Interaction of zinc oxide clusters with molecules related to the sulfur vulcanization of polyolefins ("rubber")

The vulcanization of rubber by sulfur is a large‐scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn₄O₄ (T_d) as a model species to study the thermodynamics of the initial interaction of various vulcanization‐related molecules with ZnO by DFT methods, mostly at the B3LYP/6‐31+G* level. The interaction energy of Lewis bases with Zn₄O₄ increases in the following order: CO62H₄3H₆2S₂<1,4‐C₅H₈2O2S3N?CH₃COO^?. The corresponding binding energies range from ?57 to ?262 kJ mol^?1. However, Brønsted acids react with the Zn₄O₄ cluster with proton transfer from the ligand molecule to one of the oxygen atoms of Zn₄O₄, and these reactions are all strongly exothermic binding energies kJ mol^?1] in parentheses: H₂O (?183), MeOH (?171), H₂S (?245), MeSH (?230), C₃H₆ (?121), and CH₃COOH (?255)]. The important vulcanization accelerator mercaptobenzothiazole (C₇H₅NS₂, MBT) containing several donor sites reacts with the Zn₄O₄ cluster with proton transfer from the NH group to one of the oxygen atoms of ZnO, and in addition the exocyclic thiono sulfur atom and the nitrogen atom coordinate to one and the same zinc atom, resulting in a binding energy of ?247 kJ mol^?1. A second isomer of (MBT)Zn₄O₄] with a strong O? H???N hydrogen bond rather than a Zn? N bond is only slightly less stable (binding energy ?243 kJ mol^?1). The NH form of free MBT is 36 kJ mol^?1 more stable than the tautomeric SH form, while the sulfurized MBT derivative benzothiazolyl hydrodisulfide C₇H₅NS₃ (BtSSH) is most stable with the connectivity >CSSH.