Understanding the Relationship between Catalytic Activity and Termination in photoATRP: Synthesis of Linear and Bottlebrush Polyacrylates

Linear and bottlebrush polyacrylates were prepared by photomediated atom transfer radical polymerization (photoATRP) catalyzed by either CuBr2/TPMA (tris(2-pyridylmethyl)amine) or the more active CuBr2/TPMA*3 (tris([(4-methoxy-2,5-dimethyl)-2-pyridyl] methyl)amine). The latter had a lower rate constant of photoreduction (kred) but unexpectedly enabled faster polymerization. Kinetic simulations showed that the equilibrium concentration of a Br-CuII/L deactivator was larger for CuBr2/TPMA*3, resulting in a faster reduction rate (Rred â kred[Br-CuII/L]) and higher radical concentration. At the same time, the low [CuI/TPMA*3] counterweighed its high tendency to promote catalyzed radical termination (CRT), and the CRT rate was similar for the two catalytic systems. Kinetic simulations proved that (i) relative reaction rates cannot be predicted by the rate constant alone as exhibited by the relative amount of CuI and CuII species mediated by catalyst activity and termination selectivity and (ii) the polymerization steady state is reached faster with more active catalysts. With this understanding, polyacrylate bottlebrushes were synthesized at moderately high conversion by photoATRP.