Stearate Complex in Sulfur Cross

Article pubs.acs.org/Macromolecules

Dinuclear Bridging Bidentate Zinc/Stearate Complex in Sulfur CrossLinking of Rubber Yuko Ikeda,*,† Yoritaka Yasuda,† Takumi Ohashi,† Hiroyuki Yokohama,† Shinya Minoda,† Hisayoshi Kobayashi,† and Tetsuo Honma‡ †

Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

‡

S Supporting Information *

ABSTRACT: An essential structure of the intermediate generated from zinc oxide and stearic acid during sulfur cross-linking reaction of isoprene rubber is proposed using time-resolved zinc K-edge X-ray absorption fine structure and infrared spectroscopies in situ. The structure is dominantly a bridging bidentate zinc/stearate complex, the molar ratio of the zinc ion to stearate and the coordination number of which are unexpectedly 1:1 and 4, respectively. Combination with a density functional calculation for identifying the intermediate predominantly suggests that its most possible structure is (Zn2(μ-O2CC17H35)2)2+(OH−)2·XY, where X and Y are water and/or a rubber segment. This intermediate has been unknown despite the long history of rubber science and technology. The newly observed zinc/stearate complex may play a role to accelerate the sulfur cross-linking reaction of rubber like an enzyme because of the high Lewis activity of the zinc ion.

1. INTRODUCTION The sulfur cross-linking reaction of rubber, i.e., vulcanization, is one of the most important reactive processes in polymer technology. Vulcanization is a traditional, but very sophisticated, chemical process by which to produce high performance rubbery materials, enhancing the safety and comfort of our society. Vulcanization was discovered by Goodyear in 1839 and has been advanced by ceaseless innovation of reaction modifiers, including accelerators, activators, and retarders, to improve processability and mechanical properties of rubber products.1−9 As a result, the sulfur cross-linking reaction seems to have practically become a matured technology, but further progress is still required. Contemporary tire engineering based on sulfur cross-linking, for example, has to be developed further to improve the performance of automobiles, reducing energy consumption and carbon dioxide emission to the atmosphere for creating a carbon neutral world.10 One of the methods to improve the performance may be a control of network structures in the sulfur cross-linking reactions of rubbers. Until now, many studies on the vulcanization mechanisms have been conducted from not only an academic interest but also industrial aspects.1−9 However, the details for the reactions have not yet been conclusively clarified because of the complicated chemical reactions between rubber and crosslinking reagents at each processing step. An important key to control the network formation by sulfur cross-linking is still sought for the development of the rubber industry. Nowadays, the progress of new analysis techniques based on the development of computer may also open a new window in the vulcanization. Baerends et al., for example, theoretically and © XXXX American Chemical Society

experimentally revealed a reaction mechanism of tetramethylthiuram disulfide with zinc oxide (ZnO) to generate an intermediate in the vulcanization.11,12 Recently, one of the authors of this paper was inspired to clarify the vulcanization mechanism by small-angle neutron scattering techniques and reported the effects of the combination and composition of the sulfur cross-linking reagents on rubber network formations.13 The combination and composition of ZnO with other reagents are crucial to control the structural network inhomogeneity in the N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine (CBS) accelerated vulcanization of isoprene rubber. The mesh size in a two-phase inhomogeneous structure of sulfur cross-linked isoprene rubber was unexpectedly found to be controlled by the amounts of ZnO and stearic acid (StH) used. A time-resolved zinc K-edge X-ray absorption fine structure (Zn K-edge XAFS) spectroscopy also supported the two-phase network formation.14 This observation was very important for rubber science and technology, but it has remained unclear why the combination of ZnO and StH can be a key for controlling the CBS accelerated vulcanization. Generally, it is thought that StH can be reacted with ZnO to form zinc stearate (ZnSt2) as an essential cure activator. Zinc 1,3-benzothiazole-2-thiolate with ligands has long been postulated to be generated from ZnO, StH, and CBS. It has been accepted as an active intermediate, where stearate was thought to be one of the ligands.15,16 However, the in situ details of the generated zinc Received: October 7, 2014 Revised: December 23, 2014

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DOI: 10.1021/ma502063m Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Recipe for Preparation of the Samples sample code IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5) IR-ZnO(0.5)-StH(2) IR-ZnO(1)-StH(2) IR-ZnSt2(4.5) IR-ZnSt2(2.25) IR-StH(4.5) IR-ZnO(1) IR-ZnS(3) BNf-ZnO Bn-ZnSt2 BN-ZnS

units e

phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g phr mol/g mg mol/g mg mol/g mg mol/g

isoprene rubbera

ZnO

StHb

100

0.5 5.9 × 10−5 0.5 6.0 × 10−5 1.0 11.9 × 10−5

2.0 6.7 × 10−5 2.0 6.9 × 10−5 2.0 6.8 × 10−5

100 100 100

CBSd

sulfur (S8)

1.0 3.6 × 10−5

1.5 5.6 × 10−5

ZnS

4.5 6.8 × 10−5 2.25 3.5 × 10−5

100 100 100

ZnSt2c

4.5 15.1 × 10−5 1.0 12.2 × 10−5

100

3.0 29.9 × 10−5 4.3 5.8 × 10−4 33.4 4.4 × 10−4 5.2 5.8 × 10−4

a

JSR IR2200. bStearic acid. cZinc distearate. dN-(1,3-Benzothiazol-2-ylsufanyl)cyclohexanamine. eParts per one hundred rubber by weight. fBoron nitrate. received. Each reagent was mixed with isoprene rubber on a two-roll mill. The recipes are also shown in Table 1. Abbreviations of all samples are displayed by combination of the reagents, where each reagent is abbreviated as follows: zinc oxide, stearic acid, zinc sulfide, zinc stearate, sulfur, and N-(1,3-benzothiazol2-ylsulfanyl)cyclohexanamine are “ZnO”, “StH”, “ZnS”, “ZnSt2”, “S8”, and “CBS”, respectively. A number in parentheses in the sample code shows the amount of each reagent mixed with isoprene rubber in “phr”. The last number after the hyphen in the code shows an elapsed time in minute of heating from 35 °C. For comparisons with zinc oxide and zinc sulfide in boron nitrate (BN), disk-shaped samples of BN-ZnO and BN-ZnS were prepared by pressing the powders, where a commercial BN (BBI03PB, purity >99%, Rare Metallic Co., Ltd.) was used as received. Their concentrations were 5.8 × 10−4 mol/g. BN-ZnSt2 was also prepared using a rubber reagent grade ZnSt2 (Sakai Chemical Industry Co., Ltd.). The concentration was 4.4 × 10−4 mol/g. 2.2. Zinc K-Edge XAFS Spectroscopy. In situ synchrotron XAFS measurements were carried out every 111 s at BL-14B2 beamline of SPring-8 in Harima, Japan.17 The rubber compound was sealed in a Teflon tube with Kapton windows and followed by heating in a handmade reaction cell according to a controlled program, which was set to heat at 35 °C for about 8 min, to heat from 35 to 144 °C for 10 min and to heat at 144 °C for a definite time. The temperature control was less than plus or minus 1°. A sample was a cylinder shape, whose diameter was 10 mm and length was adequately determined on the basis of molar concentrations of each element in the sample. An irradiation time of X-ray was 52 s. Si(311) crystal was used as the monochromator. The X-ray absorption spectra for the samples were recorded in a transmission mode using ionization chambers. The XAFS spectra were analyzed by ATHENA and ARTEMIS XAFS analysis package18 and FEFF6.02L.19,20 For quantitative analyses of X-ray absorption near-edge structure (XANES) spectra to evaluate a progress of reaction, a linear combination fitting method was conducted using the reference samples in an energy range from 9619 to 9739 eV using a routine linearcombo method in ATHENA. The results were plotted in a unit of “mol/g” to show an amount of substance for zinc atom. Note that the plotted values are not a concentration of each zinc compound in rubber. The R-factor is

salt of StH for sulfur cross-linking during the vulcanization have not been well understood. Most studies have focused on materials isolated from the vulcanization reactions. What is the role of the zinc salt of StH in the vulcanization reaction? On the way to determine this, the formation of a specific structural complex generated from ZnO and StH at a high temperature was found by a combination of time-resolved Zn K-edge XAFS spectroscopy and time-resolved infrared spectroscopy. A density functional theory (DFT) calculation elucidated characteristics of the complex that must be related to its role. Here, we report the first direct experimental and theoretical evidence for the unique zinc/stearate complex in the sulfur cross-linking of rubber. Our observation may be of use for producing important and necessary rubber materials in our society.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Isoprene rubber (IR2200) was supplied from JSR Co. Elemental sulfur (powder, 150 mesh), stearic acid (LUNAC S-25), zinc oxide (average diameter 0.29 μm), and N-(1,3benzothiazol-2-ylsulfanyl)cyclohexanamine (Sanceler CM-G) were commercial grades for rubber processing and used as received. They were supplied from Hosoi Chemical Industry Co., Ltd., Kao Co., Sakai Chemical Industry Co., Ltd., and Sanshin Chemical Industry Co., Ltd., respectively. According to the recipes shown in Table 1, the rubber compounds were prepared by a conventional mixing at rt for about 20 min on a two-roll mill with a water cooling system for each measurement. A sequence of mixing was in an order of ZnO, StH, CBS, and sulfur for the curing system. This order was also applied to the preparations of reference samples. Amounts of the reagents are shown in units of “mol/g” and “parts per one hundred rubbers by weight (phr)”. The latter is a conventional unit in rubber science and technology. For preparations of precise reference samples, zinc sulfide (ZN-3038-0190, purity >99.99%, Rare Metallic Co., Ltd.) and zinc stearate (purity >99.5%, Wako Pure Chemical Industries, Ltd.) were used as B


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Figure 1. Results of linear combination fitting for in situ experimental Zn K-edge XANES spectra of IR-ZnO(0.5)-StH(2) and IR-ZnO(1)-StH(2). (a) IR-ZnO(1)-StH(2)-0 at 35 °C and (b) IR-ZnO(1)-StH(2)-18.5 at 144 °C, (c) IR-ZnO(0.5)-StH(2)-0 at 35 °C, and (d) IR-ZnO(0.5)-StH(2)18.5 at 144 °C. The fractions of each reference component are shown in each color. examine the effects of polarization functions, the 6-31G(d, p) basis set was used for the carboxylate moiety (−COO−).

defined as a sum-of-squares measure of the fractional misfit as described in IFEFFIT18 to show a degree of best fitting. The smaller the R-factor is, the better the fitting accuracy is. In order to investigate extended X-ray absorption fine structure (EXAFS) spectra in detail, a parameter fitting for the EXAFS data was conducted on the basis of theoretically obtained spectra using FEFF6.02L. 2.3. Fourier-Transform Infrared Spectroscopy. In situ measurements of Fourier-transform infrared spectroscopy were carried out at 32 scans for 40 s every about 44 s in a wavenumber range from 4000 to 400 cm−1 by a single reflection ATR method on a diamond plate using GradiATR (Pike Co.) in Shimadzu Infrared Prestige-21. The resolution was 4 cm−1. Temperature was set according to a controlled program at 35 °C for about 5 min, from 35 to 144 °C for 10 min and at 144 °C for a definite time. After the heating, the sample was cooled to 35 °C and subjected to the measurement. The temperature control was less than plus or minus 1°. Sample size was about 2 × 3 mm, and its thickness was a few millimeters. Subtracted infrared spectra were obtained by subtracting a spectrum of isoprene rubber from each original spectrum. Baselines for evaluation of the carboxyl shift and methylene stretching bands were a range of 1800−1300 and 3300− 2600 cm−1, respectively. 2.4. Theoretical Calculations. The DFT calculations were carried out to confirm structures of the speculated intermediates by comparing wavenumbers between experimental and theoretical infrared spectra. The Becke three-parameter Lee−Yang−Parr hybridtype functional,21,22 and the 6-31G basis sets23−26 were used, which were implemented in the Gaussian 09 program.27 For Zn atoms, two sets of calculations were carried out with different basis: the 6-31G basis and the Los Alamos ECP28 with the double-ζ valence basis.29 To

3. RESULTS AND DISCUSSION 3.1. Generation of a Novel Zinc/Stearate Complex. Synchrotron XAFS spectroscopy is one of the most useful techniques allowing in situ study of the local structure around selected elements in organic and inorganic materials at the atomic and molecular scale.30 Normalized and backgroundsubtracted Zn K-edge XANES, EXAFS, and the Fouriertransformed EXAFS (FT-EXAFS) spectra of IR-ZnO(1)StH(2), IR-ZnO(0.5)-StH(2), and reference samples are shown with their identifications in Figure S1. To estimate the reactivity between ZnO and StH, a linear combination fitting was conducted for the XANES spectra of IR-ZnO(1)-StH(2) using IR-ZnSt2(4.5) and IR-ZnO(1) reference samples under the temperature rising from 35 to 144 °C and followed by using IR-ZnSt2(4.5)-18.5 and IR-ZnO(1)-18.5 reference samples at 144 °C (note that the reaction temperature had already reached 144 °C by 18.5 min after the start). The results fitted sufficiently well to decompose the spectra to each component. For example, two results of the fitting procedures at 35 and 144 °C are illustrated in Figures 1a and 1b, respectively, where the fractions of each component in IR-ZnO(1)-StH(2) are also shown. At 35 °C, ZnSt2 was present, which suggests that the reaction between ZnO and StH occurred by mixing with C


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Macromolecules

Figure 2. In situ infrared spectra of (a, b, c) IR-ZnO(1)-StH(2), (d, e, f) IR-ZnO(0.5)-StH(2), (g, h, i) IR-ZnSt2(2.25), and (j, k, l) IR-ZnSt2(4.5) in the range from 1800 to 1380 cm−1 for identification of the carboxyl shift. Gray arrows in (a), (d), (g), and (j) show the time progress during the heating process in (a), (d), (g), and (j). The spectra at 35 °C (red), 144 °C (blue), and 35 °C after cooling (green) of each sample are shown in (b), (e), (h), and (k), respectively. The expanded spectra from 1800 to 1680 cm−1 at 35 °C (red) and 144 °C (blue) are also shown in (c), (f), (i), and (l), respectively. Black arrows in (f), (i), and (l) show the peaks of stearic acid generated from the isolated stearate group. The detailed identification of each spectral band is summarized in the Supporting Information.

Figure 3. Variation of zinc atoms measured by in situ XAFS for (a) IR-ZnO(1)-StH(2) and (b) IR-ZnO(0.5)-StH(2) under heating. Black lines show the temperature variation. For the linear combination fittings of IR-ZnO(1)-StH(2) and IR-ZnO(0.5)-StH(2), XANES spectra of IR-ZnSt2(4.5) (red) and IR-ZnO(1) (blue) under the temperature rising from 35 to 144 °C and those of IR-ZnSt2(4.5)-18.5 and IR-ZnO(1)-18.5 reference samples at 144 °C were utilized as reference spectra, respectively. The R-factor is shown in orange. Red and green dotted lines show the calculated concentrations of zinc atoms as (ZnSt)nn+ and ZnSt2, respectively.

singlet at 1537 cm−1 gave a split to one broad peak at 1595 cm−1 and one shoulder peak around 1560 cm−1 similarly with IR-ZnSt2(2.25) and IR-ZnSt2(4.5) shown in Figure 2. All fitting results of XANES spectra of IR-ZnO(1)-StH(2) are summarized in Figure 3a, where an amount of substance for the zinc atom in the rubber matrix is plotted against time by using “mol/g” as units. The values were calculated using the

isoprene rubber on the two-roll mill. The generation of zinc stearate was supported by infrared spectroscopy. Unmistakable COO− antisymmetrical and symmetrical stretching bands of ZnSt2 were detected at 1537 and 1398 cm−1 in the spectra of IR-ZnO(1)-StH(2) at 35 °C, respectively (Figure 2a). The results show the formation of bridging bidentate structure at 35 °C.31,32 By heating, the peak at 1398 cm−1 disappeared and the D


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Macromolecules fitting results and the reagent proportions.33 In Figure 3a, a green dotted line shows the calculated concentrations of the zinc atom as ZnSt2. Unexpectedly, experimental data regarding the concentrations of zinc atoms as ZnSt2 was over the value at the green dotted line at 35 °C and moved toward the red dotted line under the temperature rising. It is notable that the red dotted line shows the calculated concentrations of zinc atoms as (ZnSt)nn+. Here, the number (n) was not clear at this stage. This observation is somewhat surprising because it means that the concentration of zinc atoms was 1.9−2.0 times larger at 144 °C than the concentration calculated based on the amounts of ZnO and StH in IR-ZnO(1)-StH(2). Namely, the molar ratio of the zinc ion to stearate was found not to be 1:2, but about 1:1 at 144 °C. Since the 1:1 association has a poor electric balance, the association may be metastable and generated temporarily, or other anions may coordinate with the zinc atom. This result was supported by our recent study14 on the twophase network formation via two different reactions in CBSaccelerated curing system with ZnO and stearic acid, where synchrotron in situ zinc K-edge XAFS measurements were conducted at about 140 °C: The in situ XANES spectra of IRZnO(1)-StH(2)-CBS(1)-S8(1.5) were reasonably decomposed using those of IR-ZnSt2(3)-CBS(1)-S8(1.5) and IR-ZnO(1)CBS(1)-S8(1.5). Unexpectedly, the fitting results were striking when the heat-pressing temperature was raised to about 125 °C. A fraction of the mesh network formation in IR-ZnO(1)StH(2)-CBS(1)-S8(1.5) became about 1.9 times larger than the calculated one on the basis of amount of zinc stearate generated from ZnO and StH assuming their quantitative reaction as shown in Figure S2.14 This observation can be explained when a concentration of the intermediate to generate the mesh network was approximately twice of that of zinc stearate. Namely, a generation of the 1:1 complex of zinc and stearate reasonably revealed the unexpected phenomenon for the initial stage of sulfur cross-linking reaction of IR-ZnO(1)-StH(2)CBS(1)-S8(1.5) at about 140 °C. To reveal more clearly the unexpectedly observed species of the zinc/stearate complex in isoprene rubber, IR-ZnO(0.5)StH(2) was also subjected to the in situ infrared and XAFS analyses. The amount of ZnO in the IR-ZnO(0.5)-StH(2) compound was decreased to the half to minimize the effect of ZnO in the analyses, where the concentration of ZnO was a little less than that of StH. As shown in Figure 2d, the formation of a bridging bidentate structure at 35 °C was confirmed by the infrared spectrum similar to that of IRZnO(1)-StH(2) at 35 °C. A combination of XANES spectra of IR-ZnSt2(4.5) and IR-ZnO(1) under the temperature rising from 35 to 144 °C and followed by using IR-ZnSt2(4.5)-18.5 and IR-ZnO(1)-18.5 reference samples at 144 °C also gave good fittings to decompose the XANES spectra of IRZnO(0.5)-StH(2) to each component. Two fitting results at 35 and 144 °C, for example, are illustrated in Figures 1c and 1d, respectively. Variations of zinc concentrations similar to those in IR-ZnO(1)-StH(2) were more clearly observed in IRZnO(0.5)-StH(2) as shown in Figure 3b. Most ZnO was found to be reacted with StH at 144 °C. The concentrations of zinc atoms at 144 °C were 1.6−1.7 times larger than that calculated as ZnSt2.33 This result corresponds to a 94%−95% conversion of ZnO into a novel zinc/stearate complex composed of one molar zinc atoms and 1.0−1.1 molar stearates in a ratio similar to that of IR-ZnO(1)-StH(2) at 144 °C.

It is noted that the isolated stearate group from ZnSt2 in IRZnO(0.5)-StH(2) was slightly detected as StH at 1757 cm−1 by infrared spectroscopy at 144 °C as shown in Figure 2f. In IRZnO(0.5)-StH(2), the concentration of ZnO was a little less than that of StH, which resulted in the small peak of carboxylic CO antisymmetrical stretching of StH at 144 °C. Since the amount of ZnO was large excess than that of StH in IRZnO(1)-StH(2), on the other hand, all stearate groups coordinated with zinc and consequently no band of StH was observed even at 144 °C as shown in Figure 2c. This novel zinc/stearate complex in the compounds were not so unstable, but its concentration was detected to slightly and gradually decrease with time at 144 °C by in situ infrared spectroscopy. Bridged bidendate stearate groups in the complex seem to enhance its stability at a high temperature. From a viewpoint of the conventional chemical wisdom, it may be difficult to accept the 1:1 complex; however, a few academic papers have shown the unexpected structures of zinc/ carboxylate complexes. For examples, Berkesi et al.34 reported that Zn4O(RCOO)6 complexes (R: C6−C18) were generated by reaction between zinc oxide and stearic acid in boiling noctane, where a presence of water was described to be the key factor to form the complexes. Recently, Nordlander et al.35 reported unsymmetrical dizinc complexes as models for the active sites of phosphohydrolases, where the tetrazinc complex was shown to dissociate into dicarboxylate dizinc complex at high pH and to transform into dihydroxido complex by cooperative deprotonation of two water molecules. In nature due to the high Lewis acidity, flexible coordination sphere, and lack of associated redox chemistry of zinc, therefore, the 1:1 complex formation of zinc and stearate may be possible in our study. When IR-ZnO(1)-StH(2) and IR-ZnO(0.5)-StH(2) were cooled down from 144 to 35 °C, the infrared spectrum of the former to show the carboxyl shift was marginally changed, but that of the latter became a complicated one as shown in Figures 2b and 2e, respectively. The latter spectrum seems to be composed of both spectra at 144 °C and rt with an enhanced peak at about 1560 cm−1. Since IR-ZnSt2(2.25) and IRZnSt2(4.5) showed the reversible changes of the spectra between 144 and 35 °C after the cooling as shown in Figures 2h and 2k, respectively, a stability of the newly observed zinc/ stearate complex seems to depend on the concentration of stearate against zinc atom. For identification of the new complex, however, its isolation and purification were difficult due to the coexistences of zinc oxide, zinc stearate, and/or the complex of IR-ZnO(1)-StH(2) at 35 °C after the cooling. Since this complex may be a key material to accelerate the sulfur cross-linking reaction of isoprene rubber at the processing temperature of 144 °C, the molecular structure of the newly observed zinc/stearate complex was further characterized in the next sections. 3.2. Coordination Number of the Zinc Atom of the Novel Zinc/Stearate Complex. The coordination number of the zinc atoms in the zinc/stearate complexes was estimated by comparing their white lines with those of BN-ZnO, BN-ZnS, IR-ZnO(1), and IR-ZnS(3). Here, the XANES spectra of zinc/ stearate complexes were obtained by normalizing and subtracting the spectra of unreacted ZnO from those of IRZnO(1)-StH(2)-18.5 and IR-ZnO(0.5)-StH(2)-18.5 based on the fitting results in Figure 1, respectively. Abbreviations for these complexes are IR-BB-ZnO(1)-StH(2)-18.5 and IR-BBZnO(0.5)-StH(2)-18.5, respectively. White line intensity for E


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Macromolecules

COO− antisymmetrical and symmetrical stretching bands at 35 °C suggests a formation of bridging bidentate structure in the both samples at 35 °C. It is because the difference of wavenumber between the COO− stretching bands is reported as one of the criterions for the coordination structure.31,32 By heating, the peak at 1398 cm−1 disappeared and the singlet at 1537 cm−1 gave a split to one broad peak at 1595 cm−1 and one shoulder peak around 1560 cm−1 similarly with IR-ZnSt2(2.25) and IR-ZnSt2(4.5).31,32 A peak at about 1625 cm−1 identified with a nonbridged monodentate structure37 was not detected in this study. Because the coordination structures of powdered ZnSt2 at rt, 131 °C, and 170 °C were reported to be the same,32,38 a bridging bidentate structure is assigned to be generated in IR-ZnO(1)-StH(2) and IR-ZnO(0.5)-StH(2) at 144 °C. However, the presence of a bridging bidentate structure cannot be singly and correctly discussed by using only the infrared spectroscopy at 144 °C because the difference between the antisymmetrical and symmetrical COO− stretching frequencies has been noticed to have low general validity and to be influenced by hydrogen bonding and electronic effects.39 Therefore, a combination of different analytical methods has been recommended for determining the mode,38,40 even though a number of studies have only used this criterion for the assignment of the carboxylate binding. Consequently, a DFT calculation was conducted in this study, results of which are reported later. 3.4. Ligands of the Novel Zinc/Stearate Complex. What kind of molecule coordinates with the zinc atom in the zinc/stearate complexes? The most likely candidate is water

Zn(II) is known to be related to its coordination number.36 Therefore, identical levels of all the intensities shown in Figure 4 suggest that the coordination number of the zinc/stearate complexes at 144 °C is 4.

Figure 4. XANES spectra of IR-BB-ZnO(0.5)-StH(2)-18.5, IR-BBZnO(1)-StH(2)-18.5, BN-ZnSt2, BN-ZnO, IR-ZnO(1)-0, IR-ZnO(1)18.5, BN-ZnS, IR-ZnS(3)-0, and IR-ZnS(3)-18.5 for comparison of their white lines.

3.3. Carboxylate Binding Mode with the Zinc Atom of the Novel Zinc/Stearate Complex. Time-resolved infrared spectra and the identification of IR-ZnO(1)-StH(2) and IRZnO(0.5)-StH(2) are shown in Figure 2 and Figure S3 where those of reference samples are also displayed. As shown in the previous section, the difference of 139 cm−1 between the

Figure 5. Experimental Fourier-transformed EXAFS spectra (red solid lines) of (a) IR-BB-ZnO(1)-StH(2)-18.5 and (c) IR-BB-ZnO(0.5)-StH(2)18.5. The Fourier transforms are k3-weighted and Δk = 3−10 Å−1. The real part of inverse Fourier-transformed EXAFS spectra (red solid lines) of (b) IR-BB-ZnO(1)-StH(2)-18.5 and (d) IR-BB-ZnO(0.5)-StH(2)-18.5. Fit results of the first shell with a Zn−O contribution are shown in black dotted lines. F


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Macromolecules

Figure 6. Comparison of experimental XAFS spectra. (a) XANES and (b) k3-weighted EXAFS spectra of IR-BB-ZnO(0.5)-StH(2)-18.5, IR-BBZnO(1)-StH(2)-18.5, IR-ZnSt2(4.5)-18.5, and IR-ZnSt2(2.25)-18.5.

Figure 7. Infrared spectra of the wagging, the stretching and the rocking band in progression regions (a and c) and symmetric and antisymmetric methylene stretching bands (b and d) of IR-ZnO(1)-StH(2) and IR-ZnO(0.5)-StH(2). The spectra were obtained by subtracting infrared spectra of isoprene rubber from raw data at same time. For (b) and (d), a baseline from 3300 to 2600 cm−1 was used.

singlet at 1587 cm−1 by hydration. In addition, the intensities of the bands were reported to vary with water content, even though the Zn−O distance and the coordination number were retained. The coordination number of Zn2+ is known to be 4 when ligands are water and strongly bound anions,43 with which the coordination number of the zinc/stearate complexes in this study is in good agreement. The hydration of the zinc/stearate complexes is also supported by FT-EXAFS analyses. Average Zn−O linkage distances in the zinc/stearate complexes were 1.98−1.99 Å at 144 °C by fitting using a FEFF6.02L technique based on the tetrahedral coordination of oxygen atoms around a zinc atom.

because water was produced by the reaction between ZnO and StH (ZnO + 2StH → ZnSt2 + H2O). Moreover, all reagents and isoprene rubber were used as received without any drying according to conventional rubber processing. Indeed, the presence of zinc-bonded water was supported by infrared spectroscopy. A peak at 1595 cm−1 appeared clearly and strongly by heating in both IR-ZnO(1)-StH(2) and IRZnO(0.5)-StH(2) as shown in Figures 2a and 2d, respectively. This phenomenon was quite similar to those seen in previous studies of ionomers of the zinc salt of poly(ethylene-comethacrylic acid),41,42 where infrared spectral peaks of carboxyl groups at 1625, 1560, and 1539 cm−1 were collapsed into a G


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Macromolecules The fitting results for the FT-EXAFS spectra of IR-BB-ZnO(1)StH(2)-18.5 and IR-BB-ZnO(0.5)-StH(2)-18.5 and their inverse FT spectra are shown in Figure 5. Both fitting results are in good agreement with the measured data. However, the distances obtained were a bit longer than that of ZnSt2 in boron nitrate (1.95 Å at rt) and that reported for ZnSt2 powder (1.95 Å at rt and 170 °C).32,38 The difference may be attributable to the coordinated water with zinc because a Zn−OH2 distance was reported to be between 2.0 and 2.1 Å in the first coordination sphere 44 and 1.987 Å for zinc acetate dihydrates.38,45 It is also noted that IR-ZnSt2(2.25) and IR-ZnSt2(4.5) showed similar infrared spectral changes to those of IR-ZnOStH by heating as shown in Figures 2g and 2j, respectively as described above, which suggests zinc/stearate complexes in IRZnSt2 are also hydrated at 144 °C. In these cases, the isolated stearate groups by the generation of novel zinc/stearate complexes were apparently detected as StH at 1759−1760 cm−1 (carboxylic CO antisymmetrical stretching) and at 1715 cm−1 (carboxylic CO antisymmetrical stretching in dimer of StH) as shown in Figures 2i and 2l. The results clearly support the generation of the novel zinc/stearate complex at 144 °C. Similar XAFS spectra of IR-ZnSt2(2.25) and IRZnSt 2 (4.5) with those of IR-ZnO-StH supported this consideration as shown in Figures 6a and 6b. Zn−O distances of IR-ZnSt2(2.25) and IR-ZnSt2(4.5) were 1.98−2.00 Å at 144 °C and a little longer than that of ZnSt2 in boron nitrate. To our knowledge, this is also the first report. 3.5. Number of Zinc Atoms in the Novel Zinc/Stearate Complex. By XAFS, we found that the molar ratio of the zinc ion to stearate in the zinc/stearate complexes at 144 °C was about 1:1. However, the skeletal structure is unclear. It may be a dinuclear, trinuclear, or multinuclear zinc complex or a mixture. Identification is difficult because an X-ray diffraction analysis is not available for the molten and low concentration of the complex in isoprene rubber at 144 °C. Because of the latter reason, unfortunately, the EXAFS data also did not give us any information on the number of zinc nuclear in the complex structure. However, results of several studies together with our study may suggest the structure. Melting behaviors of saturated fatty acid zinc soaps were studied including ZnSt2, where progression bands were utilized as a characteristic signature of all-trans chains for their quantitative measure of the fraction at different temperature.46 Melting was found to be associated with a conformational disordering of the chains as well as a change in the zinc− carboxylate coordination from the bridging bidentate to the mixture of bridging bidentate and monodentate coordinations. From these findings, the layered multinuclear zinc complexes was concluded to be abruptly collapsed upon melting. In IR-ZnO-StH of our study, on the other hand, the reaction between ZnO and StH occurred to generate ZnSt2 by mixing in isoprene rubber on the two-roll mill as mentioned above. However, at both 35 and 144 °C as shown in Figures 7a and 7c, respectively, the zinc salt of stearic acid in the isoprene rubber matrix did not show any clear progression bands that could be ascribed to the coupling of vibrational modes of all-trans methylene units in stearate. Only the symmetric and antisymmetric methylene stretching bands show a gauche disorder such as found in the alkane melt47 were detected at 2853 and 2922 cm−1, and the bands were shifted from 2848 and 2916 cm−1 by heating, respectively, as illustrated in Figures 7b and 7d. In general, a conformationally disordered chain

requires a larger lateral area as compared with an all-trans chain. In addition, the poly(isoprene) segments vibrate vigorously at 144 °C. Therefore, it is suggested that layered multinuclear zinc complexes were not formed in the isoprene rubber matrix at 144 °C because of the steric hindrance of the gauche disordered stearate chains. Furthermore, recent studies of metal−organic frameworks may provide clues to the number of nuclei for the bridging structure of the zinc/stearate complexes in IR-ZnO-StH. Paddle-wheel-type inorganic building units [M2(μ-O2CR)4] in metal−organic frameworks have been proposed.48,49 The structure of [M2(μ-O2CR)4] is well-known to be formed for many metal (M) ions including Zn, when carboxylates are used as linkers.50 To date, many studies of the metal−organic framework construction process from the building units have been conducted, especially from the dimensional viewpoint of the structure. However, the water resistance of metal−organic frameworks51 remains, and improvement of the hydrothermal stability is currently the most challenging issue for industrial implementation of these materials.52−54 In addition, carboxylate-based metal−organic frameworks are known to be especially prone to breakdown under humid conditions,50−59 a characteristic that has been attributed to the relatively weak metal−ligand bond formed between oxygen and the metal.51 A trend that a four-coordinated zinc−oxygen metal−organic framework is unstable in water has also been observed.60 Therefore, anhydrous zinc(II) carboxylate complexes are polymeric in general, although a few exceptional examples have been reported.61,62 Note that the ratios between the zinc atom and carboxylate in the recent cases were reported to be 5:8 or 2:3. However, as discussed above, the zinc/stearate complex observed in our study may have been hydrated and the ratio of zinc atoms and stearate was about 1:1. Therefore, the formation of a dinuclear-type structure may be most likely for the zinc/stearate complex during vulcanization under the presence of water in this study. Indeed, dinuclear zinc enzymes with bridging carboxylate coordination are well-known in biological systems containing water.35,63,64 For example, aminopeptidases require active sites with two zinc centers to remove C-terminal amino acid.65 The long alkyl segment of stearate and the low concentrations of ZnO and stearic acid in the rubber matrix may also support the consideration about the dinuclear structure. The former is based on a well-known phenomenon that the long alkyl segment of stearate plays a role to disperse zinc atoms in the rubber matrix. In addition, the low concentrations of ZnO and stearic acid must decrease events of the association of complex units at 144 °C. Our proposal for the dinuclear structure is also consistent with the results of our previous study on the inhomogeneity of the vulcanized networks revealed by small-angle neutron scattering.13 The mesh size in the two-phase rubber networks shown in Figure S2 was found to decrease linearly with an increase of the concentration of zinc salt of stearate under the constant amounts of sulfur and accelerator. If the zinc/stearate complex in the vulcanization was a multinuclear species or a mixture of different multinuclear species, the mesh size must have not been controlled so easily under the low concentrations of ZnO and stearic acid in the sulfur cross-linking. The difficulty of the network control is also ascribable to the effect of steric hindrance of the gauche disordered stearate groups, which may be enhanced with the increase of number of zinc atom in the complex and consequently to result in a low H


Article

Macromolecules Table 2. Parameters for Calculated Structures of Dinuclear Type Intermediates 6-31G for all atoms number of sustituted group structure

OH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

2 2 2 1 1 1

H2O 1 2 2 1 3 2 4 3 1

1 1 1 1 2

−O−a

2 1 2 2 1 1

4 3 2 1

1 2

1 2 4 3 3 3 2 1

rubber

2 1

1 1 1

1 1

1

1 1

LanL2DZ for zinc and 6-31G for others

infrared frequency of COO− (cm−1)

infrared frequency of COO− (cm−1)

charge

antisymmetric

symmetric

Δνb

bond length of Zn−O (Å)

antisymmetric

symmetric

Δνb

bond length of Zn−O (Å)

0 0 0 +2 +1 +2 +1 +2 +1 +2 +2 +1 +1 +1 +1 0 0 0 0 −2 −1 −1 −1 −2 −1

1596 1588 1584 1570 1592 1572 1588 1573 1587 1575 1574 1582 1557 1556 1553 1489 1566 1563 1562 1643 1639 1635 1614 1631 1604

1434 1417 1427 1444 1443 1449 1445 1447 1448 1438 1442 1428 1441 1441 1443 1462 1422 1424 1426 1387 1419 1412 1416 1363 1398

162 171 157 126 149 123 143 126 139 137 132 154 116 115 110 27 144 139 136 256 220 223 198 268 206

1.94 2.03 2.02 1.96 1.95 1.95 1.95 1.95 1.95 1.94 1.95 1.94 1.96 1.96 1.96 2.00 1.98 1.99 2.01 1.99 1.96 1.96 1.97 2.03 2.00

1590 1586 1586 1577 1590 1578 1589 1579 1589 1577 1576 1580 1558 1557 1555 1485 1562 1562 1560 1646 1641 1638 1614 1625 1598

1414 1423 1423 1446 1444 1450 1446 1449 1448 1440 1444 1434 1441 1441 1442 1454 1421 1424 1421 1391 1391 1421 1417 1368 1385

176 163 163 131 146 128 143 130 141 137 132 146 117 116 113 31 141 138 139 255 250 217 197 257 213

2.00 2.06 2.06 1.99 1.98 1.98 1.98 1.99 1.99 1.97 1.98 1.97 1.99 2.00 1.99 2.03 2.01 2.03 2.05 2.00 2.00 1.99 2.01 2.06 2.04

−O− is corresponding to the bridged oxygen between two zinc atoms in the intermediate. bDifferential frequency of COO− calculated by subtracting symmetric from antisymmetric frequencies.

a

reactivity of sulfur cross-linking. However, the sulfur crosslinking reaction was very fast as shown in Figure S4. These experimental results and the literatures described above may support our proposal that the presence of multinuclear type zinc/stearate complexes can be ignored and the dinuclear complex may be most likely. However, the complex is not safely assigned to be a dinuclear structure for the newly observed zinc/stearate complexes. Moreover, the assignment of the binding mode of zinc(II) carboxylate by infrared spectroscopy has been reported to be unambiguous40 as mentioned above. Consequently, a DFT calculation was conducted to identify the zinc/stearate complexes in this study, where theoretical infrared spectra obtained by the calculation were compared with the experimental ones. 3.6. Theoretical Identification of the Structure of the Novel Zinc/Stearate Complex. The DFT calculations were conducted on the basis of the structural information demonstrated by XAFS and infrared spectroscopy. The coordination number of the zinc ion is 4, and the molar ratio of the zinc ion to stearate is 1:1. The zinc/stearate complexes may be hydrated probably with two water molecules generated by the reaction between ZnO and stearic acid. The nonpolar rubber segments may promote a localization of water around ionic zinc atom. Furthermore, the water molecules are assumed to be deprotonated, and it leads to neutralization of zinc/ stearate complex because it is often reported that the Lewis acidic Zn(II) ion center activates the coordinated water toward deprotonation to generate a hydroxide species as a ligand.65,66

There may be a zinc dinuclear bridging bidentate structure. Around the zinc/stearate complex, isoprene rubber molecules and/or implicit water molecules may be also present in the rubber compounds. Thus, its possible fundamental skeleton is proposed to be a dinuclear type bridging bidentate zinc/ stearate complex composed of (Zn2(μ-O2CC17H35)2)2+·4X. Here, X may be a hydroxyl group, water, and/or rubber molecules. On the basis of this structure, the theoretical calculations were performed for 25 patterns of structures changing the configurations of hydroxyl groups and the number of hydroxyl group, rubber, and implicit water molecule. All calculated frequencies of carboxyl groups in each model are shown in Table 2, where characteristics of their carboxyl shift are summarized with the distances between Zn and O atoms in their structures. Table 2 shows two sets of frequencies with different basis sets, i.e., one set with 6-31G for all atoms and another set with LanL2DZ for zinc atom and 6-31G for others. It is clear that the two different basis sets gave similar results of frequencies. There seems no essential influences in the COO− vibrational frequencies between the two basis sets. This is quite reasonable since an even small difference in the description of Zn−O bonding affects only indirectly to the force field of COO− vibration. Furthermore, Table S1 shows the influence of the polarization functions added on COO− moiety, where the structure 1 was adopted as an example. It was found that both the symmetric and antisymmetric modes shifted to higher frequencies and deviated from the experimental values. This I


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Macromolecules

Figure 8. Experimental and calculated infrared spectra in the range between 1700 and 1300 cm−1 to show the carboxylate antisymmetric and symmetric peaks and their shifts. (a, b) The experimental spectra of IR-ZnO(0.5)-StH(2) at 144 °C (green) and IR-ZnO(1)-StH(2) at 144 °C (light green), respectively. The experimental spectra were obtained by subtracting the infrared spectra of isoprene rubber from raw data of the samples at same time. (c) The calculated spectrum of the complex structure shown in Figure S5a (black), and (d, e, f) those of the complex structures shown in Figures S5b (red), S5c (blue), and S5d (yellow), respectively. Dotted line, dashed line, and double dashed line are guides to identify the frequencies of the characteristic peaks of the intermediate in the spectrum of IR-ZnO(0.5)-StH(2) and IR-ZnO(1)-StH(2). Note that the line at 1424 cm−1 shows a peak top of the carboxylate symmetric band, which is referred to ref 46. The calculated structures are simply illustrated in each side of the spectra.

is a reasonable trend since the calculated frequencies are based on the harmonic approximation, and the polarization functions improve the bonding to a stronger direction. We are aware that good agreements to experimental frequencies with the hybrid functional are consequence in counterbalance between the errors to opposite directions. Even with the better description of the COO− bonding, some scale factor is necessary for the comparison between the calculated and observed frequencies. From the point of view of simpleness in calculations and discussion, we adopted the 6-31G basis set for all atoms and no scale factor. Among the 25 structures investigated, the three most probable structures labeled as structures 1−3 were selected with respect to both the antisymmetric and symmetric COO−

frequencies in the two basis sets, which best agreed with the experimental ones. In fact, differences in the antisymmetric and symmetric COO− frequencies (Δν) of the structures 1−3 were in better agreement with that of experimental datum (about 171 cm−1) among the 25 structures. From a viewpoint of the average Zn−O bond lengths in the zinc/stearate complexes, furthermore, calculated values of the structures 1−3 did not deviate much from the experimental one (1.98−1.99 Å). Eventually, all the selected structures were neutral species. It is interesting that the neutrality is maintained within the local reaction site without distant counterions. Their optimized structures and calculated infrared spectra are illustrated in Figures S5a, S5b, and S5c and in Figures 8c, 8d, and 8e, respectively. For comparison, the experimental spectra of IRJ


Article

Macromolecules ZnO(0.5)-StH(2) and IR-ZnO(1)-StH(2) at 144 °C are also shown in Figures 8a and 8b, respectively. Note that the infrared spectrum of isoprene rubber at 144 °C is subtracted from these experimental ones in order to clearly compare the COO− symmetrical and antisymmetrical stretching bands of the zinc/ stearate complexes. As shown in Figure 8c, bands at 1596 and 1434 cm−1 appeared when a bridging bidentate coordination was taken into account with two hydroxyl groups and two rubber segments for the calculation. In the case of two hydroxyl groups, one water molecule and one rubber segment, bands at 1588 and 1417 cm−1 were calculated as shown in Figure 8d. Furthermore, bands at 1584 and 1427 cm−1 were obtained when two hydroxyl groups and two water molecules were assumed as ligands as shown in Figure 8e. These bands were mostly in a good agreement with the experimental infrared spectra of IR-ZnO(0.5)-StH(2) and IR-ZnO(1)-StH(2) at 144 °C. These calculations, however, cannot decide one structure but may suggest a presence of a mixture of the intermediates. Therefore, the structure of the most probable intermediates is generally formulated as “(Zn2(μ-O2CC17H35)2)2+(OH−)2·XY” shown in Figure 9, where X and Y are water and/or a rubber segment. The corresponding structures are 1, 2, and 3 in Table 2.

between the experimental infrared data and theoretical ones were not easily obtained, although typical structures were selectively investigated. Only a few possibilities for the trinuclear and tetranuclear type ones were suggested as shown in Table S2. In addition, the most calculated infrared spectra of trinuclear and tetranuclear type zinc/stearate complexes showed too numerous bands due to the coordinated water molecules. These observations may also support the predominant formation of dinuclear type zinc/stearate complexes in IR-ZnO(0.5)-StH(2) and IR-ZnO(1)-StH(2) at 144 °C. At present, it cannot be rejected for trinuclear and tetranuclear complexes to be the intermediate. At least, however, the concentrations of dinuclear complexes must have been larger than those of trinuclear and tetranuclear type complexes. Furthermore, the trinuclear and tetranuclear type complexes may possess a larger stereo hindrance by stearate groups and other ligands nearby zinc atoms in these complex states and may result in their low reactivity as an intermediate for the sulfur cross-linking. The significantly fast sulfur crosslinking reaction shown in Figure S4 may support this consideration. 3.7. Dinuclear Type Bridging Bidentate Zinc/Stearate Complex as an Intermediate in the Sulfur Cross-Linking Reaction of Isoprene Rubber. To confirm the generation of the bridging bidentate zinc/stearate complex in the sulfur crosslinking reaction, a curing system of IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5) was examined in this study. Here, CBS is one of the most conventional accelerators for sulfur cross-linking reaction. A curing system composed of 0.5 phr of ZnO and 2 phr of stearic acid was selected because all most ZnO may react with stearic acid to generate the dinuclear type bridging bidentate zinc/stearate complex. This is preferable for minimizing the effect of ZnO on the identification of the zinc/stearate complex. In situ infrared spectra of IR-ZnO(0.5)-StH(2)-CBS(1)S8(1.5) showed distinct changes of spectra in the early stage of vulcanization as illustrated in Figure 10. Upon heating, the carboxyl shift to show the generation of the zinc/stearate complex was clearly observed in the initial stage of the reaction. A peak at 1398 cm−1 at 35 °C disappeared, and a singlet at 1537 cm−1 at 35 °C split to a broad peak at 1595 cm−1 and a shoulder peak at about 1560 cm−1. These variations gradually occurred during heating up to 144 °C. At the beginning of the sulfur cross-linking reaction, the infrared spectral bands became very similar to those of IR-ZnO(0.5)-StH(2) including the intensity as shown in Figure 10, where the spectra of IRZnO(0.5)-StH(2) at 11.0 and 18.3 min are indicated by red and brown lines, respectively, for comparison with the reference spectra. Furthermore, the bands varied with time at 144 °C. Peaks around 1580−1570 cm−1 appeared and then became larger with a decrease in peak intensity at about 1595 cm−1. The detail discussion on the variation will be reported with a mechanism of the sulfur cross-linking reaction in a near future. In situ Zn K-edge XANES spectra also supported the generation of the bridging bidentate zinc/stearate complex in IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5). As shown in Figure 11, the XANES spectrum of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5)18.5 was very similar to that of IR-BB-ZnO(0.5)-StH(2)-18.5 which was obtained by subtracting a spectrum of unreacted ZnO from that of IR-ZnO(0.5)-StH(2)-18.5. By a quantitative analysis using a least-squares fitting method as shown in Figure 1d, the fractions of the zinc/stearate complex and the unreacted ZnO were determined to be about 94.5% and about 5.5% in IR-

Figure 9. Most possible structure of the dinuclear type bridging bidentate zinc/stearate complex composed of (Zn 2 (μO2CC17H35)2)2+(OH−)2·XY, where X and Y are water and/or a rubber segment.

Instead of two hydroxyl groups, on the other hand, a presence of bridged oxo group between two zinc atoms may be also acceptable to identify the infrared spectral bands at about 1566 and 1422 cm−1 as shown in Figure S5d and Figure 8f. Amounts of the complexes with a bridged oxo group, however, seem to be a few from their low intensity. Note that a small band at about 1460 cm−1 was not reproduced by the zinc/ stearate complex shown in Figure S5. This band, however, may be ascribed to a CH2 scissoring.46 When the deprotonation of water did not proceed or proceeded in excess on the complex, several candidates are additionally proposed as shown in Table 2. Electron balances in these complexes are not neutralized. However, total balances as reaction systems must have been neutralized as known in a solvation of ionic sites in general. At a high temperature, the intermediates shown in Table 2 may interconvert each other. In this study, the possibility of trinuclear and tetranuclear type zinc/stearate complexes was also evaluated by using the DFT calculations. However, structures with good agreements K


Article

Macromolecules

complex in high viscous liquid of isoprene rubber at a high temperature, a density functional calculation is utilized for identifying the structural details of the newly observed zinc/ stearate complex by combining with the infrared spectra of the complexes. The results provide that the most probable structure of the complex is (Zn2(μ-O2CC17H35)2)2+(OH−)2· XY, where X and Y are (1) two rubber segments, (2) one water molecule and one rubber segment, or (3) two water molecules as ligands, respectively. The complex may be present as a mixture of them. It has been previously unknown despite the long history of rubber science and technology. In zinc enzymology, such two carboxylate-bridged dizinc ions have been known to play a number of conventional roles in catalysis, where zinc-bonded water works as an important key to control biological reactions.35,63−66 Therefore, the newly observed zinc/stearate complex may play a role to generate an active intermediate to accelerate the sulfur cross-linking reaction of rubber probably via enzyme mimic reactions. A further study is necessary to achieve a breakthrough on a traditional yet indispensable technology of vulcanization of rubber.

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ASSOCIATED CONTENT

* Supporting Information S

Characterization and computational calculation details. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 10. Variation of infrared spectra in the initial stage of the sulfur cross-linking reaction of IR-ZnO(0.5)-StH(2)-CBS(1)-S8(1.5), which are shown in black. Black numbers show the time of the measurement in minute and a green arrow shows the time progress. IR-ZnO(0.5)StH(2)-11.0 (red) and IR-ZnO(0.5)-StH(2)-18.3 (brown) are also shown for comparison.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81 75 724 7558; Fax +81 75 724 7558 (Y.I.). Notes

ZnO(0.5)-StH(2)-18.5, respectively. Here, XANES spectra of IR-ZnSt2(4.5)-18.5 and IR-ZnO(1)-18.5 were used as reference samples. Apparently, our proposed intermediate was found to be generated in the initial stage of the sulfur cross-linking reaction.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the JSPS ALCA program, JSPS KAKENHI Grant 23655214, and Izumi Science and Technology Foundation to Y.I. The XAFS experiment was performed at the BL-14B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2009A1929, 2009B2044, 2010A1778, 2010B1928, 2012A1419, 2012B1891, and 2013A1828). The authors thank Dr. Atitaya Tohsan and Mr. Yuta Sakaki for their useful support.

4. CONCLUSION We show the unprecedented essential structure of active zinc salt of stearate in sulfur cross-linking of isoprene rubber for the first time. It is a hydrated bridging zinc/stearate complex, the molar ratio of the zinc ion to stearate and the coordination number of which are 1:1 and 4, respectively. Since an X-ray crystallography analysis is difficult due to the generation of the

Figure 11. Comparison of experimental (a) XANES and (b) EXAFS spectra between IR-BB-ZnO(0.5)-StH(2)-18.5 and IR-ZnO(0.5)-StH(2)CBS(1)-S8(1.5)-18.5. L


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Macromolecules

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(33) As shown in the recipe of Table 1, zinc oxide (M = 81.41) of 1 part per weight of rubber (phr) and stearic acid (M = 284.48) of 2 phr were mixed in isoprene rubber for IR-ZnO(1)-StH(2). If zinc oxide and stearic acid are quantitatively reacted, the amounts of generated zinc stearate and remained zinc oxide will be 3.52 × 10−3 mol and 8.76 × 10−3 mol per isoprene rubber of 100 g, respectively. If the intermediate is zinc stearate, i.e., the 1:2 complex of zinc/stearate, the maximum concentration should be 3.41 × 10−5 mol/g. If the intermediate is the 1:1 complex, however, the concentration becomes 6.83 × 10−5 mol/g. On the basis of a similar calculation for IRZnO(0.5)-StH(2), the maximum concentrations of the 1:2 and the 1:1 zinc/stearate complexes become 3.43 × 10−5 and 5.99 × 10−5 mol/g, respectively. (34) Berkesi, O.; Dreveni, I.; Andor, J. A. Inorg. Acta 1992, 195, 169− 173. (35) Jarenmark, M.; Csapó, E.; Singh, J.; Wöckel, S.; Farkas, E.; Meyer, F.; Haukka, M.; Nordlander, E. Dalton Trans. 2010, 39, 8183− 8194. (36) Pan, H. K.; Knapp, G. S.; Cooper, S. L. Colloid Polym. Sci. 1984, 262, 734−746. (37) Mesubi, M. A. J. Mol. Struct. 1982, 81, 61−71. (38) Ishioka, T.; Maeda, K.; Watanabe, I.; Kawauchi, S.; Harada, M. Spectrochim. Acta, Part A 2000, 56, 1731−1737. (39) Watanabe, I.; Tanida, H.; Kawauchi, S. J. Am. Chem. Soc. 1997, 119, 12018−12019. (40) Zeleňaḱ , V.; Vargová, Z.; Györyová, K. Spectrochim. Acta, Part A 2007, 66, 262−272. (41) Ishioka, T. Polym. J. 1993, 25, 1147−1152. (42) Ishioka, T.; Shimizu, M.; Watanabe, I.; Kawauchi, S.; Harada, M. Macromolecules 2000, 33, 2722−2727. (43) Fatmi, M. Q.; Hoferb, T. S.; Rode, B. M. Phys. Chem. Chem. Phys. 2010, 12, 9713−9718. (44) Hartmann, M.; Clark, T.; Eldik, R. V. J. Am. Chem. Soc. 1997, 119, 7843−7850. (45) Ishioka, T.; Murata, A.; Kitagawa, Y.; Nakamura, K. T. Acta Crystallogr., Sect. C 1997, 53, 1029−1031. (46) Barman, S.; Vasudevan, S. J. Phys. Chem. B 2006, 110, 22407− 22414. (47) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334−341. (48) Köberl, M.; Coloja, M.; Herrmann, W. A.; Kühn, F. E. Dalton Trans. 2011, 40, 6834−6859. (49) Lawton, D.; Mason, R. J. Am. Chem. Soc. 1965, 87, 921−922. (50) Bureekaew, S.; Amirjalayer, S.; Schmid, R. J. Mater. Chem. 2012, 22, 10249−10254. (51) Li, H.; Eddaaoudi, M.; O’keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276−279. (52) Dan, M.; Rao, C. N. R. Angew. Chem., Int. Ed. 2006, 45, 281− 285. (53) Bellarosa, L.; Calero, S.; Núria López, N. Phys. Chem. Chem. Phys. 2012, 14, 7240−7245. (54) Jasuja, H.; Burtch, N. C.; Huang, Y.; Cai, Y.; Walton, K. S. Langmuir 2013, 29, 633−642. (55) Li, Y.; Yang, R. T. Langmuir 2007, 23, 12937−12944. (56) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Lomg, J. R. J. Am. Chem. Soc. 2007, 129, 14176−14177. (57) Ma, D.; Li, Y.; Li, Z. Chem. Commun. 2011, 47, 7377−7379. (58) Yang, J.; Grzech, A.; Mulder, F. M.; Dingemans, T. J. Chem. Commun. 2011, 47, 5244−5246. (59) Choi, H. J.; Dinca, M.; Dailly, A.; Long, J. R. Energy Environ. Sci. 2010, 3, 117−123. (60) Low, J. J.; Benin, A. I.; Jakubczak, P.; Abrahamian, J. F.; Faheem, S. A.; Willis, R. R. J. Am. Chem. Soc. 2009, 131, 15834−15842. (61) Nakacho, Y.; Misawa, T.; Fujiwara, T.; Wakahara, A.; Tomita, K. Bull. Chem. Soc. Jpn. 1976, 49, 595−599. (62) Clegg, W.; Harbron, D. R.; Homan, C. D.; Hunt, P. A.; Little, I. R.; Straughan, B. P. Inorg. Chim. Acta 1991, 186, 51−60.

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