tung oil can still be carried out without oxygen. This situation is driven by first LOO· from contaminations in storage, and second but more operatively, high reactive L· of tung oil. Non-conjugated lipids like sunflower oil could have LOO· because of contaminations in storage. But these peroxyl radicals do not have enough activity to propagate the radical chain reaction and initiate vulcanization at 130°C as Worthington’s work demonstrated.³⁹ The evidence that tung oil contains high reactive radicals is below. For coating applications, tung oils often cure (dry) so rapidly that a highly wrinkled surface forms due to their high reactivity. L· and LOO· in tung oil attack the double bonds directly to generate polymers. In comparison with non-conjugated oil, a lower amount of oxygen is adsorbed, and much fewer hydroperoxides LOOH are formed.⁶⁷ That said, the smaller amount of oxygen required when tung oil cures proves the high activity of L·. High reactivity L· in tung oil and LOO· excited by oxygen explain the super rate enhancement under condition A. The explanation would offer clear evidence of the radical mechanism of inverse vulcanization.

Now we contemplate the effect of moisture on the tung oil-sulfur reactions. From the data of poly (S50-TO-N) and poly (S50-TO-ND) we found that moisture helps enhance faster rate but also increases the free sulfur contents, suggesting it is adverse to reaction degree. The properties of moisture upon the tung oil-sulfur reactions would recommend the anion mechanism. The anion mechanism (nucleophilic substitution) involves the generation and separation of charged species in the transition state. For this reason, the rate depends on both the ability of the solvent to keep the charges separated and its ability to stabilize charged sites by solvation. The solvents that best solvate charges are polar protic solvents such as water. So the rates of nucleophilic substitutions are dramatically accelerated, often by several orders of magnitude compared to the same reaction in protic solvents. This explains the rate enhancement of moisture in our reactions. As for the generation pathway of anion, it’s most likely free radicals to extract allylic H atoms in lipid molecules to produce thiol groups which are then transformed into anions.

The following analyzes how the two mechanisms have different effects on free sulfur content. Think about the sulfur radicals which have the nature of linking together to become longer chains or bigger rings of sulfur. This trend with the elevating temperature makes sulfur melts viscous. Although we know that even in the most viscous sulfur melts it is not all sulfur chains, there is no doubt that sulfur radical has a tendency to form longer chains. Conversely, polysulfide anions are prone to break into fragments with a shorter length of sulfurs -- $S_{9}^{-}$ , $S_{8}^{-}$ , $S_{4}^{-}$ , $S_{2}^{-}$ . Although all of these polysulfide anions with different lengths are reactive and can undergo nucleophile substitution with double bonds, sulfur atoms in the products RSn will be less than the sulfur number added by radicals to the conjugated double bonds. Therefore, the sulfur chain produced by the free radical reaction would be longer, and the sulfur chain produced by the anion mechanism would be shorter. Under the condition that the double bond contents keep constant, the sulfur contents of polymers produced by the anion mechanism are lower (i.e. free sulfur content is high) whereas the sulfur contents of polymers from the radical mechanism are higher (less free sulfur). As such, the phenomenon that the free sulfur content of the product was larger when moisture was present would infer the anion mechanism of the inverse vulcanization.

When we observed the gel times and free sulfur content under the condition AT and NT, we found interesting effects of oxygen and moisture. When a vulcanization reaction was performed in the presence of both oxygen and moisture at the same time, the gel time was shorter but free sulfur content was more than that both of them were absent. Based on the above discussion, the effect of oxygen on gel time is consistent with that of moisture but is opposite to free sulfur content. So moisture has the leading role in the reaction under the condition of both oxygen and moisture. It can be inferred from this relationship that the anion mechanism would be a priority approach once the stimulated factors exist, like the presence of metal ions and sulfide ions.

Verification of the inverse vulcanization mechanisms

According to the aforementioned mechanism, it is known that tung oil is not only a co-monomer in the reaction but also plays an initiating role in promoting the dissociation of sulfur chains to generate free radicals for addition with non-conjugated double bonds. Others in our research group demonstrated that sunflower oil does not polymerize with S₈ at 130°C.²⁹ To confirm the initiating role of conjugated trienes in tung oil, a mixed vegetable oil with 50% tung oil (1 g) and 50% sunflower oil (1 g) was used to react with sulfur (2 g). Experiments showed that mixed vegetable oil reacted with sulfur at 130°C, and produced poly (S50-TO-50-SFO) which was lighter in color and softer in texture. The gel times under each reaction condition are shown in Table 3. The DSC curves, XRD patterns, and ¹H NMR spectra are listed as Figure S1, Figure S2, and Figure S3 in Support Information. From the gel times and free sulfur contents, it can be seen that the influence of oxygen and moisture on the vulcanization process is consistent with that of pure tung oil, proving that the mechanisms summarized in the previous section are also applicable to systems with less content of conjugated double bond.

Table 3.

Free sulfur content, gel time of poly (S50-TO-50-SFO) generated under different conditions.

	Gel time, min	Free sulfur content, w%
	Gel time, min	DSC	Titration	XRD
poly (S50-TO-50-SFO-AD)	260	3.85	14.11	9.81
poly (S50-TO-50-SFO-A)	165	4.99	15.86	11.25
poly (S50-TO-50-SFO-ND)	270	6.67	18.72	16.24
poly (S50-TO-50-SFO-NT)	255	8.91	20.72	19.29
poly (S50-TO-50-SFO-AT)	180	9.30	22.69	21.13
poly (S50-TO-50-SFO-N)	200	10.32	24.85	22.93

Conclusion

In this work, we synthesized tung oil-sulfur polymers under six conditions, determined the free sulfur contents and gel times through a series of characterizations, and analyzed the valence state of the elements in them. The above data are used to understand the influences of oxygen and moisture and therefore to exploit the mechanism of inverse vulcanization. Based on a large number of literature, two mechanisms are summed up: the free radical mechanism and the anion mechanism. The effects of oxygen on increasing reaction rates and lowering the free sulfur contents indicate the synthesis via free radical mechanism and the ease of creating radicals by tung oil (the second monomer) is the critical factor in the radical way. The roles of moisture in shortening gel times and adding the free sulfur contents infer that the anion mechanism would occur even in the absence of a catalyst. Two mechanisms would exist simultaneously but the anion mechanism may be a priority approach once the anions were excited by a given condition.

Supporting information

Estimation of free sulfur in sulfur-rich copolymers; DSC analysis of Poly (S50-TO-50-SFO) generated under different conditions; XRD patterns of Poly (S50-TO-50-SFO) generated under different conditions; ¹H NMR spectra of poly (S50-TO-50-SFO) generated under different conditions.

Footnotes

Acknowledgements

The authors gratefully acknowledge the support of CRODA China Trading Co.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ya Lyu

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The exploration of the inverse vulcanization mechanism of tung oil by controlling the oxygen and moisture presence during reactions

Abstract

Keywords

Introduction

Experimental

Materials

General procedure for the mechanism exploration: preparation of sulfur-rich copolymers based on vegetable oils

Characterization methods

Results and discussion

Synthesis of sulfur-rich copolymers

Analysis of free sulfur in copolymers by DSC, XRD, and titration

1H NMR analysis of double bonds in copolymers

Analysis of the valence state of the elements in the copolymer

Discussion of the inverse vulcanization mechanism of tung oil with sulfur

Verification of the inverse vulcanization mechanisms

Conclusion

Supporting information

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References

¹H NMR analysis of double bonds in copolymers