Synthesis and characterization of branched polyelectrolytes.: 1.: Preparation of hyperbranched poly(acrylic acid) via self-condensing atom transfer radical copolymerization

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, gamma = [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, gamma much greater than 1, the degree of branching is given as DB approximate to 2/(gamma + 1), and gamma corresponds to an average number of tBuA units between branch points. The Mark- Houwink exponents of these branched PtBuAs obtained at gamma = 0.5-100 are significantly lower (a = 0.38-0.47) than that of linear PtBuA (alpha = 0.80). The nature of the ligand and polymerization temperature affect the molecular weights and chain architectures, while the comonomer-to-catalyst ratio, mu = ([tBuA](0) + [BPEA](0))/[CuBr](0), has a slight influence only on these parameters. Subsequent cleavage of the tertbutyl ester moieties by acidic hydrolysis gave randomly branched polyelectrolytes, PAA, as confirmed by elemental analyses, H-1 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.