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Hyperbranched Polyether Polyols:A Modular Approach to Complex PolymerArchitectures**

By Alexander Sunder, Rolf Mülhaupt, Rainer Haag, andHolger Frey*

1. Introduction

In recent years, highly branched polymers have gainedwidespread attention due to their unique properties, whichdiffer significantly from their linear counterparts. Greatemphasis in this area has been placed on polymers withtreelike or ªcascade-typeº branching, with a branch-on-branch topology. These macromolecules typically exhibitcompact, globular structures in combination with a highnumber of functional groups. Since the conformation ofsuch polymers is restricted by their molecular architecture,in contrast to linear polymer chains, entanglements are ne-glectable.

Currently, there is rapidly growing interest in this type ofmaterials for numerous applications.[1] The low viscosity incombination with the high functionality may be useful for awide variety of possible uses, ranging from functional cross-linkers, additives, and rheology modifiers to components inadhesives, advanced coatings, structured hydrogels, anddental composites. Such polymers might also be of interestin nanotechnology, for example, as building blocks fornanoscale reaction compartments, as a template for nano-porous materials with low dielectric constants, or for thefabrication of defined hybrid particles (e.g., biomineraliza-tion techniques). Other fields considered are biochemistryand biomedicine, where such macromolecules could act ascarriers, either highly loaded for diagnostic purposes(e.g., for magnetic resonance imaging (MRI) of blood ves-sels) or as host compartments for controlled drug-release.Finally, their use as homogeneous supports for recyclablecatalysts and for supported organic and biochemical syn-theses has been suggested.

Commonly, the perfectly branched dendrimers havebeen discussed in this context.[2,3] However, dendrimershave to be prepared in tedious multistep syntheses, whichobviously is a limiting factor for most applications. In con-trast to dendrimers, the less structurally perfect (i.e., ran-domly branched) hyperbranched polymers synthesized viaone-step reactions have been considered as possible alter-native, since structural perfection may not be a strict prere-quisite for many applications. So far, hyperbranched poly-mers have been regarded as the ªpoor cousinsº ofdendrimers because they commonly possess broad polydis-persity (often exceeding Mw/Mn = 5!). In addition, theirrandomly branched architecture[4] (Fig. 1) has beenthought to be unsuitable for the construction of complexpolymer architectures with, for instance, core±shell topol-ogy or defined cavities that would permit the entrapmentand release of guests. Furthermore, due to intramolecularcyclization during the synthesis, hyperbranched polymerspossess no defined single focal point, such as dendrimersegments (ªdendronsº) that permits further monofunction-alization or attachment to a core as well as polymer chains.

Fig. 1. Topological classification of branched polymers by degree of branch-ing DB (D, L, T: dendritic, linear, terminal units, respectively).

Although the term hyperbranched was only introducedby Kim and Webster in the late 1980s,[5] the basic conceptis much older. Flory analyzed the polymerization of mono-mers that contain one A group and two or more comple-mentary B groups (ªABm-random polycondensatesº) in theearly 1950s.[6] Polycondensation of these ABm-type mono-mers results in two major problems: i) the step-growth ki-

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±

[*] Dr. H. Frey, A. Sunder, Prof. R. Mülhaupt, Dr. R. HaagInstitut für Makromolekulare Chemie (Hermann-Staudinger Haus)und Freiburger Materialforschungszentrum FMFStefan-Meier-Str. 21/31, D-79104 Freiburg/Brsg. (Germany)

[**] Presented at the 1st International Dendrimer Symposium, DECHEMA,Frankfurt (Germany), October 3±5, 1999.

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netics (the required high conversion leads to extremelybroad molecular weight distributions); ii) the possibility ofintramolecular cyclization (i.e., reaction of the focal Agroup with one of the large number of functional B groups,limiting control of molecular weights).

Recently, new concepts have been introduced, based onthe use of latent ABm monomers. In this case, B groups areset free only upon reaction of the A group, which permitscontrol of molecular weights.[7] As an alternative to the te-dious preparation of such peculiar monomers, we recentlydeveloped a convenient pathway to well-defined hyper-branched polyglycerol, based on the anionic polymeriza-tion of a latent AB2 monomer under slow monomer addi-tion conditions.[8] The commercially available monomerglycidol[9] represents a highly reactive hydroxy epoxide,whose polymerization has been known for a long time.[10]

Since the first systematic studies on the catalytic polymer-ization of glycidol,[11] further investigations have led to amore detailed characterization of the polymers, which stillsuffered from a lack of control of the polymerization.[12,13]

The main problem is caused by the fact that glycidol rep-resents a latent AB2 monomer that is able to propagate viaa chain-growth mechanism (via epoxide polymerization)but can at the same time initiate polymerization by its freehydroxy group, which leads to uncontrolled oligomer for-mation by ªself-condensingº steps. As shown theoreti-cally,[14] this ªinimerº character of glycidol[15] can be over-come by slow monomer addition techniques, which arewidely used in technical epoxide polymerizations.[16] Thesalient feature of this synthetic strategy is the very lowmonomer concentration present in the reaction mixture.This suppresses oligomerization, which could result in un-desired cyclization. Under such slow monomer additionconditions the reaction mechanism shown in Figure 2 isvalid. In the case of ªpseudo-chain growth conditionsº, themonomer exclusively reacts with the growing multifunc-tional hyperbranched polymer, which leads to controlledgrowth of the macromolecules in a ªlivingº type of poly-merization. A rapid proton exchange equilibrium main-tains all hydroxyl groups present as potentially active prop-agation sites, thus leading to random branching.

Remarkably, all polyglycerols possess the initiator ap-plied as incorporated core functionality. The main benefitof the approach lies in the fact that the molecular weights(Mn) can be controlled by the monomer/initiator ratio inthe range between 1000 and 10 000, hydroxyl functionalitybetween 5 and 100 hydroxyl groups. Polydispersities (Mw/Mn) below 1.5 (often below 1.3) are generally obtained.

In this brief summary we give an overview of the recentlydeveloped possibilities that emerge from the combinationof the multibranching polymerization of glycidol with well-established epoxide polymerization techniques, leading tocomplex polymer architectures based on the hyper-branched polyether polyols. By modification of the result-ing hyperbranched materials, in a second convenient syn-thetic step it is possible to tailor materials suitable for

many of the applications described above. Figure 3 sum-marizes the synthetic possibilities in the form of a modularchemistry system, detailed in the following section.

2. The Modular System for Highly BranchedPolyether Polyols

As shown in Figure 3, the choice of an initiator rep-resents the first module (I). Besides unfunctionalizedBf-type triol or polyol core molecules, such as trimethylol-propane (TMP) or fatty amine glycidol adducts, initiatorswith a second functionality can be used, thus creating thepossibility of obtaining hyperbranched analogs of dendri-mer segments (dendrons).[17] The functional groups can beprotected amines or alkenyl moieties, which are stable un-der the polymerization conditions employed, and whichmay subsequently be derivatized or used for attachment ofthe hyperbranched structures to surfaces or catalyticallyactive transition metals.

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Fig. 2. a,b) Base-catalyzed ring-opening multibranching polymerization ofglycidol and schematic structure of the resulting hyperbranched polyglycerol.

Since alkoxide catalysis is along-established standard tech-nique in polyether synthesis, awhole variety of epoxide mono-mers can be conveniently copoly-merized with glycidol either inblock or random manner (repre-senting module II in Fig. 3). Forinstance, a major problem of purepolyglycerol for further use is theextremely high polarity of thehomopolymer. In order to over-come this solubility and compati-bilization problem, the hydroxylgroups can be made hydrophobicby in-situ block copolymerizationwith a few units of propyleneoxide (PO) maintaining thehydroxyl functionality.[18] Thepropoxylation is conveniently car-ried out in the same polymeriza-tion vessel subsequent to the gly-cidol polymerization. It should be stressed that thismodification does not affect the narrow polydispersity, butTgs drop significantly, depending on the degree of prop-oxylation. Thus, the products are low viscous, highly solu-ble liquids representing potential components for polyur-ethane formulations. A simple way to extend this idea isthe subsequent block copolymerization with ethylene oxide(EO). The resulting triblock copolymers represent all-etherbased, multi-arm star polymers with narrow polydispersity,which maintains the high functionality of the polyglycerolcore. Even at low degrees of ethoxylation these materialsexhibit a strong tendency towards crystallization.[19] Ofcourse, EO and PO represent common epoxides that aresomewhat difficult to handle on the lab scale. However,other commercially available epoxides, such as glycidylethers, can be copolymerized (in either block or randomfashion) in order to vary branching density or to build morecomplex architectures.[20] Allyl glycidyl ether represents aparticularly interesting comonomer, since it allows the in-corporation of a second functionality into the polymer scaf-fold, which can be used for orthogonal synthesis on boththe hydroxyl and allyl groups of the hyperbranched struc-ture. On the other hand, the incorporation of phenyl glyci-dyl ether leads to a considerable increase of the scaffold'srigidity, resulting in considerably higher glass transitiontemperatures. Apart from this, glycidol is a chiral molecule,both enantiomers being commercially available. Under an-ionic conditions, nucleophilic attack does not affect thechiral center, and it is thus possible to produce chiral poly-glycerols with similar optical rotation as the chiral glycidolmonomers.[17]

In summary, the first two modules (Fig. 3) introduce thepossibility of tailoring polyfunctional hyperbranched poly-mersÐon the basis of well-established epoxide chemistryÐ

with specific molecular architectures. This permits varia-tion of the branching density, core and scaffold functional-ity, and chiroptical properties, as well as polarity and ther-mal characteristics.

3. Novel Topologies and Functions

Typically, when dendrimers are discussed with respect tospecific applications, each target requires a specifically tai-lored material. This usually necessitates the design of asuitable dendrimer system. However, the constructionchemistry of each system is commonly restricted with re-spect to structural alterations, thus architectural and func-tional tailoring is only possible to a limited extent. In starkcontrast, our objective is to realize a broad variety of func-tional topologies on the basis of simple modifications ofwell- defined hyperbranched (co)polymers.

The inert polyether scaffold represents an ideal systemfor further functionalization due to its stability towardsboth chemical reactions and thermal stress (e.g., polyglycer-ol can be perchlorinated in refluxing thionyl chloride, andit degrades only above 280 �C under air). As discussed inthe previous section, compact, highly functional polymersare attractive compounds for the preparation of multi-armstar polymers. As listed in Table 1, a number of well-de-fined star polymers with different kinds of polymer arms

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Fig. 3. Modular system for the synthesis of branched polyether polyols and their derivatives.

Table 1. Multi-arm star polymers based on hyperbranched polyglycerolcores.

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can be prepared from hyperbranched polyglycerols, usingprocedures based on core-first strategies.

Such star polymers are valuable building blocks for struc-tured polymer networks. For instance, we used the multi-arm stars based on EO for the preparation of structuredhydrogels via partial functionalization with methacrylateend groups and radical copolymerization with linear tele-chelics in aqueous media.[21] The materials obtained showexcellent form-stability, seen in the high compression mod-uli, and possess a large number of hydroxyl groups that canbe further modified, e.g., with cell-growth factors. Anotherwell-known pathway to well-defined star polymers is thepolymerization of e-caprolactone by Sn-catalysts. Copoly-merization of e-caprolactone in bulk, using the polyetherpolyols as initiators, proceeded smoothly, yielding multiarmstars with biodegradable poly(e-CL) arms.[22] For rheologi-cal modification of apolar polymers, vinyl-based star poly-mers have been discussed as blend components. In recentwork, we transformed the hyperbranched polyglycerolsinto suitable polyfunctional macroinitiators for atom-trans-fer radical polymerization (ATRP) and polymerizedmethyl acrylate (MA).[23] Since a drawback of polyfunc-tional initiators in radical polymerization lies in the ten-dency towards recombination side reactions, MA conver-sions were limited to 40 % in this case.

Simple esterification of the hydroxyl end groups of poly-glycerol or propoxylated polyglycerol derivatives was em-ployed to systematically study the effect of crystallizable al-kyl chains on the thermal properties of hyperbranchedpolymers. Glass transition and crystallization behaviorhave been studied with respect to the degree of substitu-tion, the length of attached alkyl chains and the polarity ofthe scaffold. We were able to identify two major factors en-hancing the rigidity (i.e., the Tg) of the polymers: one is thehydroxyl-group density (introducing rigidity by hydrogenbonding), the other is the tendency towards crystallizationof the substituents (leading to enhanced rigidity).[24] Theprinciples derived can be expected to be of general validityfor the crystallization of hyperbranched polymers.

Another intriguing class of materials is liquid-crystallinepolymers with branched scaffold. By attachment of meso-gens to polyglycerol via flexible alkyl spacers of varyinglength, we have recently been able to obtain a new class ofliquid-crystalline materials that may be considered to rep-resent the hyperbranched analogue of linear side-chain liq-uid-crystalline polymers (Fig. 4A).[25] In contrast to thesmectic phases found for similar materials based on dendri-mers, the hyperbranched polyglycerols only induced less-ordered nematic phases, which is of interest with respect toswitching behavior in electric and magnetic fields.

Simple esterification with fatty acids offers potential forthe preparation of amphiphilic core±shell structures. We ob-served unusual phase-transfer behavior for polyglycerolspartially substituted with long fatty acids. The obtained ma-terials are highly amphiphilic species with a hydrophobicshell and a hydrophilic interior due to remaining, unreacted

hydroxyl groups (Fig. 4B).[26] Such ªmolecular nanocap-sulesº do not exhibit aggregation in dilute solution, as dem-onstrated by dynamic light scattering Thus, based on theirfixed architecture, such macromolecules represent inverseunimolecular micelles. They have the ability to irreversiblytransfer a well-defined number of water-soluble guestmolecules from an aqueous into an apolar environment(e.g., dyes into solvents or polymeric matrices[27]). In contrastto the topological entrapment observed for dendrimers[28]

this kind of host±guest interaction is based on hydrogen-bonding and polar interactions only. The guests can be liber-ated by hydrolysis of the ester bonds, i.e., degradation of thenanocapsule by removal of the hydrophobic shell.

In the previous section only the general hydroxyl func-tionality of polyglycerol was discussed for modifications inthe periphery of such hyperbranched polymers. Consider-ing the structure shown in Figure 1, one realizes that theterminal groups represent 1,2-diols. By acid-catalyzed reac-tion with carbonyl compounds these can be transformed toacetals. Since this reaction is fully reversible, we use it as astrategy towards high-loading homogeneous polymer sup-ports for organic synthesis. We coupled functional ketonesonto the polyglycerol support, transformed the functional-ity of the ketones bound to the polymer and, finally,cleaved the acetals in order to isolate the reaction prod-uct.[29] All intermediates can conveniently be purified bydialysis, and the support is recycled quantitatively. On theother hand, this protection of the terminal groups can be

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Fig. 4. Novel topologies derived from hyperbranched polyglycerol: A) end-functionalized, hyperbranched liquid-crystalline polymer; boxes symbolizemesogenic units at the otherwise flexible scaffold; B) molecular nanocap-sules based on polyglycerol esterified with fatty acids: a dye-loaded partiallyesterified polyglycerol molecule is shown schematically.

used to selectively modify the hydroxyl groups attached tothe linear groups, leading to polymers with tailored internalstructures.

4. Conclusions

The ring-opening multibranching polymerization of glyci-dol can easily be combined with the well-established chem-istry of other epoxides. On this basis a versatile modular sys-tem for complex branched polymer architectures isaccessible. All necessary compounds are commerciallyavailable, and synthesis of the polyether polyols can be per-formed on a large scale (at present 300 g per batch). Inter-estingly, derivatives of these highly branched polymers ex-hibit properties that have so far been presumed to beuniquely restricted to the perfectly branched dendrimers(entrapment of small guest molecules, mono-core-func-tional/poly-periphery-functional dendrons, high-loadingcarriers).

At present, we are further investigating applications ofthe modular system presented for novel functional materi-als. We are particularly interested in the wide range oforthogonal functional groups that can easily be incorpo-rated into the polymers, either by copolymerization or post-synthetic modification (Fig. 5). Accordingly, the core func-tionality (exactly one per molecule) may be varied fromdouble bonds to amine and thiol functions. In the periphery,beside the hydroxyl groups and the 1,2-diols, a whole rangeof other functional groups is accessible, e.g., allyl groups(copolymerization with allyl glycidyl ether or post-syntheticreaction with allyl bromide), phenyl groups (copolymeriza-tion with phenyl glycidyl ether), carboxylic acids (post-synthetic reaction with succinic acid anhydride), chlorides(post-synthetic reaction with thionyl chloride) or amines(post-synthetic diol cleavage plus reductive amination).

Fig. 5. Possible orthogonal functional groups at dendron-like polyetherpolyols.

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Vögtle, K. Müllen), DECHEMA, Frankfurt 1999.[2] M. Fischer, F. Vögtle, Angew. Chem. Int. Ed. 1999, 38, 884.[3] A. W. Bosman, H. M. Janssen, E. W. Meijer, Chem. Rev. 1999, 99(7),

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[5] Y. H. Kim, O. W. Webster, Polym. Prepr. (Am. Chem. Soc., Div.Polym. Chem.) 1988, 29, 310.

[6] P. J. Flory, J. Am. Chem. Soc. 1952, 74, 2718.[7] a) M. Suzuki, A. Ii, T. Saegusa, Macromolecules 1992, 25, 7071. b) M.

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[9] A. Kleemann, R. Wagner, Glycidol: Properties; Reactions; Applica-tions, Hüthig, Heidelberg 1981.

[10] M. Hanriot, Ann. Chim. Phys. 1879, 17, 116.[11] S. R. Sandler, F. R. Berg, J. Polym. Sci., Polym. Chem. Ed. 1966, 4,

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[13] a) R. Tokar, P. Kubisa, S. Penczek, A. Dworak, Macromolecules 1994,27, 320. b) A. Dworak, W. Walach, B. Trzebicka, Macromol. Chem.Phys. 1995, 196, 1963.

[14] a) W. Radke, G. Litvinenko, A. H. E. Müller, Macromolecules 1998,31, 239. b) R. Hanselmann, D. Hölter, H. Frey, Macromolecules 1998,31, 3790.

[15] Inimer designates a monomer that bears an initiating moiety (initiator+ monomer); cf.: G. I. Litvinenko, P. F. W. Simon, A. H. E. Müller,Macromolecules 1999, 32, 2410.

[16] F. E. Bailey, J. V. Koleske, ªAlkylene Oxides and Their Polymersº, inSuface Science Series (Eds: M. J. Schick, F. M. Fowkes), Marcel Dek-ker, New York 1990, Vol. 35.

[17] A. Sunder, R. Haag, R. Mülhaupt, H. Frey, Macromolecules, in press.[18] A. Sunder, R. Mülhaupt, H. Frey, Macromolecules, in press.[19] R. Knischka, P. J. Lutz, A. Sunder, R. Mülhaupt, H. Frey, Macromole-

cules, in press.[20] A. Sunder, H. Türk, R. Mülhaupt, R. Haag, H. Frey, Polym. Prepr.

(Am. Chem. Soc., Div. Polym. Chem.), in press.[21] R. Knischka, P. J. Lutz, A. Sunder, H. Frey, unpublished.[22] A. Burgath, A. Sunder, I. Neuner, R. Mülhaupt, H. Frey, Macromol.

Chem. Phys., in press.[23] S. Maier, A. Sunder, H. Frey, R. Mülhaupt, Macromol. Rapid Com-

mun., in press.[24] A. Sunder, T. Bauer, R. Mülhaupt, H. Frey, Macromolecules, in press.[25] A. Sunder, M.-F. Quincy, R. Mülhaupt, H. Frey, Angew. Chem. 1999,

111, 3107. Angew. Chem., Int. Ed. 1999, 38, 2928.[26] A. Sunder, M. Krämer, R. Hanselmann, H. Frey, R. Mülhaupt, Angew.

Chem., 1999, 111, 3758; Angew. Chem. Int. Ed. 1999, 38, 3552.[27] Isotactic polypropylene can homogeneously be dyed with hydrophilic

colorants by extrusion with such dye-loaded nanocapsules: T. Bauer,unpublished results.

[28] J. F. G. A. Jansen, E. M. M. de Brabander-van den Berg, E. W.Meijer, Science 1994, 266, 1226.

[29] R. Haag, A. Sunder, unpublished (summary in [1], p. 74).

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