Effect of the direction of ester linkage on molecular shape selectivity through multiple carbonyl–π interaction with octadecyl chain branched polymers as organic phases in reversed-phase high-performance liquid chromatography

Poly(vinyl octadecanoate)-grafted porous silica (Sil-VODn, n = 23) was newly prepared to investigate the efficiencies of the carbonyl groups in the polymer chain for recognition of polycyclic aromatic hydrocarbons (PAHs) in RP-HPLC. In Sil-VOD23, the octadecyl side chains were connected to the polymer main chain through ester linkage in opposite direction to that in poly(octadecylacrylate)-grafted silica (Sil-ODAn, n = 25) which has been reported by us. Sil-ODAn performs enhanced molecular shape selectivity of PAHs in RP-HPLC through multiple carbonyl–π interaction of aligned carbonyl groups which are induced by the formation of highly oriented structure of side chains. Differential scanning calorimetry of VOD23 demonstrated that octadecyl alkyl chains showed crystalline to isotropic phase transition with endothermic peak at 48.7 °C which was similar to ODA25 (at 47.8 °C). After grafting of both polymers, phase transition phenomenon was completely disappeared in Sil-VOD23 whereas Sil-ODA25 still exhibits phase transition although at lower endothermic peak top temperature (38.5 °C). This indicates that the slight structural change in Sil-VODn and Sil-ODAn influence the ordered structure of side alkyl chains. Moreover, solid-state 13C NMR revealed that the long alkyl chain in Sil-VOD23 is highly disordered as compared with that of Sil-ODA25. Sil-VOD23 was applied to RP-HPLC stationary phase using PAHs as π-electron containing elutes, and compared with Sil-ODA25 and conventional monomeric octadecylated silica (ODS). Results confirmed that Sil-VOD23 showed much higher selectivity for PAH isomers than ODS, but lower than Sil-ODA25. For example, the separation factors for trans-/cis-stilbene were 1.47 (Sil-VOD23), 1.70 (Sil-ODA25) and 1.07 (ODS), respectively. These results indicate that carbonyl groups in Sil-VOD23 are effective for molecular shape recognition of PAHs through carbonyl–π interactions even in the disordered state.