AdvPolymSci(2006)198:1–49 DOI10.1007/12_059 © Springer-VerlagBerlinHeidelberg2006 Publishedonline:19January2006 HyperbranchedSurfaceGraftPolymerizations DavidE.Bergbreiter((cid:1))·AndrewM.Kippenberger DepartmentofChemistry,CollegeStation,TexasA&MUniversity, Texas,TX77842-3012,USA [email protected] 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 HyperbranchedPoly(acrylicAcid)Grafts . . . . . . . . . . . . . . . . . . 4 2.1 HyperbranchedPoly(acrylicAcid)GraftSynthesisonGoldSurfaces . . . 5 2.1.1 DerivativesofHyperbranchedPoly(acrylicAcid)Grafts . . . . . . . . . . 8 2.1.2 AqueousSolvationofHyperbranchedPoly(acrylicAcid)Films. . . . . . . 15 2.1.3 PatterningofHyperbranchedPoly(acrylicAcid)-DerivedGrafts . . . . . . 18 2.2 HyperbranchedGraftsonPolymerSurfaces . . . . . . . . . . . . . . . . . 20 2.2.1 SynthesisofHyperbranchedPoly(acrylicAcid)Grafts onPolyethyleneFilms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.2.2 HyperbranchedGraftsonPolypropyleneWafers. . . . . . . . . . . . . . . 28 2.2.3 HyperbranchedGraftsonPolyethylenePowders. . . . . . . . . . . . . . . 29 3 HyperbranchedNanocomposites . . . . . . . . . . . . . . . . . . . . . . . 32 4 HyperbranchedGrafting bySurfaceInitiatedRingOpeningPolymerization . . . . . . . . . . . . . 37 4.1 GraftingHyperbranchedPolyglycidol . . . . . . . . . . . . . . . . . . . . 37 4.2 GraftingofHyperbranchedPoly(ethyleneimine). . . . . . . . . . . . . . . 39 5 HyperbranchedGraftsofOrganic/InorganicHybridPolymers . . . . . . 43 5.1 PolysiloxaneHyperbranchedGrafts. . . . . . . . . . . . . . . . . . . . . . 43 5.2 DendriticHyperbranchedGraftsofPd(II)CoordinationPolymers . . . . 44 6 DendrimerAnalogsasHyperbranchedGrafts. . . . . . . . . . . . . . . . 47 7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Abstract This review summarizes the synthesis of irregular hyperbranched polymer graftsonvariousinorganicandorganicsubstrates.Thesynthesisofthesehyperbranched graftsaregenerallybasedon“graftonagraft”polymerizationsandincludediversesorts of graft polymers. The “graft-on-a-graft” strategies discussed here include chemistry leading to the synthesis of hyperbranched poly(acrylic acid) grafts, polysiloxane grafts, dendrimer/polyanhydride graft nanocomposites, ring-opening polymerization grafts, andpolyamidoaminegrafts.Otherrelevantchemistryofthesegraftsincludingchemistry leadingtoderivativesofhyperbranchedpoly(acrylicacid)grafts,furthermodificationby polyionicinteractions,polyvalenthydrogenbonding,andfunctionalgroupmanipulation is discussed. Examples of reactions of monomers with polyvalent surfaces that lead to hyperbranchedgraftsarealsobrieflydiscussed. 2 D.E.Bergbreiter·A.M.Kippenberger Keywords Dendrimer·Hyperbranchedgrafts·Nanocomposite·Polyvalency· Surfacemodification Abbreviations PAA poly(acrylicacid) PTBA poly(tert-buylacrylate) MUA mercaptoundecanoicacid FTIR-ERS FourierTransformInfraredexternalreflectionspectroscopy XPS X-rayphotoelectronspectroscopy PE polyethylene PP polypropylene ATR-IR attenuatedtotalreflectanceinfrared PNIPAM poly(N-isopropylacrylamide) TEA 2-thiopheneethyleneamine ROP ringopeningpolymerization APES 3-aminopropyltriethoxysilane 1 Introduction There is great interest in designing functional interfaces. Hyperbranched graftsare alternatives to existing “linear” graftsfor formationof such inter- faces. They are of interest because they can provide interfaces withdifferent sortsofproperties.Hyperbranchedgraftingisalsoconceptuallymoreattrac- tivethanotherapproachesbecausethemultiplegraftingofoligomericgraft- ingreagentscancompensateforinefficienciesinreactionsatsurfaces(Fig.1). If, for example, an initial surface graft has coverage defects or if defects are introducedduringthegraft-on-a-graftsynthesisduetoincompletereactions, subsequent hyperbranched grafting stages can “heal” these defects more ef- ficientlythanthetraditionalmonomergraftingstrategiesthatproducelinear graftchains(Fig.1bversus1a).ThissameeffectwasnotedpreviouslybyFer- gusonin layer-by-layer graftingofmicaparticles and polycationic polymers onhydrophobic surfaces like octadecyltrichlorosilanetreated Si/SiO wafers 2 and hexadecanethiol-modified silver films and is a general feature common to other layer-by-layer grafting chemistry [1,2]. As shown in Fig.1, the ad- vantages of hyperbranching are considerable. In the particular schematic drawingofthreegraftstagesshowninFig.1b,hyperbranched graftingisfar moreeffectivethanlineargraftingthroughthreestagesinFig.1aeven when thereisarelativelylow(50%)efficiencyinthefirststepofgrafting. The synthetic strategies that lead to irregularly hyperbranched grafts based on surface confined “graft-on-a-graft” polymerization reactions are the focus of this review. Limited examples of monomers reacting with poly- valent surface-bound reagents leading to hyperbranched polymers are also discussed. In general, the chemistry described here is confined to reactions HyperbranchedSurfaceGraftPolymerizations 3 Fig.1 A schematic drawing comparing linear grafting a with hyperbranched grafting bincoverageor“healing”ofsurfacedefects.Anefficiencyof100%isassumedinallthree stepsinlineargrafting(a).Inthehyperbranchedgraftexample(b),a50%efficiencyisas- sumedinthefirststepbut100%efficiencyandthreebranchespergraftstageareassumes instepstwoandthree that involvecondensation polymerization reactions orreactions that involve the reaction of an electrophile with a nucleophile. This review begins with hyperbranchedgraftingofpoly(acrylicacid)onhardinorganicormetalsur- faces and soft polymer surfaces. Methods for derivatizing these films either bycovalentmodificationorwithpolyvalentnoncovalentinteractionsaredis- cussed. Limited examples of applications of these materials are described. For example, Crooks’ group has used some of these synthetic methods to prepare patterned surfaces. In cases like this where this subject has been reviewed, it is only briefly discussed here. Other hyperbranched grafting strategies including multilayer grafting of polyvalent nucleophiles and elec- trophiles, grafting via ring opening polymerizations, and the synthesis of dendritic graftsusing polyvalent surface-bound reagents and monomers are discussed subsequently. There are other very successful synthetic strategies for preparing hyperbranched films based on free radical polymerizations that will not be a topic of discussion in this review. For example, Müller has developed anovelmethod ofhyperbranched graftpolymerization ofin- imers (initiator-monomers) by self-condensed vinyl polymerization (SCVP) via atom transfer polymerization (ATRP) [3,4]. Another example would be Matsuda’spreparationofhyperbranchedgraftsbyiniferter(initiator-transfer agent-terminator) polymerization [5,6]. A detailed description of these in- iferter polymerizations can be found in Matsuda’s contribution in this vol- ume. A similar approach by Tsubokawa is described as a post-graft poly- merization of vinyl monomers and is useful as a route to hyperbranched grafts [7–9]. Surfaces with hyperbranched grafts can also be prepared by 4 D.E.Bergbreiter·A.M.Kippenberger graftingcommerciallyavailablehyperbranchedpolymerstosurfaces.Forex- ample, Tsukruk has studied graftedhyperbranched polyesters with terminal epoxidesthatareattachedtoSi–OHsurfaces[10,11].Therearemanyexam- ples where dendrimers are attached to surfaces by covalent or non-covalent interactions[12–17].Thischemistrytooisnotdiscussedhereunlesstheden- drimers are used as reagents with linear polymers or oligomers to prepare hyperbranchedgrafts. 2 HyperbranchedPoly(acrylicAcid)Grafts The synthesis of hyperbranched grafts of poly(acrylic acid) (PAA) using a “graft-on-a-graft” strategy is a general method for modifying a variety of surfaces. It requires as a starting material a surface that contains some functional groups though the amplification of functionality inherent in the chemistry means that a surface with only a modest level of functional groups can produce an interface with a macroscopically detectable concen- tration of functional groups. Examples of surfaces that have been modi- fied include silicon (using the hydroxyl groups of the Si(OH) layer), gold x withfunctionalself-assembledmonolayers,glass,andsurface-oxidizedpoly- olefin films and powders. In each case, robust ultrathin supported-films are the products. This covalent multistep strategy is based on functional group protection/deprotection and affords modest control over the product film thickness.InPAAgrafting,thiscontrolisbasedonthenumbergraftingstages thatareused. Theproducthyperbranched graftsrangeinthicknessfromca. 30˚A to greater than 1000˚A. The film thickness initially increases rapidly in a non-linear fashion since each additional layer is added in a branching fashion multiplying thenumber ofgraftingsites (Fig.2). After several graft- ing stages the thickness increases in a linear fashion. This variable extent of progress of this grafting chemistry as measured by either ellipsometry on reflectivemetalsurfacesorasmeasuredbytitrationofthe–CO Hgroupsbe- 2 ing introduced on higher surface area materials is very similar substrate to substrate(Fig.2)[18,19]. The graft-on-a-graft strategy was conceived of as a synthetically “forgiv- ing” alternative to an attempted but ineffective borane-based radical graft polymerization onto vinyl terminated self-assembled monolayers [20] on gold and was based on earlier observations that a poly(acrylic acid) graft modifiedwithnewgraftsitescouldbeusedtoprepareamoredenseandpre- sumablythickergraftwithsubsequent polymerizationorgraftingsteps[21]. It was also conceptually more attractive than other approaches that used monomers as grafting agents because the multiple grafting of oligomeric graftingreagents couldcompensate forinefficiencies inreactions atsurfaces asdiscussedabove(Fig.1). HyperbranchedSurfaceGraftPolymerizations 5 Fig.2 Progressofhyperbranchedpoly(acrylicacid)graftformationonsmoothgoldfilms asmeasuredbyellipsometry(•)oronpolyethylenepowdersasmeasuredbytitration((cid:1)) ofthesupported–CO2Hgroups 2.1 HyperbranchedPoly(acrylicAcid)GraftSynthesisonGoldSurfaces The synthesis of surface grafted hyperbranched films of poly(acrylic acid) was first described ongoldsubstrates [18]. This synthesis of hyperbranched grafts of poly(acrylic acid) (PAA) on gold, shown in Scheme1, began with a self-assembled monolayer of mercaptoundecanoic acid (MUA). Activa- tion of the carboxylic acid groups of this monolayer was accomplished by formation of mixed anhydrides with ethyl chloroformate. While other activating agents (e.g. carbonyl diimidazole or DCC worked), the best yields were obtained with alkyl chloroformates. Subsequent amidation of this electrophilic surface by an oligomeric reagent, α,ω-diamino-poly(tert- butyl acrylate) (PTBA), yielded a 1-PTBA graft on MUA functionalized gold (1-PTBA/Au). This 1-PTBA/Au graft was initially converted to a 1- PAA/Au graft by acidolysis with p-toluene sulfonic acid/H O. Subsequent 2 work showed that this acidolysis proceeded equally well using methane- sulfonic acid (15min, room temperature). Activation of the carboxylic acid groups of this first 1-PAA/Au graft with more ethyl chloroformate followed by treatment of the new polyanhydride surface with more α,ω-diamino- poly(tert-butyl acrylate) oligomer produced a 2-PTBA/Au graft. Acidolysis of this second graft layer of PTBA produces a 2-PAA/Au graft. Repeating 6 D.E.Bergbreiter·A.M.Kippenberger Scheme1 Repetitive step-by-step synthetic scheme leading to formation of a hyper- branchedgraftofpoly(acrylicacid)onamercaptoundecanoicacidself-assembledmono- layeronasupportedgoldfilm this process for several generations produces a dense, highly functionalized surface. Films containing as many as seven graft layers were successfully prepared. The use of appropriate functionalized oligomers is a key to the success of this synthesis. The necessary functionalized oligomers were prepared by polymerization of tert-butyl acrylate (n=ca. 120) with a functional AIBN initiator(Eq.1).Since (1) HyperbranchedSurfaceGraftPolymerizations 7 tert-butyl acrylate polymerizations terminate mainly by coupling [22], the productisprincipallyadifunctionalpoly(tert-butylacrylate).Thecarboxylic acid terminated oligomers so-formed were subsequently converted into pri- mary amines (Eq.2). The product polymer was characterized by end group analysis at both the –CO H stage and at the –NH stage and had a M of 2 2 n 12000–18000Daltonsinvariouspreparations. (2) TheprogressofthissurfacegraftchemistrywasfollowedbyFouriertrans- form infrared external reflection spectroscopy (FTIR-ERS), water contact anglegoniometry,X-rayphotoelectronspectroscopy(XPS),andellipsometric analysis.Thethicknessofthesefilmsincreasedinanonlinearfashion(Fig.1). Ellipsometricanalysisshowedthethicknesschangedfromca.30˚Aforasin- gle PAA/Au graft to greater than 1000˚A for films that were prepared using morethan5graftingstages[23].ActivationoftheMUA/Aufilmbyformation of the mixed anhydride was shown to be quantitative by FTIR-ERS spec- troscopy.Upontreatmentwiththediamino-oligomerofPTBA(M =14600), n evidence for the formation of a 1-PTBA/Au graft was seen in the FTIR-ERS spectrumwhichshowedasmallamidepeakandalargetert-butylesterpeak. After acidolysis, the absorption peaks for tert-butyl esters disappeared. Wa- tercontactanglegoniometryshowedthatthe–CO H-richsurfacewasmore 2 hydrophilic as expected. The carbonyl peak intensity in the infrared spec- trum increased with each additional grafting stage. The amide peaks in the infraredspectrumfromcovalentgraftingwerealsodetectableintheIRspec- trum. These grafts were found to be stable to extensive solvent treatment. NochangeinthecarbonylintensityintheFTIR-ERSspectrumwasobserved after Soxhlet extraction with methylene chloride or sonication with acetone oracid. Tapping-mode atomic force microscopy studies showed that as these hy- perbranched PAA films became somewhat less smooth as they increased in thicknessthroughsuccessivegraftingstages[24].Forexample,averysmooth initial single-crystal Au(111) surface witha rootmean square (RMS) rough- ness of 0.2nm (over a 2µm×2µm area) had its roughness increased to 8 D.E.Bergbreiter·A.M.Kippenberger Table1 Root meansquare roughnessof PAAgraftson Au/Simeasured overa 5×5µm area Graft RMS(nm) MUA/Au/Si 2.01 1-PAA/Au/Si 1.44 2-PAA/Au/Si 1.15 3-PAA/Au/Si 1.02 4-PAA/Au/Si 1.53 0.3nm for a 1-PAA/Au graft and 0.8nm for a 3-PAA/Au graft. However, grafting on a rough Au/Ti/Si surface that was prepared from Au deposi- tiononTi/Siresultedinsomesurfacesmoothingafterseveralgraftinglayers (Table1). In this case the 1-PAA/Au, 2-PAA/Au, 3-PAA/Au, and 4-PAA/Au grafts were all smoother than the initial Au/Ti/Si surface. The PTBA grafts weregenerallylesssmooth.Therelativesmoothnessofthesesurfacesleveled off after 2 or 3 graft stages and the surface became increasingly rough with additional grafting layers following the trend noted earlier with grafting of PAAonsingle-crystalAu(111)surfaces. 2.1.1 DerivativesofHyperbranchedPoly(acrylicAcid)Grafts Hyperbranchedgraftsofpoly(acrylicacid)ongoldcaneasily bederivatized. The most common approach has been to use derivatizing agents that con- tain reactive amines or alcohols to form carboxylic acid amides or esters. Examples of compounds covalently incorporated into these PAA/Au inter- faces are shown in Fig.3. Equation 3 illustrates this general method which involves first activating the poly(acrylic acid) grafts with ethyl chlorofor- mate. Subsequent treatment (here with an amine) then produces a mix- ture of derivatized –CO H groups and unmodified –CO H groups in 2 2 the interface. The covalent amidation strategy has been used to prepare low-energy fluorinated surfaces [25–27]. Amidation or esterification has been used to incorporate pyrene fluorescence probes, molecular recogni- tion elements like crown ethers cyclodextrin [28], ferrocene, poly(ethylene glycol), and dye chromophores [23]. Other chemistry typical of –CO H 2 groups can also be carried out with the –CO H groups in these inter- 2 faces. This includes acid-base chemistry, reductions, ion exchange, and non-covalent modifications through hydrogen-bonding. Finally, the polyva- lent nature of these films can be used to advantage in molecular assembly procedures. Specifically, these films have been noncovalently functionalized using ionic entrapment of polycations [29] and enzymes [30] and through HyperbranchedSurfaceGraftPolymerizations 9 polyvalent hydrogen bonding interactions with polyvalent hydrogen-bond acceptors[31]. (3) Hydrophobic,fluorinated hyperbranchedgraftsweresynthesized froman ethylchloroformateactivatedhyperbranched3-PAA/Augraftusingthefluo- rinatedalkylamineH N(CH )(CF ) CF .Reactionofanactivated3-PAA/Au 2 2 2 6 3 filmwithH N(CH )(CF ) CF producedafluorinatedfilmthatalmostdoubled 2 2 2 6 3 inthickness.XPSanalysisshowed46atom-%Fintheproductfluoramidated graftwhichis86%ofthetheoreticalatomicconcentrationforahomogenous Fig.3 Functionalitythat has been incorporated into PAA/Au interfaces through amida- tionoresterificationchemistry 10 D.E.Bergbreiter·A.M.Kippenberger fluorinatedgraft.AsecondactivationandH N(CH )(CF ) CF treatmentin- 2 2 2 6 3 creasedthefilms’Fcontentto50atom-%,93%ofthetheoreticalconcentration forahomogenousgraft[25]. NoAupeak wasobservedfromtheunderlying goldsupportindicatinggoodcoverage.Filmswerealsosynthesizedwherethe internal graftslayers were fluorinated and the external graftslayers were ei- ther hydrophilic PAA grafts or fluorinated [26]. The control of the fluorine content of the core and shell of these grafts demonstrates the flexibility of this “graft-on-a-graft” strategy and the ability of later graft stages to cover defects in earlier grafts. This is illustrated by steps where the small amount of residual –CO H groups remaining after formation of a fluorinated graft 2 likethatdescribedabovewereusedasthebaseforformationofahydrophilic graft with a fluorinated hydrophobic interior. In this example, a film with a hydrophobic interior was first prepared by adding H N(CH )(CF ) CF to 2 2 2 6 3 agraftingsolutionofdiamino-PTBAintheearly graftingstages.Inthefinal fluorine-graftingstep,agraftingmixturecontaininga500:1mol:molratioof H N(CH )(CF ) CF todiamino-PTBAof500:1wasused. Atthispoint,the 2 2 2 6 3 ◦ supported hyperbranched graftfilmhadawater contactangleof100 .Then twoadditionalgraftingstageswerecarriedoutusingonlythediamino-PTBA polymer.Thesestepsonlyproducedhydrophilic–CO Hontheexteriororshell 2 ofthisoriginalfluorinatedgraft.Formationofthishydrophilicexteriororshell wasconfirmedbyabsenceofFpeaksintheXPSspectruminthefinalproduct ◦ ◦ andbythechangeincontactanglefrom100 to14 . Thesehydrophobic,fluorinatedhyperbranchedfilmsongoldpassivatethe underlying goldmetaltowardelectrochemicalreactions.Forexample,under basic conditions a fluorinated 3-PAA/Au interface had a measured charge- transfer resistance that was 40 times more than the resistance of an unflu- orinated 3-PAA film under the same conditions. These fluorinated hyper- branchedgraftsweresignificantlymorepassivatingthanasimplemonolayer. For example fluorinated 3-PAA/Au grafts were shown to passivate the un- derlying goldtoa104 timesgreaterextent thanaMUAcoatedelectrodeand about10timesmorethanahexadecanethiolmonolayer[25].Thefluorinated 3-PAA/Au grafts’passivation was greater under basic conditions in contrast totheunfluorinated3-PAA/Augraftsthatweremorepassivatingunderacidic conditions.Forexample,thefluorinatedgrafts’hadcharge-transferresistance that increased 10-fold as pH was changed from 3 to 10 while the unfluori- nated 3-PAA/Au grafts’ charge transfer resistance decreased 19-fold as the pH changed from 3 to 10. Presumably that reflects the fact that PAA grafts have pH-dependent swelling (vide infra) that islacking in the fluorinated 3- PAA/Augrafts[27]. Modification of PAA/Au grafts with the amine and alcohol functional- ized pyrenes 2 and 3 produced highly fluorescent films [23]. These deriva- tized films exhibited both monomer and excimer fluorescence. The relative amounts of monomer and excimer emission depended on the pyrene con- centration used in the derivatization process. When modest concentrations