A single-component, moisture-curing composition, including at least one polyisocyanate, a polyaldimine, an α-functional organoalkoxysilane, and an acid in special proportions that can be matched to one another. An exemplary composition cures by moisture as much as possible without forming bubbles. In the curing of the composition by atmospheric humidity, an anisotropic material is produced with a predominantly elastic sheath that is virtually tack-free on the surface and a predominantly plastic core. An exemplary composition can be especially well suited as a flexible sealant.