Polymers of polyacrylamide were found to possess unique properties that made them especially effective as dry-strength resins. Anionic polyacrylamide and cationic polyacrylamide is made by by copolymerizing monomers with special co-monomers which consist of anionic and cationic substitutes. Relying on the controlled ratio of positively-charged and negatively charged groups on the polymer chain, anionic polyacrylamide and cationic polyacrylamide resin can efficiently increase paper-dry strength without causing unfavourable charge imbalance in pulp.