Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. III. The reaction of 2-bisbenzothiazole-2,2′-disulfide with sulfur and ZnO in polyisoprene

The sulfur vulcanization of polyisoprene accelerated by 2-bisbenzothiazole-2,2′-disulfide (MBTS) was investigated. Rubber compounds were heated in a DSC and removed at various temperatures along the DSC thermal curve. The rubber vulcanizate was analyzed for crosslink density and for residual reactants and extractable reaction products. MBTS reacts readily with sulfur, and the polysulfidic accelerator complexes react with the rubber chain to form pendent groups. Crosslinking results from hydrogen abstraction, by the benzothiazole pendent group, from a neighboring chain. 2-Mercaptobenzothiazole, a product of crosslinking, also acts as an accelerator in the later stages of the reaction. MBTS has been shown not to react with ZnO and the higher crosslink densities obtained when ZnO is present are attributed to ZnO aiding the abstraction of the benzothiazole pendent group to give zinc mercaptobenzothiazole. A mechanism for the MBTS acceleration of sulfur vulcanization is proposed. © 1996 John Wiley & Sons, Inc.     

The interaction of sulfenamide accelerators with sulfur,ZNO, and stearic acid in the absence of rubber

The interaction of sulfur, ZnO, stearic acid, and the three sulfenamide accelerators N-cyclohexylbenzothiazole sulfenamide (CBS), 2-(4-morpholinothio) benzothiazole (MOR), and 2-t-butylaminobenzothiazole sulfenamide (TBBS) were investigated by differential scanning calorimetry in the absence of rubber. In the presence of sulfur, the same product spectrum is formed as in its absence, but at lower temperatures. Thus, CBS gives N-cyclohexylamino-2-benzothiazole polysulfides (CBP), 2-bisbenzothiazole-2,2′-disulfide (MBTS), 2-bisbenzothiazole-2,2′-polysulfides (MBTP), and 2-bisbenzothiazole-2,2′-monosulfide (MBTP), 2-mercaptobenzothiazole (MBT), and 2-N-cyclohexylaminobenzothiazole (CB). In the presence of sulfur, the amount of polysulfides formed initially is higher but the polysulfides are unstable, and on prolonged heating, only MBT and CB remain. MOR and TBBS form analogous product spectra. The sulfenamides do not react with ZnO or zinc stearate. The MBT–amine complex prevents MBT, formed on decomposition, from reacting to give zinc mercaptobenzothiazole (ZHBT). Reaction mechanisms are proposed to account for the formation of the products. © 1994 John Wiley & Sons, Inc.     

Crystallization of vulcanizates. I. Low‐temperature crystallization as a function of extent of cure for polyisoprene vulcanized with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide/sulfur

The crystallization of polyisoprene, vulcanized to various degrees of cure with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur formulations, was studied in a density column at ?25°C. The densities of vulcanizates before crystallization decrease progressively with cure time, which is ascribed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period before the onset of crystallization increases and both the rate of and the degree of crystallization decrease with extent of cure. This is attributed primarily to the presence of residual pendent groups on the polymer chain and secondly to crosslink formation. The changes are more marked with MBTS formulations where pendent groups are more bulky. MBTS compounds fail to crystallize once vulcanized to the point where a gel has formed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2565–2572, 2001     

Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Gum compounds of polyisoprene were vulcanized with a number of different curing systems to give networks with crosslink densities in two different ranges. Stress–strain curves were obtained upon rapid (500 mm/min) and slow (0.5 mm/min) extension. In tetramethylthiuram disulfide (TMTD)/sulfur and zinc dimethyldithiocarbamate/sulfur vulcanizates, which crystallize readily, failure occurred at higher tensile values upon rapid than upon slow extension and this is attributed to a greater contribution to tensile strength by a larger amount of stress‐induced crystallites. X‐ray diffraction showed that 2‐benzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates did not stress‐crystallize and failure occurred at lower tensile values. Furthermore, samples extended rapidly failed at lower tensile values than did slowly extended samples. These differences, compared to TMTD vulcanizates, are attributed to extensive main‐chain modifications (pendent groups), causing delays in the movement of sections of the chain, leading to the load being unequally distributed between chains. The fewer load‐bearing chains ensure earlier failure. The addition of zinc stearate to TMTD/sulfur and MBTS/sulfur formulations increases the ability of vulcanizates of similar crosslink density to crystallize and enhances tensile properties of vulcanizates with similar crosslink densities, outcomes that are attributed to zinc stearate's promoting crosslinking of pendent groups and reducing impediments to crystallization and chain movement. Dicumyl peroxide–cured networks crystallize readily and exhibit a very rapid upturn in the stress–strain curve. However, failure occurs at lower stress values than apply to accelerated sulfur networks and it is suggested that the distribution of subchain lengths between crosslinks may contribute to their inferior properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2587–2596, 2001     

The thermal decomposition of sulfenamide accelerators

The thermal decomposition of three sulfenamide accelerators N-cyclohexylbenzothiazole sulfenamide (CBS), 2-(4-morpholinothio) benzothiazole (MOR) and 2-t-butylaminobenzothiazole sulfenamide (TBBS) were investigated by differential scanning calorimetry. The sulfenamides decompose rapidly at 210–220°C, yielding a number of products, including reactive polysulfidic complexes. Thus, CBS gives N-cyclohexylamino-2-benzothiazole polysulfides (CBP), 2-bisbenzothiazole-2,2′-disulfide (MBTS), 2-bisbenzothiazole-2,2′-polysulfides (MBTP), 2-bisbenzothiazole-2,2′-monosulfide (MBTM), 2-mercaptobenzothiazole (MBT), and 2-N-cyclohexylaminobenzothiazole (CB). The polysulfides are unstable, and on prolonged heating, only MBT and CB remain. The amine fragment of the accelerator is present as the amine salt of MBT. At lower temperatures, the sulfenamides are relatively stable. MOR forms an analogous product spectrum. The decomposition of TBBS is endothermic, in contrast to the exothermic reaction observed with CBS and MOR, and the concentrations of the various polysulfides do not decrease on prolonged heating. Reaction mechanisms are proposed to account for the formation of the products. © 1994 John Wiley & Sons, Inc.     

N,N′‐Pentamethylenethiuram disulfide and N,N′‐pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. III. Vulcanization of polyisoprene and 2,3‐dimethyl‐2‐butene in the absence of ZnO

Polyisoprene and model compound, 2,3‐dimethyl‐2‐butene, were vulcanized with N,N′‐dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,N′‐dipentamethylenethiuram hexasulfide (CPTP6) in the absence of ZnO and residual extractable curatives and reaction intermediates analyzed by HPLC at various stages of the reaction. Accelerator polysulfides, required for the formation of accelerator‐terminated polysulfidic pendent groups, form rapidly, or are present from the outset in the case of CPTP6. Model compounds confirm the formation of thiuram‐terminated polysulfidic pendent groups as intermediates in the vulcanization process. Removal of pentamethylenedithiocarbamic acid (Hpmtc) from the system during heating delays the onset of vulcanization and leads to very low crosslink densities. Rubbers heated under vacuum can subsequently be crosslinked by the addition of zinc stearate, pointing to the presence in the compound of thiuram‐terminated pendent groups. However, such pendent groups do not readily crosslink on their own, and hydrogen‐terminated polysulfidic pendent groups, formed by the reaction of sulfurated Hpmtc with the polymer, are suggested to be involved in the crosslink formation. N,N′‐Pentamethylenethiurea forms in the vulcanizate, but is not as product of crosslinking reactions, rather of CPTD degradation. The data are discussed with respect to mechanisms proposed in the literature for crosslinking, and it is concluded that the data support recently formulated mechanisms in which crosslinking involves reaction between thiuram and thiol‐terminated pendent groups, with Hmptc playing and essential role in the overall process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1100–1111, 2000     

Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide,tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide,and zinc–accelerator complexes

Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at ?16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn₂(dmtc)₄]– and zinc mercaptobenzothiazole [Zn(mbt)₂]–accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn₂(dmtc)₄ compounds crystallize extensively on cooling, pointing to limited main‐chain modification. It is suggested that main‐chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt)₂ compounds crystallize less readily than Zn₂(dmtc)₄ compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2573–2586, 2001     

Effect of carboxylic acids on 2‐bisbenzothiazole‐2,2′‐disulfide‐ and tetramethylthiuram disulfide‐accelerated sulfur vulcanization. II. Vulcanization of polyisoprene in the absence of ZnO

Polyisoprene was vulcanized by 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur and tetramethylthiuram disulfide (TMTD)/sulfur in the absence and presence of benzoic and stearic acids. It was found that the crosslink density of MBTS vulcanizates is halved by the addition of carboxylic acids and this can be explained in terms of the attack of the acids on the accelerator polysulfides. TMTD polysulfides are more reactive toward polyisoprene than are MBTS polysulfides, and their addition to the polymer chain occurs before significant attack by the carboxylic acids can reduce the polysulfide concentration. Consequently, the acids have little effect on the crosslink density of TMTD vulcanizates. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1007–1012, 1999     

Differential scanning calorimetric study of the interaction of 2,2′-dibenzothiazole,sulfur, zinc oxide,and stearic acid in the presence of polyisoprene

The interactions between mixtures of 2,2'-dibenzothiazole (MBTS), sulfur, ZnO, and stearic acid were studied by DSC in the presence of polyisoprene (IR). In the absence of ZnO, the onset of vulcanization was at a lower temperature than in its presnece, and the crosslink densities were relatively low. ZnO as well as stearic acid did not influence the consumption of MBTS, but the amount of MBT in the sample after vulcanization increased in the presence of ZnO and still more when ZnO and stearic acid were present—the same applied for the crosslink densities. In view of these results, interaction mechanisms are proposed for the different systems.     

Benzothiazole‐accelerated sulfur vulcanization. I. 2‐Mercaptobenzothiazole as accelerator for 2,3‐dimethyl‐2‐butene

2,3‐Dimethyl‐2‐butene (TME) was used as a model compound for polyisoprene in a study of 2‐mercaptobenzothiazole (MBT)‐accelerated sulfur vulcanization. Mixes that contained curatives only were heated in a DSC to various temperatures, while those that also contained TME were heated isothermally at 150°C in evacuated, sealed glass ampules. Heated mixtures were analyzed for residual curatives, intermediates, and reaction products by HPLC. It is proposed that MBT forms polysulfidic species (BtS_xH) in the presence of sulfur and that these react with TME via a concerted, substitutive reaction pathway to form polysulfidic hydrogen‐terminated pendent groups of varying sulfur rank (TME–S_xH). MBT is released as a by‐product of this reaction. Crosslinking occurs slowly as a result of the interaction of polythiol pendent groups, the rate being dependent on the pendent group concentration. H₂S is released on crosslinking. 2,3‐Dimethyl‐2‐butene–1‐thiol was synthesized and reacted in the presence of sulfur to confirm the formation of crosslinked products (TME–S_x–TME). Benzothiazole‐terminated pendent groups (TME–S_xBt) were not observed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1377–1385, 2000     

The role of thiol intermediates in 2‐mercaptobenzothiazole accelerated sulfur vulcanization of rubber model compounds

The model compound, 2,3‐dimethyl‐2‐butene (TME), was vulcanized using 2‐mercaptobenzothiazole (MBT) and sulfur. MBT was not consumed during the vulcanization reaction. The resultant crosslink products were bis(alkenyl) in nature. 2,3‐Dimethyl‐2‐buten‐1‐thiol (TME‐SH) was identified as being present in the vulcanization mixture by a postcolumn derivatization technique. The appearance of thiol was coincident with crosslinking. Polysulfanes (H₂S_n) were formed on crosslinking. Studies of the reaction of TME‐SH and sulfur indicated a rapid reaction to form crosslink products and polysulfanes. No monosulfidic crosslink species were formed in these reactions. Closer investigation revealed the presence of small quantities of what appeared to be highly reactive polysulfidic thiols. This is the first time that such species have been identified in vulcanization systems. Consequently, MBT‐accelerated vulcanization of TME is proposed to occur via the reaction of MBT and S₈ to form polysulfidic MBT, which then reacts with TME to form polysulfidic thiols. These thiols then rapidly react via a metathesis reaction pathway to provide crosslink products and polysulfanes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 47–54, 2003     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. I. Comparison of sulfur and 2-mercaptobenzothiazole accelerated reactions

Polyisoprene compounds with sulfur and with sulfur and 2-mercaptobenzothiazole (MBT) were vulcanized by heating in a differential scanning calorimeter (DSC) at a programmed rate. The reaction was stopped at various temperatures along the thermogram and the product analyzed by determining the crosslink density and crosslink type, and by determining the amount of extractable curatives and soluble reaction products by high-performance liquid chromatography. DSC cure curves and plots of crosslink density and extractable sulfur vs. temperature for the unaccelerated and MBT accelerated compounds can be made to coincide by shifting them along the temperature axis. It is suggested that MBT accelerated sulfur vulcanization occurs by the same mechanism as unaccelerated sulfur vulcanization, with SH⁺ ions from MBT adding across the carbon–carbon double bond, thus increasing the rate at which the reaction is initiated. © 1995 John Wiley & Sons, Inc.     

A DSC study of curative interactions. II. The interaction of 2,2′-dibenzothiazole with ZnO,sulfur, and stearic acid

The interaction of combinations of sulfur, 2,2′-dibenzothiazole (MBTS), ZnO, and stearic acid were studied by differential scanning calorimetry. A MBTS/stearic acid interaction was indicated as evidenced by the effect the MBTS/stearic acid combination had on the melting of sulfur, the S_α → S_β transition being suppressed in favor of a S_α → S_γ transition. The dissolution/interaction of MBTS in molten sulfur was also delayed by the MBTS/stearic acid interaction, which, it is proposed, involved protonation of the N atom in MBTS by stearic acid. MBTS did not affect the formation of zinc stearate from ZnO and stearic acid, but when sulfure was added to the mixture, the ZnO/stearic acid reaction did not go to completion. No direct evidence for the formation of 2,2′-dibenzothiazole polysulphides was found, but the absence of the S_γ → S_μ transition in sulfur/MBTS mixes was interpreted as indirect evidence of a reaction between these curatives. There was no evidence for the formation of a sulfur/MBTS/ZnO compound of the type generally attributed the role of an active sulfurating agent in accelerated sulfur vulcanization.     

The reactions of 2‐(4‐morpholinothio)‐ and 2‐(4‐morpholinodithio) benzothiazole in the absence of polyisoprene

2‐(4‐morpholinothio)benzothiazole (MOR) and 2‐(4‐morpholinodithio)benzothiazole (MDB) were heated with sulfur and ZnO in a DSC. The products formed at various temperatures were identified and analyzed by HPLC. At temperatures below 200°C, decomposition of the accelerator in the absence of other curatives was slow, degradation products being mainly 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS) and 2‐mercaptobenzothiazole (MBT). A rapid exothermic decomposition above 200°C resulted in the formation of MBT (or its amine salt) and 2‐(4‐morpholino)benzothiazole (MB). MOR and MDB reacted with sulfur to form higher polysulfides. MDB was shown to react more readily with sulfur than MOR and the delayed action of MOR in rubber can therefore not be ascribed to a stable polysulfide as suggested by other authors. Neither MOR nor MDB was found to react with ZnO. A limited reaction between MBT and ZnO was observed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1235–1240, 1999     

Rate of formation of polysulfides of 2-bisbenzothiazole-2-2′-disulfide in the presence of sulfur and 2-mercaptobenzothiazole

2-Bisbenzothiazole-2,2′-disulfide was heated isothermally with and without sulfur at temperatures between 130 and 178°C for various periods of time and the mixture analyzed by high-pressure liquid chromatography (HPLC). 2-Bisbenzothiazole-2,2′-monosulfide formed rapidly and 2-bisbenzothiazole-2,2′-polysulfides after an induction period of about 5 min. Polysulfides of higher sulfur rank are formed by the sequential addition of sulfur atoms. 2-Mercaptobenzothiazole catalyzes the formation of 2-bisbenzothiazole-2,2′-polysulfides and elimates the induction period. Mechanisms for the decomposition of 2-bisbenzothiazole-2,2′-disulfide and the catalytic role of 2-mercaptobenzothiazole are proposed. © 1995 John Wiley & Sons, Inc.     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. II. Reaction of 2-mercaptobenzothiazole and its zinc salt with sulfur and ZnO in polyisoprene

Compounds of polyisoprene with sulfur and bis(2-mercaptobenzothiazole)zinc(II) (Zn(mbt)₂) or ZnO and 2-mercaptobenzothiazole (MBT) were vulcanized by heating in a differential scanning calorimeter. The reaction was stopped at points along the thermogram and the product analyzed. ZnO and MBT readily react, the reaction going to completion during compounding. The presence of Zn(mbt)₂ delays the onset of crosslinking compared to compounds without zinc. It is suggested that the induction period prior to crosslinking is occasioned by the inactivity of Zn(mbt)₂, which must breakdown to MBT before it can participate in the vulcanization process. Such decomposition results from attack by anions generated when polysulfidic crosslinks, formed in the unaccelerated sulfur that occurs in the early stages of crosslinking, undergo scission. The effect of MBT, not bound to zinc, on the mechanism of the reaction is discussed. © 1995 John Wiley & Sons, Inc.     

The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

Polyisoprene/tetramethylthiuram disulfide (TMTD)/sulfur compounds were vulcanized under a variety of conditions. TMTD does not decompose to tetramethylthiourea (TMTU) at vulcanization temperatures as has been suggested, neither is it formed as an integral part of the crosslinking process. Instead, it results from the attack of dimethylamine, released on decomposition of dimethyldithiocarbamic acid (Hdmtc), on TMTD. It is demonstrated that the formation of TMTU in vulcanizates may be overlooked, as it is readily lost in the work‐up for HPLC analysis. Hdmtc is shown to play an essential role in the crosslinking process in polyisoprene/TMTD/sulfur formulations, and its removal from the system during vulcanization severely impedes crosslinking. Polysulfidic thiuram‐terminated pendent groups are formed, in part, by the interaction of tetramethylthiuram polysulfides with the polymer chain, but largely by an exchange between Hdmtc and polysulfidic thiol pendent groups. The latter are formed when sulfurated Hdmtc reacts with the polymer chain. Crosslinking of thiuram‐terminated pendent groups is slow, and in the absence of ZnO crosslinking results from reaction between polysulfidic thiuram pendent groups and thiols. Crosslinking is delayed until the bulk of the accelerator is bound to the polymer chain, at which point the concentration of free thiuram groups, in the form of Hdmtc, is low, and exchanges between newly formed thiol pendent groups and Hdmtc is less frequent, permitting crosslinking of thiuram pendent groups with these newly formed thiol pendent groups. Data to support the proposed reaction mechanism is presented. Hdmtc on its own accelerates sulfur vulcanization and acts as a catalyst for the reaction, being regenerated in the crosslinking process. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1371–1379, 1999     

Reactions of 2-(4-morpholinothio)- and 2-(4-morpholinodithio)benzothiazole in the presence of polyisoprene

2-(4-Morpholinothio)benzothiazole (MOR) and 2-(4-morpholinodithio)-benzothiazole (MDB) were reacted, in combination with sulfur and ZnO, in the presence of polyisoprene (IR). Samples were heated in a DSC at 2.5°C/min and characterized by swelling experiments. The products formed at various temperatures were analyzed by HPLC. Crosslinking only occurred once all the benzothiazole sulfenamide had been consumed, the onset of vulcanization characterized by a considerable increase in 2-mercaptobenzothiazole (MBT) concentration. Crosslinking occurred earlier in all corresponding MDB formulations. Higher crosslink densities were recorded with addition of ZnO. The delayed action experienced in MOR systems was attributed to an exchange reaction between benzothiazole-terminated pendent groups and MOR and not due to the stability of the disulfide, MDB. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1093–1099, 1997     

Studies on sulfur vulcanization of natural rubber accelerated combinedly with 2-mercaptobenzothiazole and diphenylguanidine both in presence and absence of dicumyl peroxide

Sulfuration of natural rubber (NR) by the binary accelerator 2-mercaptobenzothiazole (MBT) and diphenylguanidine (DPG) both in presence and in absence of ZnO and stearic acid with or without dicumylperoxide (DCP) was studied in detail. It was observed that the rate of decomposition of DCP in presence of both MBT and DPG is quite similar to that with MBT alone. The reduction of crosslinking depends also on MBT only. Through DPG has no influence on the decomposition rate, it reacts with MBT during the vulcanization process and suppresses the retardation caused by MBT on the DCP vulcanization. In accordance with the initial additiveness of crosslinking in systems containing DCP, the free sulfur decrease, and the rapidity of crosslink formation the vulcanization process of MBT-DPG-S-NR systems was interpreted in terms of a polar mechanism induced by the complex MSH₂NR′R″. In mixtures containing DCP together with sulfur, MBT, DPG, ZnO, and stearic acid, the initial stage of crosslinking is additive as indicated by a mixed reaction as well as by a methyl iodide treatment of the vulcanizates. Comparison with single accelerators shows a pronounced synergistic effect. This is because of the enhanced activity of the MBT-ZnO-stearic acid complex due to DPG which also induces polar sulfuration of NR by forming the active complex MSH₂NR′R″. In presence of ZnO and stearic acid, DCP cannot increase the net crosslink density but suppresses the reversion so much pronounced in its absence.     

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. II. Vulcanization of rubbers and model compound 2,3‐dimethyl‐2‐butene

Rubber and model compound 2,3‐dimethyl‐2‐butene were vulcanized for various times with dimethylammonium dimethyldithiocarbamate [(dma)dmtc]‐accelerated sulfur formulations in the absence of ZnO. Model compound systems were analyzed by HPLC, and no reaction intermediates containing pendent groups were found. Crosslinked sulfides, characterized by ¹H‐NMR, were found to be essentially bis(alkenyl). Residual curatives were extracted from rubber compounds vulcanized for various times and analyzed by HPLC. Compounds, cured to various crosslink densities, were found to crystallize readily in a density column at subambient temperatures. This supports evidence from model compound systems that pendent groups are largely absent from vulcanizates. It is suggested that a reaction mechanism, similar to that applicable to zinc dimethyldithiocarbamate‐accelerated sulfur vulcanization, may be applicable with (dma)dmtc accelerated formulations. Very limited crosslinking occurred on heating compounds under vacuum, and this can be attributed largely to the rapid loss of (dma)dmtc from rubber at elevated temperatures. However, the slower rate of crystallization on cooling of the gels, compared to the rate in press‐cured vulcanizates of similar crosslink density, was interpreted as evidence that some pendent groups did form during heating with (dma)dmtc/sulfur. Crosslinking of such pendent groups may be inhibited by the loss of (dma)dmtc, that, like zinc dimethyldithiocarbamate, may catalyze their crosslinking, and/or to the loss under vacuum of dimethyldithiocarbamic acid that would form thiol pendent groups that would rapidly crosslink with thiuram pendent groups. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3074–3083, 2001