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 1 and 2 are formed, respectively. The reactions of the poly(methacrylic acid) radicals were investigated by pulse radiolysis (using optical and conductometric detection), EPR, product analysis, and kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under conditions in which the polymer is largely deprotonated, the primary radical 1 abstracts a hydrogen (k= 3.5 x 10(2)s(-1)) from the methylene group, and this yields the more stable secondary radical 2. This radical undergoes chain scission by beta-fragmentation (k= 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k= 80 dm3mol(-1)s(-1)). This allows effective depolymerization (depropagation) to take place (k=0.1 s(-1)). The yield of monomer release is higher than the original radical yield by up to two orders of magnitude. Once monomer is formed, it reacts with 3 (propagation, k= 15 dm3mol(-1)s(-1)), and a situation close to an equilibrium radical polymerization is approached. From these data, the equilibrium monomer concentration is calculated at 6.7 x 10(-3) mol dm(-3) at room temperature. The standard entropy of propagation is estimated at -185 to -150 J mol(-1)K(-1). Because the monomer reaches concentrations in the millimolar range, the *OH radicals increasingly react with monomers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prolonged irradiation. In acid solutions, the decay of the polymer radicals becomes much faster (estimated at about 10(7)dm3mol(-1)s(-1) at pH3.5), and monomer release is no longer observed.