Pulsed laser polymerization–size exclusion chromatography investigations into backbiting in ethylhexyl acrylate polymerization

Abstract

Free radical acrylate polymerization is associated with a considerable number of side reactions occurring during chain growth. Most notably, secondary chain propagating radicals can undergo intramolecular transfer reactions, forming more stable, tertiary midchain radicals. These midchain radicals can transform back into rapidly propagating secondary radicals by monomer addition. In practice, under steady-state polymerization conditions, an average propagation rate will be observed that is described by three individual rate coefficients. These are the coefficients for secondary radical propagation, k^SPR_p, the backbiting reaction, k_bb, and the tertiary radical monomer addition rate, k^tert_p. Based on our previous work on tert-butyl acrylate, we determined these rate coefficients in the temperature range of 5 to 60 °C via pulsed laser polymerization experiments in a laser frequency range of 1 to 1000 Hz for the monomer 2-ethylhexyl acrylate. Parameter estimations via Predici allowed all rate coefficients to be determined directly from the apparent propagation rate coefficients determined at various frequencies. The obtained Arrhenius relations are E_A(k^SPR_p) = 16.0 kJ mol⁻¹ and ln(A(k^SPR_p)/L mol⁻¹ s⁻¹) = 16.1, E_A(k_bb) = 32.2 kJ mol⁻¹ and ln(A(k_bb)/L mol⁻¹ s⁻¹) = 16.5 and E_A(k^tert_p) = 33.3 kJ mol⁻¹ and ln(A(k^tert_p)/L mol⁻¹ s⁻¹) = 15.1.