This invention relates to ethylenically unsaturated macromers which contain reactive unsaturation, and to a process for preparing these ethylenicallly unsaturated macromers which contain reactive unsaturation. The process of preparing these ethylenically unsaturated macromers comprises reacting a monofunctional compound which corresponds to a specific formula with at least one alkylene oxide, in the presence of at least one alkoxylation catalyst. The ethylenically unsaturated macromers correspond to the formula set forth herein. This invention also relates to preformed stabilizers comprising ethylenically unsaturated macromers, to a process for preparing these preformed stabilizers, to polymer polyols comprising these preformed stabilizers, and to a process of preparing these polymer polyols which comprise these preformed stabilizers. Polymer polyols comprising ethylenically unsaturated macromers and a process for their preparation are also described.