UV-curable resins and fiber-reinforced plastics thereof are particularly suitable for repairing defects due to their short curing time. They are already being used in the construction sector (e.g. sewer rehabilitation), the automotive industry (e.g. refinishing business) and in medicine (e.g. tooth repair). One type of UV curing polymerization is the radical polymerization. Radical polymerizing systems may undergo a considerable volume shrinkage. In this work, polymeric diphenylmethane diisocyanate (PMDI) was employed as a primary prepolymer building block. This still allows the formation of highly crosslinked plastics for mechanically demanding applications which nevertheless possess strongly reduced shrinkage. A novel method of refractometry, the Temperature Modulated Optical Refractometry (TMOR) was employed to very accurately assess and monitor volume changes in situ during the polymerization process.