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 5 0.01 and r DEF 5 0.81 at 60 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 a relaxation (T a 5 180 and 100 C, respectively), lower elastic moduli at the rubbery plateau (G 0 5 10 8 and 10 7 Pa) and lower mechanical properties as measured by three points bending tests. Strain at break (e f 5 1.8 6 0.2%) and Charpy impact strength (2.7 6 0.3 kJ m 22 ) are comparable independently of the RD chemical nature.
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