Effect of SnCl4 on the radical‐initiated polymerization of diethyl itaconate: Kinetical and ESR studies

The effect of SnCl4 on the polymerization of diethyl itaconate (1) with dimethyl 2,2′‐azoisobutyrate (2) in benzene was investigated kinetically and ESR spectroscopically. The polymerization rate (Rp) at 50°C shows a flat maximum on varying the SnCl4 concentration. The molecular weight of the resulting polymer decreases with increasing SnCl4 concentration. The overall activation energy of the polymerization is lowered from 52 to 33 kJ · mol−1 by the presence of SnCl4, (0,342 mol · L−1). An NMR study revealed that 1 and SnCl4 form 1:1 and 2:1 complexes with a large stability constant in benzene. The propagating polymer radicals in the absence and presence of SnCl4 are ESR‐observable as a five‐line spectrum under the actual polymerization conditions. The complexed polymer radicals show further three‐line splitting due to two methylene hydrogens of the ethyl ester group. The polymer radical concentration increases with the SnCl4 concentration. The rate constant (kp) of propagation was determined using Rp and the polymer radical concentration. kp (6,3–2,9 L · mol−1 · s−1 at 50°C) decreases with increasing SnCl4 concentration. The presence of SnCl4 (0,342 mol · L−1) reduces the activation energy of propagation from 29 to 21 kJ · mol−1. The rate constant (kt) of termination was estimated from the decay curve of the polymer radicals, kt (3,1–1,1 · 105 L · mol−1 s−1) also decreases with the SnCl4 concentration. The activation energies of termination in the absence and presence of SnCl4 (0,342 mol · L−1) are 30 and 24 kJ · mol−1, respectively. Suppression of propagation and termination by SnCl4 seems to be explicable in terms of an entropy factor.