Functional Poly(ethylene oxide) Multiarm Star Polymers: Core-First Synthesis Using Hyperbranched Polyglycerol Initiators Ralf Knischka and Pierre J. Lutz* Institut Charles Sadron, (CNRS), 6, rue Boussingault, F-67083 Strasbourg Cedex, France Alexander Sunder, Rolf Mu 1 lhaupt, and Holger Frey* Institut fu ¨ r Makromolekulare Chemie und Freiburger Materialforschungszentrum FMF der Albert-Ludwigs-Universita ¨ t, Stefan-Meier-Str. 21/31, D-79104 Freiburg, Germany Received July 20, 1999; Revised Manuscript Received October 26, 1999 ABSTRACT: Hyperbranched polyglycerol as well as polyglycerol modified with short apolar oligo- (propylene oxide) segments (DP n ) 23-52; Mw/Mn ) 1.2-1.4) was deprotonated with diphenylmethylpo- tassium and used as polyfunctional initiators for the anionic polymerization of ethylene oxide to prepare poly(ethylene oxide) (PEO) multiarm star polymers. In the case of unmodified polyglycerol, after metalation, aggregation occurred, preventing efficient initiation and propagation. Using the apolarly modified polyglycerols with terminal oligo(propylene oxide) segments, hydroxyfunctional PEO multiarm star polymers with Mn values in the range 34 000-95 000 g/mol, arm numbers in the range 26-55, and narrow polydispersity (Mw/Mn < 1.5) were obtained in a core-first strategy. 1 H and 13 C NMR measurements evidenced complete conversion of all end groups of the propylene oxide-capped end groups of the initiator. Reinitiation of the multiarm PEO stars by deprotonation was possible and afforded star polymers with considerably larger molecular weights (M n ) 180 000 g/mol) and identical functionality. The thermal properties of the stars (DSC) were found to depend strongly on the arm length. The novel multiarm star architectures prepared consist of polyether structures only and are thus of interest for biomedical applications, e.g., in hydrogels. Introduction Multiarm star polymers are three-dimensional mac- romolecules, in which a large number of linear arms of similar molecular weight and narrow molecular weight distribution emanate from a central core. 1 This class of star polymers recently attracts increasing interest due to the compact structure, which may lead to peculiar rheological properties. 2 In addition, end-functional mul- tiarm star polymers possess unusually high functional- ity that permits further modification or cross-linking. 3 Functional poly(ethylene oxide) (PEO) star polymers are regarded as a particularly promising class of ma- terials, since they represent variable building blocks for structured polymer networks, e.g., hydrogels 4 or am- phiphilic network systems. 5 Due to the excellent bio- compatibility of PEO, both the star precursors and the resulting networks are of interest for biomedical and pharmaceutical applications. In this context, Merrill et al. recently demonstrated that due to their high func- tionality PEO stars attached to a surface permit size- selective protein adsorption. 6 Generally, star polymers can be prepared by two different routes: the “arm-first” 7-10 strategy and the “core-first” 11,12,15 approach on the basis of a multifunc- tional core used as initiator. In the latter case the arm length can be tailored by the ratio of active sites to the amount of added monomer. Well-defined PEO star polymers with three or four arms have been obtained in a core-first manner on the basis of trimethylolpropane or pentaerythrol cores, respectively. For the synthesis of PEO multiarm stars with considerably higher func- tionality, the core-first method employing a poly(divi- nylbenzene) core (DVB) is now commonly used. 13 The main disadvantage of the materials obtained by this procedure is the relatively large polydispersity both concerning molecular weights of the arms as well as functionality. This leads to undesired aggregation in aqueous media or in methanol because of insufficient shielding of the apolar hydrocarbon cores by PEO chains, 14 which induces amphiphilic properties of the resulting stars. An improved route along this line has been presented recently, based on a poly(1,3-diisopro- penylbenzene) core, permitting better control of the functionality. 15 A novel route toward PEO multiarm stars based on dendrimers was presented in recent elegant work by Merrill et al. 9 as well as by Roovers et al., 16 using poly- (amidoamine) (PAMAM; arm-first) as well as carbosi- lane dendrimers (core-first) with functional end groups as core molecules, respectively. In these works, the number of PEO chains per star molecule was as high as 32. 9 However, a drawback of this route lies in the tedious preparation of the dendrimer core molecules, which has to be carried out in a multistep synthesis. 17 An alternative strategy for the preparation of multiarm star polymers has been introduced recently, which relies on the use of hyperbranched 18-20 polyester core mol- ecules. Unfortunately, hyperbranched polymers usually exhibit broad polydispersity, limiting the potential of this method for the preparation of well-defined stars. Furthermore, polyester structures are not stable under the conditions of the living anionic polymerization of ethylene oxide. In a recent publication, we described the controlled synthesis of hyperbranched polyglycerol, based on the anionic ring-opening multibranching polymerization (ROMBP) of glycidol. 21 This route permits to tailor the degree of polymerization (DP n ) 15-100) and leads to polydispersities below 1.5 (mostly below 1.3). In subse- quent work, we have been able to demonstrate that the 315 Macromolecules 2000, 33, 315-320 10.1021/ma991192p CCC: $19.00 © 2000 American Chemical Society Published on Web 12/29/1999