Poly(amido-amine)s with novel molecular architecture: synthesis and thermodynamic studies of protonation and metal (Zn, Cu) ion complexes

Abstract

New short chain poly(amido-amine)s (SPAAs) carrying amido- and tertiary amino groups along the polymer chain separated by a single methylene group have been synthesized by polycondensation of N,N'-bis-(2-chloro- or bromo-acetyl)-piperazine with N,N'-dimethyl-hexamethylenediamine or 2-methyl-piperazine. The basicity of the SPAAs was lower compared to the PAAs obtained previously by polyaddition of bis-amines to bis-acrylamides. The shortening of the methylene chain from two (PAAs) to one (SPAAs) reduced the logK values of both tertiary amino groups present in the repeating unit due to inductive effects. Furthermore, the shortening resulted in basicity constants, that depend on pH, with respect to the first step of protonation. The real changes in enthalpy (-Delta H-0) of protonation of the SPAAs are greater than those found for PAAs due to the fact that protonated nitrogens and amido C=O groups are closer, favouring the formation of five-member rings. Five-member rings are formed also with metal(II) ions, such as Cu(II) and Zn(II). giving complex species more stable than those of PAAs, with higher log beta and larger -Delta H-0 values. Low entropy changes (Delta S-0) and molar absorption coefficient (epsilon) values support strongly a well ordered chelate structure involving three condensed five-membered rings in an octahedral distorted geometry.