The promising potential of dendrimers in a variety of areas, such as catalysis, materials science and biomedicine is related to their globular shape, large number of modifiable surface functionalities and the presence of internal reservoirs.1 Their use in liquidliquid-phase transfer protocols, based on the encapsulation of guest molecules as drug delivery vehicles for pharmaceutical application, represents an important issue.2 Unfortunately, dendrimer synthesis is timeconsuming, which currently limits practical use to laboratory scale. For that reason, hyperbranched polymers prepared from ABm-type monomers in one-step processes have gained increasing interest.3 The development of the slow monomer addition technique has resulted in well-defined hyperbranched polymers (1.3 < Mw/Mn < 1.5) of narrow polydispersity with peculiar functional group distribution throughout the polymer scaffold, as demonstrated in detailed studies for hyperbranched polyglycerols.4a Partial modification of these hyperbranched polyethers with fatty acids or ketones leads to amphiphilic hyperbranched materials with compact core-shell structures. These structures exhibit dendrimer-like properties, such as the formation of unimolecular micelles.4b,c Hyperbranched poly(ethylenimine)s (PEI) have been used for a long time for various industrial purposes, e.g., as flocculating agents, thickeners, and dispersion stabilizers, and have recently been studied as a prototype for “weak” polyelectrolytes, as a component in hydrogenation processes, and for quantum dot nanocomposites.5 Recently, partial functionalization of PEI via amidation with long alkyl chains afforded amphiphilic hyperbranched polymers with core-shelltype architectures, capable of stabilizing silver nanoparticles and transferring polar dyes into organic solvents.6 In this paper, we describe a comparison of the amidated hyperbranched PEI with the linear analogue with respect to solution properties and phase transfer. Furthermore, the influence of the polarity difference of “core” and “shell” components in the specific function of a “molecular nanocapsule” prepared from hyperbranched polymers has not yet been detailed. Herein we report a comparative analysis of amidated hyperbranched PEI vs the linear analogue in solution and summarize the results of a study of the effect of core polarity on guest encapsulation, using quaternized, amidated-hyperbranched PEI.7 The combination of selective external and internal functionalization of hyperbranched polymers to fine-tune their properties has hardly been addressed. In one previous paper a selective core-functionalization route for hyperbranched polyglycerol cores based on acetal formation has been presented; however, no detailed study of encapsulation properties was carried out.4e Two commercially available hyperbranched PEI samples, namely PEI10K (Mh n ) 104 g/mol, Mh w/Mh n ) 2.5) and PEI25K (Mh n ) 2.5 × 104 g/mol, Mh w/Mh n ) 2.5) have been partially amidated with palmitoyl chloride as previously reported,6a affording organo-soluble, hyperbranched PEI10K)C160.6 and PEI25K)C160.6, respectively (Scheme 1). FT-IR spectra clearly show the typical band of the amide group (υ ) 1635 cm-1). 1H NMR analysis confirmed a degree of amidation of 60%. To render the interior of PEI10K)C160.6 and PEI25K) C160.6 more polar, further quaternization of the residual amine groups (40%) with methyl iodide afforded the fully modified and organo-soluble hyperbranched polymers, namely, PEI10K)C160.6N0.4 and PEI25K) C160.6N0.4 as indicated by the downfield shift of adjacent CH2 protons of the amine groups in the 1H NMR spectra (Supporting Information). The linear analogue with high molecular weight, LPEI15K (Mh n ) 1.5 × 104 g/mol), was obtained after hydrolysis of poly(2-ethyl-2oxazoline) and subsequent partial amidation (60%) of the secondary amine groups, yielding LPEI15K)C160.6 (Scheme 1).8 Covalently modified hyperbranched polymers with hydrophobic shell have been shown to sequester polar dyes from the aqueous phase into organic media.6b To confirm the micellar properties of the amidated PEIs with both neutral and cationic cores, their capacity for polar guest encapsulation have been evaluated and compared with that of the modified linear PEI. To this end, four water-soluble dye probes, namely Eosin Y (EY), Fluorescein Sodium (FS), Methyl Orange (MO) and Congo Red (CR) have been utilized (Figure 1). The encapsulation results are summarized in Table 1. Whereas the linear polymer LPEI15K)C160.6 only transports dyes to an insignificant extent, the hyperbranched amidated PEI10K)C160.6 and PEI25K) C160.6 encapsulate all dyes in large amounts more efficiently than the previously reported hyperbranched PEI containing an imine-bound shell.6b The higher encapsulation capacities of the amide shell in the hyperbranched PEI-based systems is ascribed to the additional contribution of the amide functional groups via hydrogen-bonding interactions. Such multiple secondary interactions originating from the presence of amide groups have been previously used to encapsulate and assemble end groups in a reversible way at the interior as well as at the periphery of dendrimers.2e-f,9 Remarkably, the number of encapsulated dye molecules increased by a factor of about 3 after quaternization. For instance, the loading of encapsulated CR changed from 14.4 in PEI10K)C160.6 to 41.2 dye molecules in the sample PEI10K)C160.6N0.4. The low loading of dyes by the linear amidated polymer was not unexpected, since a previous study on hyperbranched and linear polyglycerols had confirmed the peculiarity of the hyperbranched structure in the context of phase transfer.7c In the case of LPEI, we attribute the low, but not negligible fraction of dye * Corresponding author. E-mail: salah.stiriba@uv.es. † Johannes Gutenberg-Universitat Mainz. ‡ Universidad de Valencia. 227 Macromolecules 2005, 38, 227-229