The specification is concerned with sealant compositions comprising prepolymers which are capable of being cured by atmospheric moisture. The compositions provide a desirable blend of properties with regard to application and use, using prepolymers derived from comparatively inexpensive raw materials. The prepolymers correspond to a reaction product of an at least substantially linear acrylate ester polymer having a Tg of not greater than 10.degree. C. with a hydrolysable silane having a saturated organo-functional group, the reaction product being a fluid material soluble in organic solvents and curable by exposure to atmospheric moisture to a resilient solid. The prepolymer is a silane-modified polymer which preferably has from 2 to 3 hydrolysable silane groups per molecule of prepolymer and from 1 to 3 hydrolysable groups per silane group. Preferred acrylate ester polymers are homo- or co-polymers of butyl acrylate; copolymers may contain other acrylate monomers and or may contain up to 50 wt % vinyl monomer e.g. styrene. Preferred silanes are amino-silanes particularly those having 2 or 3 hydrolysable alkoxy groups. Also described and claimed are methods of making the prepolymers. The compositions are suited for construction and glazing sealants.