Vinyl levulinate (VL) is used as a biobased reactive diluent in styrene (St)-free unsaturated polyester resins (UPR). The reactivity ratios for the radical copolymerization of VL with diethyl fumarate (DEF) are determined by the Jaacks method (r(VL)=0.01 and r(DEF)=0.81 at 60 degrees C in DMSO-d(6)). The properties of UPRs having a stoichiometric ratio between unsaturated groups from the UPR and either St or VL are compared. Defect-free, slightly yellow, transparent, and rigid thermosets are obtained after a mild curing cycle. Due to unfavorable reactivity ratios about 5.5 wt % of unpolymerized VL remains inside the network and acts as plasticizer. Consequently, compared with St-based ones, VL-based UPRs exhibit lower relaxation (T=180 and 100 degrees C, respectively), lower elastic moduli at the rubbery plateau (G=10(8) and 10(7) Pa) and lower mechanical properties as measured by three points bending tests. Strain at break (epsilon(f)=1.8 +/- 0.2%) and Charpy impact strength (approximate to 2.7 +/- 0.3 kJ m(-2)) are comparable independently of the RD chemical nature. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3356-3364