The invention relates to the carbonylation of cross linked substituted and unsubstituted polysaccharides, their copolymers with macroporous synthetic polymers, macroporous synthetic polymers and rigid supports with pendant hydroxyalkyl groups. The carbonylated product can be used to prepare an affinity chromatography matrix which remains an uncharged species at varying pH's. The carbonylated product is also useful for the preparation of other compounds: Typical polysaccharides are agarose, starch, dextran, cellulose and regenerated cellulose, typical macroporous synthetic polymers are acrylamides, acrylates and methacrylates, typical rigid supports are silica beads coated with hydroxy alkyl groups and typical cabonylating agents are N,N'-carbonyl diimidazole; N,N'carbonyl di-1,2,3-benzotriazole; and N,N'-carbonyl di-1,2,4-triazole.