A method for making a copolymer polyol includes (a) forming a pre-compounded thermoplastic polymeric composition by compounding from 10 wt % to 90 wt % of a solid functional additive component with from 10 wt % to 90 wt % of a polystyrene component, (b) melting the pre-compounded thermoplastic polymeric composition to form a melted thermoplastic polymeric composition, (c) combining the melted thermoplastic polymeric composition with a carrier polyol component in the presence of a stabilizer component to form a pre-mixture that includes the pre-compounded thermoplastic polymeric composition dispersed within a continuous phase of the carrier polyol component, and (d) cooling the pre-mixture to form the copolymer polyol.