A process for preparing a reinforced resin material comprising the steps of: (i) providing an inorganic substrate, said inorganic substrate being glass, basalt, carbon fibres, carbon nanotubes, inorganic nanotubes, or metal fibres (ii) applying to the inorganic substrate a sizing composition comprising a coupling activator compound of the formula: S-X-A and S represents an organosilane group, through which the compound is bonded to the surface of the inorganic substrate, of the formula: wherein X is as defined below and R 1, R 2 and R 3 may be the same or different and each may represent alkyl, aryl, alkoxy, halogen, hydroxy, or a cyclic structure wherein X is connected with one or more of R 1, R 2 and R 3, A represents a blocked isocyanate precursor of a ring-opening polymerization activator moiety, which is capable of participating in an in situ ring-opening polymerization of a monomer in the presence of a polymerization catalyst when exposed to ring-opening polymerization conditions, and X represents a linking moiety capable of linking the S moiety and the A moiety. (iii) mixing the sized inorganic substrate with a monomer and a ring-opening polymerization catalyst to form a polymerization mixture; and (iv) exposing the polymerization mixture to conditions sufficient to cause an in situ ring-opening polymerization of the monomer to form a reinforced resin in which the inorganic substrate is grafted onto a polymer. (i) providing an inorganic substrate, said inorganic substrate being glass, basalt, carbon fibres, carbon nanotubes, inorganic nanotubes, or metal fibres (ii) applying to the inorganic substrate a sizing composition comprising a coupling activator compound of the formula: S-X-A and S represents an organosilane group, through which the compound is bonded to the surface of the inorganic substrate, of the formula: wherein X is as defined below and R 1, R 2 and R 3 may be the same or different and each may represent alkyl, aryl, alkoxy, halogen, hydroxy, or a cyclic structure wherein X is connected with one or more of R 1, R 2 and R 3, A represents a blocked isocyanate precursor of a ring-opening polymerization activator moiety, which is capable of participating in an in situ ring-opening polymerization of a monomer in the presence of a polymerization catalyst when exposed to ring-opening polymerization conditions, and X represents a linking moiety capable of linking the S moiety and the A moiety. S represents an organosilane group, through which the compound is bonded to the surface of the inorganic substrate, of the formula: wherein X is as defined below and R 1, R 2 and R 3 may be the same or different and each may represent alkyl, aryl, alkoxy, halogen, hydroxy, or a cyclic structure wherein X is connected with one or more of R 1, R 2 and R 3, A represents a blocked isocyanate precursor of a ring-opening polymerization activator moiety, which is capable of participating in an in situ ring-opening polymerization of a monomer in the presence of a polymerization catalyst when exposed to ring-opening polymerization conditions, and X represents a linking moiety capable of linking the S moiety and the A moiety. (iii) mixing the sized inorganic substrate with a monomer and a ring-opening polymerization catalyst to form a polymerization mixture; and (iv) exposing the polymerization mixture to conditions sufficient to cause an in situ ring-opening polymerization of the monomer to form a reinforced resin in which the inorganic substrate is grafted onto a polymer.