New developments in hyperbranched polymers

HIGHLIGHT

New Developments in Hyperbranched Polymers

BRIGITTE VOITInstitut fur Polymerforschung Dresden e.V., Hohe Strasse 6, D-01069 Dresden, Germany;e-mail: [email protected]

Received 3 April 2000; accepted 10 April 2000

ABSTRACT: In the last 12 yearsthe field of hyperbranched poly-mers has been well establishedwith a large variety of syntheticapproaches and fundamental stud-ies on structure and properties ofthese unique materials. However,new developments involving hy-perbranched materials appeared re-cently, for example, different syn-thetic strategies, new reaction

mechanisms, formation of morecomplex architectures, a deeperunderstanding of the branchedstructure and their kinetic develop-ment, and intensive studies on thematerial properties and possibleapplications. This demonstratesthe high versatility and the possi-bilities that are still involved inhyperbranched polymers and ren-

der it one of the most active fieldsin polymer science with a verypromising future.© 2000 John Wiley

& Sons, Inc. J Polym Sci A: Polym Chem

38: 2505–2525, 2000

Keywords: hyperbranched poly-mers; dendritic polymers; synthe-sis; characterization; application

Professor Dr. Brigitte Voit obtained the diploma and Ph.D. degree inChemistry at the University Bayreuth with Professor Dr. Oskar Nuykenon the subject of diazosulfonate-containing polymers for application asphoto resins. Her first contact with hyperbranched polyesters was duringthe postdoctoral work with Dr. Richard Turner at Eastman Kodak in1991. After this, she moved to the Technical University Munich whereshe obtained her habilitation degree on the topic of dendritic macromol-ecules in 1996. At present she is a full professor for Organic Chemistryof Polymers at the Technical University Dresden and she is heading theSubinstitute for Macromolecular Chemistry at the Institute of PolymerResearch in Dresden. Her major interests are in polymers with specialarchitectures, for example, hyperbranched polymers, block and graftcopolymers, as well as in photo- and thermolabile polymers.BRIGITTE VOIT

2505Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 2505–2525 (2000)© 2000 John Wiley & Sons, Inc.

INTRODUCTION

Two years ago, Young Kim1 presented an excellentoverview on “Hyperbranched Polymers 10 Years After”at the same location, which demonstrated very well thathyperbranched polymers can now be considered an es-tablished field in polymers science. Today, worldwide,the activities in this area are still extremely high asdemonstrated by numerous reviews on hyperbranchedpolymers,2–7 dendritic molecules,8–17 or particularly ondendrimers18 are numerous. It became difficult to followup all the activities and to keep track of the new struc-tures appearing in the literature. Clearly, different as-pects of synthesis, preparation of new structures, struc-ture–property relationships, application prospects, andalso theoretical aspects of kinetics and the degree ofbranching dominate. Recent developments in this fieldfrom my personal view will be the focus of the presentreview. But before starting with this, a brief journey backwill be allowed.

When I was introduced into the field of hyper-branched polymers in 1991 it was symptomatic for thetime that the initiative came not from academia but fromindustry. Richard Turner at Eastman Kodak was lookingfor a postdoc who was going to study the properties ofhyperbranched polyesters. His interest was certainly in-fluenced by recent publications in the field of dendrim-ers13,19–22and a close cooperation with the group of JeanFrechet. But, while perfectly branched dendrimersemerged from academia, from the beginning hyper-branched polymers were related to activities at industry23

as was also pointed out by Kim.1 The reasons for this arequite clear: it was obvious from the beginning that thestep-by-step synthesis of dendrimers will restrict indus-trial application on a larger scale, whereas the randomintroduction of branching, for example, into polyconden-sates, was a well-established technique, which could becombined with known technical processes. In addition,the concept of highly branched, but irregular structureswas well known for biopolymers, for example, polysac-charides,24 for low-density polyethylene,25 poly(ethyl-eneimine)26 and, of course, for polymer networks.27,28

But at this time, little was known on the properties ofsynthetic polymer structures with branching in each re-peating unit (branch-on-branch topology) but withoutresulting in a crosslinked system. Everybody referred tothe theoretical work of Flory28 who described the ran-dom ABx polycondensation theoretically already in1952. Only few experimental data29–34 were alreadyavailable, some of them even neglected or missed at thistime since the term hyperbranched polymers was notused. Therefore, it was understandable that the maininterest in the early 90th was to produce a reasonableamount of a hyperbranched polymer for being able to

study the branched structure and especially the relatedmaterial properties. The question arose whether hyper-branched polymers are something special compared toknown branched polymers and further, is it possible toachieve new, interesting material properties, which canbe clearly correlated to the highly branched and assumedglobular structure. Today these questions still cannot beconsidered as fully answered, even though several appli-cation aspects of hyperbranched polymers, for example,in blends35–43 and coatings44–46 have been exploredsuccessfully by now.

In 1991, dendrimers entitled by different names (star-burst dendrimers,13 arborols,21 cascade polymers22) werealready established and the differentiation towards the“dirty” imperfect hyperbranched structures prepared in aone-step synthesis was finalized. The name “hyper-branched polymers” for the latter as given by Kim andWebster32 indicated clearly that there is only a limitedanalogy to dendrimers and a close relation to the classi-cal introduction of branching into a polymer chain. To-day, however, both structures, dendrimers as well ashyperbranched polymers, are often discussed togetherusing the term dendritic polymers. In many cases, thehyperbranched polymers are discussed as a more rapidlyprepared and more economical replacement for perfectdendrimers in special applications. Today it is knownthat this is true for many aspects, for example, both typesof branched structures exhibit a higher solubility and alower solution viscosity compared to linear ana-logues.47,48 In addition, the large number of functionalgroups offers the possibility for further modification andspecial applications, and the type of end groups deter-mines to a considerable extent the properties, for exam-ple, glass-transition temperature,49–52 of both classes ofbranched polymers. Nevertheless, there are also distinctdifferences between dendrimers and hyperbranchedpolymers. The most prominent feature of hyperbranchedpolymers is their “degree of branching”34 DB or the“branching factor”,33 which defines the ratio ofbranched, terminal, and linear units in the polymer struc-ture. By definition DB is 100% for dendrimers and,100% (50% for a statistical growth) for hyperbranchedstructures. Furthermore, hyperbranched polymers exhibittypical polymer features, as a broad molar mass distri-bution, isomerism, and an irregular growth with a statis-tical distribution of the functional groups throughout thestructure. Often, they are even considered as havingmore resemblance with networks (just before the gelpoint)53–55than with dendrimers. The high symmetry ofthe perfectly branched dendrimers, which allows a fullmathematical description of the molecules, their repeti-tive synthesis, and the lack of a molar mass distributionassuming ideal growth, shifts dendrimers more towards

2506 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 38 (2000)

the area of large organic molecules or biomolecules thantowards classical polymers.

The Classical Synthetic Approach

In the classical approach towards hyperbranchedpolymers, which goes back to Flory’s28 early de-scription as a special type of polycondensation,ABx monomers with equal reactivity of the Bfunctionalities are condensed (Scheme 1). The re-action involves the typical features of a stepgrowth reaction of multifunctional monomers andthe formed oligomers but without the danger ofcrosslinking. Dendritic (fully reacted B func-tions), terminal (no reacted B function), and lin-ear (one reacted B function) units and one focalunit (A function) should be present in the result-ing, highly branched macromolecule.

The use of AB2 monomers predominates thesynthetic approaches leading to a very broadstructural variety in hyperbranched products.AB3

29,56–58 and very few AB459,60 and even AB6

60

monomers are also reported in order to control thebranching pattern. Even though a COC couplingreaction23 was used for one of the first publishedAB2-type polymers, polyester structures were fa-vored clearly by many authors3,4,30,31,34,61–68 dueto the availability of suitable monomers. Poly-amides,69–73 polyurethanes,74,75 polyethers,76

poly(ethersulfone)s and -ketones,77–81 polycar-bonates,82 poly(phenylacetylene)s,83 and also poly-siloxy- or polycarbosilanes56–58,84–87 were alsoreadily synthesized. For successful synthesis ofclassical hyperbranched polymers from ABxmonomers, certain requirements have to be ful-filled, as the absence of side reactions, equal re-

activity of the two B functionalities, and no inter-nal cyclization reactions limiting the achievablemolar mass. It was found that the occurrence ofcyclization reactions depends strongly on themonomer structure.60,67,88 Whereas the aliphaticmonomer 2,2-dihydroxymethyl propionic acidleads to hyperbranched polymers with up to 92%cyclics instead of the focal A unit, as verified byMALDI-TOF measurements,88 Hawker et al.67

could verify that up to 95% of the hyperbranchedmacromolecules prepared from 4,49-dihydroxy-phenyl-propionic acid still contain the acid focalunit.

Continuously, many new aspects are beingfound in the synthesis of hyperbranched polymersfollowing the classical approach (Scheme 1), forexample, improvement of the speed of the reac-tion by new reaction conditions and activation ofmonomers as described by Moore et al. for hyper-branched aromatic poly(etherimide)s.89 Afteronly 2.5 min reaction time the hyperbranchedproduct could be isolated using a rapid catalyticarylation method and the t-butyldimethylsilylprotected diphenol 1. The degree of branching inthis product strongly deviated with 67% from thestatistical value of 50% for equal reactivity of theB groups in AB2 monomers. DSM company devel-oped a new hyperbranched poly(esteramide) (Hy-brane™) with alcoholic end groups from AB2monomers (e.g., 2) based on different anhydridesand diisopropanolamine.90 The polycondensationproceeds when a slight excess of diisopropylamineis added without any catalyst at 140 °C. Thematerial properties, especially the glass-transi-tion temperature, is controlled by the used anhy-drides and by end group modification. Hyper-

Scheme 1. Synthetic strategies towards networks and hyperbranched polymersbased on the classic branched polycondensation approach of Flory.28

HIGHLIGHT 2507

branched poly(e-caprolactone)s91 were achievedby esterification of AB2 macromonomers 3 and 4(Scheme 2). Compound 3 resulted in hyper-branched structures with a degree of branching ofonly 37%, whereas the polymer based on 4 cameclose to a perfectly branched dendrimer.91 A novelAB2 monomers, p-(chloromethyl)phenylacetoni-trile, in which the OCH2CN groups representsthe B2 function and OCH2Cl the A group, wasdescribed by Jin et al.92 Using suitable reactionconditions, a soluble, hyperbranched polymerwith cyano end groups was obtained. A very un-usual hyperbranched polymer based on the re-versible self-assembly of a organopalladiummethylcyano complex has been reported byRheinhoudt et al.93 Large organopalladiumspheres with diameters of approximately 200 nmwere obtained, which can be “degraded” to themonomer by addition of acetonitrile. These exam-ples demonstrate that Flory’s presumption of anequal reactivity of the B functions might not bemet in all synthetic examples.

Early on, the addition of a “core” molecule Bx (x$ 2) was explored (Scheme 1), mainly for bettercontrol over molar mass but also for control of theresulting geometrical shape.2,9–11,65,94 The mostprominent example is the polycondensation of di-hydroxymethylpropionic acid 6 in the presence oftrimethylolpropane65 5 (Scheme 3). In this case, asuccessive addition of the monomer also led to anincrease in the degree of branching up to 67%.This effect was later described theoretically under

the name “slow monomer addition” and verifiedby additional experiments by Frey et al.95–97

Most recently, several groups recalled that theusually not commercially available AB2 can besubstituted by conventional combination of A2and B3 monomers98–101 (Scheme 1). Hyper-branched aromatic polyamides and polyethers(Scheme 4) have been obtained using combina-tions of monomers 718 and 915 when the reac-tion was stopped prior to the gel point. However,the critical conversion in these condensations isdependent on many factors, for example, the ratioof functionalities, purity of solvent and reagents,reaction time, and temperature a.o.102 Therefore,it is very difficult to fully control the reaction andto obtain hyperbranched polymers with high mo-lar mass without the need of separation of the solfrom the resulting gel fraction. In addition, itcould not be verified up to now that the growthand structure of the products are comparable tothat of ABx polymers.

From the beginning, modification of the func-tional groups of hyperbranched polymers was ofhigh interest in order to optimize material prop-erties.49,50,80,103,104 This is primarily achieved bypolymer analogous reactions on a preformed hy-perbranched polymer, for example, as demon-strated in the alkyl modification of aliphatic51 andaromatic polyesters52 with a strong effect on thethermal and rheological properties of the result-ing polymers. But also in situ end-group modifi-cation94 by direct addition of the modifying agentat the end of the polycondensation process and, asspecialty, the internal functionalization105 of hy-perbranched polymers by use of acetal-containingmonomer units, which could later be deprotectedleading to internal keto and hydroxyl groups,were reported.

Scheme 2. Examples for new AB2 monomers.89–91

Scheme 3. AB2 1 B3 approach exemplified on dihy-droxymethylpropionic acid 6 and trimethylolpropane 5as described by Hult and coworkers.65

Scheme 4. Hyperbranched polymers via combinationof A2 and B3 monomers.98,99,101


Addition Reactions

Besides classical polycondensation reactions more andmore addition reaction mechanism have been involved inthe synthesis of hyperbranched polymers. This includesthe above-mentioned reactions towards polyurethanesand polycarbosilanes involving the classical reactions ofisocyanates and the hydrosilylation, but also more un-usual addition reaction types. In Scheme 5, some mono-mers are listed, which have drawn my attention. The AB2

monomer10 can undergo a Diels–Alder [214] cycload-dition reaction leading to hyperbranched polyphe-nylenes,106,107which can be planarized by a subsequentintramolecular cyclodehydration reaction. The buildingblock 10—when the triple bond is blocked by triisopro-pylsilyl substitutents—is also the basis for perfectlybranched polyphenylene dendrimers.106

Weber et al. used the ruthenium-catalyzed addition ofortho COH bonds of acetophenone across a triple bond(11)108 or a vinyl group (12),109 the latter yielding onlya DB of 12%. Scheme 6 pictures the proposed hyper-branched polymer structure from11, which had aDBclose to 60%.

We were able to prepare hyperbranched poly(ether-amide)s without encountering any side reactions via nu-cleophilic ring-opening addition reaction of phenols onoxazolines110 using the AB2 monomer13.111 The ther-mally induced reaction proceeds inN-methylcaprolac-tame solution above 190 °C. The products are randomlybranched with a degree of branching of 50% verified byhigh resolution NMR studies.

Hobson and Feast112 used a Michael addition reactionof the AB2 molecule14, which leads to a hyperbranchedpolymer nearly identical to the PAMAM dendrimers of

Tomalia.13 Surprisingly, the authors reported for theresulting hyperbranched poly(amidoamine)s only termi-nal and dendritic units. No evidence for linear units couldbe found meaning aDB of 100% by the conventionaldefinitions! Therefore, it was suggested that the forma-tion of dendritic units is predominant due to specialsterically or thermodynamically favored intermediates.This report started an intensive discussion on the degreeof branching as a characteristic feature of hyperbranchedpolymers. CanDB be raised easily above the statisticalvalue of 50%? Are hyperbranched polymers well de-scribed byDB? What are the topological differencesbetween hyperbranched polymers with aDB of 100%and perfect dendrimers? Is it worthwhile to try to reacha DB of 100% from the material properties point ofview?

We addressed this problem with the goal to intention-ally prepare hyperbranched polymers that cannot containany linear units. Again, an addition reaction was applied,the so-called [212] criss-cross cycloaddition of the bis-azine15 (Scheme 7).113 Because of the instability of theintermediate azomethinimine, which would represent thelinear unit, the isolation of molecules with linear units isnot possible in this case. The azomethinimine has to reactback to a terminal unit or will add a second monomer toform a new dendritic unit. In the absence of any reagents,which can trap the unstable intermediate and thereforewould lead to side products, a hyperbranched polymerwith a degree of branching of 100% is formed. We couldverify by intensive NMR studies that only the proposeddendritic and terminal units are present in the product.However, as expected a broad molar mass distribution(PD5 2.2) was obtained via SEC analysis withMn in therange of 6000 g/mol. These materials can now be used totry to answer some of the above-posed questions.

Scheme 6. Proposed structure from the reaction of11.108

Scheme 5. AB2 monomers in addition reac-tions.106,108,109,111,112

HIGHLIGHT 2509

Ring Opening

The example of the AB2 monomer 13 demon-strated already, that ring- opening reactions havea substantial potential for the preparation of hy-perbranched polymers. But, whereas still a clas-sical random step growth reaction proceeds via 13with equal reactivity of the functional groups Band following a known addition reaction mecha-nism, new monomers have been explored re-

cently, which involve also new mechanisms, someof them closer to chain growth than step growth.These approaches have their origin in classicalring-opening reaction mechanism towards linearpolymers, especially polyethers and polyesters.Chang and Frechet114 reacted the bisepoxide 16(Scheme 8) involving a proton transfer mecha-nism. In the first step of the polyreaction a protonis abstracted from the phenol group in 16 by an

Scheme 7. First steps of the criss-cross cycloaddition reaction of 15 with indication ofthe instable intermediate, which would represent a linear unit.113

Scheme 8. Monomers for ring-opening multibranching reactions.26,114,116–118,126,127


OH to yield the phenolate. The nucleophile phe-nolate then adds to a second molecule 16 andopens one epoxide ring forming a dimer with asecondary alkoxide. This dimer does not propa-gate directly, first a proton exchange takes placewith an unreacted monomer 16 yielding again thephenolate of 16, which reacts as nucleophile. Animportant feature in order to achieve this type ofgrowth without undesired propagation throughthe nucleophilic center of the secondary alkoxideis that the phenolate formation in 16 is signifi-cantly faster than the nucleophilic propagationstep. The much lower pKa (pKa ; 10) of the phe-nolic group relative to that of the secondary alkox-ide obtained by epoxide ring opening (pKa ; 17)enables the fast proton exchange. Interestingly,the molar mass increases still exponentially withconversion as observed for the classical hyper-branched polycondensations.28,55,88,115 The au-thors stated that the polymerization mechanismis much more complicated than that of a classicalpolycondensation especially due to the increasedpossibility of intramolecular cyclization, a prob-lem that was addressed generally for all hyper-branched polymers by Dusek et al.88

As mentioned before, commercial poly(ethyl-eneimine)26 prepared by self-condensing, ring-opening reaction of aziridine 17 is a branchedpolymer due to further reaction of the NH groupsin the formed polymer chain with the cyclic mono-mer. Following the same principle, nearly at thesame time Penczek et al.116 and Hult et al.117

published the successful cationic ring-openingpolymerization of 3-ethyl-3-(hydroxymethyl)ox-etane 18 leading to hyperbranched aliphaticpolyethers with a degree of branching in therange of 41%117 and molar masses Mn around2000 to 5000 g/mol. Some preliminary resultson this reaction could also be obtained by Kim.1

Benzyltetramethylenesulfonium hexafluoroanti-monate, BF3O(C2H5)2 or CF3SO3H have been ap-plied as initiators and trimethylolpropane 5 wasadded as core molecule. The reaction proceeds via

protonation of the oxygen in the oxetane ring,followed by ring opening due to nucleophilic at-tack of a second monomer (active chain end mech-anism).116 However, in addition, two hydroxy-methylene functions can be condensed under acidcatalysis forming an ether bond (activated mono-mer mechanism),116 which results in identical re-peating units. The important reaction leading tobranched units is a chain transfer process wherethe protonated oxetane ring can react with anyhydroxyl group present in the system (pending inthe linear repeating units or in a monomer). TheDB value below 50% indicates that this chaintransfer process proceeds with a lower probabilitythan the growth reaction step.

Following the same line but using a differentmechanism, Frechet and coworkers118 publishedthe formation of hyperbranched polyesters basedon 5-(2-hydroxyethyl)-e-caprolactone 19. Themonomer looks first like an AB monomer andchanges to a AB2 type after the first addition of asecond monomer (Scheme 9). The propagationand initiation proceeds entirely through one typeof reactive nucleophil: a primary alcohol. Bulkpolymerization of 19 in the presence of stannousoctoate as catalyst yielded in hyperbranched poly-mers of Mw 5 65,000–85,000 g/mol (Mw/Mn 5 3.2)and a DB of 50% indicating equal reactivity ofboth primary alcohol groups.

It has also been known for some time that thepolyreaction of glycidol 22 results in branchedpolymers119,120 (Scheme 10). Frey et al.121 de-scribed now the anionic ROMBP (5 ring-openingmultibranching polymerization) of 22, which theyconsider a latent AB2. The polymerization provedto be very versatile and leads to hyperbranchedpolymers with rather narrow molar mass distri-bution (Mw/Mn 5 1.1–1.4) due to a livinglike char-acter of the reaction when only partial deprotona-tion to the initiating alkoxide (initiating site) oftriol 5 was performed. This leads to a more or lesssimultaneous growth of all chain ends. After de-protonation of 5, the resulting alkoxide reacts

Scheme 9. Ring-opening multibranching polymerization of glycidol.121

HIGHLIGHT 2511

with the unsubstituted end of 22 and therebygenerates a secondary alkoxide. In contrast to thecationic polymerization of 22 described by Penc-zek and Dworak,120 a nucleophilic attack on thesubstituted end of the epoxide ring was observed.By use of the trifunctional initiator (core mole-cule) and slow monomer addition, cyclization wassuppressed and the molar mass and polydisper-sity was controlled. The degree of branching wasdetermined to be 53 to 59%, which is somewhatlower as expected for the slow monomer approach(67%).95 Molar masses Mn between 2000 and12,000 g/mol and polydispersity , 1.5 are re-ported. This approach opens now a pathway tovery interesting, water soluble and biocompat-ible, aliphatic polyethers,122 which can be readilymodified for use, for example, in star polymers,123

as nanocapsules124 for dye carriage, or as chiraland liquid crystalline material.125

In this respect, it should be mentioned that avery interesting approach towards hyper-branched polyamines using a multibranching po-lymerization type ring-opening reaction has beenpublished by Suzuki et al.126 already in 1992,based on the cyclic carbamate 20 (Scheme 8). Theapproach was further extended towards 21 in1998.127 In this case, the monomer has no resem-blance to any ABB9 or AB2 monomer discussed upto now. Growth occurs only when an initiator withan active B group is added together with a cata-lyst. Scheme 11 gives the reaction pathway asproposed by the authors.126 Upon Pd-catalyzed

activation of the allyl functionality, an amine at-tacks nucleophilicly the allylic position, the cyclicstructures is opened under evolution of carbondioxide; this liberates a free amino function,which is now equivalent to a B2 group. Secondaryand primary amines can react with the allylgroup leading to linear or dendritic structuralunits. Further modification of the primary andsecondary amino functions with n-BuNCO facili-tated storage and characterization of the result-ing products from 21 and 22.127 Thus, molar massdetermination became possible resulting in Mnvalues between 1800 and 5300 g/mol (GPC orVPO) and rather narrow molar mass distribu-tions (Mw/Mn 5 1.3–1.5). DB was calculated fromNMR data to be 60–80%. The authors state thatthe positive deviation of the DB is a result of thehigher nucleophilicity of the secondary amine inpolar solvents compared to the primary ones.

These examples demonstrate the trend in thesynthesis of hyperbranched polymers towards theexploration of new reactions types and integrationof chain growth mechanism. Reasons for this arethe strive for better control of structure and poly-dispersity in hyperbranched polymers. Monomersas those described by Suzuki et al. allow even tofully suppress the self-condensation of the mono-mers and strictly force a growth at the chain end.

Self-Condensing Vinyl Polymerization

The trend towards chain growth mechanism is well dem-onstrated by the growing importance of the self-condens-

Scheme 10. First step of the ring-opening reaction of 19 leading to hyperbranchedpolyesters.118


ing vinyl polymerization (SCVP), described the first timeby Freche´t et al.128 in 1995 for the synthesis of hyper-branched polymers (Scheme 12). SCVP is based on avinyl monomer that additionally bears an initiating group(“inimer” 5 initiator 1 monomer). These monomersallow propagation through the double bond (5 chaingrowth) and condensation of the initiating site with thedouble bond (5 step growth). This potential has beenobserved already by Nuyken et al.129 in the living cat-ionic copolymerization of 4-(chloromethyl)styrene23(Scheme 13) and isobutylene since a significant amountof branch points had been found in the attempted linearmacroinitiator. The living cationic polymerization of3-(1-chloroethyl)ethenylbenzene24 at low temperatureusing SnCl4 was also the basis for the first successfulsynthesis of hyperbranched polymers.128 Polymers withlow Mark–Houwink constants were achieved as typical

for globular structures. More recently, 1-[(2-vinyloxy)-ethoxy]ethyl acetate130 25 was polymerized cationicallyusing ZnCl2 as activator leading to hyperbranched poly-mers ofMn , 5000 g/mol and very broad polydispersi-ties of 7–10; no information onDB is given in this case.

This process has been readily extended towards othercontrolled chain growth mechanism, especially grouptransfer polymerization131,132 using 26, and controlledradical polymerization both via TEMPO133 (27) and viaatom transfer radical polymerization (ATRP)134–136(23,28, 29). Scheme 13 depicts a selection of used SCVPmonomers. A general feature of SCVP is the unequalreactivity between chain growth of the vinyl group andstep growth through the initiating site. Therefore, alsothe degree of branching can differ strongly from thevalue 50% of the random AB2 condensation. Freche´t134

demonstrated that just by varying the reaction conditionsthe DB achieved in the metal catalyzed “living” radicalpolymerization of 4-chloromethylstyrene23 can be var-ied from nearly zero (linear polymer) to highly branched.Matyjaszewski et al.135 used preferentially the acrylatemonomer28 compared to29 in ATRP since29 mightlead to a lowerDB. The danger of side reactions (elim-ination, radical coupling) limits also the use of controlledradical processes. Especially at longer reaction times agel fraction can no longer be avoided. Nuyken and Wie-land137recently developed a concept to use the azo groupcontaining vinyl monomers30 for the preparation ofhighly branched polymers also via a radical process. It isproposed that the methylmalonodinitrile radical, which isset free after azo decomposition, acts as reversible ter-minating agent but not as initiator, therefore no indica-

Scheme 11. Proposed reaction scheme for 20 by Suzuki and coworkers.126

Scheme 12. Scheme of the self condensing vinyl po-lymerization SCVP (adapted from Frechet et al.128).

HIGHLIGHT 2513

tion of crosslinking was observed even in the homopo-lymerization of30. Light scattering and viscosity mea-surements indicated a globular structure of the products.

Often in SCVP it is not possible to determine theDBdirectly via NMR analysis. Therefore, indirect methods,for example, viscosity measurements and light scatteringmethods selective towards the more globular structure ofa hyperbranched polymer, have to be used. The polydis-persities are usually very high and represent the presum-ably nonliving character of the reaction. However, thebig advantage is the extension of the concept of hyper-branched polymers towards vinyl monomers and chaingrowth processes, which opens unexpected possibilities.In addition, the mechanism and kinetics138 of SCVP,molar mass, and polydispersity development with con-version,139 the degree of branching,140 influence of acore molecule141 as well as effect of different reactionrate constants142 has been treated theoretically quite ex-tensively. These efforts are representative for the in-creased interest in theoretical aspects of hyperbranchedpolymers.

Theoretical Treatments

First theoretical considerations especially on statistics,on molar mass evolution, and polydispersity of hyper-branched polymers based on ABx monomers were pub-lished by Flory.28 These studies were deepened by Bur-chard in the 70s.55 But with the increasing interest insynthetic aspects of hyperbranched polymers also thetheoretical aspects had been reconsidered. As mentionedabove, this was of special interest for the SCVP136,138–142

since in this case two different growth mechanisms are

involved and the monomers are of ABB9 type. Never-theless, the obtained conclusions can be transferredreadily also towards classical ABx polyreactions as com-parison of results obtained for SCVP with those derivedfor ABx systems143 prove. Dusˇek88 could show how themolar mass and the polydispersity can be limited byinternal cyclization reactions. Intensively, the degree ofbranching and the influence of different reaction param-eters, core molecules, copolymerization, and post modi-fication steps were studied by Frey et al.95,144–146,Thus,under special conditions, for example, slow monomeraddition, it was found that the degree of branching can beincreased from 50 to 67% even for a statistical process.Post modification of polymers with branched units evenallows to “disguise” a linear polymer as a branchedone.146,147This demonstrates that the valueDB follow-ing the accepted definitions34,43,144is not fully suitablefor the description of the microstructural features and thetopology of hyperbranched polymers.

We were able to verify the proposed development ofthe branched units with conversion by experimental stud-ies on the polycondensation of 4,4-bis-(49-hydroxyphe-nyl)pentanoic acid, a monomer that was known for a lowtendency of internal cyclization67 and a high probabilityfor a undisturbed statistical growth with equal reactivityof the B functions (DB 5 50%). The development ofdifferent structural units with conversion and even thedevelopment of structural diads could be followed by useof high resolution NMR analysis.148 However, this idealcase of equal reactivity of the B functions cannot beassumed for all real hyperbranched reactions, particu-larly if SCVP or new addition and ring-opening reactions(latent AB2 momomers) are applied as shown above.

Scheme 13. Examples for monomers for SCVP.128,130–133,135,137


Therefore, unequal rate constants have to be consideredand incorporated into theoretical treatments. This wasalready discussed by Frey95 and Muller.138 A detailedtheoretical treatment, which can be applied to any reac-tion with unequal reaction constants was developed alsorecently in our own group.149 An experimental applica-tion of this theoretical derivation has been done on theexample of the polycondensation of 3,5-bis(trimethylsi-lyloxy)benzoyl chloride, which usually leads to aDB of60 to 65%.150

Hyperbranched Polyethylene

Branching is a very well known aspect in polyethylenechemistry, either as uncontrolled long chain branching inlow density polyethylene LDPE (high pressure, radicalprocess) or as intended short chain branching in linearlow density polyethylene LLDPE (copolymerization).151

Recently, Brookhart et al.152 and then Guan et al.153

reported the formation of highly branched (“hyper-branched”) polyethylene at low pressure by a so-called“chain walking” process using palladium(II) and nick-el(II) catalysts, which contained very bulky chelatingdiimine ligands. Even higher branching was observed bySen et al.154 with a [Ni(p-methallyl)(Br)]2

3 or a Pd(1,5-cyclooctadiene)(Me)(Cl)4 catalyst. “Chain walking” orbetter isomerization of the active site to the internalbackbone during polymerization was reported already byFink and coworkers155 in 1985 for a-olefins. Now, itcould be shown that the polyethylene topology can bewell controlled from nearly linear to hyperbranched just

by the reaction pressure. Scheme 14 summarizes theschematic mechanism as proposed by Guan.153 Thechange in topology could be verified by differences inhydrodynamic radius and solution viscosity for samplesprepared at different pressure but with similarMw asdetermined by MALS.

The example of these highly branched polyolefinsdemonstrates the trend to broaden the expression hyper-branched or dendritic to much more complex architec-tures no longer solely based on ABx or related mono-mers.

Arborescent Graft and Comburst Polymers

The concept of very highly, irregularly branched poly-mer topologies is realized intriguingly in the so-called“comburst” (by Tomalia156) and arborescent graft poly-mers (by Gauthier157). Here, first linear polymer chainswith functional sites are prepared. By subsequent“branch on grafting” preferably with polymer chains ofdefined molar mass, very rapidly products with ex-tremely high molar mass are obtained. Both types ofgrafting are possible: “grafting onto” based on preformedliving polymer chains and “grafting from” using poly-meric bound initiating sites [Scheme 15 (a)].

By living anionic “grafting onto” polystyrene,157 po-lybutadiene,158 (and polyethylenes after hydrogenation),poly(ethyleneoxide)159 and also polystyrene-co-polyiso-prene160 arborescent graft polymers could be achieved.Core-shell molecules with amphiphilic properties arereported by polystyrene-co-poly(ethyleneoxide)161,162

Scheme 14. Highly branched polyethylene by the “chain walking” mechanism asadapted from Guan et al.153

HIGHLIGHT 2515

arborescent polymers. These polymers with molarmassesMw up to 7 3 107 and Mw/Mn , 1.3 are com-pletely soluble and exhibit comparable low solution vis-cosities. In melt rheology studies these macromoleculesshowed a viscous flow behavior and resembled gels justbefore the gel point.163 The most highly branched arbo-rescent graft polymers display a frequency dependencesimilar to crosslinked networks or microgels.163 Theoverall shape of the molecules can be influenced bychanging the arm length or the grafting density. Theformer can be readily controlled by varying the ratiomonomer to initiator. Light scattering measurementsshowed that the polymers behave in solution as hardspheres. Sheiko et al.164could visualize arborescent graftpolystyrenes with different branching density as mono-layer films cast on mica. The polymers were depicted asdistinct hexagonally packed globuli (Scheme 16) whosesize was consistent with molecular dimensions obtainedfrom viscosity and dynamic light scattering measure-ments. The layer thickness and the particle dimensionsdepend on the branching density. Molecules with a highbranching density (on average 500 g/mol between branchpoints) recovered a spherical geometry after annealingabove the glass-transition temperature. In the case oflower branching density (on average 2000 g/mol be-tween branch points) a pancake structure remained stableafter annealing. This demonstrates that arborescent graftpolymers represent a peculiar type of colloidal particlespossessing a topological defined surface.

Comburst poly(ethyleneimine)s have been preparedby cationic polymerization of oxazolines and subsequenthydrolysis of the acylimine groups.156 Grafting of livingpolyoxazolines onto the resulting linear poly(ethylenei-mine) chains led to the first grafting generation. Subse-quent hydrolysis and grafting of polyoxazoline leads to

the second grafting generation and so on. More recentlythe radical “grafting from” reaction involving ATRP orthe TEMPO method have been applied for preparation of“dendrigraft” polymers.165–167In this case the introduc-tion of chloromethyl groups into polystyrene, previouslyused for the anionic “grafting onto” reaction157 results insuitable initiating sites, for example, for the metal as-sisted ATRP.

An alternative route to highly branched polymers witholigomeric chains between the branch points has been de-veloped recently by the use of AB2 type macroinitiators[Scheme 15(b) and Scheme 2, monomers3, 4).91,168,169

However, very high molar mass products with extremelydense packing and the characteristics of a hard sphere wereonly reported for the “grafting onto” route.

Hyperbranched-Linear Combinations

The combination of a dendritic unit with a linear polymerchain has been extensively studied for perfectly brancheddendrons leading to complex architectures, for example,dendron-linear block copolymers,170–173dendronized ormonodendron jacketed linear polymers,174–176 and tostar polymers177–182with a dendritic core. Especially theattachment of monodendrons to a linear polymer chainby postmodification or by polymerization of monoden-dron-containing macromomomers draw much attentiondue to the visualization of these stiffened polymer chainsas single molecules by AFM174,176,183–185(Scheme 17).

The preparation of more complex architectures usinghyperbranched macromolecules, however, is a muchmore recent trend. Here, especially the field of star poly-mers with a hyperbranched core has been ex-plored.43,123,178,186–190Free radical, controlled radical,ring-opening, anionic, and cationic polymerizations have

Scheme 15. Schematic outline of the possible synthetic routes towards arborescentgraft polymer (a) via branch-on “grafting onto” or “grafting from” leading to very dense,high molar mass polymers, or (b) via AB2 macromonomers leading to more classicalhyperbranched structures but with oligomeric units between branch points.


been applied using hyperbranched macroinitiators. Theresulting polymers show typical behavior of star poly-mers, for example, low solution viscosities nearly inde-pendent from molar mass. By combination of suitablecore/arm combinations, structures with amphiphilicproperties can be obtained. Scheme 18 depicts the syn-thesis of star polymers with a relatively unpolar hyper-branched polyester core and polar poly(methyloxazoline)arms.188 Wang et al.191 produced electrically conductingstar polymers with a hyperbranched polyphenylene coreand polythiophene arms.

A triblock copolymer A-B-A with A consisting ofhyperbranched polyesters from 3,5-dihydroxybenzoicacid and B being an oligo(etherketone) was reported byKricheldorf et al.192 The triblock was prepared by co-polycondensation of the telechelic oligomer with AB2

monomers. The polymer proved to be fully soluble inTHF due to the hyperbranched structural components.

Hyperbranched Polymers and Surfaces

The modification of surfaces and thin polymer filmsdominates recent activities in polymer science. There-

fore, it is not surprising that dendrimers—and in somecases also hyperbranched polymers—have been studiedon surfaces. This covers the field of the visualization ofsingle dendritic macromolecules in order to get informa-tion on their shape and size (see sections above and, e.g.,refs.193–196), but also the use of dendrimers as carriers formetal ions,197 change of surface adhesion by graftingwith dendrons198 or studies on thin films (monolayers,self-assemblies) of dendrimers with electrical, catalytic,or sensoric properties.199

The properties of hyperbranched polymers on sur-faces have been studied up to now only seldom, but it canbe expected that this aspect will develop rapidly. Oneexample is the study of surface morphology and frictionof thin films from hyperbranched perfluorinated poly-mers in dependence of the length of perfluoroalkylchains by AFM.200Quite intensively, arborescent surfacegrafted poly(acrylic acid) has been explored by Crooksand Bergbreiter201 for different applications (Scheme19). Their interest covers surface patterning, biocompat-ibility, ion binding ability, and sensoric properties.

Following this line, we prepared recently thin poly-mer layers of aromatic hyperbranched polyesters with

Scheme 16. Monolayer structure of arborescent graft polystyrenes of differentbranching densities adsorbed on mica.164 (a) Spherical particles correspond to individ-ual molecules of Mw 5 2.8 z 107 g/mol and N 5 5. (b) Disklike structures correspond toarborescent graft polymers with Mw 5 5.5 z 107 g/mol and N 5 20. (The pictures havebeen supplied very kindly by S. Sheiko and M. Moller).

HIGHLIGHT 2517

different end groups on silica substrate and studied wet-ting and swelling behavior.202

Various Applications

Hyperbranched polymers were considered early on assuitable for any application where the low viscosity andthe large number of functional groups are of advantage.Studies on material properties proved that the highlybranched structure does not favor the formation of en-tanglements. Therefore, hyperbranched polymers arebrittle and the determination of mechanical properties ismostly not possible due to difficulties in sample prepa-ration. This limits certainly their application as bulkmaterial, for example, as engineering plastics.

Application fields discussed for perfectly brancheddendrimers, for example, in medicine as drug carriermolecules or in gene delivery, as standards or models forbiomolecules or as catalytic active molecules, were alsonot considered seriously for hyperbranched polymers,since the lack of a well-defined structure and molar massis a disadvantage in these sensitive areas.

But, as pointed out earlier, hyperbranched polymersdraw much attention as blend components, additives, andprimarily, as coating components. In the later, the highfunctionality (after modification, for example, withcrosslinkable groups) in combination with the high sol-ubility and low viscosity are excellent prerequisites foruse in high solid coatings or powder coatings. Hyper-branched polymers with acrylate, vinyl ether, allyl ether,

Scheme 17. (a) Suggested conformation of monodendron jacketed linear chains; (b)SFM-micrograph of single monodendron jacketed (polyether, G1) polystyrene moleculeswith (Mmonomer 5 1194 g/mol; Mnpolymer 5 1.1 3 106 g/mol, Mw/Mn 5 2.6) on highlyoriented pyrolitic graphite.176 (The pictures have been supplied very kindly by S.Sheiko and M. Moller).

Scheme 18. Amphiphilic star polymers via “grafting from” with a hyperbranchedpolyester core and polar poly(methyloxazoline) arms.188


epoxy, and OH-functions were studied as multifunctionalcrosslinker44–46,203in coatings and in thermosets.204,205

Recently, also the application of OH terminated hyper-branched polyesters in low volatile organic compounds(VOC) polyurethane coatings has been explored.206 As ageneral feature of hyperbranched polymers in crosslink-ing reactions we concluded46 that a very high function-ality number can lead to a too fast crosslinking reactionand therefore solidification of the material before allreactive functions had the chance to react. Therefore, the

number of reactive groups has to be balanced and reac-tive diluents should be used in coatings with hyper-branched components.

The application of hyperbranched polymers (and alsoarborescent graft polymers163,207) in processing as meltmodifier, additive, or as blend component is also wide-spread. Kim et al. showed that blends of hyperbranchedpolyphenylene and linear polystyrene exhibit a reducedviscosity at high temperatures and shear rates and animproved thermal stability compared to pure polysty-rene.43 When hyperbranched polyesters were blendedwith linear polyesters, polyamides, and polycarbonate asdescribed by Massa and coworkers,42 mostly full misci-bility was observed due to strong hydrogen bonding. Thepartially miscible polycarbonate blends exhibited an in-crease in the tensile and compression modulus but loss intoughness. Modified hyperbranched polyesters were usedas dye carrier in polyolefine blends.37 The resulting ma-terials showed a reduced melt viscosity and a homoge-neous distribution of the dye in the matrix. The fact thatalkyl modified hyperbranched polyesters37,51,52,208 orpolyethers124 show amphiphilic character and can act ascarrier molecules for physically enclosed organic mole-cules (for example, organic dyes) like in nanocapsules orin the dendritic box209opens many new application fieldsnot only as additive carrier for thermoplastics (Scheme20).

Scheme 19. Surface arborescent graft polyacrylicacid on a self-assembly monolayer confined to a goldsubstrate as depicted by Crooks and coworkers [MUA5 mercapto undecanoic acid; PAAM-c-PAA 5 randomcopolymer of poly(acrylic acid) and poly(acrylamide)formed from PAA carboxylic acid groups and theamines of hydrolyzed amine terminated poly(t-butylacrylate)].

Scheme 20. Schematic representation of the diffusion of organic dyes into an am-phiphilic hyperbranched polyester (aromatic hb polyester modified with C12-alkyl sidechains, D 5 organic dye) and the subsequent incorporation into a polyolefin matrix.37

HIGHLIGHT 2519

Hong et al.39 found a strong effect of alkyl modifiedaliphatic hyperbranched polyesters on the rheologicalbehavior of polyethylene (LLDPE). Mulkern et al.210

reported a lubricant effect of hyperbranched polymersalso in polystyrene blends. Recently Wahlen et al.211

observed the reinforcement of PA6 when hyperbranchedfully aromatic polyamides were added during the poly-condensation.

We studied blends of hyperbranched aromatic-ali-phatic poly(etheramide)s111 with commercial linearpolyamide PA6.38 All blends showed full miscibility.Furthermore, the hyperbranched polymer had no signif-icant negative influence on the melting and crystalliza-tion behavior of the PA6 matrix, the shear storage mod-ulus was even increased. But the rheological investiga-tions exhibited an unexpected strong reduction in meltviscosity even when only 0.1 wt % of hyperbranchedpolymer was added. With regard to the processing ofPA6 the reduced melt viscosity without loss of mechan-ical properties implies interesting application aspects ofhyperbranched polymers as processing aids.

Besides those more classical application aspects incommodity and engineering plastics, also new areas areexplored for the use of hyperbranched polymers. Onecovers the formation of nanofoams. Globular, hyper-branched molecules, which are degradable can act aspore forming material for a nanoporous system. For this,however, dispersion of the hyperbranched polymer on amolecular level in a stable matrix is required. Nanopo-rous material is for example, of interest in chromatogra-phy and for the formation of aerogels and xerogels.Hence, Boury et al.212 described the use of dendrimersand arboroles with carbosilane cores for the preparationof hybrid xerogels. In this regard, Muzafarov et al.213

studied the degradation behavior of hyperbranched po-ly[bis(undecenyloxy)methylsilane]s. Of high interest isalso the preparation of nanoporous polymers as lowdielectric constant material for use as novel interlayerdielectric material (ILD).214Already realized porous sys-

tems have reached dielectric constants fromer 5 2.5215

to approximately 1 as found for aerogels.216 The de-crease ofer is given by the inclusion of air filled cavities,which might be formed by degradable star or dendriticpolymers (Dendriglas 30™)217 or by phase separatedblock copolymers with labile segments.218–220 Hyper-branched poly(aryl ether phenylquinoxaline)s221 wereused for structure control in organic–inorganic hybridsfor application in microelectronics. We selected as labilemoiety for degradable hyperbranched polymers, the tria-zene group,222 which is known to be photolabile and canbe degraded into volatiles for example, upon laser abla-tion.223 As depicted in Scheme 21, the AB2 monomer31containing the labile unit was condensed under mild poly-condensation conditions to a degradable polyester.224

Recently, also the use of hyperbranched polymerswere reported in sensorics,201,202 as nonlinear optic225

and LC material,125,226,227 for ion conductivity,228 inmolecular imprinting,229 in catalysis,230 and also as sol-uble functional supports.122,231These examples demon-strate the very broad perspectives of hyperbranchedpolymers in all areas of modern polymer science.

Where to Go?

Only very few areas in the field of polymer science havedrawn that much interest in the last decade as dendriticmacromolecules. These branched structures have a largeinput not only on organic, polymer, and physical chem-ists but also to a large extent on physicists, biologists andbiochemists, biomedical groups, and theoreticians. Theyopened a new view towards the possibilities of branchingand topology in polymers with regard to synthetic chal-lenges but also for new application areas.

But, one can clearly see that we just left the startingline in the field of hyperbranched polymers. In the lastfew years the goal to prepare new and more complexhyperbranched structures to be able to explore the fullpotential of this polymer class, led to significant new

Scheme 21. Hyperbranched polyesters with labile triazene groups in the main chainforming labile, globular molecules, which can be incorporated into a thermally stablematrix as pore-forming molecules.224


synthetic approaches, even those never used for the prep-aration of linear polymers. Similarly as in the rapiddevelopment of the controlled radical polymerization,new and surprising concepts rose for the preparation ofhighly branched but soluble polymers. Especially inter-esting is the combination of controlled and ring-openingtechniques with the branching concept. Whereas, origi-nally only step growth reactions were considered suitablefor the preparation of hyperbranched polymers, more andmore chain growth is involved, too. In addition, morecomplex architectures involving dendritic and linear seg-ments start to dominate.

One can observe that there is a strong tendency tobring hyperbranched polymers closer towards perfectlybranched dendrimers, first by increasing the degree ofbranching and by having better control over the branch-ing, the molar mass distribution, and the architecture;and second, by opening application fields for hyper-branched polymers, which were up to now preservedmainly for dendrimers.

The future of hyperbranched polymers is still fullyopen and good for surprises, but one can predict that thepresent activities driven by the attraction of hyper-branched structures will also bring important new as-pects to other fields of polymers science.

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HIGHLIGHT 2525

New developments in hyperbranched polymers

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