Effect of initiation conditions on the uniformity of three-arm star molecular brushes

A series of three-arm star densely grafted molecular brushes with poly(n-butyl acrylate) side chains were prepared by atom transfer radical polymerization (ATRP). The three-arm macroinitiators of poly(2-bromopropionyloxyethyl methacrylate) were prepared by ATRP of 2-trimethylsilyloxyethyl methacrylate using trifunctional initiators 1, 1, 1-tris(4-(2-bromoisobutyryloxy)phenyl)ethane and different catalytic systems, i.e., a (CuX)-X-I/dNbpy complex (X = Br, Cl) with and without (CuX2)-X-II (X = Br, Cl) as deactivators, followed by the subsequent esterification with 2-bromopropionyl bromide. The kinetics exhibited first-order reaction with respect to the monomer concentration in all systems. Polymerization rates decreased with the catalyst in the order of CuBr > CuBr/CuBr2 > CuCl > CuCl/CuCl2. The different initiation conditions also affect the polydispersities of the final brushes, although in all cases the number-average molecular weight (M-n) increases with the increasing monomer conversion. AFM measurements of multiarm brushes enabled quantitative analysis of the length distribution. Because of visualization of individual starlike molecules, one could separately analyze the brush arms and the whole molecule. The polydispersities of the arm length decreased in the order of CuBr > CuBr/CuBr2 > Cucl > CuCl/CuCl2. The CuCl/CuCl2 system seems to be the best catalyst for the star synthesis yielding brushes with narrow length distribution. CuBr is a relative "worst" catalyst system in which several coupled molecular brushes were observed, while some arms were missing. These led to the broadest distribution of arm length. Similar analysis for four-arm molecular brushes was performed. The polydispersity of the total length was significantly lower than those of the arm length. Here, the total length polydispersity displayed amazing agreement with Schulz-Flory theory for chain coupling.