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, γ = [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, γ ≫ 1, the degree of branching is given as DB ≈ 2/(γ + 1), and γ corresponds to an average number of tBuA units between branch points. The Mark−Houwink exponents of these branched PtBuAs obtained at γ = 0.5−100 are significantly lower (α = 0.38−0.47) than that of linear PtBuA (α = 0.80). The nature of the ligand and polymerization temperature affect the molecular weights and chain architectures, while the comonomer-to-catalyst ratio, μ = ([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, 1 H 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.