We have modified the SAFT-EOS by using a temperature and density-dependent segment diameter based on the Weeks, Chandler and Andersen (WCA) perturbation theory of dense fluids. The performance of the resulting equation of state (SAFT-WCA) was investigated through the prediction of high-pressure experimental data of isotactic polypropylene in n-butane, 1-butene, propane and propylene. Results show that the SAFT-WCA EOS improves the prediction of liquid to liquid-liquid phase transitions when compared to the original SAFT-EOS and correctly accounts for the solvent effect on phase equilibrium for all polymer-solvent systems studied, especially at polymer concentrations greater than I wt.%.