Positional and compositional heterogeneity of partially modified poly(propyleneimine) dendrimers

The partial end group modification of dendrimers leads to two types of heterogeneity in the product's structure: compositional heterogeneity (distribution of degree of substitution) and positional heterogeneity (spatial distribution of the substitutents over the dendrimer molecule). Poly(propyleneimine) dendrimers were partially modified by direct amidation at 150°C with stearic acid. The compositional distribution (analysed by HPLC and ES-MS) follows a random pattern, governed by a binomial distribution. The positional distribution can be expressed as the distribution of dyads AA (two unreacted end groups), AB (one unreacted, one reacted) and BB (both reacted), where a dyad consists of two end groups originating at a common final branching point in the dendrimer. 13C NMR and a Cu(II) complexation titration were used for determining the dyad distribution. Lower generations of dendrimers give a random distribution of dyads. In higher generations a marked preference for single substitution of dyads was found, possibly caused by intramolecular interactions or steric hindrance.