Study of the composition of low-molecular compounds in the oligomethylhydridesiloxanes

The presence of reactive hydrogen-silyl (SiH) bonds in oligomethylhydrosiloxanes provided their widespread use in the building, paper, textile and rubber industries. Low molecular weight fractions of methylhydrosiloxanes, both cyclic and linear, negatively affect the performance of oligomer-based materials. The main goal of the present study is detection and identification of these fractions in the commercial product 136-157M, as well as in the co-oligomers synthesized from it. A method of gas chromatography-mass spectrometry was used in the study. First, we proved that an increase in the temperature of the column and evaporator from 60°C to 300°C does not change the composition of the low molecular weight fractions. Consequently, no depolymerization processes related to the formation of low-molecular-weight compounds occur during the analysis. To perform a qualitative analysis, the components of low-molecular-weight fraction were separated on a capillary column and then identified using a mass detector with an ion trap. It is shown that electron ionization triggers fragmentation of molecules and the formation of positively charged ions. As for methylhydrosiloxanes, cations with a molecular weight of [M – 15]⁺ arising from the rupture of the carbon-silyl bond are formed first. Moreover, when such a rupture occurs in cyclosiloxanes with a total number of units of more than four, further dissociation processes can occur with the detachment of the neutral fragment (methylsilane) and the formation of bicyclosiloxane cations. Since the set of positive ions is individual for each of the components and depends on [M]^+•, a component-wise description of the entire low molecular weight fraction becomes possible with a high degree of the reliability. It is sown that for the considered commercial sample it can be described by two homologous series — cyclic where m ≥ 4, and linear siloxanes where n ≥ 1

gas chromatography, mass spectrometry, methylhydrosiloxanes, homologous series, electronic ionization, fragmentation

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