Improving imprinting effect by reducing sites embedding: Selective extraction of 1,2,3,4,6-penta-O-galloyl-β-d-glucose from Paeonia lactiflora Pall by hydrophilic molecularly imprinted polymers based on macromonomer and metal ion pivot

•Combination of macromonomer and metal ion pivot was used for reducing sites embedding of MIP.•OEG was used as macromonomer and Ni2+ was adopted as metallic pivot.•Necessity of simultaneous use of OEG macromonomer and Ni2+ was proved.•The Ni2+ OEG MIP made can be used for SPE to purify β-PGG from crude extract.