The present invention relates to an ultra low monomer PU prepolymer containing free NCO groups obtainable from polyols, diisocyanates and NCO-reactive compounds, wherein at least one polyol is reacted with at least one polyisocyanate, wherein at least one polyisocyanate is In molar excess with respect to the hydroxyl groups of at least one polyol to obtain a polyurethane prepolymer containing free isocyanate groups; At least one compound having at least one H-acidic functional group is reacted in a molar ratio (NCO: XH) of free isocyanate groups to H-acidic functional groups of 2 to 15, preferably 2.5 to 10, more preferably 3 to 8 To a polyurethane prepolymer containing a free isocyanate group in an amount sufficient to give a polyisocyanate group. Also included are methods of making the same, laminating adhesives containing the prepolymer, bonding methods of the substrates and the use of the adhesives described for laminating two or more films.
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