AbstractThe copolymerization ofp‐tert‐butoxystyrene (1) (M1) and dibutyl fumarate (2) (M2) initiated with dimethyl 2,2′‐azobisisobutyrate (3) was studied in benzene at 60°C kinetically and by means of electron paramagnetic resonance (EPR) spectroscopy. The monomer reactivity ratios were determined to ber1= 0,18 andr2= 0,01, indicating that homopropagation of M2is almost negligible in the copolymerization. The copolymerization system was revealed to involve EPR‐observable propagating polymer radicals under practical copolymerization conditions. The apparent rate constants of propagation (kp) and termination (kt) determined by EPR show a rapid increase in the range from 0,9 to 1,0 of feed composition (f1= {[M1]/([M1] + [M2])}) of M1. From the relationship betweenkpandf1based on Fukuda's penultimate model, the rate constants of propagation of copolymerization were evaluated;k111= 140 L · mol−1· s−1,k211= 4,3 L · mol−1· s−1,k112= 778 L · mol−1· s−1,k212= 24 L · mol−1s−1and k121= 19 L · mol−1· s−1, sug