Compounds of the general formula I′ in which R1 is H, C1–C12alkyl; C5–C12cycloalkyl; C2–C18alkyl which is interrupted by one or more O; SR4; OR5; or is a group of formula II or III R2, R21 and R3 independently of one another, are H or —Y—T as defined in claim 1, where T is a reactive group selected from OH, acryl- and methacryloxy and arylcarbonate groups, and further symbols are as defined in claim 1, are effective as crosslinking agents for polymeric networks, especially as in flexographic printing plates, coatings, and plastic containers or films. Further provided is a method of protecting the content of a clear or lightly colored plastic container or film against the deleterious effects of ultraviolet radiation, which method comprises permanently and covalently bonding one or more UV absorbing moieties of a durable s-triazine UV absorber via condensation to a suitable polymer component.