WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
Synthesis,Purification,andCharacterizationof‘‘Perfect’’StarPolymersvia‘‘Click’’Coupling
YejiaLi,BoyuZhang,JessicaN.Hoskins,ScottM.Grayson
DepartmentofChemistry,TulaneUniversity,NewOrleans,Louisiana70118Correspondenceto:S.M.Grayson(E-mail:sgrayson@tulane.edu)
Received6October2011;accepted7November2011;publishedonline15December2011DOI:10.1002/pola.25864
ABSTRACT:Thecopper(I)-catalyzedazide-alkynecycloaddition
‘‘click’’reactionwassuccessfullyappliedtopreparewell-defined3,6,and12-armspolystyreneandpolyethyleneglycolstars.Thisstudyfocusedparticularlyonmaking‘‘perfect’’starpolymerswithanexactnumberofarms,aswellasdevelopingtechniquesfortheirpurification.Variousmethodsofcharacteri-zationconfirmedthestarpolymershighpurity,andthestruc-turaluniformityofthegeneratedstarpolymers.Inparticular,matrix-assistedlaserdesorptionionization-time-of-flightmassspectrometryrevealedthequantitativetransformationofthe
endgroupsonthelinearpolymerprecursorsandconfirmedtheirquantitativecouplingtothedendriticcorestoyieldstarpolymerswithanexactnumberofarms.Inadditiontoprepar-ingwell-definedpolystyreneandpoly(ethyleneglycol)homo-polymerstars,thistechniquewasalsosuccessfullyappliedto
C2011WileyPeriodi-amphiphilic,PCL-b-PEGstarpolymers.V
cals,Inc.JPolymSciPartA:PolymChem50:1086–1101,2012
KEYWORDS:atom
transferradicalpolymerization(ATRP);
MALDI-TOFMS;separationofpolymers;starpolymers
INTRODUCTIONStarpolymersarebranchedpolymersthat
consistofmultiplelinearchainsconnectedtoacentralcore.Starpolymers’multiarmstructure,globularshape,andmul-tiplicityofendgroupsimpartsonthemauniquesetofprop-erties(e.g.,crystalline,mechanical,andviscoelasticproper-ties),whencomparedwiththeirlinearanalogs.Perhapsmostsignificantly,thethree-dimensionalarchitectureofappropriatelydesignedamphiphilicstarpolymerscanbetai-loredtoencapsulateguestmoleculesprovidingrelativelyrapidaccesstowell-definednanocarriersforapplicationsindrugdelivery.2–4Therearethreegeneralstrategiesforthesynthesisofstarpolymers,eachofwhichexhibitsparticularadvantagesanddisadvantages:the‘‘corefirst’’approach,the‘‘armfirst’’approach,andthe‘‘graftonto’’approach.5–8The‘‘corefirst’’approachinitiatespolymerizationofthearmsfromapolyfunctionalcore,whichlikelyresultsinnonuni-formarmslengthsbecauseofpotentialdifferencesinthereactivityofeachinitiatinggroup.Likewise,whencomparingasetofstarpolymerspreparedfromdifferentcores,itisdif-ficulttoensureuniformityofarmlengthfrombatchtobatch.The‘‘armfirst’’approachinvolvesfirstthepolymeriza-tionofthearmsfollowedbytheircoupling,usuallybycon-tinuationofthepolymerizationwithadditionofacrosslink-ingmonomer.Althoughtechnicallysimple,thistypicallyleadstomaterialswithaninexactnumberofarmsandbroaderpolydispersities.Thethirdmajorroute,the‘‘graftonto’’approachinvolvestheattachmentofpreformedarms
1ontocoremoleculesviaanefficientcouplingreaction.Thisapproachmakestheconjugationofarmsofuniformlengthontodifferentcorespossiblebutrequireshighlyefficientconjugationreactionstocompletethecouplingtothecore.Thisroutealsorequiresanexcesslinearpolymerarmstodrivethecouplingreactiontocompletion,andthereforeinvolvestediouspurificationtoremovetheunreactedlinearreactant.The‘‘graftonto’’methodtypicallyaffordsthemoststructurallyuniformstarpolymersandprovidesaccesstocomparablelibrarieswithdifferentcoresyetidenticalarmlengths,however,trulywell-definedstructurescanonlybeaccessedwithhighlyefficientcouplingreactionsandsub-stantialpurificationefforts.
Muchoftheearlyworkpreparingwell-definedstarpolymersusedanionicpolymerizationmethodsowingtotheexcep-tionalcontrolofthistechniquebutsufferedfromlimitedmonomercompatibilityandextremelytediousexperimentalpreparationduetothetechnique’ssensitivitytotraceimpur-ities.9Recently,thedevelopmentofcontrolled/livingradicalpolymerization,10especiallyatomtransferradicalpolymer-ization(ATRP),11,12enabledthefacilepreparationofnarrowdispersedpolymerswithbroadmonomercompatibilityyetbearinguniformlyend-functionalizedpolymergroups.Aroundthesametime,theoptimizationofdendrimerchem-istryaffordedanumberofefficientsynthesesthatprovideaccesstowell-defineddendriticcores,withexactnumbersoffunctionalgroups.13Thecombinationofthesetwo
AdditionalSupportingInformationmaybefoundintheonlineversionofthisarticle.
C2011WileyPeriodicals,Inc.V
1086JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
approachesprovidesrelativelystraightforwardroutestothesynthesizedbydescribedprocedure.35Thedetailcanbewell-definedprecursorstomakehighpuritystarpolymersfoundintheSupportingInformation.
viathe‘‘graftonto’’approach.
Theonlyremainingrequirementforpreparingwell-definedCharacterization
starpolymers,ahighlyefficientcouplingreaction,ispro-All1HNMR(400MHz)and13CNMR(100MHz)wereobtainedvidedbythecopper-catalyzedazide-alkynecycloadditionusingaVarianMercuryspectrometer(PaloAlto,CA),usingTMS(CuAAC)‘‘click’’couplingreaction.14–16Thisparticularreac-¼0.00ppmfor1Hcalibration.Gelpermeationchromatographytionisexceptionallypowerfulinovercomingthestericinhi-(GPC)wasperformedonaWatersmodel1515seriespumpbitionthatfrequentlypreventsthequantitativecouplingof(Milford,MA)withthreecolumnseriesfromPolymerLaborato-polymericsubstrates.Itsusageforpolymerconjugationshasries,consistingofPLgel5lmMixedD(300mmÂ7.5mm,mo-beendemonstratedwidely17–20butisperhapsmostappa-lecularweightrange200–400,000),PLgel5lm500Å(300rentinthenearquantitativecouplingofhinderedmacromo-mmÂ7.5mm,molecularweightrange500–30,000),andPLgellecularcomponents.Representativeexamplesincludethe5lm50Å(300mmÂ7.5mm,molecularweightrangeuptosynthesisofdendronizedpolymersbycouplingthirdgenera-2000)columns.ThesystemwasfittedwithaModel2487dif-tiondendronsontoeachrepeatunitofeitheralinearpoly(-ferentialrefractometerdetectorandanhydrousTHFwasusedvinylacetylene)backbone21oracyclicstyrenicbackbone;22asthemobilephase(1mLminÀ1flowrate).Theresultingmo-thesynthesisofpolymer-peptideconjugatesofbovineserumlecularweightwasbasedoncalibrationusinglinearpolysty-albuminwithpolystyrene(4.15kDa)23orpoly(N-isopropyla-renestandards.DatawerecollectedandprocessedusingPreci-crylamide)(PNIPAM)(16.3kDa);24andthesynthesisofgraftsionAcquiresoftware.Massspectraldatawasacquiredusingapolymersviathecouplingof775DaPEGchainsontoa27.5BrukerAutoflexIIImatrix-assistedlaserdesorptionionization-kMWpoly(hydroxyethylmethacrylate)backbone.25Thetime-of-flightmassspectrometer(MALDI-TOFMS)withdelayedCuAACreactionhasalreadybeenusedinnumerousexam-extractionusingbothpositiveionandreflectordetectionplestomakestarpolymers,includingpoly(styrene)stars,25modes.ForallPCLpolymers,THFstocksolutionsof9-nitroan-poly(tert-butylacrylate)andpoly(ethyleneglycol)stars,26thraceneasthematrix(20mgmLÀ1)andNaIasthecounterioncyclodextrin-coreseven-armstarpoly(e-caprolactone)(10mgmLÀ1)wereused.Thepolymersamplewaspreparedat(PCL),27cyclodextrin-coreseven-armand21-armstarPNI-a2mgmLÀ1concentrationinTHF.MALDIsampleswerepre-PAM,28POSS-coreblockcopolymerstars,29,30andsoforth.Inparedbycombining50lLofpolymersolution,100lLofcoun-addition,othertypesof‘‘click’’reactions,suchasthiol-eneterionsolution,and200lLofmatrixsolution.ForallPEGpoly-reaction31andDiels-Alder32reactionshavealsobeenusedmers,THFstocksolutionsofalpha-cyano4-hydroxycinnamictosynthesizestarpolymersviathe‘‘graftonto’’method.acid(20mgmLÀ1)andsodiumtrifluoroacetateasthecounter-However,mostofthesematerialswerenotanalyzedinsuffi-ion(1mgmLÀ1)wereused.Thepolymersamplewaspreparedcientdetailtojudgetheirpuritywithregardstomissingata2mgmLÀ1concentrationinTHF.MALDIsampleswerepre-arms.Becauseoftheinterestinstarpolymersforbiomedicalparedbycombining20lLofpolymersolution,0.5lLofcoun-applicationssuchastransdermaldrugcarriers,4,33,34itisterionsolution,and10lLofmatrixsolution.ForallPStpoly-criticaltodevelopsynthetic,purification,andcharacteriza-mers,THFstocksolutionsofdithranol(20mgmLÀ1)andsilvertionmethodologies,whichcanreproduciblyyield‘‘perfect’’trifluoroacetateasthecounterion(1mgmLÀ1)wereused.Thestarpolymers,thosewhichexhibittheexactnumberofarmspolymersamplewaspreparedata2mgmLÀ1concentrationinasweretargetedintheirsynthesis.Forthesereasons,aTHF.MALDIsampleswerepreparedbycombining20lLofdetailedstudyofthecoupling,purification,andcharacteriza-polymersolution,0.5lLofcounterionsolution,and10lLoftionofstarpolymerswasperformedanddescribedbelow.
matrixsolution.
EXPERIMENTAL
GeneralProcedureforSynthesisofAlkynylatedDendronsThealkynylateddendronsweresynthesizedaccordingtoaMaterials
previouslyreportedprocedure.36DendrimerswithhydroxylAllreagentswerepurchasedfromAldrichandusedwithoutgroupontheperiphery,pentynoicacidanhydride,4-Dime-furtherpurification,unlessotherwisenoted.e-CLwasdriedthylaminopyridine(DMAP),andpyridine(1hydroxide:1.67overcalciumhydrideatroomtemperatureforatleast8hpentynoicacidanhydride:0.33DMAP:6pyridine)wereaddedanddistilledunderreducedpressurejustbeforeuse.Sol-toaroundbottomflaskwithdichloromethane.AfterstirringventswerepurchasedfromPharmacoAaper,Aldrich,andfor12hinroomtemperatureundernitrogen,themixtureFisherScientific.Tetrahydrofuran(THF)wasdriedoverso-wasquenchedbyDIwaterforabout2h.Sequentiallythediumrefluxingovernight.Dichloromethaneandhexanewerereactionwaswashedthreetimeswith1MNaHSO4solution,driedovercalciumhydrideatroomtemperatureovernight.1MNaHCO3solutionandofbrine.TheorganiclayerwasAllothersolventswerereagentgradeandusedwithoutfur-driedoveranhydrousMgSO4,filtered,andconcentratedtherdistillationorpurification.Bio-BeadsVRS-Xbeadsarepo-beforedissolveindiethylether.Afterfiltration,thefiltraterouscrosslinkedpolystyrenepolymersusedforgelpermea-wasconcentratedandprecipitatedinto40mLofcoldhex-tionseparationsofhydrophobicpolymersinthepresenceofanes.Theproductwasisolatedasoilanddriedinvacuo.Theorganicsolvents.Thedendrimerswithhydroxylgroupwere
specificdetailsaredescribedintheSupportingInformation.
WWW.MATERIALSVIEWS.COM
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
1087
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
SynthesisofPolystyreneBromide
A500mLroundbottomedflaskcontaining2.73g(19.06mmol)CuBr,0.75g(3.37mmol)CuBr2,4.66g(26.9mmol)PMDETA,and300g(2.88mol)styrenewasdegassedusingtwofreeze/pump/thawcycles.Afterwarmingtoroomtem-perature,4.15g(22.4mmol)oftheinitiatorwasaddedviasyringe.Thereactionmixturewasplacedinapreheated75
Coilbathandallowedtostirundernitrogenfor56min.Thereactionmixturewasthencooledtoroomtemperatureandpurifiedbyextractionfromwaterintodichloromethanefollowedbyprecipitationintomethanoltoyieldthepolymerasawhitesolid:26.9g(yield8.86%).1HNMR(400MHz,CDCl3)d0.9–1.1(br,3),1.3–2.6(br,72),4.4–4.6(br,1),6.4–7.4(br,120).MS(MALDI-TOF):(m/z),Mn¼2450;PDI:1.07.GPC:Mn¼2800;PDI¼1.12.FTIR:3082,3061,3026,2923,2850,1943,1872,1803,1685,1601,1493,1453,1373,1181,1154,1070,1028,907,759,698cmÀ1.
SynthesisofPolystyreneAzide
Toa500mLroundbottomedflaskwasadded26.9g(10.4mmol)PS-Br.Thepolymerwasthendissolvedinto60mLdimethylformamide(DMF),and3.37g(0.51.8mmol)sodiumazidewasaddedasasolid.Thesolutionwasallowedtostirovernightatroomtemperaturebeforepurificationbyprecip-itationintomethanoltogiveawhitesolid:22.3g(yield83%).1HNMR(400MHz,CDCl3)d0.9–1.1(br,3),1.3–2.6(br,72),3.8–4.1(br,1),6.4–7.4(br,120).MS(MALDI-TOF):(m/z),Mn¼2500;PDI:1.04.GPC:Mn¼2800;PDI¼1.12.FTIR:3082,3060,3026,2923,2849,2094,1943,1872,1803,1745,1601,1493,1453,1374,1181,1154,1069,1028,907,760,698cmÀ1.
SynthesisofPEG-OMs
AtypicalprocedureofsynthesisofPEG-OMswaspreviouslyreported.37PEG-OH(Mn¼2000,4g,2mmol)washeatedto50Cinoilbathanddriedinvacuofor12h.Itwascooledtoroomtemperaturebeforedissolvedin100mLofdrieddichloromethane.Thesolutionwascooledto0Cinice-waterbath,anddistilledtriethylamine(1.53mL,11.0mmol)andmethanesulfonylanhydride(1.74g,10mmol)weresequentiallyslowlyadded.Afterstirringfor12hinroomtemperatureundernitrogen,themixturewasfilteredandsequentiallywashedthreetimeswith100mLof1MNaHSO4solution,100mLof1MNaHCO3solutionand50mLofbrine.TheorganiclayerwasdriedoveranhydrousMgSO4,filtered,andconcentratedbeforeprecipitationfromdichloromethaneinto40mLofcoldethylether.Theproductwasisolatedviafiltrationanddriedinvacuo;72%yield.1HNMR(400MHz,CDCl3)d3.06(s,3H),3.35(s,3H),3.51–3.53(m,2H),3.61–3.63(br,189H),4.36–4.37(m,2H).13CNMR(100MHz,CDCl3)d37.91,59.23,61.80,69.20,69.52,70.47,72.08,72.84.MS(MALDI-TOF):(m/z),Mn¼2260;PDI:1.01.GPC:Mn¼2800;PDI¼1.02.
SynthesisofPEG-N3AtypicalprocedureofsynthesisofPEG-N3waspreviouslyreported.20PEG-OMs(2.00g,1.00mmol)andsodiumazide(0.33g,5.00mmol)wereseparatelyheatedto50Cundervacuumfor12h,andthencooledtoroomtemperature.The
1088
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
PEG-OMswasdissolvedin25mLofdriedDMF,followedbytheadditionofsodiumazide.Thereactionwasheatedto50
Candstirredundernitrogenfor12h.Thereactionwasdis-solvedin50mLofDCM,andthenwashedthreetimeswith100mLofDIH2O,andthreetimeswith100mLofbrine.TheorganiclayerwasdriedoveranhydrousMgSO4,filtered,andconcentratedbeforeprecipitationfromdichloromethaneinto20mLofcoldethylether.Theproductwasisolatedviafiltrationanddriedinvacuo;78%yield:1HNMR(400MHz,CDCl3)d1.99(s,2H),3.35(s,3H),3.35–3.37(m,4H),3.62–3.64(br,189H).13CNMR(100MHz,CDCl3,)d50.87,59.27,70.25,70.78,72.13.MS(MALDI-TOF):(m/z),Mn¼2100;PDI¼1.01.GPC:Mn¼2700;PDI¼1.02.
SynthesisofThreeArmPCL-OH
1,1,1-tris(hydroxymethyl)ethane(0.421g,3.5mmol)wasplacedinapreviouslydried25mLtwoneckroundbottomflaskequippedwithastirbaranddriedundervacuumforabout1h.e-Caprolactone(10g,87.6mmol)wasaddedthroughtherubberseptumviasyringetotheinitiator.Afterallinitiatorwasdissolvedinthemonomer,tin(II)ethylhexa-noate(0.177g,0.4mmol)wasaddedviasyringe,andthereactionflaskwasimmediatelysubmergedinanoilbathat130C.After3h10min,theflaskwasremovedfromtheoilbath,andthecontentsweredilutedwithdichloromethane,precipitatedtwiceintoa1:1mixtureofhexanesanddiethylether,recoveredbyfiltration,anddriedinvacuobeforechar-acterization.Characterizationdata:1HNMR(400MHz,CDCl3)d1.31(m,2n),1.57(m,4n),1.88(m,2,J¼6.3Hz),2.27(t,2n,J¼7.6),3.37(t,2,J¼6.3),3.60(t,2,J¼6.6Hz),4.03(t,2n,J¼6.3Hz),4.14(t,2,J¼6.2Hz).CalcdMn¼4600.13CNMR(CDCl3,d,ppm)24.78,25.73,28.54,34.32,64.36,173.79.MS(MALDI-TOF):(m/z),Mn¼2220,PDI¼1.06.GPC:Mn¼4040,PDI¼1.08.
SynthesisofSixArmPCL-OH
G1(OH)6(0.295g,0.63mmol)wasplacedinapreviouslydried25mLtwoneckroundbottomflaskequippedwithastirbaranddriedundervacuumat110Cfor24h.Thetemperatureoftheflaskwasthenraisedto130Cundervacuumandbackfilledwithnitrogen.e-Caprolactone(2.67g,25.2mmol)wasaddedthroughtherubberseptumviasy-ringetotheinitiator.Afterallinitiatorwasdissolvedinmonomer,tin(II)ethylhexanoate(0.025g,0.07mmol)wasaddedviasyringe.After40minaftertheadditionofthecat-alyst,theflaskwasremovedfromtheoilbath,andthecon-tentsweredilutedwithdichloromethane,precipitatedtwiceintoa1:1mixtureofhexanesanddiethylether,recoveredbyfiltration,anddriedinvacuobeforecharacterization.Charac-terizationdata:1HNMR(CDCl3,d,ppm)1.31(m,2n),1.57(m,4n),1.88(m,2,J¼6.3Hz),2.27(t,2n,J¼7.6),3.37(t,2,J¼6.3),3.60(t,2,J¼6.6Hz),4.03(t,2n,J¼6.3Hz),4.14(t,2,J¼6.2Hz).calcdMn¼4600.13CNMR(CDCl3,d,ppm)24.78,25.73,28.54,34.32,64.36,173.79.MS(MALDI-TOF):(m/z),Mn¼5800,PDI¼1.06.GPC:Mn¼7760,PDI¼1.14.
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
SCHEME1Synthesisofalkynylateddendrimercores2,3,and4.
ThePreparationofThreeArmsPCL-Alkynes
(1.000g,5.61mmol),pyridine(0.755g,9.51mmol),and4-ThepolymerthreearmsPCL-OH(1.122g,0.51mmol)was(dimethylamino)pyridine(0.069g,0.561mmol)wereaddeddissolvein10mLofdichloromethane.4-pentynoicanhydride
tothereactionflask.Thereactionwasstirredatroom
WWW.MATERIALSVIEWS.COM
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
1089
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
temperaturefor10h.ThenthecrudereactionmixturewasextractedthreetimesfromsaturatedaqueousNaHSO4andthreetimesfromsaturatedaqueousNaHCO3intodichloro-methane.TheorganiclayerwasdriedoveranhydrousNa2SO4.Afterremovingmostofthesolvent,theproductwasisolatedbyprecipitationfromdichloromethaneintocoldmethanol.Yield(89%).1HNMR(400MHz,CDCl3)d1.31(m,2n),1.57(m,4n),1.88(m,2),1.98(s,1),2.27(t,2n,),2.51(t,4,),4.03(t,2n).MS(MALDI-TOF)(m/z),Mn¼2660,PDI¼1.04).GPC:Mn¼4600,PDI¼1.08.
PreparationofSixArmsPCL-Alkynes
ThepolymersixarmsPCL-OH(0.200g)wasdissolvein10mLofdichloromethane.4-pentynoicanhydride(0.114g),pyridine(0.098g),and4-(dimethylamino)pyridine(0.008g)wereaddedtothereactionflask.Thereactionwasstirredatroomtemperaturefor10h.Then,thecrudereactionmixturewasextractedthreetimesfromsaturatedaqueousNaHSO4andthreetimesfromsaturatedaqueousNaHCO3intodichloromethane.Theorganiclayerwasdriedoveranhy-drousNa2SO4.Afterremovingmostofthesolvent,theprod-uctwasgotbyprecipitationfromdichloromethaneintocoldmethanol.Yield(87%)1HNMR(400MHz,CDCl3)d1.31(m,2n),1.57(m,4n),1.98(s,1),2.27(t,2n),2.51(t,4),4.03(t,2n),4.14(t,2).MS(MALDI-TOF)(m/z),Mn¼6330,PDI¼1.04).GPC:Mn¼8600,PDI¼1.08.
GeneralProcedureofCu(I)-Catalyzed‘‘Click’’AssemblyofStarPolymers
Alkynefunctionalizedstarcores,azidefunctionalizedmacro-molecule,andPMDETA(1alkyne:1.05azido-polymer:2PMDETA)wereaddedtoaroundbottomflaskwithdichloro-methane.Aftertwofreeze-pump-thawcycles,Cu(I)Br(1:1withPMDETA)wasadded,andthethirdfreeze-pump-thawcyclewasperformed.Thereactionwasstirredfor16hatroomtemperature,thenwaswashedwithde-ionizedwaterandbrine,andfinallywasdriedwithNa2SO4.Followingcon-centrationinvacuo,theproductswererunthroughashortsilicagelcolumntoremoveresidualcopper.Thedetailpro-cedureforsynthesisofstarpolymersisdescribedintheSupportingInformation.
RESULTSANDDISCUSSION
Toprepare‘‘perfect’’starpolymersviathe‘‘graftonto’’approach,well-definedpolymerarmsanddendriticcoresmustfirstbepreparedwithcomplimentaryfunctionalities.Thedendriticcoreswerepreparedviathehighlyefficientdi-vergentsyntheticapproach,usingbenzylideneprotectedbis-MPA.38Afteraseriesofmonodispersedendrimerswith3,6,and12hydroxylgroupswereobtained(‘‘generations’’G0,G1,andG2,respectively),theywerereactedwithpentynoicacidanhydridetoquantitativelyintroducealkynegroupsontotheirperipherytoyieldcore2,3,4(Scheme1).Theacquired1Hand13CNMRspectraverifiedthepurityoftheproducts,whichexhibitingexceptionalsymmetry.Thequantitativeesterificationoftheperipheralhydroxylscouldbeconfirmedbycomparingtheintegrationoftheresonancesoftheterminalalkyne(1.98ppm)andthoseofthemethyl-1090
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
FIGURE1MALDI-TOFmassspectrumofdendriticcore2,3,and4.
eneadjacenttotheesteroxygen(4.05ppm)tothoseofthemethylgroupsatthecore(G0:1.03ppm;G1:1.05ppm;G2:1.12ppm)(SupportingInformationFig.S1andTableS1).Elementalanalysiswasalsousedtoconfirmthepurityof
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
SCHEME2Synthesisof3,6,and12-armstarPSandPEG.
thedendriticcoresusedinthisstudy.However,thecores’Withapolymerofunknownendgroups,itisimpossibletoMALDI-TOFmassspectraweremostvaluableforconfirmingcalculateitsendgroupmassexactly,exclusivelyfromthethetrulymonodispersenatureofalkynylateddendrimersasmassspectra,becauseitisunknownhowmanymonomeronlyonesignalwasobservedineachoneoftheirmassspec-unitscontributetoagivenn-mersmass,andhowmuchistra(Fig.1).
contributedbytheendgroups.Atechniquehasbeendevel-Becausethelineararmsmustalsoexhibitnarrowpolydis-oped,however,todefinethesetofpossibleendgrouppersityandpreciseendgroupcontrol,‘‘living’’polymeriza-masses.Thisisperformedbydeterminingthesmallestpossi-tiontechniqueswereusedtopreparetheazido-functionalbleendgroupmass,Mres,byextrapolationfromeveryn-merarms.Thelinearpolystyreneprecursorsweresynthesizedinthemassspectra.FromthisvalueofMres,theseriesofviaATRPfroma2-ethylbromobenzeneinitiator5,usingpossibleendgroupmassescanbeidentified(byaddingmul-Cu(I)Br/Cu(II)Br2ascatalystandPMDETAasaligandtiplesofthemonomermass).Becausethehypotheticalend(Scheme2).
groupstructureisknownfortheselinearandstarpolymers,thetheoreticalendgroupmass(MToensurenear-quantitativefunctionalizationwithabromideend)canbeeasily
groupattheendofeachpolymerchain,thereactionwasstoppedatalowconversionofthemonomer(20%)yieldingpolystyrene6withaMnof2800,andPDI¼1.12byGPC.TheirMALDI-TOFmassspectraexhibitedmonomodaldistri-butionsandtheobservedsignalsmatchedtheexpecteddis-tributioncausedbycomplexationtoAgþinadditiontothepreviouslyreportedeliminationofHBr[PSBrþAgþ-HBr].39CalculationsfromtheMALDI-TOFmassspectraconfirmedthewell-definedcharacterofthepolymers,andagreedcloselywiththeGPCdata(Mn¼2500,PDI¼1.12;Fig.2andTable1).
Toconfirmtheendgroupidentity,apreviouslyreportedpro-cedurewasusedtocalculatefrommassspectraldatathe‘‘residualmass(Mres)’’andthe‘‘endgroupmass(Mend).’’
FIGURE2MALDI-TOFmassspectrumofpolystyrene6.
WWW.MATERIALSVIEWS.COM
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011091
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
TABLE1RepeatUnit(Mru)andEndGroupMass(Mres)DataforPolystyrene,6
Theoretical(6þAg-HBr)þ1459.721563.781667.841771.911875.971980.032084.092188.162292.222396.282500.342604.412708.472812.532916.603020.663124.723228.78
Observed
(2.1aþAg-HBr)þ1459.671563.791667.811771.881875.931979.992084.082188.122292.172396.212500.272604.322708.392812.442916.493020.543124.593228.68AverageTheoretical
104.12104.03104.07104.05104.05104.09104.04104.05104.04104.07104.05104.07104.05104.05104.05104.05104.09104.06104.06Mru(6n-6nMresfor6104.02104.07104.04104.05104.03104.03104.06104.04104.02104.00104.00103.99104.00103.99103.97103.96103.95103.97104.01104.06
N121314151617181920212223242526272829
À1)
casen¼0yields104.01,whichagreescloselywiththetheo-reticalvalueof104.06.Forthelarger12-armstarpolymers,thesubstantialmassofthecoreyieldssolutionswheren>>0.
Toenabletheirclickconjugation,thebromineendgroupswerethenconvertedtotheazidebyreactionwith$5equiv-alentsofsodiumazideinDMF40Theproductretainedthenarrowpolydispersityoftheprecursor,asjudgedbyGPC(Mn¼2800,PDI¼1.12).TheMALDI-TOFmassspectrapro-videdadditionalstrongevidenceforthenear-quantitativeconversiontotheazideendgroup,asthepreviouslydescribedbromideresonanceswerecompletelylost,andanewsetofresonances,dominatedbytheuniquemetastablefragmentationoftheazidefunctionality[PSN3þAg-N2]þ(metastableionobservedas[PSN3-23]þ)wereobservedinreflectormode.Furtherconfirmationofthemetastablena-tureofthesesignalswasgainedbyacquiringtheMALDI-MSspectrainlinearmode(whichdoesnotexhibittheeffectsofmetastablefragmentation),yieldingastrongsetofsignalscorrespondingto[PSN3þNa]inadditiontootherin-sourcedecayfragments(e.g.,[PSN3þAg-N2(insource)]þobservedas[PSN3-28.0]þ(SupportingInformationFig.S2andTableS2).Theunusualbehaviorhasbeenobservedformultipleazidecontainingpolymersandexaminedindetailelsewhere.41Theazido-PEGwaspreparedfollowingthepreviouslyreportedprocedure(Scheme2).42Byreactingwithmethane-sulfonylanhydridewithmonomethylpolyethyleneglycol(PEG-OH)(Mn¼2090),thehydroxylendgroupofPEG-OH8couldbequantitativelyconvertedtobemesylategroup.ThisresultwasverifiedbytheirMALDI-TOFspectra,whichexhib-itedamassincreaseof78.07Da(Dtheo.¼77.98Da)foreachpeak(Fig.3).
Subsequently,themesylatedPEG9wasreactedwith5equivalentofsodiumazide40toaffordPEGwithasingleaz-ideendgroup.Again,MALDI-TOFrevealedthattheazido-PEG10wasformednearquantitativelywiththeexpectshiftofmolecularweight(Dobs.¼À52.03Da,Dtheo.¼À52.97
determinedandcomparedwiththecalculatedmass.Forexample,inthecaseofthebromideterminatedpolymer,whichwasknowntoionizeviaeliminationofHBrandcom-plexationwithAgþ,themassofeachmonoisotopicpeakwasdetermined,andthetrendextrapolatedtothesmallestpossi-blepositivevalue,whichyieldedandMresof104.01,andarepeatunitmassof104.06.Therepeatunitmassagreeswiththeexpectedvalue(104.0626),whiletheendgroupmasscouldbeoneoftheseries104.01þn(104.06).Inthis
FIGURE3MALDI-TOFmassspectraofthePEGmethylether2000,8,mesylatedPEG,9,andazido-PEG,10.
1092JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
TABLE2MALDI-TOFandGPCCharacterizationofPurifiedPolystyreneStarswithCalculatedValueinParenthesisCompoundPSStarsMnbyMALDIPDIMnbyGPCPDIMruMres7PS-N32,5001.042,8001.12104.1(104.1)À0.1(0)113-arm8,110(7,860)1.028,1001.07104.2(104.1)72.9(72.9)126-arm15,000(15,900)1.0113,6001.04104.2(104.1)60.9(61.5)13
12-arm
30,000(32,100)
1.02
18,300
1.05
N/A
N/A
Da)andtheappearanceofthecharacteristicazidemetasta-confirmedbythefactthattheproductsnolongerexhibitbleionusingreflectormode.Theseendgrouptransforma-anymetastablefragmentation,consistentwithprevioustionwerealsoconfirmedbythe13CNMRspectrum,whereobservations,whichconfirmthatthemetastabledecaychar-thesignalofCH2OH(59ppm)disappearedinmesylatedPEGacteristicofpolymerazidesislostwhentheyareconverted9spectrumandthesignalof–OSO2CH3(38ppm)disap-totherelativelyheartytriazolelinkages.43Notably,nosignalpearedin10spectrum(SupportingInformationFig.S3).isobservedinthemolecularweightrangeof3000to5000TheassemblyofstarpolymerscouldthenbeperformedthatwouldcorrespondtothestarwithonlytwocoupledusingstandardCuAAC‘‘click’’reactioncondition(Schemearms.
2).42AslightexcessoflinearpolymerwasusedtoensureBecauseofthedifficultyinpreciselydeterminingtheaveragethecompleteformationofstarpolymers,typicallyusingmolecularweightoftheazidefunctionalizedarms,aslight1:1.05moleratioofalkynetoazide.Becausethisapproachexcesswasrequiredtoensuresufficientcouplingtoyieldrequiresverycarefullymeasuredratiosoflinearpolymerquantitativelycoupledcores.Asaresult,amethodofpurifi-anddendriticcore,themolecularweightofthepolymercationmustbeoptimizedtoisolatethedesiredstarpolymerarmsmustbedeterminedwithhighaccuracy.Forthebelowproductfromtraceamounts(typically$5%)ofunreactedexamples,themolecularweightusedforstoichiometriccal-polymerarms.Manydifferenttechnologieshavebeendevel-culationswasdeterminedfromMALDI-TOFMS.AllGPCandopedtopurifiedpolymers,suchasfractionation,37dialysis,44MALDI-TOFresultssuggestthattheresultantcrudestarGPC,45preparativeGPC,46liquidchromatographyatcriticalpolymersampleexhibitedquantitativecouplingofpolymercondition,47modifiedhighperformanceliquidchromatogra-toeachofthealkynefunctionalitiesonthecoremoleculesphy,48silicagelchromatography,27solidresinasscavenger,49(Tables2and3).
andsoforth.Inthisstudy,manyofthesehavebeeninvesti-Forthesynthesisofthepolystyrenethree-armstarpolymers,gatedtoascertainthemosteffectiveandhigh-throughputtheGPCtraceofthecrudeproductexhibitamajordistribu-techniqueforisolating‘‘pure’’starpolymers.GPCandprepa-tionthatexhibitedashifttoalargerhydrodynamicvolume,rativeGPChavedemonstratedtheirabilitytoisolatetheandonlyasmalltraceofresidual‘‘one-arm’’linearpolymer.purestarpolymersbutareonlyamenabletopurificationsTheMALDI-TOFmassspectraoftheproductexhibitedanap-onsmallscales,thereforeothertechniquesweresought.Di-proximatethreefoldincreaseinthemolecularweight(Tablealysisisattractiveforseparatingcompoundsofdisparate4).
sizes(e.g.,removalofresidualmonomerfromapolymer)butislesseffectivewhenthereisalesssubstantialsizedif-Inaddition,endgroupanalysisoftheproductagreeswithference.Eventhoughthemolecularweightincreasessub-theexpectedmassincreaseforthreecouplingstothetris-stantiallyforthesix-armand12-armstarsrelativetothealkynecore,forexample,theexpectedexactmassforathree‘‘one-arm’’linearprecursors,themorecompactconfirmationarmpolymerwith24repeatunitperarm(n¼72)isoftheresultingstarpolymersreducestheirsizesubstantially8096.09(includingsilvercounterion),andasignalisinsolution,complicatingtheirseparationbydialysis.Inobservedat8096.08.Alternatively,usingpreviouslyreportedtheory,scavengingresinsfunctionalizedwithcomplimentarytechniques,the‘‘residualmass’’ofthe‘‘endgroups’’(allpor-‘‘click’’functionalitiesofferarelativelysimpleandappealingtionsofthestarthatcannotbeaccountedforbystyrenemethodforremovingunattachedarms.50,51Thismethodwasrepeatunits)canbecalculatedfromtheMALDI-TOFMSasinvestigatedforanumberofthecrude‘‘click’’star-formation72.9Da,whichmatchesthetheoreticalvalueof72.9Da.reactionmixtures,however,provedineffectiveatremovingFurthermore,thequantitativereactionoftheazidescanbe
allunreactedlineararms.Forexampleduringthesynthesis
TABLE3MALDI-TOFandGPCCharacterizationofPurifiedPEGStarswithCalculatedValueinParenthesisCompoundPEGStarsMnbyMALDIPDIMnbyGPCPDIMruMres10PEG-N32,1101.012,7001.0244.0(44.1)13.0(13.0)143-arm6,600(6,690)1.019,6001.0344.1(44.1)2.8(2.8)156-arm13,600(13,600)1.0114,1001.0444.0(44.1)13.7(13.8)16
12-arm
29,000(27,400)
1.01
14,200
1.12
N/A
N/A
WWW.MATERIALSVIEWS.COMJOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011093
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
TABLE4RepeatUnit(Mru)andResidualMass(Mres)DataforPurified3-ArmStarPolystyrene,11
Theoretical(11þAg)þ7471.27575.37679.57783.67887.87991.98096.18200.28304.48408.58512.7
Observed(11þAg)þ7470.97575.17679.27783.57887.67992.08096.18200.38304.68408.88513.1AverageTheoretical
104.1104.2104.3104.1104.4104.1104.2104.3104.2104.2104.2104.1Mru(11n-11nMresfor1172.672.672.672.872.772.972.973.073.173.273.372.972.9
n6667686970717273747576
À1)
Purificationbyfractionationwasanattractivealternative,asitshouldbeinsensitivetoincompleteendgroupfunctionali-zations,however,oftenrequiresmultipleinefficientprecipita-tions,andthereforeasubstantialreductioninyields.Usingaseparatoryfunnel,thecrude-armstarPSreactionmixturewasdissolvedintoluene,andthenmethanolwasaddeddropwiseuntilthesolventturnedcloudy.Thesolutionwassetovernightresultingintheformationofanoilylayerinthebottomofthefunnel.CharacterizationbyGPCandMALDITOFMSverifiedthatthebottomlayerwasenrichedwiththehigh-molecularweightportionsofthecrudereac-tionmixture.Aftertwofractionations,theGPCtraceverifiedtheremovalofthesmallamountthelinearstartingmaterial,andpurestarpolymerswereobtainedwithlowPDI(Mn¼8100;PDI¼1.07;Fig.5).
Theremovalofthelinearpolymerdistributionaroundm/z¼2600intheMALDI-TOFmassspectrawasparticularlycon-vincing,asitiswellknownthatlowermolecularweightana-lytesexhibitmuchstrongersignalinMALDI-TOFMSanalysis.Finally,althoughthewell-definedone-armdistribution(MW$2000–4000withamonomerspacingof104)hasbeenremoved,closeexaminationofthebaselineshowsaweak,broaddistribution(withunresolvedmonomerspacing)thatiscalculatedtohaveaMnofabout3100.Theseobservationsareconsistentwithtriplychargedpolymer(Fig.6).52Usinganalogoussyntheticandpurificationprocedures,thesix-armand12-armpolystyrenestarswerealsoprepared.ThecrudeproductsofbothexhibitedalargeincreaseinsizeasjudgedbyGPCandtheexpectedincreasesinmolecularweightascalculatedbytheirMALDI-TOFmassspectra(Figs.5and6).Inthecaseofthesix-armpolymer,theobservedMnwaswithin10%oftheexpectedvalue(Mntheo¼15,900,Mnobs¼15,000)andasimilartrendwasnotedforthe12-armpolymer(Mntheo¼32,100,Mnobs¼30,000).MSendgroupanalysisagainconfirmedthatthesignaldistributionsagreedwiththosecalculated,asfurtherconfirmedbytheobservedresidualmasses(SupportingInformationTableS3).Afterpurificationbyfractionalprecipitation,thesix-armand12-armstarpolymersexhibitamonomodalproductwitha
ofthethree-armstarpolystyrene,asolidresinbearingalkynefunctionalgroupswasaddedtothecrudestarpoly-merproductsandthe‘‘click’’conditions(Cu(I)Br/PMDETA)werereapplied(Scheme3).
TheGPCtracesofthescavengerproductshowedonlyaslightreductioninthelinearimpurities(Fig.4).
Additionalscavengingattemptsonthesamesampledidnotseemtofurtherreducetheamountoflinearimpurity.Simi-larresultswereobservedforotherPSandPEGstars,sug-gestingthatatraceamountofthelinearpolymerhadinac-tiveendgroups,thuspreventingitfromcouplingtoboththemultiarmcoresandthesolidphaseresinduringsubsequentattemptstoscavenge.Thiswasnotunexpectedconsideringthelikelihoodoflosingatraceamountofendgroupfunc-tionalityduringATRP,orinthecaseofPEG,thelikelihoodofatraceamountofasidereaction(e.g.,hydrolysis),duringthetwo-stependgrouptransformation.
SCHEME3Purificationofthree-armstarPSbyscavengingwithalkynefunctionalizedsolidphaseresin.
1094JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
generallylow(around20%)andthetechniqueverytime-consuming.ForthePEGstars,preparativesizeexclusionchromatographicseparationusingapackedcolumnof‘‘Bio-Beads’’provedtobearelativelyfastandeffectivetechnique,alsoaffordedsubstantiallyimprovedyields.27Theseweregenerallyperformedona20–100mgscaleusingTHFasthemobilephase,whilecollectingfractionsinwhichthelowermolecularweightlinearimpuritieselutedlast.Parallelanaly-sisofeachfractionbyGPCandMALDIprovidecomprehen-sivecharacterization(Fig.7).Intheearlyfractions,Figure7(c)oligomericimpuritieswereobservedintheGPC,whichFIGURE4GPCspectrumofscavengedthree-armstarpolysty-werenotobservedinthecrudeprecursor.Itispresumedrene7and11.
thattraceamountsofhighermolecularweightimpuritiesonlybecamevisiblebecausetheywereconcentratedduringverynarrowpolydispersitybothbyGPC(six-armPDI¼chromatographicpurification,thoughthepossibilityofcon-1.04,12-armPDI¼1.05)andbyMALDI-TOFMS(six-armtaminationcannotberuledout.ThemiddlefractionsexhibitPDI¼1.01,12-armPDI¼1.02).MALDI-MSanalysiscon-averypristinethree-armstarPEG,asjudgedbothMALDI-firmsthelossofthe‘‘one-arm’’PSprecursor,(Mn$2000–TOFMSandGPC,withtheonlytraceamountsoflowmolec-4000)butagainshowstraceamountsoftriplychargedspe-ularweightsignalinthemassspectraconfidentlyassigned
ciesinadistinctlydifferentmassrange(e.g.,forthesix-armstarMW¼4500–6500inFig.6).ItshouldbenotedthataslightincreaseintheMnofthepurifiedproductwasobserved,mostlikelyaresultofthefractionationprocedurefavoringtheprecipitationofhighermolecularweightstarpolymers.Inaddition,lowMWanalysisofthe12-armprod-uctwasnotpossible,becausethelaserpowerrequiredforionizationofthehighMWproductresultedinsubstantialmatrixnoisebelowm/z¼15,000.
Toaccesswell-definedhydrophilicstars,theattachmentofmono-azidefunctionalizedPEGtothesamedendriticcoreswasalsoexplored.Usinganalogousclickcouplingconditions,thePEG-N3lineararmsandthetrialkynecore,werecoupledtogeneratethree-armstarpolymers.Beforepurification,theGPCtracerevealedanearlymonomodalproductwiththeexpectedincreaseinhydrodynamicvolume,thoughthecal-culatedMnvalueswereinaccuratebecausetheywerecali-bratedagainstlinearpolystyrenestandards(Mn¼9660,PDI¼1.03;Fig.7andTable3).
TheMALDI-TOFmassspectraexhibitedasingledistributionwithaMn6570,closetothecalculatedvalueoftheexpectedthreearmstar,6680.Again,rigorouscalibrationusingden-driticstandardsenablestheaccurateverificationofeachsig-nalwithintheobservedmassspectra.Forexample,thecal-culatedmassofthethree-armstarpolymerwithexactly138ethyleneglycolrepeatunits(correspondingtoanaverageof46repeatingunitsforeacharm)whencomplexedtosodiumis6633.8Da,whiletheobservedsignalwas6633.6Da(Ta-ble5).
Furtherverificationacrosstheentiredistributionwasachievedbycomparingthecalculatedresidualmassof2.8withthetheoreticalvalueof2.8.Despitetheincreasedsignalstrengthforlowermolecularweightanalytes,noMSsignalswereobservedthatcorrespondedtostarpolymerswithoneormorefailedclickcouplingreactions.
Althoughthefractionationapproachenabledsuccessfulisola-FIGURE5GPCspectraofthree-armstarPS(11),six-armsstartionofthepurepolystyrenestarpolymers,theyieldswere
PS(12),and12-armsstarPS(13).
WWW.MATERIALSVIEWS.COM
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011095
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
FIGURE6MALDI-TOFmassspectrumofthree-armPS11(a),six-armstarPS12(b),and12-armstarPS13(c).
todoubleandtriplechargedspecies.Asexpected,thelatefractionsshowamixtureofthree-armstarandlinearpre-cursor,butagainthelinearprecursorcanbedistinguishedfrommultiplechargedspeciesbecauseoftheclear44.0Da(monomermass)spacingbetweeneachsignal.Impurityofone,two-armproductswassynthesizedpur-poselyusing2equivalentofazido-PEGtotris-alkyne.MALDI-TOFMSshowedtheresidualmassmatchedtheexpectedbyproducts.Toconfirmtheabilitytoidentifyalllikelyimpuritiesduringthecouplingreaction,acoupling
FIGURE7MALDI-TOFandGPCcharacterizationofthree-armstarPEGfractionsfrom‘‘Bio-Bead’’chromatography.
1096JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
TABLE5RepeatUnit(Mru)andResidualMass(Mres)DataforPurified3-ArmPEG,14
TheoreticalObservedMruMresn(14þNa)þ(14þNa)þ(14n-14nÀ1)
for141316325.46325.32.81326369.56369.243.92.61336413.56413.344.12.71346457.66457.344.02.61356501.76501.444.12.61366545.76545.444.12.71376589.86589.544.12.71386633.86633.644.12.71396677.96677.744.12.81406721.96721.744.02.71416766.06765.643.92.61426810.06810.044.42.91436854.16853.943.82.71446898.16897.944.12.81456942.26941.943.92.61466986.26986.144.22.81477030.37030.043.92.7148
7074.3
7074.244.12.8Average
44.12.7
withaninsufficientamountoflinear‘‘arms’’($2equivalentsperthree-armcore)waspurposelyperformed.Theresultingmixtureofone-,two-andthree-armedstarswascharacter-izedbyMALDITOFMStoconfirmtheunambiguousidentifi-cationofeachmajorcomponent(Fig.8andSupportingIn-formationTablesS4andS5).
FIGURE9GPCofthree-armstarPEG14,six-armsstarPEG15,and12armsstarPEG16.
Forone-armimpurity,theMreswas20.9,closetothetheo-reticalvalueof21.0.TheMresoftwo-armimpuritywas33.9,Usinganalogoustechniques,thePEGsix-armand12-armwhiletheexpectedvalueis34.0.
starswerepreparedandpurified.CharacterizationbyGPCandMALDI-TOFprovidedstrongevidenceforaquantitativecouplingofsix-armPEG,withtheproductsafterBiobead
FIGURE8MALDI-TOFspectrumofincompletelycoupledstarswithonlyonearm(left),twoarms(middle)andallthreearmsFIGURE10MALDI-TOFmassspectraofthree-armPEG14(a),(right)ofPEGonthetrisalkynecore,2.
six-armstarPEG15(b),and12-armstarPEG16(c).
WWW.MATERIALSVIEWS.COMJOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011097
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
SCHEME4Synthesisofthree-andsix-armstarPCL-b-PEG19and23.
FIGURE11MALDI-TOFMSspectrumofthree-armstarPCL17anditsalkynylatedproduct18.
purificationexhibitinganarrowmonomodaldistributioninitsGPCtrace(PDI¼1.04)aswellasitsMALDImassspec-trum(PDI¼1.01)[Figs.9and10(b)andSupportingInfor-mationTableS6].
Aspredicted,theMncalculatedfromtheMALDI-TOFmassspectraprovidesamoreaccuratevalue(Mnobs¼13,600)thanGPC(Mnobs¼14,100)forthemolecularweightofthePEGsix-armstar(Mntheo¼13,600).Thepurified12-armstarPEGalsoexhibitedmonomodalproductsbasedontheGPCtrace(Mn¼14,200,PDI¼1.12)[Figs.9and10(c)].TheMALDI-TOFmassspectrumenabledtheverificationofthequantitativeformationofpure12-armstarwithMnof29,100(Mntheo¼27400)andPDIof1.01withoutevidenceof11-armor10-armimpurities.53Similarwiththe12-armstarPS,theMALDI-TOFmassspectraof12-armPEGstarscouldnotenablevisualizationoflow-molecularweightrangeduetomatrixnoise.
Well-definedthree-armstarblockcopolymerscouldbesyn-thesizedinatleastthreedifferentways:diblockarmscouldbegrowndivergentlyfromacoremolecule,preformedlineardiblockscouldbecoupledtoafunctionalizedcore,orhomo-polymerarmscouldbecoupledtotheendsofafunctional-izedthree-armstarhomopolymer.54ThelastofthesethreeapproacheswastestedbycouplinglinearPEGtoaPCLthree-armstarcore(Scheme4)
ThePCLthree-armstarwaspreparedviapolymerizationofe-caprolactoneinbulkfrom1,1,1-tris(hydroxymethyl)ethaneusingSn(Oct)2catalystsat130C.Subsequently,the
1098
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
hydroxylendgroupsof17wereesterifiedwithpentynoicacidanhydridetoquantitativelyfunctionalizeeachendwiththerequisitealkyneforclickcoupling.TheGPCtraceofthe
TABLE6MALDI-TOFDatafor3-ArmPCL-OH(17)and3-ArmPCL-Alkyne(18)
Theoretical(17þNa)þ1511.8851625.9531740.0221854.091968.1582082.2262196.2942310.3622424.432538.4982652.5662766.6342880.7022994.773108.8383222.9073336.9753451.043
Observed(17þNa)þ1511.9141626.0081740.0871854.1651968.2362082.3092196.3792310.4492424.5152538.5792652.6462766.7152880.7792994.8473108.9093222.9863337.0493451.132
Theoretical(18þNa)þ1751.9641866.0321980.12094.1682208.2362322.3042436.3732550.4412664.5092778.5772892.6453006.7133120.7813234.8493348.9173462.9853577.0533691.121
Observed(18þNa)þ1751.9481866.0371980.1292094.1952208.272322.3382436.4042550.4722664.5362778.6012892.6713006.7393120.8063234.8663348.9343463.0063577.0833691.154
n121314151617181920212223242526272829
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE12MALDI-TOFMSspectrumofsix-armstarPCL21anditsalkynylatedproduct22.
hydroxylterminatedstar17,showedamonomodaldistribu-cule.Thesix-armstarPCLwaspreparedviapolymerizationtionwithanarrowpolydispersityandaMnof2230whenofe-caprolactoneinbulkusingSn(Oct)2catalystsat130C.correctedforPCL(4650whencalibratedagainstPSstand-From1HNMRdata,wecanseeallpeakof–CH2OHgroupinards)TheMALDI-TOFmassspectraofthehydroxyltermi-coreinitiatorshiftedfrom3.70ppmdoublepeakto4.03natedpolymerconfirmedthesynthesisofwell-definedthree-ppmconfirmingthateveryhydroxylgroupfromthecoreini-armstarPCLwithoutanymeasurableamountoflinearpoly-tiatethepolymerizationofaPCLarm(SupportingInforma-merbyproductsthatmightoccurfrominitiationfromwatertionFigs.S5andS9).TheGPCshowsamonomodaldistribu-orothersmallmoleculeimpurities[Fig.11(a)].
tionwithanarrowpolydispersity(Mn¼7760,PDI¼1.11).Theobservedrepeatunitmolecularweight(114.07)agreedTheMALDI-TOFmassspectraalsoconfirmthesynthesisofcloselywiththetheoreticalvalue(114.07)andtheMncalcu-well-definedsix-armsPCLstars[Fig.12(a)]andtheirquanti-latedbyMALDI-TOFMS(2220)agreedcloselywiththeGPCtativefunctionalizationwithpentynoateesters(480.48values.Theidentityofthecorecouldalsobeconfirmedbyincreaseinmass;Fig.12andSupportingInformationTableMScalculationsoftheresidualmass(6.1Da),whichagreesS7).
closelywiththetheoreticalvalue(6.0Da).Theattachmentofthethreepentynoateestersviaesterificationwith4-penty-noicacidanhydridecouldbeconfirmedbymonitoringthemassshiftsofaparticularsignalintheMALDI-TOFMS,forexample,the19-mer.Theioncorrespondingtothesodiumadductofthehydroxylfunctionalizedstarwithatotalof19repeatunitsofcaprolactonehasatheoreticalexactmolecu-larmassof2310.4Da(observed2310.4Da).Onfunctionali-zationwith3pentynoateesters,themolecularweightfor18isexpectedtoincreaseby240.04Da(additionof3penty-noicgroupseachweighing80.01Da),whichyieldsatheoret-icalmolecularmassof2550.4(observed2550.5)[Fig.11(b)andTable6].
ThealkynylatedPCLstar18wasthenreactedwith2kDaPEG-N310understandardclickconditionstoyieldthethree-armstarPCL-b-PEG19.Afterpurification,theGPCtracerevealedthehighermolecularweightstarblockcopoly-merswereformedwithnarrowPDI(Mn¼13,600,PDI¼1.03)(Fig.13).TheMALDI-TOFmassspectraexhibitedaMnof8970,closetocalculatedvalueof9720.Detailedendgroupanalysisoftheblockcopolymerwasnotpossible,however,becausethenon-equivalentmassesofthedifferentmonomersleadtoacomplexdistributioninwhichindividualsignalsarenolongerresolved(Fig.14).
Thesix-armPCL-PEGblockcopolymerwassynthesizedbyfollowingthesameprocedurebutusing20asthecoremole-FIGURE13GPCofthree-armstarandsix-armstarPCL-b-PEG.
WWW.MATERIALSVIEWS.COM
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011099
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
JOURNALOFPOLYMERSCIENCE
REFERENCESANDNOTES
1Roovers,J.InStarandHyperbranchedPolymers;MarcelDekker:NewYork,1998.
2Quaglia,F.;Ostacolo,L.;Nese,G.;Canciello,M.;De,R.;Ungaro,F.;Palumbo,R.;La,R.;Maglio,G.J.Biomed.Mater.Res.PartA2008,87A,563–574.
3Aryal,S.;Prabaharan,M.;Pilla,S.;Gong,S.Int.J.Biol.Mac-romol.2009,44,346–352.
FIGURE14MALDI-TOFMSspectrumofthree-arm(19)andsix-arms(23)starPCL-b-PEG.
4Poree,D.E.;Giles,M.D.;Lawson,L.B.;He,J.;Grayson,S.M.Biomacromolecules2011,12,898–906.
5Hadjichristidis,N.;Pitsikalis,M.;Iatrou,H.Macromol.Eng.;Wiley-VCHVerlagGmbH&Co.KGaA,2007;Vol.2,pp909–972.6Gao,H.;Matyjaszewski,K.Prog.Polym.Sci.2009,34,317–350.7Wu,Z.;Liang,H.;Lu,J.Macromolecules2010,43,5699–5705.8Wu,Z.;Liang,H.;Lu,J.;Deng,W.J.Polym.Sci.PartA:Polym.Chem.2010,48,3323–3330.
9Hadjichristidis,N.;Pitsikalis,M.;Pispas,S.;Iatrou,H.Chem.Rev.2001,101,3747–3792.
10Matyjaszewski,K.;Davis,T.P.HandbookofRadicalPoly-merization;Wiley:Hoboken,NJ,2002.
11Wang,J.-S.;Matyjaszewski,K.J.Am.Chem.Soc.1995,117,5614–5615.
12Kato,M.;Kamigaito,M.;Sawamoto,M.;Higashimura,T.Macromolecules1995,28,1721–1723.
13Grayson,S.M.;Frechet,J.M.Chem.Rev.2001,101,3819–3868.
CONCLUSIONS
14Kolb,H.C.;Finn,M.G.;Sharpless,K.B.Angew.Chem.Int.Ed.Engl.2001,40,2004–2021.
15Tornoe,C.W.;Christensen,C.;Meldal,M.J.Org.Chem.2002,67,3057–3064.
16Rostovtsev,V.V.;Green,L.G.;Fokin,V.V.;Sharpless,K.B.Angew.Chem.Int.Ed.Engl.2002,41,2596–2599.
17Lahann,J.ClickChemistryforBiotechnologyandMaterialScience;Wiley:Chickester,UK,2009.
18Golas,P.L.;Matyjaszewski,K.Chem.Soc.Rev.2010,39,1338–1354.
19Iha,R.K.;Wooley,K.L.;Nystro¨m,A.M.;Burke,D.J.;Kade,M.J.;Hawker,C.J.Chem.Rev.2009,109,5620–5686.
20Fournier,D.;Hoogenboom,R.;Schubert,U.S.Chem.Soc.Rev.2007,36,1369–1380.
21Helms,B.;Mynar,J.L.;Hawker,C.J.;Frechet,J.M.J.J.Am.Chem.Soc.2004,126,15020–15021.
22Laurent,B.A.;Grayson,S.M.J.Am.Chem.Soc.2011,133,13421–13429.
23Dirks,A.J.;Van,B.;Hatzakis,N.S.;Opsteen,J.A.;Van,D.;Cornelissen,J.J.L.M.;Rowan,A.E.;Van,H.;Rutjes,F.P.J.T.;Nolte,R.J.M.Chem.Commun.(Cambridge,U.K.)2005,4172–4174.
24Li,M.;De,P.;Gondi,S.R.;Sumerlin,B.S.Macromol.RapidCommun.2008,29,1172–1176.
25Gao,H.;Matyjaszewski,K.Macromolecules2006,39,4960–4965.
26Altintas,O.;Yankul,B.;Hizal,G.;Tunca,U.J.Polym.Sci.PartA:Polym.Chem.2006,44,6458–6465.
27Hoogenboom,R.;Moore,B.C.;Schubert,U.S.Chem.Com-mun.2006,4010–4012.
28Xu,J.;Liu,S.J.Polym.Sci.PartA:Polym.Chem.2009,47,404–419.
29Gungor,E.;Bilir,C.;Durmaz,H.;Hizal,G.;Tunca,U.J.Polym.Sci.PartA:Polym.Chem.2009,47,5947–5953.
ThealkynylatedPCLstar22wasthenreactedwith2kDaPEG-N310understandardclickconditionstoyieldthesix-armstarPCL-b-PEG.TheGPCtraceofthecrudeproductcon-firmedasubstantialincreaseinthemolecularweight,aswellasatraceofunreactedPEG.AfterusingBio-beadmethodtoremovetheunreactedPEG,ahighpuresix-armsstarpolymerwithnarrowPDI(GPC:Mn¼23,700,PDI¼1.07)wasisolated(Fig.13)
TheMALDI-TOFmassspectraexhibitedaMnof18,700,closetotheoreticalvalueof18,980(Fig.14).
Withtheevolutionofoptimizeddendronizationmethodsandlivingpolymerizationtechniques,well-definedpolymericcomponentscanbepreparedwithanexactnumberoffunc-tionalgroups.Therecentfocusonhighlyefficientcouplings,chiefamongthemtheCuAACcouplingreaction,enablescom-plexarchitecturestobeassembledfromthesecomponentsyieldingcomplexmacromoleculararchitectureswithunprec-edentedstructuralpurity.Inthecaseofstarpolymers,thishasbeendemonstratedwithbothhydrophobicandhydro-philicarmsattachedviathe‘‘graftonto’’methodwithdendri-ticcores.Althoughmanypurificationmethodswereinvesti-gated,fractionalprecipitationandchromatographicpurificationseemthemostgeneral,scalable,andsuccessfulinremovingresiduallinearstartingmaterials.Theexcep-tionalpurityofthestarswereconfirmedthroughdetailedcharacterizationbybothGPCandMALDI-TOFMS.Inaddi-tion,thisapproachtomakingmultiarmstarpolymersshowspromisetowardmakingwell-definedblockcopolymerstarsviathesameclickroute.Theabilitytosynthesizewell-definedstarpolymersandconfirmtheirpurityiscriticaltoadvancingtheirbiomedicalapplications.
TheauthorsacknowledgetheNationalInstituteofHealth(EB006493),forfinancialsupportofthisresearch,andtheNationalScienceFoundation(MRI-0619770)isacknowledgedforenablingMALDI-TOFMScharacterization.Inaddition,thisworkwassupportedbytheU.S.ArmyMedicalResearchandMaterielCommandunderAwardNo.W81XWH-10-1-0377.Opinions,interpretations,conclusions,andrecommendationsarethoseoftheauthorandarenotnecessarilyendorsedbytheU.S.Army.
1100
JOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–1101
POLYMERJOURNALSCIENCE
OFWWW.POLYMERCHEMISTRY.ORG
ARTICLE
30Gungor,E.;Bilir,C.;Hizal,G.;Tunca,U.J.Polym.Sci.Part43Hoskins,J.N.;Grayson,S.M.Macromolecules2009,42,A:Polym.Chem.2010,48,4835–4841.
6406–6413.
31Chan,J.W.;Yu,B.;Hoyle,C.E.;Lowe,A.B.Chem.Com-mun.2008,4959–4961.
44Parrish,B.;Breitenkamp,R.B.;Emrick,T.J.Am.Chem.Soc.2005,127,7404–7410.
32Dag,A.;Durmaz,H.;Tunca,U.;Hizal,G.J.Polym.Sci.PartA:Polym.Chem.2009,47,178–187.
45Narasimhan,V.;Burns,C.M.;Huang,R.Y.M.;Lloyd,D.R.33Meier,M.A.R.;Gohy,J.-F.;Fustin,C.-A.;Schubert,U.S.Adv.Chem.Ser.1984,206,3–15.
J.Am.Chem.Soc.2004,126,11517–11521.
46Ekmanis,J.L.ACSSymp.Ser.1987,352,47–58.
34Kul,D.;VanRenterghem,L.M.;Meier,M.A.R.;Strandman,47Hunkeler,D.;Macko,T.;Berek,D.ACSSymp.Ser.1993,S.;Tenhu,H.;YIlmaz,S.S.;Schubert,U.S.;DuPrez,F.E.521,90–102.
J.Polym.Sci.PartA:Polym.Chem.2008,46,650–660.
35Ihre,H.;PadillaDeJesu´s,O.L.;Fre
chet,J.M.J.J.Am.48Park,S.;Park,I.;Chang,T.;Ryu,C.Y.J.Am.Chem.Soc.Chem.Soc.2001,123,5908–5917.
2004,126,8906–8907.
36Malkoch,M.;Schleicher,K.;Drockenmuller,E.;Hawker,C.49Leeb,L.;Gmeiner,P.;Loeber,S.QSARComb.Sci.2007,26,J.;Russell,T.P.;Wu,P.;Fokin,V.V.Macromolecules2005,38,1145–1150.
3663–3678.
50Urbani,C.N.;Bell,C.A.;Lonsdale,D.E.;Whittaker,M.R.;37Knauss,D.M.;Huang,T.Macromolecules2002,35,2055–2062.Monteiro,M.J.Macromolecules2007,40,7056–7059.
38Ihre,H.;Johansson,M.;Malmstroem,E.;Hult,A.Advances51Clark,P.G.;Guidry,E.N.;Chan,W.Y.;Steinmetz,W.E.;inDendriticMacromolecules;JAIPress:Stamford,CT,1996,Grubbs,R.H.J.Am.Chem.Soc.2010,132,3405–3412.
pp1–25.
52Althoughthepresenceofthedoublechargedspeciesis39Li,Y.;Hoskins,J.N.;Sreerama,S.G.;Grayson,M.A.;Gray-alsoexpected,theoverlapwiththelowMWportionsoftheson,S.M.J.MassSpectrom.2010,587–611.
singlychargedpolymermakesthisdistributiondifficultto40Theuseofexcesssodiumaziderequirescareduringworkup,resolve.
anddisposalofwaste,inadditiontoavoidinguseofhalogenated53Becauseofthelowionizationpotentialinthismolecularsolventswhichmayreactwithazidetoformexplosivebyproducts.weightrange,increasedlaserpowerisrequiredtoprovidesuf-41Li,Y.;Hoskins,J.N.;Sreerama,S.G.;Grayson,S.M.Mac-ficientsignal,andsubstantiallowmolecularweightnoiseisromolecules2010,43,6225–6228.
observedpreventingaccurateexaminationoflowermass42Li,Y.;Giles,M.D.;Liu,S.;Laurent,B.A.;Hoskins,J.N.;rangesinthesamespectra.
Cortez,M.A.;Sreerama,S.G.;Gibb,B.C.;Grayson,S.M.54Lammens,M.;Fournier,D.;Fijten,M.W.M.;Hoogenboom,Chem.Commun.2011,47,9036–9038.
R.;Prez,F.D.Macromol.RapidCommun.2009,30,2049–2055.
WWW.MATERIALSVIEWS.COMJOURNALOFPOLYMERSCIENCEPARTA:POLYMERCHEMISTRY2012,50,1086–11011101
因篇幅问题不能全部显示，请点此查看更多更全内容