In this paper, I will describe the crosslinked structure and the mechanism of the crosslinking commonly used. The crosslinking mechanism and the crosslinked structure have not been elucidated clearly.
The crosslinking of rubber with sulfur was discovered 175 years ago. Since then, organic peroxide, quinoid and resin crosslinkings have been put to practical use. Further, the development of special purpose rubbers has been accompanied by that of new crosslinking methods.

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Crosslinking reaction of sulfur-based rubber compounds with sulfur, accelerators and zinc oxide (ZnO) as crosslinking agents was studied by a time-resolved X-ray adsorption fine structure (XAFS) method. The Zn K-edge XANES (X-ray absorption near-edge structure) spectra obtained during the process were properly decomposed into two components, assigned to ZnO and zinc sulfide (ZnS). The [ZnO]/[Zn] ratio was found to decrease with the increased amounts of sulfur and accelerators, or by the addition of surface active-carbon black. It was confirmed that the decreasing behavior of the amount of ZnO gives a good indicator of the crosslinking reaction by changing of ZnO into ZnS through the chemical reactions with sulfur-containing reagents.

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Tetra-PEG gel has drawn much attention as a polymer gel with extremely suppressed heterogeneity. Heterogeneity is one of the intrinsic characteristics of polymer networks, and is difficult to control. Tetra-PEG gel is formed by a novel fabrication method, i.e., AB-type cross-end coupling, which is polycondensation between mutually reactive multi-armed polymers forming a polymer network. AB-type cross-end coupling has advantages on suppressing the heterogeneity over conventional polycondensation. The reaction achieved up to 90% of reaction conversion. The homogeneity of Tetra-PEG gel was confirmed by small angle neutron scattering; no excess scattering was observed in the range below 200 nm. The models of elastic modulus shift from the phantom network model to the affine network model with an increase in polymer volume fraction. The fracture energy was well predicted by the Lake-Thomas model. The ultimate elongation ratio was not predicted by the Kuhn model, but predicted by our semi-empirical model. These experimental studies suggested that Tetra-PEG gel has extremely homogeneous network structure and is a candidate material for evaluating the basic physical properties of polymer gels.

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This review summarizes the catalyst- and solvent-free grafting and cross-linking of common polymers with unsaturated bonds exploiting kinetically stabilized nitrile N-oxides via click process. The nitrile N-oxides enable the efficient 1,3-dipolar cycloaddition to internal olefins, nitrile groups, and alkynes incorporated into versatile polymers such as polyacrylonitrile (PAN), natural rubber (NR), acrylonitrile butadiene rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and internal alkyne-containing polymer to give the corresponding functionalized polymers. The reaction features i) catalyst-free procedure, ii) versatile applicability to various polymers, iii) high yield, iv) no byproduct, and v) remarkable acceleration of reaction rate under solvent-free condition. The applications of the stable nitrile N-oxide-based reactions to cross-linking reaction are also discussed.

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Polymerization of a monomer in the presence of cyclic macromonomer in high concentration leads mechanically linked three-dimensional network polymer. The network structure is formed by the threading of the cyclic moiety by a segment of another polymer chain during the copolymerization process. This class of materials has attracted a great deal of interest as new functional materials with unusual chemical, physical and mechanical properties due to the high degrees of freedom in segmental movement since cross-linking points can move in these three-dimensional structures.

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The mechanism of the chemical deterioration of black-filled ethylene propylene diene terpolymer (EPDM) in sodium hypochlorite (NaOCl) solution of pH 4.5 was studied at various temperatures and free available chlorine (FAC) concentrations. At pH 4.5, FAC exists predominantly as undissociated hypochlorous acid (HOCl). At relatively low temperatures, HOCl diffused into and reacted with EPDM without changes in appearance of EPDM surface. Morphological changes of EPDM surface, and diffusion and reaction of HOCl were accelerated at elevated temperatures and FAC concentrations. X-ray photoelectron spectroscopy (XPS) analysis showed that C-Cl, C-O, C=O and COO groups were formed on EPDM surface. When the pH of HOCl solution became less than 2.5 after immersion of EPDM, the content of C-Cl group increased markedly, suggesting that chlorine radical might involve in the formation of C-Cl group. These results indicated that chlorination, hydroxylation, oxidization, crosslinking and chain scission reactions occurred between HOCl and EPDM, depending on temperature and FAC concentration. On the basis of the above data, we proposed deterioration processes of EPDM by HOCl.

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