The effect of size on the rate of an aminolysis reaction using a series of amine terminated PAMAM dendrimers

When compared to an equivalent amine concentration of ethylenediamine, the initial rate of a simple aminolysis reaction in water (at pH 8.5) was found to be greatly enhanced by the use of amine terminated PAMAM dendrimers of generations 1–5 (with 4, 8, 16, 32 and 64 terminal amines, respectively). The fourth generation dendrimer gives the maximum rate enhancement, even when compared to the larger 64 amine terminated dendrimer. The observed increase in aminolysis rates is due to hydrophobic binding of the substrate within the outer region of the larger dendrimers, which results in the substrate being held in close proximity to the reactive amine groups on the surface. Another contributory factor is transition state stabilisation by the internal amide groups within the dendrimers. The trend in reactivities of dendrimers is discussed and provides strong evidence that both these effects are responsible for the enhanced rates of aminolysis.