The invention relates to a method for producing NCO-modified polyoxymethylene block copolymers comprising the step of polymerizing formaldehyde in a reaction vessel in the presence of a catalyst, the polymerization of formaldehyde in addition taking place in the presence of a starter compound comprising at least 2 Zerewitinoff active H atoms to obtain an intermediate product. The intermediate product obtained is reacted further with an isocyanate to obtain an NCO-modified polyoxymethylene block copolymer. The invention also relates to NCO-modified polyoxymethylene block copolymers obtained by a method of this type and to the use of said copolymers for producing polyurethane polymers.

The present invention relates to a composition comprising a mixture of a first and a second hydrophobically modified alkylene oxide polymer, wherein the first hydrophobically modified alkylene oxide polymer is endcapped with at least one first hydrophobic group functionalized with a secondary amine or a salt thereof, or a tertiary amine or a salt thereof; and wherein the second hydrophobically modified alkylene oxide polymer is endcapped with at least one second hydrophobic group, structure I: ##STR00001##
where R.sup.1, R.sup.2, m, and n are as defined herein. The present invention also relates to a method for preparing the composition. The composition of the present invention provides an associative thickener with an excellent balance of as-is viscosity and temperature stability over a wide temperature range.