Stabilized, finely disperse low-viscosity polymer polyols with a high content of polystyrene or polystyrene copolymers

A process for the preparation of the finely dispersed, low-viscosity polymer polyols, comprising

• (I) polymerizing (B) a polymer comprising (1) styrene, or (2) a mixture of styrene and at least one other ethylenically unsaturated monomer which can be copolymerized with styrene,
• in the presence of (A) a base polyol, and (C) a polymer containing mercaptan groups and/or disulfide groups which is compatible with said base polyol (A) and has a number average molecular weight of about 500 to 100,000 and contains an average of at least 0.5 mercaptan groups or 1 disulfide group per molecule, and (D) an active amount of a free-radical initiator, and (E) a polar solvent, wherein component (C) is present in an amount of 1 to 6% by weight, and component (B) is present in an amount of 30 to 60% by weight, with the %'s by weight being based on the combined weight of components (A), (B) and (C).