Free-radical-induced chain breakage and depolymerization of poly(methacrylic acid): Equilibrium polymerization in aqueous solution at room temperature

Hydroxyl radicals, generated by ionizing radiation in N2O saturated aqueous solutions, abstract H atoms from poly(methacrylic acid) at the methyl and methylene groups, and radicals I and 2 an formed, respectively. The reactio ns of the poly(methacrylic acid) radicals were investigated by pulse radiol ysis (using optical and conductometric detection), EPRI product analysis, a nd kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under renditions in which t he polymer is largely deprotonated, the primary radical 1 abstracts a hydro gen (k = 3.5 x 10(2) s(-1)) from the methylene group, and this yields the m ore stable secondary radical 2. This radical undergoes chain scission by be ta -fragmentation (k = 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k = SO dm(3)mol(- 1)s(-1)). This allows effective depolymerization (depropagation) to take pl ace (k = 0.1 s(-1)). The yield of monomer release is higher than the origin al radical yield by up to two orders of magnitude, Once monomer is formed, it reacts with 3 (propagation, k = 15 dm(3)mol(-1)s(-1)), and a situation c lose to an equilibrium radical polymerization is approached. From these dat a, the equilibrium monomer concentration is calculated at 6.7x10(-3) moldm( -3) at room temperature. The standard entropy of propagation is estimated a t - 185 to - 150 Jmol(-1) K-1. Because the monomer reaches concentrations i n the millimolar range, the (OH)-O-. radicals increasingly react with monom ers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prol onged irradiation. In acid solutions, the decay of the polymer radicals bec omes much faster (estimated at about 10(7)dm(3)mol(-1)s(-1) at pH 3.5), and monomer release is no longer observed.