The invention relates to a cross-linking agent for the cross-linking of matrix materials, the matrix which results therefrom, the method used therefor, and the use of said cross-linking agent. Said cross-linking agent contains functionalised polyhedral oligomer silicon-oxygen cluster units, according to formula (I) [(RaXbSiO1,5)m (RcXdSiO)n (ReXfSi2O2,5)o (RgXhSi2O2)p], wherein a, b, c are equal to 0-1; d is equal to 1-2; e, f, g are equal to 0-3; h is equal to 1-4; m+n+o+p is greater than or equal to 4; a+b is equal to 1; c+d is equal to 2; e+f is equal to 3 and g+h is equal to 4; R represents a hydrogen atom, an alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkinyl, cycloalkinyl, aryl, or heteroaryl group or a polymer unit which are respectively substituted or non-substituted, or other functionalised polyhedral oligomer silicon-oxygen cluster units which are bound by a polymer unit or a bridge unit; X represents an oxy, hydroxy, alkoxy, carboxy, silyl, alkylsilyl, alkoxysilyl, siloxy, alkylsiloxy, alkoxysiloxy, silylalkyl, alkoxysilylalkyl, alkylsilylalkyl, halogen, epoxy, ester, fluoroalkyl, isocyanate, blocked isocyanate, acrylate, methacrylate, nitrile, amino, or phosphine group or substituents of type R comprising at least one such group of type X, the substituents of type R being the same or different and the substituents of type X being the same or different.