The free energy of mixing during cure has been modeled for different blends of linear unsaturated polyester, styrene and allyl ether functional hyperbranched polymers. The molecular weight of the blend components has been implemented as measured by gel permeation chromatography (GPC) into the Flory-Huggins equation. This approach enables the distinction to be made between the contributions of enthalpy and entropy of mixing, the latter being automatically followed during curing by the development of average molecular weights. The model allows gaining an idea of the factors acting as driving forces for chemically induced phase separation in these blends.