A process for preparing guanidino-functional, free radically polymerizable compounds comprises (a) combining (1) an amine compound comprising (i) at least one primary aliphatic amino group and (ii) at least one secondary aliphatic amino group, primary aromatic amino group, or secondary aromatic amino group, and (2) a guanylating agent; (b) allowing or inducing reaction of the amine compound and the guanylating agent to form a guanylated amine compound; (c) combining (1) the guanylated amine compound, and (2) a reactive monomer comprising (i) at least one ethylenically unsaturated group and (ii) at least one group that is reactive with an amino group; and (d) allowing or inducing reaction of the guanylated amine compound and the reactive monomer to form a guanidino-functional, free radically polymerizable compound.