Development of a novel synthetic method for aliphatic ester dendrimers

Abstract

Aliphatic ester dendrimers based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) growth units were successfully synthesized as a form of dendritic hybrids up to G4. Linear polystyrene supports in the dendritic hybrids were obtained through atom transfer radical polymerization at 130 degrees C for 12 h using the CuCl/PMDETA catalyst/ligand system with 4-(chloromethyl)benzyl alcohol as the initiator. An optimized molecular weight of 8700 was chosen as the support, considering the difference in solubility between the dendritic hybrids and the growth units as well as the effective characterization of the end functional groups in the H-1 NMR analysis. The dendritic repeating units were grown from the benzyl alcohol end functional group of the support via a typical esterification process in methylene chloride for 12-24 h at 25 degrees C. The resulting hybrids were purified by a simple precipitation into methanol without any chromatographic separation, utilizing the difference in solubility between the poorly soluble dendritic hybrids and highly soluble growth units. At the final step, the dendrimers were obtained via the selective cleavage of the benzyl ester group connecting the polymeric supports and the dendrimers using a paladium(II) acetate catalyst in DMF (10 equiv volume of dendritic hybrid) for 24 h under the hydrogen pressure of 6 atm. The dendritic hybrids and dendrimers obtained with 100% conversion yield were characterized by H-1 NMR, C-13 NMR, and gel permeation chromatography. It is believed that this simplified purification procedure in the proposed synthesis will be suitable for the production of high-generation dendrimers on a large scale.