Influence of mid-chain radicals on acrylate free radical polymerization: Effect of ester alkyl group

The benzene solution polymerizations of tertbutyl acrylate (tBA) and 2-ethylhexyl acrylate (EHA) have been investigated with respect to the effects of mid-chain radicals (MCRs) on the rate of polymerization, the total radical concentration, the apparent rate coefficients for propagation, termination, and beta-fragmentation, the contents of unsaturated end groups and branching, and the molecular weight distribution. The EHA polymerization involves a considerably higher concentration of MCR and is more significantly affected by MCR than the tBA polymerization. The MCR content as estimated by electron paramagnetic resonance (EPR) spectroscopy during photosensitized EHA polymerization was as high as 70% of the total radical concentration at 25 degreesC. However, no H-1 and C-13 NMR resonances due to unsaturated ends and branching, respectively, were detected for the poly(EHA) obtained at 25 degreesC. These findings indicate that a high MCR content does not directly correspond to significant MCR effects on the polymer structure. The rate constants for mutual reaction (k(t)(')) and beta-fragmentation (k(f)) of MCR were estimated for the two assumed cases where MCR was consumed solely by (i) mutual reaction, and (ii) beta-fragmentation, based on the aftereffect of the photosensitized EHA polymerization monitored by EPR spectroscopy at 25 degreesC; k(t)(')= 3.5 x 10(3) L . mol . s(-1) and k(f) = 4.2 x 10(-2) s(-1). These rate coefficients were compared with those for reactions of structurally similar radicals.