Characterization of the vulcanization products of squalene by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Model studies on the vulcanization of natural rubber

A method for the characterization of the vulcanization reaction products of squalene using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was developed. Squalene was chosen as a model molecule for natural rubber. Different matrix-solvent pairs were evaluated with the spectrometer functioning in the linear mode, working with positive/negative high voltages and different laser powers. The study led to the following important conclusions. The identification of squalene requires a cationizing agent whereas the identification of the two families of curing systems used, a sulfenamide and a thiuram, requires only the use of a matrix (dithranol and dihydroxybenzoic acid, respectively). For the first time, the intermediate compounds such as squalene derivatives with pendant sulfur-containing groups and interacting with zinc as well as cross-linked squalene were identified. Only a matrix is required and the solvent acetonitrile leads to better spectra than THF (which oligomerizes during sample preparation). Adequate sample treatment is critical to the success of the method. Suitable conditions were found for the characterization of the vulcanization reaction. MALDI-TOFMS appears to be an important tool for the analysis and deconvolution of the mechanisms of the vulcanization of natural rubber.