III. Structures of Organic Polysulfanes 3922 A. General 3922 B. Structures of Chain-Like R−Sn−R Molecules 3923 C. Structures of Cyclic R−Sn−R Molecules 3924 D. Conformational Studies 3925 IV. Analysis of Organic Polysulfanes 3925 A. General 3925 B. NMR Spectroscopy 3926 C. Chromatography 3926 D. Raman Spectroscopy 3928 E. XANES Spectroscopy 3928 V. Reactions of Organic Polysulfanes 3928 A. Interconversion Reactions 3928 B. Sulfur Transfer Reactions 3930 C. Replacement Reactions 3931 D. Nucleophilic Displacement Reactions at the Sulfur−Sulfur Bond 3931

The chemistry of organic polysulfanes R-Sn-R (n > 2)

The mechanism of cross-link formation in sulfur vulcanization mediated by bis(dimethyldithiocar- bamato)zinc(II), ZDMC, has been uncovered, utilizing a combination of Density-Functional calculations and model experiments. These studies have revealed that, in a three-stage process, ZDMC exhibits a unique combination of catalytic activity: (1) It mediates the reaction between sulfur and rubber. This is achieved by incorporating sulfur atoms in the zinc-dithiocarbamate ring and inducing their insertion into an allylic C -H bond via an ene-like reaction. This ene reaction yields a rubber-bound polythiothiol and is only slightly endothermic, even though an activation energy of 90 kJ mol -1 is required. (2) The resulting polythiothiols engage in equilibrated metathesis reactions to yield polysulfides, the initial sulfur cross-links. (3) In a hitherto unsuspected mechanistic step ZDMC has been found to shift the metathesis equilibrium to the side of cross- links by mediating desulfhydration of the polythiothiols, producing sulfides and H 2S. Thus, the combined results of theoretical and experimental work have allowed to put forward a novel mechanism for ZDMC- mediated sulfur cross-link formation that successively comprises (a) homogeneous catalysis of thiol formation from sulfur and rubber, (b) an equilibrium between polythiothiol intermediates and cross-links and (c) ZDMC- induced desulfhydration.

The Mechanism of Zinc(II)-Dithiocarbamate-Accelerated Vulcanization Uncovered; Theoretical and Experimental Evidence

The addition of zinc oxide (ZnO) as an activator for the sulfur vulcanization of rubbers enhances the vulcanization efficiency and vulcanizate properties and reduces the vulcanization time. The first part of this article deals with the reduction and optimization of the amount of ZnO. Two different rubbers, solution-styrene-butadiene rubber and ethylene-propylene-diene rubber, have been selected for this study. The results demonstrate that the curing and physical properties can be retained when the level of ZnO (Red Seal) is reduced to 1 or 2 phr, respectively. Of particular interest is nano-ZnO, characterized by a nanoscale particle distribution. The cure characteristics indicate that with nano-ZnO, a reduction of zinc by a factor of 10 can be obtained. In the second part, model compound vulcanization is introduced to investigate the effects of ZnO during the different stages of vulcanization. Experiments are described with two models, squalene and 2,3-dimethyl-2-butene, both with benzothiazolesulfenamide-accelerated vulcanization systems. The results demonstrate the influence of ZnO during the different stages of the vulcanization. With ZnO present, a marked decrease can be observed in the sulfur concentration during an early stage of vulcanization, along with a slight delay in the disappearance of the crosslink precursor. The crosslinked product distribution is influenced as well.

Influence of zinc oxide during different stages of sulfur vulcanization. Elucidated by model compound studies

This paper reviews homogeneous catalysis during the sulfur vulcanization of rubber by zinc dithiocarbamates and zinc mercaptobenzothiazolates. By means of quantumchemical calculations and model studies, individual reaction steps of the vulcanization reaction sequence have been studied in detail. This has revealed that zinc accelerator complexes, such as the zinc dithiocarbamates and mercaptobenzothiazolates homogeneously catalyze a multitude of reactions during sulfur vulcanization. Among these are the formation, desulfuration, but also the degradation, of sulfur cross-links. In addition it could be shown that under oxidative conditions these zinc complexes catalyze the formation of diene and triene structures in the rubber. The zinc complexes are able to effect these reactions by (a) their ability to incorporate and activate sulfur atoms and (b) their Lewis acidity, which enhances the reactivity of sulfur vulcanization intermediates.

Zinc accelerator complexes.: Versatile homogeneous catalysts in sulfur vulcanization

The efficiency of sulfur vulcanization reaction in rubber industry is generally improved thanks to the combined use of accelerators (as sulphenamides), activators (inorganic oxides), and co-activators (fatty acids). The interaction among these species is responsible for the formation of intermediate metal complexes, which are able to increase the reactivity of sulfur towards the polymer and to promote the chemical cross-links between the rubber chains. The high number of species and reactions that are involved contemporarily in the process hinders the complete understanding of its mechanism despite the long history of vulcanization. In this process, ZnO is considered to be the most efficient and major employed activator and zinc-based complexes that formed during the first steps of the reaction are recognized to play a main role in determining both the kinetic and the nature of the cross-linked products. However, the low affinity of ZnO towards the rubber entails its high consumption (3–5 parts per hundred, phr) to achieve a good distribution in the matrix, leading to a possible zinc leaching in the environment during the life cycle of rubber products (i.e., tires). Thanks to the recent recognition of ZnO ecotoxicity, especially towards the aquatic environment, these aspects gain a critical importance in view of the urgent need to reduce or possibly substitute the ZnO employed in rubber vulcanization. In this review, the reactivity of ZnO as curing activator and its role in the vulcanization mechanism are highlighted and deeply discussed. A complete overview of the recent strategies that have been proposed in the literature to improve the vulcanization efficiency by reducing the amount of zinc that is used in the process is also reported.

Zinc-Based Curing Activators: New Trends for Reducing Zinc Content in Rubber Vulcanization Process

To study the mechanism of the sulfur vulcanization of rubber, 2,3-dimethyl-2-butene (C 612 ) was used as a simple, low-molecular-weight model alkene. Only equivalent allylic positions are present in this alkene. Treating C 6 H 12 with a mixture of Zn, S 8 , and the accelerator tetramethylthiuramdisulfide at 140°C yields a mixture of addition products (C 6 H 11 -S n -C 6 H 11 ). RP-HPLC in combination with MS and H-NMR shows that the products differ only in the length of the sulfur bridge. Small quantities of isomerized products have been found, in which a 1,3-shift of the double bonf has occured

Sulfur Vulcanization of Simple Model Olefins, Part I: Characterization of Vulcanization Products of 2,3-Dimethyl-2-Butene

In a study of the mechanism of the sulfur vulcanization of unsaturated rubber, 2,3-dimethyl-2-butene (C6H12) was used as a simple, low-molecular model alkene. Only equivalent allylic positions are present in this alkene. Treating C6H12 with a mixture of ZnO, S8 and the accelerator tetramethylthiuramdisulflde at 140°C for 20 minutes yields a mixture of addition products (C6H11—Sn—C6H11) and also intermediate products (C6H11—Sn—S(S)CN(CH3)2). The formation of C6H11—Sn—C6H11 from these intermediate products only proceeds in the presence of the zinc dimethyldithiocarbamate complex and free alkene.

Sulfur Vulcanization of Simple Model Olefins, Part II: Characterization and Reactivity of Intermediate Vulcanization Products of 2,3-Dimethyl-2-Butene

Vulcanization of ENBH (C9H14), a model for ENB containing EPDM rubber, with a system consisting of zinc oxide, stearic acid, sulfur, TMTD, and MBT at 140°C for 1 h yields slightly more than 30 different crosslinked products C9H13-Sn-C9H13 where n=2,3,4, and 5. In all of the products, the original ENBH structure is maintained (no shift of the double bond) and attachment of the sulfur bridge occurs only at the two allylic positions 3 and 9. Monosulfides (n=1) are probably also formed but no structures could be determined. Only small amounts of the MBT-Sn-C9H13 coupling products and noncrosslinked cyclic sulfides are formed.

Model Vulcanization of EPDM Compounds—Part I: Structure Determination of Vulcanization Products from Ethylidene Norbornane

Influence of vulcanization temperature and time on the sulfur crosslinks in vulcanized EPDM (third monomer ENB) was studied using the low-moleular model compound ENBH (C9H14). For each vulcanization condition (60 min at 180°, 160°, 140°, and 120° and 60, 30, 15, and 5 min at 140°), the nature of the crosslinks is represented by the composition of the C9H13−Sn−C9H13 vulcanization mixture as indicated by its HPLC chromatogram. In this way it is shown that increase in temperature favors formation of disulfidic crosslinks at the expense of the higher-sulfide crosslinks. Under the mildest vulcanization conditions (60 min at 120° or 5 min at 140°), disulfidic crosslinks are nearly absent and penta-sulfidic crosslinks are the most important.

Model Vulcanization of EPDM Compounds—Part II: Influence of Temperature and Time on the Vulcanization Products from Ethylidene Norbornane

The vulcanization of ENBH (C9H14), a model for vulcanization of EPDM containing ENB, was studied (140°, 1 h) using four different vulcanization systems (conventional, sulfur free, low-sulfur and high-sulfur). The influence of the system on the total yield of crosslinked products (C9H13—Sn—C9H13) is considerable, low-sulfur giving the lowest and high-sulfur giving the highest yield. The influence of the system on the composition of the crosslinked products as determined by HPLC is surprisingly small. By using 1H-NMR it was shown that formation of monosulfides is almost negligible under our conditions with all four vulcanization systems.

E. F. J. Duynstee

Papers

Sulfur Vulcanization of Simple Model Olefins, Part I: Characterization of Vulcanization Products of 2,3-Dimethyl-2-Butene

Sulfur Vulcanization of Simple Model Olefins, Part II: Characterization and Reactivity of Intermediate Vulcanization Products of 2,3-Dimethyl-2-Butene

Model Vulcanization of EPDM Compounds—Part I: Structure Determination of Vulcanization Products from Ethylidene Norbornane

Model Vulcanization of EPDM Compounds—Part II: Influence of Temperature and Time on the Vulcanization Products from Ethylidene Norbornane

Model Vulcanization of EPDM Compounds—Part III. Influence of Vulcanization System on the Vulcanization Products from Ethylidene Norbornane