Synthesis of first- and second-generation imidazole-terminated POSS-core dendrimers and their pH responsive and coordination properties

Imidazole-terminated first- and second-generation poly(amidoamine)-type oligomeric silsesquioxanes (POSS)-core dendrimers (denoted as POSS-Im16 and POSS-Im32, respectively) were synthesized by the ester-amide exchange reaction from first- and second-generation methyl ester-terminated POSS-core dendrimers, respectively. Transmittances of the aqueous POSS-Im32 solution drastically decreased at pH > 7.2, but the change was reversible without hysteresis. The results of pH titrations for POSS-Im16 and POSS-Im32 showed well-defined one-step titration curves identical to those of monomeric imidazole compounds, such as 1-methylimidazole. Such simple acid-base behavior caused phase transition in the aqueous dendrimer solution to be highly sensitive to pH changes. The pKa values of 1-methylimidazole, POSS-Im16 and POSS-Im32 were 7.3, 7.0 and 6.7, respectively. Spectrophotometric titrations of the aqueous dendrimer solutions with Cu2+ ions indicated that coordination modes of POSS-Im16 changed from the Cu2+-N2O2 complex to the Cu2+-N-4 complex as the concentration of the dendrimers increased; however, only one complexation mode (Cu2+-N-4) existed between Cu2+ ions and POSS-Im32. Polymer Journal (2012) 44, 353-359; doi:10.1038/pj.2011.145; published online 18 January 2012