The present invention relates to novel polyurethane prepolymers (PU prepolymers) of low viscosity containing NCO groups blocked with compounds containing phenol groups and to their use for producing polyurethane polymers. The PU prepolymers are prepared by reacting one or more diisocyanates or polyisocyanates with at least one isocyanate-reactive compound with a functionality of at least 1.5 to form an NCO-functional polyurethane prepolymer and subsequently blocking the free NCO groups with at least one hydrocarbon resin containing phenolic OH groups and/or one optionally substituted phenol in the presence of a catalyst which comprises at least one triorganoamine.