The effects of three series of thermoplastic polyurethane-based (PU) low-profile additives (LPA) with different chemical structures and molecular weights on the glass transition temperatures and mechanical properties for thermoset polymer blends made from styrene (ST), unsaturated polyester (UP), and LPA have been investigated by an integrated approach of static phase characteristics-cured sample morphology-reaction conversion-property measurements. The three series of PU used were made from 2,4-tolylene di-isocyanate (2,4-TDI) and varied diols, namely polycaprolactone diol (PCL), poly(diethylene adipate) diol (PDEA), and poly(propylene glycol) diol (PPG), respectively, while the two UP resins employed were synthesized from maleic anhydride (MA) and 1,2-propylene glycol (PG) with and without modification by phthalic anhydride (PA). Based on the Takayanagi mechanical models, factors that control the glass transition temperature in each phase region of cured samples, as identified by the method of thermally stimulated currents (TSC), and mechanical properties will be discussed.