Initiator efficiency of 2,2′-azobis(isobutyronitrile) in bulk dodecyl acrylate free-radical polymerizations over a wide conversion and molecular weight range

The initiator efficiency, f, of 2,2′-azobis(isobutyronitrile) (AIBN) in dodecyl acrylate (DA) bulk free-radical polymerizations has been determined over a wide range of monomer conversion in high-molecular-weight regimes (M _n cong; 10⁶ g mol^-1 [≅ 4160 units of DA)] with time-dependent conversion data obtained via online Fourier transform near infrared spectroscopy (FTNIR) at 60°C. In addition, the required initiator decomposition rate coefficient, k_d, was determined via online UV spectrometry and was found to be 8.4 · 10^-6 s^-1 (±0.5 · 10^-6 s^-1) in dodecane, n-butyl acetate, and n-dodecyl acetate at 60°C. The initiator efficiency at low monomer conversions is relatively low (f = 0.13) and decreases with increasing monomer to polymer conversions. The evolution of f with monomer conversion (in high-molecular-weight regimes), x, at 60°C can be summarized by the following functionality: f^60°c (x) = 0.13-0.22 · x + 0.25 · x² (for x ≤ 0.45). The reported efficiency data are believed to have an error of >50%. The ratio of the initiator efficiency and the average termination rate coefficient, (k_t±, (f/〈k_t〉) has been determined at various molecular weights for the generated polydodecyl acrylate (M_n = 1900 g mol^-1 (≅ 8 units of DA) up to M_n = 36,500 g mol^-1 (≅ 152 units of DA). The (f/〈k_t〉) data may be indicative of a chain length-dependent termination rate coefficient decreasing with (average) chain length.