Core-Shell-Structured Highly Branched Poly(ethylenimine amide)s: Synthesis and Structure Lydie Antonietti, † Cyril Aymonier, ‡ Ulf Schlotterbeck, ‡ Vasil M. Garamus, § Tatiana Maksimova, ⊥ Walter Richtering, ⊥ and Stefan Mecking* ,† Fachbereich Chemie, Universita ¨ t Konstanz, D-78457 Konstanz, Germany; Institut fu ¨ r Makromolekulare Chemie und Freiburger Materialforschungszentrum der Albert-Ludwigs-Universita ¨ t Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany; GKSS Research Centre, Max Planck Str., D-21502 Geesthacht, Germany; and Institut fu ¨ r Physikalische Chemie, RWTH Aachen, Templergraben 59, D-52056 Aachen, Germany Received December 10, 2004; Revised Manuscript Received March 21, 2005 ABSTRACT: The synthesis of amphiphilic macromolecules by amidation of hyperbranched polyethyl- enimine was studied. Amidation with palmitic acid or the methyl ester proceeds up to 84% degree of amidation (140 °C, vacuum). The primary amine end groups react preferentially. With carbonyldiimidazole (CDI)-activated acids nearly complete conversion of all primary amine end groups and secondary amine linear units can be achieved; with a corresponding limited amount of CDI the end groups can be amidated selectively. The products of these reactions are free of any unreacted carboxylic acid or other impurities ( 1 H and 13 C NMR) and can be optionally purified by pressure ultrafiltration washing with a toluene/ amine solution. Side-chain crystallization is observed (DSC), which can be supressed entirely employing branched alkyl moieties (2-hexyldecanoic acid as amidating agent). Solution structures were studied by SANS. In C 6D6 radii of gyration of 2-5 nm (Guinier analysis) were observed for samples differing in PEI core molecular weight. These sizes and their observed independence of concentration in the range of 5-40 g L -1 indicate the presence of nonaggregated unimolecular inverted micelles. Introduction Defined micellar structures are of interest from a fundamental perspective, as well as for applications. For example, such structures can be used as hosts for the encapsulation and subsequent controlled release of guest molecules 1 or as templates for the formation of nanoscale particles. 2 Block copolymer micelles can allow for a precise control of metal nanoparticle size and arrangement in thin films. 3 However, such self-as- sembled micelles are by their nature dynamic in solu- tion, and they can be shear sensitive. Therefore, uni- molecular polymeric micelles are of interest for particle formation and stabilization. 4 For particle size control, polymeric micelles of defined structure are required. Polymer-analogous amphiphilic modification of a highly branched scaffold represents a convenient route, if a scaffold of defined molecular weight distribution, branch- ing, and chemical structure is utilized in combination with effective modification reactions. We have investi- gated hybrids of amphiphilically modified hyperbranched polyglycerols 5,6 with metal nanoparticles as soluble catalysts for hydrogenation and C-C coupling reac- tions. 7 Polyethylenimines (PEI) represent another class of readily available, highly branched molecules. 8 By comparison to modified polyglycerols (polyether polyols), such polyamines will coordinate much more strongly to metal salts or other precursors used for metal particle synthesis and also to the surface of the final metal nanoparticles. Amidation of PEI with apolar aliphatic acid derivatives can afford polymers that are soluble in apolar organic solvents. Silver nanoparticles stabilized by such amphiphilic poly(ethylenimine amide)s (PEI- amides) possess antimicrobial properties. 9 The PEI- amide can act as a unimolecular “nanoreactor” for the synthesis of a silver particle. 10 Despite their apparent attractiveness due to their accessibility from cheap starting materials, the synthesis of amphiphilic poly- (ethylenimine amide)s and characterization with respect to microstructure have not been reported to date. Also, solution structures have not been studied. Because of their amphiphilicity, these hyperbranched polymers could form aggregates in solution. SANS studies were carried out in dilute solutions to this end. Results and Discusssion Polymer Synthesis and Characterization. Poly- ethylenimine is prepared by cationic ring-opening po- lymerization of aziridine on a large scale. Depending on the reaction conditions, linear or highly branched products can be obtained. 11 For the present study, commercially available highly branched polyethylen- imine (PEI) was employed. The material contains primary amine end groups, secondary amine linear units, and tertiary amine branched (i.e., dendritic) units in a 34:36:30 ratio for PEI with a molecular weight of M w 5000 g mol -1 and in a 31:39:30 ratio for PEI with a molecular weight of M w 25000 g mol -1 , as determined by inverse gated 13 C NMR spectroscopy (relaxation time ) 4 s). These values are similar to data previously published for branched poly- ethylenimine. 12 The degree of branching (DB) of these polymers can be defined as DB ) 2D/(2D + L)(D ) dendritic units; L ) linear units). 13 The values obtained by inverse gated 13 C NMR spectroscopy correspond to a degree of branching of 63 and 61%, respectively, by comparison to 100% for a perfect dendrimer and 0% for a linear polymer. For the introduction of apolar hydro- phobic functionalities, amidation 14 with long-chain car- † Universita ¨ t Konstanz. ‡ Albert-Ludwigs-Universita ¨ t Freiburg. § GKSS Research Centre. ⊥ RWTH Aachen. * To whom correspondence should be addressed. E-mail: stefan.mecking@uni-konstanz.de. 5914 Macromolecules 2005, 38, 5914-5920 10.1021/ma047458w CCC: $30.25 © 2005 American Chemical Society Published on Web 06/14/2005