IGC investigation of the effect of the length of the n-alkyl substituent in 5-alkylsubstituted norbornenes on solute retention.

Polymerization of 5-n-alkyl-substituted 2-norbornenes synthesized a series of polymers having the same structure of the main polymer chain, but differing in the length of the alkyl substituent (up to 14 methylene units). The obtained polymers were studied by the capillary IGC method as a stationary phase during separation of a mixture of normal hydrocarbons C6-C10. Retention data in the form of a logarithm of the retention factor lnk were correlated with the size of the sorbate (via the carbon number of the alkane ZS) and with the size of the n-alkyl substituent in the polymer chain (via the carbon number of the polymer ZP). Correlation of lnk vs. ZS turned out to be linear for all polymers, but the angle of the slope of linear dependence dlnk/dZS increases with a decrease in the carbon number of the polymer ZP. Dependency of dlnk/dZS vs. ZP is not linear and indicates an increase in the retention of sorbates by the stationary phase with a decrease in the length of the alkyl substituent in the polymer chain. The correlation of the retention of lnk analytes with the carbon number of the polymer ZP is not linear and indicates an increase in the sorbate/sorbent interaction with a decrease in the length of the alkyl substituent. Inflection points were found at both correlations with ZP in the region of ZP = 8, which indicates a possible change in the sorption mechanism or a change in the phase state of the polymer. In polymer chemistry, the phase state of a polymer is characterized by the glass transition temperature Tg, the dependence of which vs. ZP turned out to be nonlinear with an inflection point at ZP ∼11. Thus, a decrease in the length of the alkyl substituent leads to the transition of the polymer from a rubbery state to a glassy one at ZP ∼ 11, which in turn, with a further decrease in the carbon number of the polymer to ZP ∼ 8, causes a change in the sorption mechanism from bulk sorption to surface sorption. The change in the sorption mechanism is accompanied by an increase in the interaction of the sorbate with the stationary phase, which manifests itself both in an increase in the retention time of analytes and in an increase in the enthalpy and entropy of sorption. The reason for this increase can be seen in the formation of a microporous structure in 5-alkyl-substituted polynorbornenes in a glassy state.