Initiation efficiency in the synthesis of molecular brushes by grafting from via atom transfer radical polymerization

Grafting from via atom transfer radical polymerization (ATRP) was employed for the synthesis of densely grafted molecular brushes. A poly(2-(2-bromopropionyl)oxyethyl methacrylate) (PBPEM) macroinitiator was used to polymerize n-butyl acrylate (BA) with various degrees of polymerization. The brush syntheses proceeded with first-order kinetic behavior, and the molecular weights determined by gel permeation chromatography coupled with multiangle laser light scattering (GPC-MALLS) increased linearly with conversion. The PBA side chains (SCs) were cleaved by acid solvolysis, and the resulting products were characterized in order to study the initiation process in the synthesis of molecular brushes. By comparing the molecular weight of the cleaved SCs to their theoretical molecular weights (calculated assuming quantitative initiation), the grafting efficiency was determined as a function of monomer conversion. In the early stages of polymerization, the initiation efficiency is low but gradually increases to 87% at 12% monomer conversion. The results were compared to an analogous linear ATRP conducted under identical conditions, except that ethyl 2-bromopropionate was employed as the initiator. The initiation efficiency for the linear polymerization was consistently higher than that observed for the brushes. The difference was attributed to the congested environment (high local concentration of initiation sites) encountered in the case of grafting from a macroinitiator backbone, which led to slower deactivation of growing radicals at low conversion. The initiation efficiency was enhanced by increasing the rate of deactivation of the growing species or decreasing the rate of propagation.