a, Synthetic route to mannose-6-phosphonate-serine N-carboxyanhydride (NCA). b, Synthetic route to M6P-NCA, followed by Ni-catalysed polymerization. Polymerization reactions were carried out in a N2 glovebox for 48 h in tetrahydrofuran. c, d, General synthetic schemes for the polymerization of mannose-NCA (c) and GalNAc-NCA (d). DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; Fmoc, fluorenylmethyloxycarbonyl; mCPBA, meta-chloroperoxybenzoic acid; OTf, triflate.