|168. Mori, H.; Chan Seng, D.; Lechner, H.; Zhang, M.; Müller, A.H.E.: Synthesis and Characterization of Branched Polyelectrolytes 1. Preparation of Highly Branched Poly(acrylic acid) via Self-Condensing Atom Transfer Radical Copolymerization, Macromolecules 35, 9270 (2002)|
We report on the synthesis of randomly branched (arborescent) poly(acrylic acid) (PAA) by self-condensing vinyl copolymerization (SCVCP) of an acrylic AB* inimer, 2-(2-bromopropionyloxy)ethyl acrylate (BPEA), with tert-butyl acrylate (tBuA) via atom transfer radical polymerization (ATRP), followed by hydrolysis of tert-butyl groups. Depending on the comonomer ratio, = [tBuA]0/[BPEA]0, branched PtBuAs with number-average molecular weights between 8000 and 76 000 and degrees of branching (DB) between 0.48 and 0.02 were obtained by SCVCP, as evidenced by GPC, GPC/viscosity, GPC/MALS, and NMR analysis. For the case of high comonomer ratios, 1, the degree of branching is given as DB 2/( + 1), and corresponds to an average number of tBuA units between branch points. The Mark-Houwink exponents of these branched PtBuAs obtained at = 0.5-100 are significantly lower ( = 0.38-0.47) than that of linear PtBuA ( = 0.80). The nature of the ligand and polymerization temperature affect the molecular weights and chain architectures, while the comonomer-to-catalyst ratio, = ([tBuA]0 + [BPEA]0)/[CuBr]0, has a slight influence only on these parameters. Subsequent cleavage of the tert-butyl ester moieties by acidic hydrolysis gave randomly branched polyelectrolytes, PAA, as confirmed by elemental analyses, 1H NMR, and FT-IR measurements. Aqueous-phase GPC and dynamic light scattering confirm the compact structure of the branched PAAs. Their water solubility and their size depend on the degree of branching and on pH.