ARGET ATRP Una Herramienta Para La Modificacion de La Celulosa Tesis(1)

7/24/2019 ARGET ATRP Una Herramienta Para La Modificacion de La Celulosa Tesis(1)

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ARGETATRPASATOOLFORCELLULOSEMODIFICATION

SusanneHanssonDoctoralThesis

KungligaTekniskahgskolan,Stockholm2012

AKADEMISKAVHANDLING

sommed tillstndavKungligaTekniskahgskolan iStockholm framlgges tilloffentliggranskning fravlggandeav teknologiedoktorsexamen fredagenden14 december 2012, kl. 10.00 i sal K2, Teknikringen 28, KTH, Stockholm.Avhandlingen frsvaraspengelska.Fakultetsopponent:Dr.SteveEdmondson,LoughboroughUniversity,UK.


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Copyright2012SusanneHanssonAllrightsreserved

PaperI2009TheAmericanChemicalSocietyPaperII 2011TheRoyalSocietyofChemistryPaperIII2012TheRoyalSocietyofChemistryPaperIV2012TheAmericanChemicalSociety

TRITACHEReport2012:60

ISSN16541081

ISBN9789175015446


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To

my

family


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ABSTRACTThe importance of finding new applications for cellulosebased products hasincreased, especially to meet the demand for new environmentally friendlymaterials,butalsosincethedigitalizationofoursocietywilleventuallydecreasetheneed forpaper.Toexpand theapplicationareaofcellulose,modification toimproveand/or introducenewpropertiescanbea requisite.Thus, the focusofthisstudyhasbeentoachievefundamentalknowledgeaboutpolymergraftingofcelluloseviawellcontrolledradicalpolymerization.Cellulose, in the form of filter paper, has successfully been grafted via

activators regeneratedbyelectron transferatom transfer radicalpolymerization(ARGET ATRP) of themonomers:methylmethacrylate, styrene, and glycidylmethacrylate.TheadvantagesofARGETATRParethatonlyasmallamountofacoppercatalystisrequiredandthereactioncanbeperformedinlimitedamountofair;yet,providing for relativelywellcontrolled reactions.Thesebenefits canrenderARGETATRPanattractivemethodforindustrialutilization.Thecontactanglemeasurementsofthegraftedfilterpapersconfirmedthatthe

hydrophobicity of cellulosewas significantly increased, even for shorter graftlengths.FTIRspectroscopyestablishedthattheamountofpolymersuccessively

increasedwithmonomerconversion.HighresolutionFTIRmicroscopy(FTIRM)wasproventobeaveryusefultechniquefortheanalysisofcellulosesubstrates,displaying the spatial distribution of polymer content on cellulose fibers. Thepolymerwasshowntobefairlyhomogenouslydistributedonthefiber.Aninitiatorwithareducibledisulfidebondrenderedcleavageofthepolymer

graftspossible,employingmild reaction conditions.The cleavedgraftsand thefreepolymersformedfromasacrificialinitiatorinparalleltothegraftingwereshown tohavesimilarmolarmassesanddispersities,confirming that thegraftscanbetailoredbyutilizingasacrificialinitiator.Moreover,theinitiatorcontenton

filterpaperandmicrocrystallinecellulosewasassessed.Acomparisonbetweenthetwograftingtechniques,graftingfromcelluloseviaARGET ATRP and graftingto cellulose via copper(I)catalyzed alkyneazidecycloaddition,was performed. To achieve a trustworthy comparison, the freepolymerformedinparalleltothegraftingfromreactionwasemployedastheprepolymer inthegraftingtoapproach,resultinginnearly identicalgraftlengthonthesubstratesforthetwograftingmethods.FTIRManalysesverifiedthatundertheselectedconditions, thegraftingfrom technique issuperior to thegraftingtoapproachwith respect tocontrolling thedistributionof thepolymercontenton

thesurface.TheresultswerecorroboratedwithXrayphotoelectronspectroscopy.


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SAMMANFATTNINGBehovetavatthittanya tillmpningarfrcellulosabaseradeprodukterharkat,framfrallt fr attmta efterfrganpnyamiljvnligamaterial,men venddigitaliseringenavvrtsamhllepsiktkommerattminskapappersbehovet.Fratt ka cellulosans anvndningsomrde kan modifiering, fr att frbttraoch/eller introducera nya egenskaper, vara en nyckelfrga. Denna studie harsledes fokuserat p att erhlla grundlggande kunskaper om polymeraympningsprocesseravcellulosagenomvlkontrolleradradikalpolymerisation.Cellulosa, i form av filterpapper, ympades framgngsriktmedmonomeren

metylmetakrylat, styren samt glycidylmetakrylat viaARGETATRP (activatorsregenerated by electron transfer atom transfer radical polymerization).Frdelarnamed ARGET ATRP r att endast smmngder kopparkatalysatorkrvssamtattreaktionenkanutfrasinrvaroavbegrnsademngderluft,ochtrotsdettauppnsrelativtvlkontrolleradereaktioner.DessafrdelargrARGETATRPtillenattraktivmetodattanvndaindustriellt.Kontaktvinkelmtningarna av de ympade filterpapperna bekrftade att

cellulosanshydrofobicitetkadekraftigt,venfrdekortarekedjelngderna. FTIR spektroskopi bekrftade att polymermngden successivt kade med

monomeromsttningen.HgupplsandeFTIRmikroskop(FTIRM)visadesigattvaraenmycketanvndbarteknikfranalysavcellulosasubstrat,ddenspatialafrdelningenavpolymermngdenpcellulosafibrerkundeuppvisas.Polymerenvisadesigvaraganskahomogentfrdeladpfibern.En initiatormeden reducerbardisulfidbindningmjliggjordeklyvningen av

deympadepolymerkedjornaundermildabetingelser.Deklyvdakedjornaochdefriapolymerernapolymeriseradefrnenfriinitiatorparallelltmedympningenhade liknandemolmassorochdispersiteter,vilketvisarattdeympadekedjorkanskrddarsysgenomanvndandetavdenfriainitiatorn.Vidareuppskattades

veninitiatormngdenpfilterpappersamtpmikrokristallincellulosa.Enjmfrelsemellande tvympningsteknikerna,ympningfrn cellulosaviaARGETATRP samtympning till cellulosa via koppar(I)katalyserad alkynazidcykloadditionutfrdes.Fratterhllaen relevantjmfrelseanvndesden friapolymeren, bildad parallellt med ympningfrnreaktion, som prepolymer iympningtillmetoden, vilket resulterade i nst intill identiska kedjelngder psubstraten fr de tv ympningsmetoderna. FTIRM verifierade att under degllande reaktionsbetingelserna s r ympningfrntekniken verlgsenympningtillteknikenmed avseende p kontroll ver polymerfrdelningen p

ytan.DessaresultatbekrftadesmedXPS.


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LISTOFPAPERSThisthesisisasummaryofthefollowingpapers:

I. ARGET ATRP for versatile grafting of cellulose using variousmonomers SusanneHansson, Emmastmark,Anna Carlmark, andEvaMalmstrm,ACSAppliedMaterials&Interfaces2009,1,26512659.

II. Selectivecleavageofpolymergraftsfromsolidsurfaces:Assessmentofinitiator content and polymer characteristics Susanne Hansson, PerAntoni, Helena Bergenudd, and Eva Malmstrm, Polymer Chemistry2011,2,556558.

III.

Visualization of poly(methyl methacrylate) (PMMA) grafts oncellulose via highresolution FTIR microscopy imaging SusanneHansson, Thomas Tischer, Anja S. Goldmann, Anna Carlmark,Christopher BarnerKowollik, and EvaMalmstrm,PolymerChemistry2012,3,307309

IV.

Grafting efficiency of synthetic polymers onto biomaterials: Acomparative study of grafting from versus grafting to SusanneHansson,VanessaTrouillet,ThomasTischer,AnjaS.Goldmann,AnnaCarlmark, Christopher BarnerKowollik, and Eva Malmstrm,Biomacromolecules2012,

DOI:10.1021/bm3013132

Mycontributiontotheappendedpapers:

I.

Alloftheexperimentalwork,alloftheanalyses,andmostofthepreparationofthemanuscript.II. Mostoftheexperimentalwork,alloftheanalyses,andmostofthe

preparationofthemanuscript.III.

Alloftheexperimentalwork,mostoftheanalyses,andmostofthepreparationofthemanuscript.

IV.

Alloftheexperimentalwork,mostoftheanalyses,andmostofthepreparationofthemanuscript.

Thisthesisalsocontainsunpublishedresults.


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Scientificcontributionsnotincludedinthisthesis:

V.

Thermoresponsivecellulosebasedarchitectures:tailoringLCSTusingpoly(ethylene glycol) methacrylates Christian Porsch, SusanneHansson, Niklas Nordgren, and Eva Malmstrm, Polymer Chemistry2011,2,11141123.

VI. Surfacegrafted conjugated polymers for hybrid cellulosematerialsJosephPeterson,MarkusWillgert,SusanneHansson,EvaMalmstrm,andKennethCarter,JournalofPolymerScience,PartA:PolymerChemistry

2011,49,30043013.

VII.

Linearvs.hyperbranchedpolymersinthepreparationofpolymer/claynanocompositesLindaFogelstrm,SusanneHansson,AnnaCarlmark,AndersHult,andEvaMalmstrm,manuscript


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ABBREVIATIONSAGU anhydroglucoseAFM atomicforcemicroscopyAl2O3 aluminumoxideARGET activatorsregeneratedbyelectrontransferAsAc ascorbicacidATR attenuatedtotalreflectanceATRA atomtransferradicaladditionATRP atomtransferradicalpolymerizationBiB bromoisobutyrylbromideCA contactangleCRP controlledradicalpolymerizationCuAAC copper(I)catalyzedazidealkynecycloadditionCu(II)Br2 copper(II)bromideCuSO4 5H2O coppersulfatepentahydrateDCM dichloromethaneDMAP 4dimethylaminopyridineDMF N,Ndimethylformamide

DP degreeofpolymerizationDS degreeofsubstitutionDTT 1,4dithiothreitolM molarmassdispersityEBiB ethyl2bromoisobutyrateFESEM fieldemissionscanningelectronmicroscopyFTIR FouriertransformationinfraredFTIRM FTIRmicroscopyGMA glycidylmethacrylate

HDA heteroDielsAlderIE initiatorefficiencykact rateconstantofactivationkdeact rateconstantofdeactivationKeq equilibriumrateconstantkpapp apparentrateconstantofpropagationkp rateconstantofpropagationkt rateconstantofterminationMMA methylmethacrylate

MCC microcrystallinecellulose


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NaAsc sodiumascorbateNaN3 sodiumazideNCC nanocrystallinecellulose

NFC nanofibrillatedcelluloseNMP nitroxidemediatedpolymerizationNMR nuclearmagneticresonancePMDETA N,N,N ,N ,N pentamethyldiethylenetriamineRAFT reversibleadditionfragmentationchaintransferRDRP reversibledeactivationradicalpolymerizationRp rateofpolymerizationSETLRP singleelectrontransfermediatedlivingradical

polymerization

SI surfaceinitiatedSn(EH)2 tin(II)2ethylhexanoateSt styreneTBAF 3H2O tetrabutylammoniumfluoridetrihydrateTEA triethylamineTHF tetrahydrofuranIUPAC internationalunionofpureandappliedchemistryXPS Xrayphotoelectronspectroscopy


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TABLEOFCONTENTS

1.

PURPOSE OF THE STUDY .................................................................................... 1

2.

INTRODUCTION ..................................................................................................... 2

2.1 CELLULOSE.......................................................................................................... 2

2.1.1 Structure of cellulose ................................................................................... 3

2.1.2 Cellulose types ............................................................................................. 4

2.1.3 Modification of cellulose.............................................................................. 4

2.2

REVERSIBLE-DEACTIVATIONRADICALPOLYMERIZATION(RDRP) .. 6

2.2.1

Atom transfer radical polymerization (ATRP) ............................................. 7

2.2.2 Activators regenerated by electron transfer (ARGET) ATRP .................... 12

2.3 CLICKCHEMISTRYPOLYMERCONJUGATIONREACTIONS........... 13

2.3.1 Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) ........................ 13

2.4 GRAFTINGOFCELLULOSEVIAWELL-CONTROLLED

POLYMERIZATIONREACTIONS ................................................................. 14

2.5

CHARACTERIZATIONOFMODIFIEDCELLULOSESUBSTRATES ........ 15

2.5.1 High-resolution FT-IR microscopy (FT-IRM) ........................................... 16

3.

EXPERIMENTAL ................................................................................................... 18

3.1 MATERIALS .................................................................................................... 18

3.2 INSTRUMENTATION ..................................................................................... 18

3.3 INITIATORSYNTHESES ................................................................................ 21

3.3.1 Synthesis of disulfide-containing sacrificial initiator ................................ 21

3.3.2 Synthesis of disulfide-containing initiator for immobilization onto cellulose

substrates ................................................................................................................... 22

3.3.3 Synthesis of silane-protected alkyne-functional sacrificial initiator .......... 22

3.3.4

Synthesis of bromine-functional initiator for immobilization onto cellulosesubstrates ................................................................................................................... 23

3.4 IMMOBILIZATIONOFINITIATORSONTOCELLULOSE ......................... 23

3.5 GRAFTINGOFVARIOUSMONOMERSFROMCELLULOSEVIAARGET

ATRP ................................................................................................................. 24

3.6 CLEAVAGEOFPMMAGRAFTSFROMMODIFIEDCELLULOSE ........... 25

3.7 GRAFTINGOFALKYNE-FUNCTIONALPMMATOCELLULOSEVIA

CUAAC ............................................................................................................. 26

3.7.1 Activation of alkyne-functional PMMA ...................................................... 26

3.7.2

Grafting to cellulose .................................................................................. 27


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4.

RESULTS AND DISCUSSION .............................................................................. 28

4.1 GRAFTINGOFVARIOUSMONOMERSFROMCELLULOSEVIAARGET

ATRP ................................................................................................................. 28

4.2

SELECTIVECLEAVAGEOFPOLYMERGRAFTS ...................................... 34

4.3 ASSESSINGINITIATORCONTENT .............................................................. 35

4.4 COMPARISONBETWEENGRAFTING-FROMANDGRAFTING-TO

CELLULOSE .................................................................................................... 37

4.4.1

Grafting from and grafting to cellulose ..................................................... 38

4.4.2 XPS analysis of modified cellulose substrates ........................................... 40

4.4.3 FT-IRM of modified cellulose substrates ................................................... 44

4.5 TOWARDSINDUSTRIALGRAFTINGOFCELLULOSE ............................. 47

5. CONCLUSIONS ...................................................................................................... 50

6.

FUTURE WORK ..................................................................................................... 52

7.

ACKNOWLEDGEMENTS .................................................................................... 53

8.

REFERENCES ........................................................................................................ 56


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PurposeoftheStudy

1

1.PURPOSEOFTHESTUDY

The interest incellulosearenewableand inexpensiveresourcewithastiffnessalmost comparable to steel has increased significantly the lastdecades.Somerequestsare toachievingmoreenvironmentally friendlymaterials,aswellas toincrease the utilization of cellulose inmore advanced applications, comparedwithmoretraditionallycommoditiessuchaspaper.However,toexploitthefullpotentialofcellulose,modificationofthenativeresourceisoftenaprerequisite,inorder to improve the properties of cellulose and/or to introduce newfunctionalities. The modification in this study was performed by graftingpolymersfrom and to cellulose;moreover, to have the possibility to tailor thesurfaceproperties, thepolymerizationshouldbeperformed inawellcontrolled

manner.

Thepurposeofthisstudywastoinvestigatethegraftingprocessbyapplyingthemethod activators regenerated by electron transfer atom transfer radicalpolymerization(ARGETATRP)whengraftingfromcellulosesubstrates.Anotherimportantaimwastodevelopanapproachtodetachthegraftedpolymerchainsunder mild reaction conditions; thereby, rendering their postcharacterizationpossiblewhilekeepingthesubstrateintact.Theamountofavailablereactivesitespresenton thecellulosesubstratewasalso theobjectof investigation,aswellas

thepolymerdistributiononthesubstratesurface,withthepurposeofobtainingvaluableknowledgeaboutthegraftingprocess.

Anotherobjectivewastosystematicallystudyandcomparethegraftingefficiencyofthetwograftingapproachesgraftingfromandgraftingtobyinvestigatingthepolymericamountonthesurface,employingdifferentgraftlengths.

TheARGETATRPprocesswasalsoinvestigatedwiththepurposeofmakingthegraftingreactionmoreapplicableinanindustrialcontext.


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Introduction

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2.INTRODUCTION

2.1CELLULOSE

Cellulosethemaincomponentinplantstogetherwithligninandhemicelluloses is themost abundant natural resource in theworld. It is an attractive andinexpensive polymeric rawmaterial that is renewable,biocompatible, and hasexcellent mechanical properties. The modulus of native cellulose is 138 GPa,whichcanbecompared to themodulusofsteel:200GPa.1Thus, the interestofutilizingcellulosehasincreasedsignificantlytherecentdecades,especiallyduetoenvironmental concerns. The focus has latelybeen to replace petroleumbasedmaterials with renewable resources like cellulose, employing cellulose in a

broader range of products than only in commodities such as papers and cardboards. Nevertheless, the digitalization of our society will in the long termdecrease the utilization of paper, increasing the importance of finding newapplicationsforcellulosebasedproducts.Asanexample,cellulose fiberscanbeemployedasafillerorreinforcementinapolymericmatrix,formingacomposite,e.g. sawdust in polypropylene2. The large advantages of cellulose fiberscomparedtotraditionalfillers,suchascarbonfibersorglassfibers,arelowercostanddensity,highspecificstrength,renewability,degradability,andnontoxicity.35However,commondrawbacksaresensitivitytomoisture,bacteria,androt,but

it is especially the poor interfacial adhesionbetween the hydrophilic cellulosefibers and the hydrophobic polymermatrix that results inweakenedmaterialperformance of the composite.5, 6 Poor compatibilitybetween filler andmatrixrendersthecompositeinhomogeneous,causinglossofmechanicalstrengthsinceit is more difficult to transfer the applied load throughout the material.3Therefore,surfacemodificationofcelluloseisarequisite.


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Introduction

3

2.1.1

Structureofcellulose

Cellulose is composed of Dglucose units that are linked togetherwith 14

glycosidicbonds, forming a linear polysaccharidewith one primary and twosecondaryhydroxyl groups on each anhydroglucoseunit (AGU), see Figure 1.

The degree of polymerization (DP), i.e., the number of constituent AGUs, of

nativecelluloseisdependentonitsorigin.Themostcommonsourcesarewoodor

cotton, and the DP can then range from 300 to 10,000.7 Cellulose exhibits a

fascinatinghierarchical structure from amolecular tomacroscopic scale,which

providesfortheoutstandingpropertiesofcellulosefibers,seeFigure1.Inplants,

theextendedcellulosechainsarealignedinsheetsthatarestabilizedbyintra and

intermolecularhydrogenbonds.Thesecrystallinesheetsarecloselypacked,with

secondary forces that are holding them together, forming threedimensionalmicrofibrils that haveboth ordered and less ordered regions of cellulose. The

microfibrils are arranged in bundles of microfibrils that together with the

hemicellulosesandligninbuildupthecellwall.Thewoodcellconsistsofseveral

cellwall layers with various cellulosic compositions and with the cellulose

microfibrilsarrangedindifferentorientation,whichisvitalforthehighstrength

of thecellulose fiber.Themicrofibrilsand themicrofibrilbundleshavea lateral

dimensionof1.53.5nmand1030nm,respectively,andthelengthcanbeupto

afewmicrometers.7Thecellulosefiberhasgenerallythewidthofsometensofa

micrometer,andlengthsofafewmillimeters.8

Figure1.Theidealizedschematicpictureofthehierarchicalstructureofcellulose,adopted

fromIsogaietal.8


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Introduction

4

2.1.2 Cellulosetypes

Cellulosecanbeextractedformwoodbyremovingtheligninandhemicelluloses,

resulting in cellulose pulp. By producing cellulose from different cellulosesources,e.g.flax,hemp,andjute,andnotonlywood,andbyemployingchemicaland/or mechanical methods, it is possible to obtain many different types ofproducts.7, 9Cellulosecanalsobeobtained fromaerobicbacteria.9Recently, thefocushasbeen to achievenanosized cellulose components, e.g.,nanofibrillatedcellulose (NFC) andnanocrystalline cellulose (NCC).9NFC andNCC canhaverathersimilarwidthsof570nm,butthelengthofNFCisgenerallymuchlonger,up toseveralmicrometers,compared to the lengthofNCC thatoriginates fromplantcelluloses:100 to250nm.Furthermore,NCCconsistsofelongatedrodlike

crystalsandhasnoamorphousregionslikeNFC,limitingtheflexibilityofNCC.Apartfromnanocellulose,microcrystallinecellulose(MCC)isanotherinterestingand commercially available substrate. It is derived from cellulose pulp byhydrolysiswithmineralacids,resulting inparticlesintherangeof10100 m.10Another common cellulose substrate isWhatman filter paper that isproducedfrom highquality cotton linters, resulting in a very pure product with highcellulosecontent(>98%)andwithacrystallinityof68%.11

2.1.3

Modificationofcellulose

Themanyhydroxylgroupsalongthebackboneofcellulosearesuitablesitesforchemicalmodification.However, the number of available hydroxyl groups isdependentonhowthecellulosesubstrate ispretreated.Treatmentwithastrongbase such asNaOH (mercerization)breaks the hydrogenbonds, swelling andincreasing the activity of cellulose. Nevertheless, cellulose exists in differentcrystallineformsanduponmercerizationthecrystalstructurecelluloseI,whichisthenative form, is converted to cellulose II.Cellulose IIhas antiparallel chains

and ismore thermodynamicallystable thancelluloseI.7The functionalizationofthe available hydroxyl group may have a slight impact on the mechanicalproperties, especially if the strong hydrogenbonds thatbuildup the cellulosestructure are affected. The challenge when modifying cellulose fibers canthereforebetopreservetheinternalstructureofthefiber.

Cellulose canbemodifiedby grafting synthetic polymers onto its surface, anapproachthathasgainedincreasinginterestforthepastyears.1215Thetwomostcommongraftingapproachesarethegraftingfromandgraftingtotechniques,see


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Introduction

5

Figure 2. In the graftingfrom method, i.e., the surfaceinitiated (SI)

polymerization, the polymer chains arebuilt up through successivemonomer

additionfrominitiatingsitesonthesurface.Comparatively,whengraftingtothe

surface, a preformed polymer with suitable functionality is covalently orphysicallyattached toreactivesiteson the surface.Thegraftingfrommethod is

assumed to result inahighergraftingefficiencydue to less sterichindranceof

diffusing smallmonomers, compared to the graftingto approachwhere larger

polymerchainshavetodiffusetothereactivesites,whichmaybeshieldedbythe

already grafted polymer chains.12, 13, 16 In themajority of all grafting studies,

grafting from is reported to yield higher grafting densities,12, 14, 1722 although

examples have been reported where graftingto approaches have provided

graftingdensitiesinthesamerange.2325

Figure2.Theschematicpictureofthegraftingfromandthegraftingtomethod.

The advantage with the graftingto method is that the employed preformed

polymercanbeeasilycharacterized,andtherebythemolarmassandmolarmass

dispersity (M) canbedeterminedprior to the grafting reaction; therefore, thegraftingtomethodispreferredfromanindustrialpointofview.

Thechallengewiththegraftingfromapproachistoknowthecharacteristicsofthe

polymergraftsonthesurface.However,thiscanbecircumventedbyutilizinga

sacrificialinitiatorthatformsanunbound,freepolymerinthereactionmediain

parallel tothegraftingprocess.Thefreely formedpolymer,whicheasilycanbe

characterized, has shown to have comparable characteristics to the polymer

grafts,18, 2628 also rendering this an applicable approach to control the polymer

graftsonthesurface,byemployingdifferentratiosbetweenthemonomerandthe


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Introduction

6

initiator (targeted degree of polymerization (DPtarget) = [M]0/[I]). However, toevaluatethegraftproperties,detachmentofthecovalentlylinkedpolymerchainsand their subsequent analysis is vital.Acidic hydrolysis of cellulose hasbeen

employedasamethodtoisolatethegraftsforsubsequentcharacterization,17,21,29,30resultinginacompletedegradationofthesubstrate.Nevertheless,thismethodmaynotbe applicable forpolymers containing sensitive functionalgroups likeesters,suchas(meth)acrylatepolymers.

2.2REVERSIBLEDEACTIVATIONRADICALPOLYMERIZATION(RDRP)

Freeradicalpolymerizationisthemostcommonpolymerizationmethodappliedinthe industryfortheproductionofpolymers,since it isthemosteconomicallyfavorable approach to produce large volumes. One advantage is the facilereactionconditionsthatcanbeemployedonavarietyofmonomers,withoutanyspecific demands for high purity.However, due to the high concentration ofradicals formed during the reaction, many chaintransfer and terminationreactionstakeplace,resultinginpoorcontroloverthepolymerization.Toobtainmorecomplexandwelldefinedpolymers,itiscrucialtobeabletopredetermine

themolarmass,themolarmassdispersity(M),andthechainendfunctionality.Therefore,polymerizationmethodsthatsuppresschaintransferandterminationreactionsarerequired.Inalivingpolymerizationsystem,noirreversibletransferor termination reactions occur. The initiation is fast and quantitative and thereaction will continue until all the monomers are consumed. Reversibledeactivationradicalpolymerization(RDRP)techniquesalmostfollowtheconceptofatruelivingsystem;however,thesemethodsarediscouragedtobereferredtoaslivingbyIUPAC.31Inaddition,RDRPwaspreviouslycalledcontrolledradicalpolymerization(CRP)buttheterminologyRDRPisnowbeingencouraged.

The RDRP methods can be divided into nitroxide mediated polymerization(NMP),32, 33 atom transfer radical polymerization (ATRP)3436, and reversibleaddition fragmentation chain transfer (RAFT) polymerization37, 38. NMP andATRParebasedonadynamicequilibriumbetweena largemajorityofdormantspeciesandonlyafewactivespecies,keepingtheradicalconcentrationlow,seeScheme1.InRAFTpolymerization,theequilibrium isbasedonreversiblechaintransfer. NMPwasthefirstmethodtosuccessfullyscavengeradicalsbyutilizinganitroxideleavinggroupthatreversiblyterminatethepropagatingspecies.RAFT

polymerization relies on a chaintransfer agent that captures and releases


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Introduction

7

propagating radicals reversibly. ATRP, discussed in more detail in the nextsection, isbasedon the transferofanatombetweena transitionmetal catalystcomplex and the propagating chain. The RDRP methods diverge in their

applicability fordifferentmonomer systems,andallhave theiradvantagesanddisadvantages.

Scheme1.ThedynamicequilibriumpresentinRDRPtechniques.

2.2.1 Atomtransferradicalpolymerization(ATRP)

ATRPisthemoststudiedRDRPmethodowingtoitsversatilityandcompatibilitywithavarietyofmonomers,renderingthesynthesisoffunctionalpolymerswithwelldefined compositions possible.39 In 1995, ATRP was independentlydiscovered by Sawamoto et al.34 and Wang and Matyjaszewski35, 36. ATRPoriginates from the atom transfer radical addition (ATRA)40, which is a

modificationoftheKharaschadditionreaction41.ThereversibleredoxprocessinATRP involves theatom transferofahalogenatombetween theactiveand thedormantchain,catalyzedbyatransitionmetal/ligandcomplex.Intheactivationprocess, this complex abstracts thehalide from thedormant chain, forming anactivechainbearingaradical.Theactivespeciescantheneitheraddamonomerin a propagating step or react with the oxidized metal complex via backtransferringofthehalogenatomtothepropagatingchainend,whichdeactivatesthe radical.Toachievepropercontrol, it iscrucial that the initiation is fastandquantitative and that the reversibledeactivation occurs rapidly.However, it isimpossibletoachieveatrulylivingsystem,andterminationreactionsalsooccur,principally via radical coupling and disproportionation. Yet, just a fewpropagatingchainsundergo terminationduring the initial,nonstationarystageof the polymerization. During this stage, the concentration of the deactivatorincreases, shifting the equilibrium towards the dormant side, and therebyinhibiting the termination reactions. This phenomenon is called the persistentradicaleffect.42


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Introduction

8

2.2.1.1

Components

of

ATRP

AnATRPsystemisgenerallycomposedofamonomer,aninitiator,andacatalyst

basedona transitionmetal/ligandcomplex.Thechoiceofmonomerdetermineswhich initiatorandcatalystaswellas temperatureand solvent thatwillbeutilized.43 Monomers that successfully havebeen polymerized via ATRP arestyrenics,35, 44 (meth)acrylates,4547 meth(acrylamides),48, 49 and acrylonitriles,50, 51which all are effective in stabilizing the propagating radical.43, 52ATRP is alsotolerant to several functional groups such as epoxides,5355 hydroxyl groups,amines, and cyanides.43 Vinylpyridines can be polymerized if a stronglycoordinating ligand is employed, suppressing the monomers ability tocoordinate to the transition metal. Conversely, acidic monomers cannot be

polymerized via an ordinaryATRP system due to protonation of the ligand.43

However,Janaetal.haverecentlyreportedaunimolecularligandinitiatordualfunctionalsystemthatrendersATRPof(meth)acrylicacidspossible.56

The initiatordetermines thenumberofpropagating chains,andhence also theDP.Arapid initiationandnegligibletransferandterminationreactionsresultinthat the number of propagating chains is equal to the initial initiatorconcentration;thus,themolarmassorDPcanbeestimatedaccordingtoEq.1

.

initiatormonomerDP

0

0 conv (1)

The initiator is commonly composedof an alkylhalide,where thehalidemostfrequentlyisabromideorchloride.Thehomolyticcleavageofthelabilebondontheinitiatorresultsinafreeradicalthatcaninitiatethepolymerization.Thebonddissociation energy for bromide halides is lower than for chloride halides,suggesting thatbromidehalidesaremoreefficient.Itcanbebeneficial toutilizeaninitiatorthatresemblesthemonomerinstructure,e.g.abenzylichalideforthe

polymerizationofstyrene,butitisnotnecessary.Tertiaryalkylhalidesarebetterat stabilizing the radical than secondary alkyl halides, resulting in a fasterinitiationforthetertiarycompound.43

The catalyst has a central role in ATRP, since it determines the equilibriumbetween the active and dormant species. Ideally, the catalyst shouldbe veryselectiveforatomtransferandnotparticipateinotherreactions;furthermore,thedeactivation shouldbe extremely rapid. The transition metal must have twoaccessibleoxidationstatesandaffinitytothehalogenatom,andtheligandshould


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Introduction

9

complextothemetalrelativelystrongly.43Themostfrequentlyutilizedtransitionmetal is copper, due to its low cost and applicability to several differentmonomers.Many othermetals have alsobeen employed: titanium, chromium,

molybdenum,rhenium, iron,ruthenium,osmium,rhodium,nickel,palladium,43,57andcobalt58.Thecatalyticactivityandselectivityisstronglyliganddependent,since itcontrols the redoxpotentialof themetalcenter.Another important rolefor the ligand is to solubilize themetal in the organicmedia.Commonly, theligands are based on nitrogen or phosphorus. Nitrogenbased ligands areespeciallyapplicabletocoppermediatedsystems.39,43,52

ATRP canbe performed inbulk or in solution.Various organic solvents havebeen employed for different monomers, and it is especially essential if the

polymer is insoluble in the monomer. The structure of the catalyst maybeaffectedbydifferent solvent, resulting indifferent reaction rates.ATRP canbeperformed in aqueous medium,59, 60 which is of interest both from anenvironmental as well as industrial perspective. Polar media has shown toenhance the reaction rate since it canpromote the solubilityof the catalyst,59, 61which has resulted in the development of an alternative mechanism,6264comparedwiththeoriginalmechanismpresentedbyMatyjaszewskietal.52.Bothmechanismswillbediscussedinthenextsection.

2.2.1.2 SuggestedmechanismofATRP

Infreeradicalpolymerization,thepolymerizationoccursinthreesteps:initiation,propagation,and termination. InATRP,mainly initiationandpropagation takeplacesince the terminationreactionsaresuppressed.Themechanism,presentedinScheme2,isexemplifiedwithcopper(I)ascatalyst.Theinitiationinvolvesthehomolytic cleavage of the alkyl halide (RX), transferring the halogen radicalatom to the activator Cu(I)X/L, forming the radical R and the deactivator

Cu(II)X2/L,whereLrepresenttheligand.TheATRPequilibriumrateconstantofthe initiation is denoted as K eq. The active species can react with either amonomer or the deactivatorbybacktransfer of the halogen in the reversibleredox reaction. The latter reactionmustbe favored, and the rate constant ofdeactivation (kdeact) shouldbehigher than thepropagation rate constant (kp), toachieveacontrolledpolymerization.52 In the initial stageof thepolymerization,equalamountsofradicalsandCu(II)areformed,whichresultsinsimilarratesofactivationanddeactivation.Duetothepersistentradicaleffectthatoccursintheearly stage of the polymerization, the concentration of theCu(II)X2/L complex

increases and shifts the equilibrium to the dormant side, maintaining the


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Introduction

10

concentrationoftheactivespecieslow.39,43Ifappropriateconditionsareobtained,the few terminationreactions thatoccur in the initialstageof thereaction,withtherateconstantkt,willnothaveansignificantimpacttothesystem.43

Scheme2.ThesuggestedmechanismforcoppermediatedATRPinaproticsolvents.

The ATRP process occurs with a homolytic innersphere electrontransfermechanism.62 The alternative mechanism, the heterolytic outersphere singleelectrontransfermediatedlivingradicalpolymerization(SETLRP),wasreportedbyPercecetal.,6264andisclaimedtobevalidwhenpolarsolvents,e.g.,waterandalcohols,polaraproticsolvents,or ionic liquids,areemployed. InSETLRP, theCu(I)X/L species disproportionate spontaneously to Cu(0) and Cu(II)X2/L.Accordingly,theCu(I)X/LspeciesisinactivemeanwhiletheelectrondonorCu(0)isextremelyreactive.ThedeactivatorCu(II)X2/L issuggested tobe formedonly

due todisproportionation,making thepersistent radicaleffectunnecessary, i.e.,terminationreactionsareessentiallyabsent.

2.2.1.3 KineticsofATRP

The kinetics of the ATRP mechanism can be derived by assuming that theinitiation is fast and quantitative and that termination reactions are absent;consequently, the concentration of radicals will be constant. The rate ofpolymerization, Rp, is dependent on the apparent rate constant, kpapp, and theconcentrationofthemonomer[M],andhence,Rpisoffirstorderwithrespectto


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Introduction

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the[M],activator[Cu(I)X],andinitiator[PX],whileitisofnegativelyfirstorderwithrespecttothedeactivatorconcentration[Cu(II)X],seeEq.2.However,duetothepersistentradicaleffect,somedeactivatorsareformedintheinitialperiodof

thereaction,makingtheactualkineticsmorecomplicated.

2eqpp

appp Cu(II)X

MCu(I)XPXMPM KkkkR

p

(2)

where

Cu(I)XPX

Cu(II)XP 2

deact

acteq

k

kK (3)

According to Eq. 2, an increased initiator concentration, resulting in a highernumber of propagating radicals and lower DPtarget, will increase thepolymerizationrate.65 Ifnotall initiators initiate, the initiatorefficiency (IE)willbelessthan1,andthemolarmassoftheformedpolymerwillbehigherthantheDPtarget.TheIEcanbeestimatedby

n

theo

n M

M

M

conv

initiatormonomertarget

M.MIE

DP (4)

whereMn is the obtainedby size exclusion chromatography (SEC).The IEhasshown togradually increasewith reaction time formethacrylatesystems,whenethyl 2bromoisobuturate (EBiB) is utilized as the initiator and anisole as thesolvent, implying that the initiationdonotonlyoccurat the initialstageof thepolymerization.65,66TheIEcanbeincreasedbyemployingasolventthatsolubilizethe deactivator complexbetter, increasing the deactivation rate. Accordingly,acetoneandDMFwereshowntoyieldhigherIEthananisole.66The transferand terminationreactions,whichcanbecomplicated tocompletely

avoid, canbe suppressedby reducing theDPtarget

and limit the conversion, i.e.,aimforshorterpolymerchains.65Therateofpropagationissignificantlyreducedathighconversionsas themonomerconcentrationdecreases(Eq.2).Therateofthe side reactions, on the other hand, is not as affected by the monomerconcentrationandcantherebybeessentiallyunchanged.43


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Introduction

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2.2.2 Activatorsregeneratedbyelectrontransfer(ARGET)ATRP

AlthoughATRPhasmanyadvantages incomparisonwithconventional radicalpolymerization, itsuffersfromsomedrawbacks.Therequirementofarelativelylarge amount of the transition metal catalyst, which subsequently requirespurification of the final product, complicates and increases the industrialproductions costs. Furthermore, residual copper tracesmay color the product,and can further cause severe problems in some application, e.g., biologicalsystems,duetoitstoxicity.Anotherdisadvantageisthehighsensitivitytowardsoxygen,whichneedstoberemovedbythoroughdegassing,furthercomplicatingandincreasingthepriceofthereactionprocess.Tocircumventthesedrawbacks,

Matyjaszewski and coworkers67 developed a new approach to conductATRP:activators regeneratedby electron transfer (ARGET) ATRP. ARGET ATRP isbasedontheregenerationof theactivespeciesCu(I)from thedeactivatorCu(II)bytheutilizationofareducingagent,seeScheme3.Thesystemiscomposedofamonomer,aninitiator,thetransitionmetalinitsinactiveform:Cu(II)X2,aligand,andareducingagent.Owingtothereducingagent,theoxidativelystableCu(II)complex is continuously reduced to Cu(I) throughout the polymerization,significantlyreducingtheamountofcoppertoonlyafewppm.

Scheme

3.ThemechanismofARGETATRP.67

Ascorbic acid (AsAc) is frequently utilized as an environmentally friendlyreducing agent. AsAc is a strong reducing agent, so to avoid a too highconcentration of radicals, which would reduce the control over thepolymerization, the solubility in the reaction medium can be limited byemploying aheterogeneous systemwith for example anisole as solvent.68Two


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Introduction

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othernotasstrongly reducingagents thatcanbeutilizedaresodiumascorbate(NaAsc), the saltofAsAc,aswellas tin(II)2ethylhexanoate (Sn(EH)2).67, 69ThebeautyofemployingareducingagentinARGETATRPisthatthereactioncanbe

conductedinthepresenceoflimitedamountsofair,sincethereducingagentalsoscavengeoxygen.Thereby,thedifficultywithtediousdeoxygenationproceduresis reduced or avoided. Moreover, Matyjaszewski and coworkers recentlypresentedanaqueousARGETATRPsystem,wheretheadditionofahalidesalt,e.g.NaCl, promotedan increasedconcentrationofdeactivators to improve thecontroloverthepolymerization.70

2.3CLICKCHEMISTRYPOLYMERCONJUGATION

REACTIONS

TheclickconceptthatwasintroducedbySharplessandcoworkers71in2001wasfirstly intended for organic chemistry,but it has also had a great impact onpolymer chemistry and the synthesis of macromolecules with welldefinedarchitecturesandfunctionalities.72Therequirementsofaclickreactionarethatitmustgiveveryhighyields,bemodular,wideinscope,andorthogonal,andmustalsoproceedbyasinglereactiontrajectory.However,nottoomanyreactions inpolymerchemistrycanbereferredtoasatrueclickreaction,andthetermclickis commonlymisused.Therefore,BarnerKowollik et al. introduced amodifieddefinition adapted to polymer chemistry, including other parameters such asequimolarityand largescalepurification.Polymerlinking reactions thatdonotfulfillthesecriteriaarenot clickreactionsbut(sometimesefficient)conjugationprocesses.73 Examples of reactions that canbe classified as click reactions arethiolenereactions,7476heteroDielsAlder(HDA)chemistry,7779andazidealkynecycloaddition80.

2.3.1 Copper(I)catalyzedazidealkynecycloaddition(CuAAC)

The most frequently utilized click reaction is the copper(I)catalyzed azidealkyne cycloaddition,81, 82which is amodification of the classicalHuisgen 1,3dipolar cycloaddition between alkynes and azides83. The 1,4cycloadditionbetweenanazideandalkyne,catalyzedbyCu(I),formingthe1,2,3triazolering,is presented in Scheme 4. A common Cusystem to employ is the Cu(II)SO4combinedwithNaAscthatreducesCu(II)intoCu(I),butothersystemsutilizing

e.g.,Cu(I)BrorCu(I)Ialsoexist.84


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Scheme4.The1,4cycloadditioncatalyzedbyCu(I)formingthe1,2,3triazole.

2.4GRAFTINGOFCELLULOSEVIAWELLCONTROLLED

POLYMERIZATIONREACTIONS

Asmentionedbefore, an approach to expand the utilization of cellulose isbygrafting polymers onto the cellulose surface.However, to understand and to

tailor the surface properties for a given application, a deeper and more

fundamental knowledge about chemical modifications, such as grafting, is

required. If cellulose is tobe applied formore sophisticated applications, e.g.,

superhydrophobic,85stimuliresponsive,8688orantibacterialsurfaces,30,89sensors,90,

91 and biomedical applications,88, 9295 the demands on understanding well

controlledreactions,suchasRDRP,arehighlysignificant.

To obtain a controlled polymerizationwhen grafting polymersfrom a surface,surfaceinitiated (SI)ATRP,NMP,andRAFTpolymerization,canbeemployed.

Among these methods SIATRP is the most studied one.13, 88, 96 In SIATRP

systems,asacrificialinitiatorcanbeemployedtocontrolthegraftlength,forming

a freepolymer in thesolution,asmentionedabove.Thesacrificial initiatoralso

keeps the concentration of radicals low, since the overall concentration of

dormantspeciesonthesurfaceistoolowtoachieveacontrolledpolymerization.

However, from a processing point of view, the sacrificial initiator can be a

disadvantagesincethefreelyformedpolymerhastoberemoved.Thus,another

usefulapproach to finetunetheSIATRPcanbebyadding thedeactivator, i.e.,Cu(II), to the solution, shifting the dynamic equilibrium towards the dormant

sidewithouttheformationoffreepolymer.16,88,97

RAFTandATRPare frequentlyutilizedwhengraftingpolymersfrom cellulose

substrates.1214 When SIRAFT polymerization is employed, pretreatment of

cellulosewithNaOH, to increase the surface area, has shown tobe crucial to

obtainsufficientgraftingefficiency.21ThefirstSIATRPfromcellulosefilterpaper

wasreported in2002byCarlmarkandMalmstrm.19Since then,severalreports

on the graftingfrom approach of various cellulose substrates employingATRP


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Introduction

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havebeendescribed.98101Aproblem correlatedwith SIATRP from cellulose isthat it can occasionallybe problematic to remove the copper catalyst, whichmightresult incoloredproductsdespite thoroughwashing.14However, thiscan

be circumventedbyutilizingARGETATRP.ARGETATRPhasproven tobeavery efficient tool when grafting polymersfrom cellulose substrates.26, 98, 102104ARGETATRPhasbeenemployedforgraftingofnanotubes,withoutthepresenceof a sacrificial initiator, showing that themolarmass couldbe controlledbyvarying thereaction time.105Furthermore,SIARGETATRPhasbeenperformedfrom a siliconwaferwithout the requirement for any deoxygenation,69whichmostcertainlycanincreaseitscommercialsignificance.

ATRP and RAFT polymerization are frequently utilized to produce the pre

polymerfortheensuinggraftingtoreaction.14AgreatadvantageofemployingaRDRP is thepreservedendgroup functionality.ForanATRPsystem, theactiveendgroup,which commonly has abromine or chlorine functionality, canbetransformedintoanazidemoiety.39,80Anazideoralkynefunctionalitycanalsobeincorporated into the ATRP initiator prior to the polymerization,106 formingfunctionalizedprepolymers thatcanbegraftedbyconjugatationviaCuAAC toanalkyne orazidefunctional surface.Cellulose substrateshavebeenmodifiedby employing efficient conjugation reactions such as CuAAC,106108 thiolenechemistry,109 andHDA chemistry110, 111. The assumed lower grafting efficiency

connectedtothegraftingtoproceduremaybeenhancedbyutilizingthesehighlyefficientconjugationreactions.

2.5CHARACTERIZATIONOFMODIFIEDCELLULOSESUBSTRATES

Apersistent challengewhenmodifying solid cellulose substrates is to actually

knowthecharacteristicsofthegraftedsurfaces,i.e.,polymergraftcharacteristics,polymerdistribution,degreeofsubstitution (DS), initiatorcontent,andgraftingdensity. Several surface characterization methods exist, such as: Fouriertransformation infrared (FTIR) spectroscopy, contact angle (CA)measurement,fieldemissionscanningelectronmicroscopy (FESEM),atomic forcemicroscopy(AFM),Xrayphotoelectronspectroscopy(XPS),andellipsometry.However,theinherentsurfaceroughnessofcellulosesubstrates,suchasfilterpaper,combinedwith its porosity renders cellulose a tricky substrate to analyze properly.Comparatively,flatsurfacessuchassiliconandgoldwafersaremoreconvenient

substrates to characterize, especiallywithmethods like XPS and ellipsometry.


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Introduction

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Withthelatter,itispossibletodeterminethethicknessofthinpolymerlayersonsmooth surfaces,but not on cellulose substrates due to its roughness.On thecontrary, XPS analysis, which can provide the elemental composition on the

surface, can be utilized on cellulose;18, 112 however, it is difficult to obtainquantitative information about modified cellulose substrates due to theinhomogeneoussurface,butthesuccessofthemodificationcanbeconfirmedbythecomparisonofspectrafromdifferentsurfaces.113

2.5.1 HighresolutionFTIRmicroscopy(FTIRM)

Anadvantageousmethod toemployforcharacterizationofgraftedsubstrates is

highresolutionFTIRmicroscopy (FTIRM) imaging.110, 111, 114, 115 Itcombines thebenefits of IR spectroscopy and microscopy, yielding information about thechemical composition on the surface combined with the surface topography.Thereby,thespatialdistributionofthegraftedpolymercontentcanbevisualized.A focal plane array detector spatially resolves the incomingbeam of theATRcrystal, acquiring 4096 FTIR spectra over an area of 32 32 m. Themeasurement only takes a few minutes and can be achieved without anyrequirementsof special samplepreparation.FTIRMhasproven tobe ahighlyapplicable technique for the characterization of modified cellulose surfaces,

allowing foranalysisofsinglecellulose fibers.110, 111, 114, 115The topographyof thecellulosefiberscanbeobtainedbyintegrationofthecelluloseabsorptionintensityfrom the FTIR spectra from 950 to 1200 cm1,which originates from theCOvibrationincellulose.Iftheintensityofacharacteristicpolymersignalisutilized,e.g.,between1700and1760cm1thatcorrespondstothestretchingvibrationofacarbonylbond,thespatialdistributionofthepolymercontentonthefibercanbevisualized.ThisispresentedinFigure3,wherethecorrespondingregionsofthespectraresultintwoFTIRMmicrographsofasinglecellulosefiber,respectively.Noteworthy, themicrographs correspond to the same fiber,butwith different

intensityscalesofthecorrespondingregions.


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Introduction

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Figure 3. The characteristic carbonyl (polymer) and cellulose region as a single FTIR

spectrum(top)andasFTIRmicrographs(bottom),where4096FTIRspectraareacquired

overanareaof3232mofaPMMAgraftedfilterpaper.Themicrographscorrespondto

thesamefiber,butwithdifferentintensityscalesofthecorrespondingregions.


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Experimental

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3.EXPERIMENTAL

3.1MATERIALS

Bromoisobutyryl bromide (BiB, 98 %), 2bromoethanol (95 %), copper(II)bromide (Cu(II)Br2, 99 %), 4dimethylaminopyridine (DMAP, 99 %), oxalylchloride(98%),N,N,N ,N ,N pentamethyldiethylenetriamine(PMDETA,99%),3(trimethylsilyl)propargylalcohol (99%),andWhatmanno.1 filterpaperwerepurchased from SigmaAldrich.Ascorbic acid (AsAc, 99%), sodiumbisulfate(NaHSO4), and sodium carbonate (Na2CO3) were purchased from Fluka.Triethylamine(TEA)andsuccinicanhydride(99%)werepurchasedfromMerck.Tetrabutylammonium fluoride trihydrate (TBAF 3H2O), copper sulfate

pentahydrate(CuSO4 5H2O),sodiumazide(NaN3),sodiumascorbate(NaAsc),and 1,4dithiothreitol (DTT, 99 %) were purchased from ABCR. Methylmethacrylate(MMA,99%,SigmaAldrich),styrene(St,99%,Fluka),andglycidylmethacrylate (GMA, 97%, Fluka),werepassed through a column of activatedneutralaluminumoxide (Al2O3,SigmaAldrich)prior touse inorder to removetheinhibitor.

3.2INSTRUMENTATION

1HNMRspectrawererecordedonaBrukerAvance400MHzNMRinstrument,usingCDCl3assolvent.Thesolventresidualpeakwasusedasinternalstandard.

TheCP/MAS13CNMRspectrawererecordedinaBrukerAvanceIIIAQS400SBinstrumentoperatingat9.4T.Thefilterpaperwaswettedwithdeionizedwaterto 4060% water content and packed uniformly in a zirconium oxide rotor.Recording spectra on wet rather than dry samples gives a higher apparentresolution.116Allmeasurementsweremadeat295 (1)KwithaMASrateof10

kHz.A 4mm double airbearing probewas used.Acquisitionwas performed


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Experimental

19

usingaCPpulsesequence, i.e.,a2.95microsecondsproton90pulseanda800microsecondsramped (10050%)fallingcontactpulse,witha2.5secondsdelaybetween repetitions.ASPINAL64pulse sequencewasused for1Hdecoupling.

TheHartmannHahnmatchingprocedureisbasedonglycine.Thechemicalshiftscalewas calibrated to theTMS ((CH3)4Si) scaleby assigning thedatapoint ofmaximumintensityintheglycinecarbonylsignaltoashiftof176.03ppm.Atotalof 4096 transientswere recordedon each sample,givinganacquisition timeofapproximately3h.Thesoftware forspectral fittingwasdevelopedat InnventiaAB and isbasedon aLevenbergMarquardt algorithm.117All computations arebasedonintegratedsignalintensitiesasobtainedfromthespectralfitting.118

Size exclusion chromatography (SEC) using THF (1.0mLmin1) as themobile

phase(inpaperI,II,andII)wasperformedat35CusingaViscotekTDAmodel301 equipped with two T5000 columns with porous styrene divinylbenzenecopolymer(300mmL7.8mmID,exclusionlimitMWpolystyrene:400,000,000Da) fromMalvern (UK), aVE 2500GPC autosampler, aVE 1121GPC solventpump, and aVE 5710GPCdegasser fromViscotekCorp. (theNetherlands).Aconventional calibrationmethodwas created using narrow linear polystyrenestandards.Correctionsfortheflowratefluctuationsweremadebyusingtolueneas an internal standard. Viscotek OmniSEC version 4.0 softwarewas used toprocessdata.

SECmeasurementswerealsoperformedusingTHF (1.0mLmin1 )at35 C (inpaper IV) on a Polymer Laboratories PLGPC 50 Plus Integrated System,comprisinganautosampler,aPLgel5 mbeadsizeguardcolumn(507.5mm)followedbythreePLgel5 mMixedCcolumns(3007.5mm)andadifferentialrefractiveindexdetector.TheSECsystemwascalibratedusinglinearpoly(methylmethacrylate) standards ranging from 700 to 2,000,000 gmol1 (MarkHouwinkparametersK=12.8105dLg1,=0.69).119

SECmeasurementswereperformedusingDMF(0.2mLmin1)with0.01MLiBras the mobile phase at 50 C on a TOSOH EcoSEC HLC8320GPC systemequipped with an EcoSEC RI detector and three columns (PSS PFG 5m;Microguard,100,and300)(MWresolvingrange:300to100,000Da)fromPSSGmbH. A conventional calibration method was created using narrow linearpoly(methylmethacrylate)standards.Correctionsforflowratefluctuationsweremadeusingtolueneasaninternalstandard.PSSWinGPCUnitysoftwareversion7.2wasusedtoprocessdata.


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InfraredspectrawererecordedonaPerkinElmerSpectrum2000FTIRequippedwith aMKIIGoldenGateTM, Single ReflectionATR System (from Specac Ltd,London,UK).TheATRcrystalwasaMKIIheatedDiamond45ATRTopPlate.

Fieldemissionscanningelectronmicroscope(FESEM)imageswererecordedonaHitachiS4800FESEM.Thesamplesweremountedonasubstratewithcarbontape and coated 3 s of a carbon coater (Cressington 108carbon/A) andsubsequently2x4nmofagold/palladiumsputtercoater(Cressington208HR).

ACary100UV/VISspectrophotometer(Varian,PaloAlto,CA,USA)wasusedtorecordtheabsorbanceincreaseat412nm.Thecellulosesubstratefromwhichthepolymer hadbeen cleavedoff was immersed in 20 mL of a 3.0 mM DTNB

phosphatebuffersolution,pH7.0,for30min tomakesurethatall thiolsonthesurfacewereaccessible.1mLof thesolutionwascharacterizedemployinga3.0mM solution of DTNB medium as a reference (1 mL). Prior to the DTNBtreatment,eachsubstratewasweighed(seeSampleweightinTable2),inordertoassesstheinitiatorcontent.

FTIRmicroscopy(FTIRM)imagingmeasurementsofthecellulosesampleshavebeenperformedusingaBrukerFTIRmicroscopeHYPERION3000coupledtoaresearchspectrometerVERTEX80.TheHYPERION3000microscopeisequipped

with two typesofdetectors:asingleelementMCTdetector (MercuryCadmiumTelluride) for the conventional mapping approach and a multielement FPAdetector (Focal Plane Array) for imaging. The multielement FPAdetectorconsistsof6464elements.This factallowsforthesimultaneousacquisitionof4096spectracoveringasampleareaof3232 m(forATRdetection).Forpostprocessing,baselinecorrectionandatmospheric compensationwereused.WiththeFPAdetectorincombinationwiththe20GermaniumATRlens,atheoreticallateralresolutionof0.25m2perpixelisachieved.

Xrayphotoelectron spectroscopy (XPS)measurementswereperformedusingaKAlphaXPS spectrometer (ThermoFisher Scientific,EastGrinstead,UK).Dataacquisition and processing using the Thermo Avantage software is describedelsewhere.120Allsampleswereanalyzedusingamicrofocused,monochromatedAlK Xraysource(400mspotsize).TheKAlphachargecompensationsystemwasemployedduringanalysis,usingelectronsof8eVenergy,and lowenergyargonionstopreventanylocalizedchargebuildup.Thespectrawerefittedwithone or more Voigt profiles (BE uncertainty: +0.2eV) and Scofield sensitivityfactorswereapplied forquantification.121Allspectrawerereferenced to theC1s


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Experimental

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peakassumed tooriginate fromsurfacehydrocarboncontaminationat285.0eVbinding energy controlledbymeans of thewellknownphotoelectronpeaks ofmetallicCu,Ag,andAu,respectively.

Contactanglemeasurementswereperformedat50%relativehumidityand23Cand conducted on aKSV instrumentsCAM 200 equippedwith aBaslerA602fcamera,using5 Ldropletsofdeionizedwater.ThewatercontactanglesweredeterminedusingtheCAMsoftware.

3.3INITIATORSYNTHESES

InitiatorssuitableforARGETATRPweredesignedandsynthesizedwiththeaimtoalsoholdcertain functionalities toenableensuing reactions;e.g.,adisulfidecontaining initiator for immobilization onto cellulose, prior to grafting, wasproduced, rendering postcleavage of the polymer grafts from the surfacepossible.Furthermore,by introducingalkyne functionality toanATRP initiator,the freely formed polymer will bear this functionality, and can thereby beemployed in a subsequent conjugation reaction when grafting to a surface.Noteworthy,alldetailsregardingtheexperimentalreactionscanbefoundinthecorrespondingarticles.

3.3.1 Synthesisofdisulfidecontainingsacrificialinitiator

Byreacting2,2dithiodiethanolwith bromoisobutyrylbromide(BiB)for3hatambienttemperature,thedisulfidecontainingsacrificial initiatorOHdiSBrwassynthesized,seeScheme5andpaperII.26

Scheme5.ThesynthesisofthedisulfidecontainingsacrificialinitiatorOHdiSBr.


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Experimental

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3.3.2 Synthesisofdisulfidecontaininginitiatorforimmobilizationontocellulosesubstrates

Thedisulfidecontainingsacrificial initiatorOhdiSBrwasreactedwithsuccinicanhydridefor18hatambienttemperaturetointroduceacarboxylicacidmoietythatsubsequentlywastransformedintoanacidchloride,utilizingoxalylchloridefor3hatambienttemperature,formingCldiSBr,seeScheme6.The1HNMRofCldiSBrispresentedinFigure9andpaperII.26

Scheme6.ThesynthesisofthedisulfidecontaininginitiatorCldiSBrfor immobilizationontocellulose.

3.3.3 Synthesisofsilaneprotectedalkynefunctionalsacrificialinitiator

3(Trimethylsilyl)propargyl alcohol and BiBwere reacted for 18 h at ambienttemperaturetoformthesilaneprotectedalkynefunctionalsacrificialinitiatorSiAlkBr,seeScheme7andpaperIV.115


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Experimental

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Scheme

7.The synthesis of the silaneprotected alkynefunctional sacrificial initiator SiAlkBr.

3.3.4 Synthesisofbrominefunctionalinitiatorforimmobilizationontocellulosesubstrates

Abrominefunctional initiatorwas producedby reacting 2bromoethanol andsuccinic anhydride for 18 h at ambient temperature. The carboxylic acidwas

subsequently reacted with oxalyl chloride for 3 h at ambient temperature,formingClRBr(R=CO(CH2)2COO(CH2)2),seeScheme8andpaperIV.115

Scheme 8. The synthesis of thebrominefunctional initiatorClRBr for immobilizationontocellulose.

3.4IMMOBILIZATIONOFINITIATORSONTOCELLULOSE

Whatmanno.1filterpapersofvarioussizes(2x3cm,1x1cm,or=4cm)werewashedwith ethanol, acetone, and THF and ultrasonicated for 2min in eachsolvent, prior to the immobilization of the initiator. The available hydroxylgroups on the surface were converted into ARGET ATRP initiators by the

reactionwithaninitiatorholdingeitheranacidbromideoracidchloridemoiety:


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Experimental

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BiB,CldiSBr,orClRBr.Thereactionswereallowedtoproceedfor0.25 18hinTHF at ambient temperature on a shaking device. Scheme 9 shows theimmobilization of ClRBr and the posttransformation of the bromines into

azidesemployingNaN3(seealsopaperIV).115Scheme10depictsBiB(asinpaperIand III)102, 114 and CldiSBr (as in paper II and IV)26, 115 immobilized onto thecellulosesubstrate.

Scheme9.ImmobilizationofbrominefunctionalinitiatorClRBrontocelluloseandposttransformationofthebrominesintoazides.

3.5GRAFTINGOFVARIOUSMONOMERSFROMCELLULOSEVIAARGETATRP

ThegeneralpolymerizationsystememployedwhengraftingMMA(inpaperI,II,III,andIV),26,102,114,115styrene(inpaperI),102orGMA(inpaperI)102frominitiatorfunctionalizedcelluloseconsistedofasacrificialinitiator(EBiB,OHdiSBr,orSiAlkBr),PMDETA,Cu(II)Br2,AsAcorNaAsc,andanisole,inaninertatmosphere,

seeScheme10 for furtherdetails.Different ratiosof initiator tomonomerwereutilizedobtaindifferentDPtarget,andtheotheremployedratioswere:[MMA]:[I]:[Cu(II)Br2]:[PMDETA]:[AsAc/NaAsc]=DPtarget:1:0.1:1:1,exceptforPGMAwhere the ratioofAsAcwas0.5.The reactionwasmonitoredby 1HNMRandterminatedwhen thedesired conversionwas reached or after apredeterminedtime. The polymer was precipitated and the grafted cellulose substrate wasthoroughlywashed.


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Experimental

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Scheme 10. The surfaceinitiated ARGET ATRP of MMA, St, or GMA from initiatorfunctionalizedcellulosesubstrate(CelBiBorCeldiSBr)inparalleltothepolymerizationinitiatedfromasacrificialinitiator(EBiB,OHdiSBR,orSiAlkBr).

In addition, grafting ofMMA from BiBfunctionalized filter paper (2 3 cm)without any deoxygenation was also investigated (unpublished results). Thesame ratios as above were employed, except that the amount of AsAc waschanged for some polymerization systems. The reaction took place in either aroundbottomflaskcapsuledwitharubberseptumorinaglassjarwithascrewlid,bothwithandwithouttheadditionofsolvent.ExperimentswiththeadditionofasmallamountofH2O( 0.1mLto25gMMA)tothebulkreactionswerealsoperformed.

3.6CLEAVAGEOFPMMAGRAFTSFROMMODIFIEDCELLULOSE

The PMMA grafts, which had been grafted from the cellulose substratefunctionalized with the disulfidecontaining initiator, were cleaved off byemploying 1,4dithiothreitol (DTT) for five days at ambient temperature, seeScheme11andpaperII.26,115


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Experimental

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Scheme11.CleavageofgraftsfromPMMAmodifiedcellulosesubstrate.

3.7GRAFTINGOFALKYNEFUNCTIONALPMMATOCELLULOSEVIACUAAC

ThePMMApolymerSiAlkPMMAxBr(wherexdenotestheDPcalculatedfrom1HNMR),formedfromthesacrificialinitiatorSiAlkBrinparalleltothegraftingfromcellulose,wasutilizedastheprepolymerforthegraftingtoreaction.Priortothegraftingreaction,thesilaneprotectedPMMAhastobedeprotected.

3.7.1

ActivationofalkynefunctionalPMMAThe silaneprotected PMMA was deprotected by subjecting it totetrabutylammoniumfluoridetrihydrate(TBAF 3H2O)underinertatmosphere,seeScheme12andpaperIV.115


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Experimental

27

Scheme12.DeprotectionofsilaneprotectedPMMA,formingAlkPMMAxBr.

3.7.2 Graftingtocellulose

The graftingto reactionwas conductedby immersing the azidefunctionalizedcellulose substrate into a solution containing the alkynefunctional PMMA,

coppersulfatepentahydrate(Cu(II)SO4 5H2O),NaAsc,asmallamountofH2O,andDMF,employinganinertatmosphere,seeScheme13andpaperIV.115

Scheme13.ThegraftingofalkynefunctionalPMMA toanazidefunctionalizedcellulosesubstrateviaCuAAC.


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4.RESULTSANDDISCUSSION

Celluloseisafascinatingnaturalresourceinmanyperspectives.Bymodifyingtheavailablehydroxylgroupsoncelluloseviagraftingofpolymer, theapplicabilityofcellulosecanbeenhanced.Totrulytailorthesurfacecharacteristics,itiscrucialtoobtainknowledge about thegraftingproperties; furthermore, certain controlover the reactionmechanism isessential.However,due to the inherent surfaceroughness of cellulose substrates, such as filter paper, it is challenging tocharacterizethegraftedsubstratequantitatively.Nevertheless,tobeabletoapplythemodification industrially,simpleandnottooharshreactionconditionsareaprerequisite. Therefore, the development of straightforward surfaceinitiatedARGETATRPgraftingsystems,with lowamountofcoppercombinedwith low

sensitivitytowardsoxygen,canbeofgreatimportance.

4.1GRAFTINGOFVARIOUSMONOMERSFROMCELLULOSEVIAARGETATRP

ARGET ATRPwas employed for surfaceinitiated polymerization of cellulosesubstrates, utilizing the variousmonomers:MMA, St, and GMA. Commonly,finetuning of ATRP systems for different monomers is required for the

polymerizationtosucceed,butonlyminoradjustmentstothereactionconditionsfor these three rather different monomers were performed. The changes inbetween the systems were the reaction temperature as well as the choice ofreducing agent (see Scheme 10). Furthermore, the grafting procedure isperformed straightforwardly by just immersing the initiatorfunctionalizedsubstrateinthereactionmixture,followedbyafewminutesofpurgingwithinertgas. The graft length was controlledby employing a sacrificial initiator, alsoallowing for themonitoring of thekineticsby 1HNMR andSEC.Noteworthy,parallel grafting experiments were conducted and quenched after a


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predeterminedtime,sincethewithdrawalofaliquotsmayaffectthekinetics.Theunbound polymer, freely formed from the sacrificial initiator, was visuallycolorlessafterprecipitation,without the requirement for removalof theCusalt

bypassing thepolymer solution through anAl3O2 column;nevertheless, smallamounts of Cu canbe remaining. Therefore, the polymer was purified fromcopperwhenbeingemployedinsubsequentreactions.

The free polymer formed simultaneously in the solution showed firstorderkinetics with respect to monomer conversion up to at least 20 % monomerconversion for all three systems; e.g., see Figure 4 for the kinetic plot of thePMMA system, targeting two different DPs. For the PS system, a distinctinduction period in thebeginning of the reaction,when the Cu(II) species is

transformedtoCu(I)species,couldbeobserved.ForthePMMAandPGMAsystems this effectwas not as clearly observed,which canbe due to that thestronger reducing agent AsAc was employed instead of NaAsc as for PS.Moreover, thepropagation rate constant (kp) ismuch larger forMMA than forstyrene.39,43

Figure

4.Kineticplot forARGETATRPofPMMA.Experimental conditions:T = 40 C,anisole(50wt%), [MMA]:[EBiB]:[Cu(II)Br2]:[PMDETA]:[AsAc]=400:1:0.1:1:1)and[MMA]:[EBiB]:[Cu(II)Br2]:[PMDETA]:[AsAc]=800:1:0.1:1:1).102

Figure 5 illustrates themolarmass anddispersity of crude samples ofPMMA(DPtarget = 800) as a function ofmonomer conversion.At low conversions, themolarmassesobtainedfromSECarehigherthanthetheoreticalvaluescalculatedfrom1HNMR(seeEq.4),suggestingthatthe initiatorefficiencyisbelow1.Theincomplete initiation and the fact that the endgroupof aPMMA chainhas an


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activation rateconstant (kact) that isapproximatelyeight timeshighercomparedwithkactofEBiB,66,122,123seeScheme2andEq.3,explainswhythemolarmassishigherat lowmonomerconversion.Athighermonomerconversions, themolar

massislowerthanthetheoreticalvalue,whichwepreviouslyexplainedtobedueto that the surfacepropagationcontributes to theconsumptionofmonomers tolargeextent,whenaimingforhigherDPtarget.102However,atthattime,wedidnotknow that on the cellulose substrate employed (filter paper: 2 3 cm)approximately1.1molinitiatorsarepresent26(discussedindetailinsection4.3).In the same system, the DPtarget of 800 with a monomer amount of 10 gcorresponds to125 molofEBiB; thus, the sacrificial initiator is in largeexcesscomparedwiththeinitiatoronthesurface.Therefore,themonomerconsumedbythe cellulose substrate cannot be the only explanation, also suggesting the

occurrenceof transferreactions,suchas intra and intermolecularchaintransferas well as transfer to monomer or solvent. Transfer reaction results in anincreased number of total chains. The chain transfer to polymer results inbranching124, 125,whichdecreases thehydrodynamicvolumeof thepolymer,andtherebyloweringtheobtainedMnsincetheSECseparatesmoleculesaccordingtothehydrodynamicvolume.125

Figure

5.Molarmassanddispersity(PDI)ofPMMAasafunctionofmonomerconversion.The continuous line represents the theoretical values that were calculated from themonomerconversiondeterminedby1HNMR.Experimentalconditions:T=40C,anisole(50wt%),[MMA]:[EBiB]:[Cu(II)Br2]:[PMDETA]:[AcAs]=800:1:0.1:1:1.102

Anotherpossibility canbe that as the reactionproceeds, the initiator efficiencyincreases as reported previously,65, 66 formingmore propagating specieswhichwill lower the molar mass. Matyjaszewski et al. have reported the same


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phenomenonseeninFigure5forMMAwithEBiBasaninitiator,alsosuggesting

thatthereasonistransferreactionsandslowerinitiation.126Aschematicgraphof

how thedifferent featuresaffect themolarmassasa functionof themonomer

conversionispresentedinFigure6.Nevertheless,thesamedeviationofMnfromMtheowasobservedforPSandPGMA,butthedifferenceathigherconversionwas

notaspronouncedas for thePMMA system.102Figure5alsodemonstrates that

the M (named here as PDI) slightly decreases with increasing monomer

conversion, which is contradictive to the presence of chaintransfer reactions

whichwouldincreasethedispersity.125

Figure6.AschematicgraphtoillustratehowMnvariesasafunctionoftheconversion,for

differentreactionfeaturesinthepolymerizationsystem.127

The cellulose substratesgraftedwithPMMA,PS,orPGMAwere characterized

withFTIRtoverifythesuccessfulgrafting.Itcouldbeconcludedthattheamount

of polymer successively increasedwithmonomer conversion.As an example,

Figure 7 depicts the increase in the carbonyl region for the PMMAgrafted

cellulose substrateswith increasing reaction time.However, it is important tokeep in mind that the method is semiquantitative, since the spectra are

normalized against the noise arising from the ATR crystal (23001950 cm1).

Furthermore,whenanalyzingaroughsubstratesuchascellulosefilterpaper,the

absorbance can vary slightly depending onwhich point on the surface that is

beingexamined.Thiseffectcanalsobeduetothatthepolymerlayermaynotbe

completely homogenous.Nevertheless, FTIR is a versatilemethod to prove a

successfulgrafting,but itdoesnotgive the spatialdistributionof thepolymer

graftsonthesurface.


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Figure

7.

FTIR spectraofPMMAgraftedcelluloseatdifferent reaction times (DPtarget=800), showing the increaseof carbonylpeakat1732 cm1aswell as theboundwater in

cellulose(approx.16001700cm1).102

Todeterminethespatialdistributionofthepolymercontentonthesurface,high

resolutionFTIRMwasemployed.Figure8depicts theFTIRMmicrographsof

thegraftedcellulosesubstrateswithincreasinggraftlengthforPMMAx(wherex

denotestheDPcalculatedfrom1HNMR).Thesixupperimagescorrespondtothe

same substrates as in Figure 7, except that the substratemodified for 0.5 h is

excluded. As can be seen, the polymer distribution is relatively uniform,especially as the graft length increases.However, a tendency to form smaller

regionswithhigherpolymerconcentrationcanalsobeobserved.These smaller

inhomogeneitiesmaybe a result of the surface roughness. Still, FTIRMwas

proventobeaveryusefulmethodforthecharacterizationofcellulosesubstrates.


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Figure8.FalsecolorhighresolutionFTIRmicrographs(4cm1spectralresolutionwitha

0.25 m2theoreticalspatialpixelresolutionandanopticalresolutionofcloseto1 m)of

initiatorfunctionalizedcellulose(withBiB)andPMMAxgraftedcellulose(wherexdenotestheDPcalculatedfrom1HNMR),displayingthecarbonylregionbyintegrationoftheC=O

stretchingvibration(between17001760cm1).Regionswithdarkbluecolorrepresentlow

intensityandpinkcolordepictshighintensityregions.114

The hydrophobicity of the cellulose surface was investigated utilizing CA

measurements.TheCAisaffectedbythechemicalcompositionofthesurfaceas

well as the surface roughness.Due to the inherent surface roughness of filter

paper, it is difficult to determine absolute values; hence, the CAs are only

approximations. An applied water droplet is immediately absorbed by anunmodified filterpaperdue to thehydrophiliccharacterof cellulose.Upon the

grafting of ahydrophobic polymer, theCA is significantly increased, even for

cellulosesubstrateswithshorterpolymergrafts;e.g.,aDPofaround50resultsin

CAsof1093 forPMMAgraftedand1373 forPSgraftedcellulose. In fact,

shorterpolymer chainsmay result in ahigherCAdue to a rougher surface,128

since longer polymer grafts tend to smoothen the surface (see Figure 10b in

section4.2),whichisalsocorroboratedbytheFTIRMresultsthatsuggestamore

homogeneous topography for longer polymer grafts. TheCAs of the cellulose

substratesgraftedwithhigherDPs,e.g.,DP=384forPMMAandDP=288forPS,


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wereessentially the sameas for thosewith shortergrafts:1122and1323,respectively.

4.2SELECTIVECLEAVAGEOFPOLYMERGRAFTS

An initiator, employed for immobilization onto cellulose prior to the graftingreaction,wasdesignedwithadisulfidelinker(Scheme6)toallowforcleavageofpolymer grafts, as well as for an estimation of the initiator content. Mildconditions of the cleavage reaction were a prerequisite, in order to avoiddegradationofasensitivesubstratelikecellulose,andalsotobeabletoperformthe subsequent surface analysis. The 1H NMR of the initiator CldiSBr is

presentedinFigure9.Theinitiatorwasimmobilizedfor0.25,0.50,1.0,and15hatambient temperature, prior to the grafting of MMA via ARGET ATRP. Asacrificial initiator (OHdiSBr) was employed to be able to compare thecharacteristics of the grafted polymers with the free polymer. The graftedsubstrates were subjected to DTT to cleave the disulfide bond at ambienttemperature for five days (see Scheme 11). FTIR analyses of the cleavedsubstrates showed thatmost polymer chainswere cleaved off after only 18h,suggesting that the disulfidebonds are readily accessible.However, to obtaincomplete cleavage,which is crucial for the following analysis, longer reaction

timescombinedwithalargeramountofDTTwererequired.

Figure9. 1HNMRspectrumof thedisulfidecontaining initiatorCldiSBremployed forimmobilizationontocelluloseinCDCl3.

Theamountoftheobtainedpolymergraftswasinsufficientforprecipitation,whycrude sampleswere analyzedby SEC. Themolarmasses of the free and thecleaved polymers are in good agreement Table 1 suggesting that thepolymerization occurs at similar rate from the surface as from the sacrificial

initiator,corroboratingthehypothesisthatgraftscanbetailoredbyemployinga


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sacrificial initiator.The freepolymershave slightlyhighermolarmasseswhich

areprobablyduetoremovalof lowmolarmassmoleculesduringprecipitation,

resultinginanoverallhigheraveragemolarmassandlowerM.

Table1.Propertiesofthefreepolymerandthecleavedgrafts,DPtarget=800.

Freepolymer Cleavedgrafts

Samplea Mnb Mb Mnb Mb

PMMA240 20900 1.14 19000c 1.21c

PMMA432 47000 1.18 41200c 1.22c

a PMMAxwhere x represents theDP calculated from the conversion obtained from 1H

NMR.bObtainedfromTHFSEC(PSstandards).cSECofacrudesample.

FESEM was employed to study the surface topography of the modified

substrates. The initiatorfunctionalized cellulose substrate displays the typical

finestructureofanunmodifiedcellulosesubstrate,Figure10a.Onthecontrary,

the PMMAgrafted cellulose substrate in Figure 10b gives rise to a smoother

surface.AftercleavageofthePMMAgrafts,thefinestructureofcellulosecanbe

perceived again (Figure 10c), indicating that the grafts havebeen successfully

removed. The initiatorfunctionalized andDTTtreated cellulose structures are

very similar to theunmodified substrate, establishing thatboth treatments are

essentiallynondestructive.

Figure

10.

FESEM micrographs of cellulose substrate a) initiatorfunctionalized, b)PMMAgrafted,andc)aftercleavage,magnified10000x.26

4.3ASSESSINGINITIATORCONTENT

After the reduction of the disulfidebonds, thiolmoieties are present on the

surface,which can readilybe quantifiedby Ellmans reagent (5,5dithiobis2

nitrobenzoicacid(DTNB)).129132ThereactionbetweenDTNBandathiolgenerates

thechromophore5thio2nitrobenzoicacid(TNB),whichabsorbsstronglyat412


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nm,providingforthepossibilitytoassesstheinitiatorcontentofthesubstrateviaUVanalysis.Themeasuredabsorbancewasutilized to calculate thenumberofmoles of thiols present on the substrate, see Table 2. To evaluate the initiator

content, the number ofmoles of thiolswas normalized to the sampleweight(Table 2), resulting inavalueof approximately21 mol initiatorspergramofcellulose. As a comparison, the initiator content of silica nanoparticles withvarious sizes hasbeen reported tobe in the range of 35 to 260 mol/g,28, 133suggestingthatthevaluesarereasonable.Tofurtherverifytheversatilityofthismethod,microcrystallinecellulose(MCC)withanaveragediameterof20 mwas utilized as an additional substrate. The initiator contentwas found toberoughly twice as high as for filter papers, see Table 2. Comparatively, silicaparticles in the same size asMCC,where theOHgroupswere reactedwith a

monofunctionalsilylchloride to immobilize the initiator,have initiatorcontentsof135mol/g134and260mol/g27.

The initiatordensityof the filterpaperwasestimatedbyemploying thespecificsurface area of cellulose,which is 0.59m2/g according to BETmeasurementsreported in the literature.135 Thus, the number of moles initiator per squarecentimeterofcellulosecanbeassessed,seeTable2.

The results inTable 2alsodemonstrate that the immobilizationof the initiator

onto the surface is a fast reaction when a large surplus of the immobilizedinitiatorwasutilized: 1.33mmol ofCldiSBr to approximately 1 mol reactedhydroxyl groups.After 30min of immobilization, the initiator content on thesurfacewasessentiallythesameasafter15h.

Table2.DataassessedfromUVanalysisofthecleavedcellulosesubstrates.

Samplea Immob.[h]

Abs.(=412nm)

nthiols[mol]

Sampleweight[mg]

Initiatorcontent[mol/g]

Initiatordensityb

[nmol/cm2]

Initiator0.25 0.25 0.6530.003 1.0050.004 53.9 18.60.1 3.4

Initiator0.5 0.50 0.6860.002 1.0550.003 51.2 20.60.1 3.5

Initiator1.0 1.0 0.6810.003 1.0480.005 52.0 20.20.1 3.5

PMMA240 15 0.6850.001 1.0530.001 51.1 20.60.1 3.5

PMMA432 15 0.7010.005 1.0780.008 51.3 21.00.2 3.6

MCC 3.5 0.6610.003 1.0170.004 25.6 39.70.2

aInitiatorywhereyrepresent the immobilization timeof the initiator. bCalculationbased

onthespecificsurfaceareaofcellulosefilterpaper:0.59m2

/g.135


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Todeterminethenumberofavailablehydroxylgrouponfilterpaper,solidstateNMR (CP/MAS 13CNMR)wasemployed,suggesting that filterpaperhas7.1 (0.4)%ofavailableOHgroupswhenit isswelledinwater.136Themolarmassof

theAGUof cellulose (C6H10O5) is162.2g/mol.Thus, 1.0gof cellulose contains18.5mmolofhydroxylgroups,where1.31mmoloftheseareactive,accordingtotheNMRanalysis.Uponimmobilizationofthe initiatorontocellulose,THFwasemployedasasolvent;thus,thecelluloseisnotasswollenasinwater,resultingin feweravailablehydroxylgroups.However, if thehighervalueofactiveOHgroups isutilized, the initiatorcontentofapproximately20 mol/gcorrespondsto1.5%oftheavailablehydroxylgroupshavingreacted.

4.4

COMPARISONBETWEENGRAFTINGFROMANDGRAFTINGTOCELLULOSE

Thegraftingfromand thegraftingtoapproachesbothhave theirprosandcons,but thegeneral comprehension is that the surfaceinitiatedpolymerization, i.e.,thegraftingfromreaction,resultsinahighergraftingdensity.12,13,16However,tothebestofourknowledge,acomparativestudy investigating thishasnotbeenconducteduntilnow.115ThedifferenceingraftingefficiencywhengraftingfromcelluloseviaARGETATRPandgraftingtocelluloseviaCuAACwasevaluatedbyFTIRMandXPS.Thegraftingfromreactionwasperformedinparalleltotheinsitu polymerization from a sacrificial initiator with a protected alkynefunctionality.Owingtothealkynefunctionalinitiator,thefreepolymer,bearingthis functionality, can subsequentlybe utilized in the graftingto reaction viaconjugation to an azidefunctionalized cellulose substrate. Thus, the polymersgraftedfromandtothesurfacewereessentiallyidentical,intermsofmolarmassanddispersity.Thiswasfurthervalidatedbyemployingthedisulfidecontaininginitiator for immobilization onto the cellulose surface, rendering cleavage and

ensuingcharacterizationofthegraftedpolymerspossible.SeeScheme14fortheschematicsyntheticpathway.


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Scheme 14. The schematic synthetic pathway of the grafting strategy,where the freely

formed PMMA in the graftingfrom reactionwas employed as the prepolymer in the

graftingtoreactionafterthedeprotectionoftheprotectedalkynefunctionalinitiator.115

4.4.1 Graftingfromandgraftingtocellulose

The graftingfrom cellulose and the insitu formation of the free polymerwere

performedbyARGETATRPofMMA,targetingthefinalDPof1200(Scheme10).

Fourpolymerizationswereconductedat50Ctodifferentconversions:11,39,51,

and80%,inordertoobtaindifferentgraftlengths.Thesilaneprotectedalkyne

functionalinitiatorSiAlkBr,seeScheme7,wasutilizedasasacrificialinitiator.

ThefreepolymerSiAlkPMMAxBr(wherexdenotestheDPcalculatedfrom1H

NMR)wasprecipitatedandcharacterizedwithSECtoverifythatwellcontrolled

reactionswereobtained, seeTable3.The slightlyhigherdispersity (M) forSi

AlkPMMA960Brcanbeexplainedbythehighconversion,whichprobablyledto

ahigherdegreeof termination, compared to theother reactions; still, theM is

below1.3.


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Table3.Characteristicsof the freepolymersand thecleavedpolymergraftsobtained inthegraftingfromreactions.

Freepolymers Cleavedpolymergrafts

Namea pb

(%)Mtheoc

(g/mol)Mnd

(g/mol)Md Namea M

nd

(g/mol)Md

SiAlkPMMA132Br 11 13100 21000 1.07 SHPMMA132Br 23200 1.08




a PMMAxwhere x represents theDP calculated from the conversion obtained from 1HNMR.bConversioncalculatedfrom1HNMR.c Mtheo=DPtargetpMMMA+MSiAlkBrbased

on1HNMRconversion.dObtainedfromTHFSEC(PMMAstandards).

ThePMMAgrafts(SHPMMAxBr)werecleavedoffthesurface(Scheme11),andsubsequently precipitated and isolated by centrifugation, due to very smallobtainedamounts(afewmg).ThedriedpolymerwascharacterizedbySEC,andthe results compared to those of the free polymer formed from the sacrificialinitiator,seeTable3.Ascanbeseen,themolarmassesandthedispersitieswereingoodagreement,whichalsohasbeensuggestedbeothers,18,2628proposingthatthegraftlengthcanbetailoredbytheutilizationofasacrificialinitiator.

The silaneprotecting group of the free polymers (SiAlkPMMAxBr) wasremoved utilizing TBAF, resulting in the alkynefunctional polymers AlkPMMAxBr(Scheme12).ThepolymerswereprecipitatedandcharacterizedwithSEC,seeTable4.Themolarmassesforthesepreformedgraftsforthegraftingtoreactionwere in the same range as the cleaved grafts from the graftingfromsurfaces, presented in Table 3, which are of utmost importance to obtain asystematiccomparisonofthegraftingtechniques.

ToconjugatethesepolymersontocellulosesubstratesviaCuAAC,theinitiatorClRBr was first immobilized onto cellulose prior to the transformation of thebrominestoazides,formingazidefunctionalizedcellulose:CelAz,seeScheme9.The same conditions were employed as when immobilizing CldiSBr to thesurface,most likely resulting in the same amount of initiator on the cellulosesubstrate.ThegraftingtoreactionwasperformedbyutilizingCu(II)SO4 5H2OandNaAscinDMF(Scheme13);furthermore,asmallamountofH2Owasaddedto the solution to facilitate the dissolution of the catalyst. The polymerconcentrationwaskeptconstant(1.5mM)toobtainthesamenumberofmolesof


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alkyneendgroupsforthefourdifferentpolymerlengths(Table4).Theresultingcellulose substrates CelAzPMMAxBr were thoroughly rinsed andultrasonicated to remove any polymer residues that were not covalently

conjugatedtothesubstrate.

Table4.Thepolymer characteristics afterdeprotectionof the silaneprotectinggroupofthefreepolymers.

Prepolymer

NameaMnb

(g/mol)Mb

AlkPMMA132Br 18500 1.07

AlkPMMA468Br 51800 1.07

AlkPMMA612Br 66700 1.06

AlkPMMA960Br 89400 1.26

a PMMAxwhere x represents theDP calculated from the conversion obtained from 1HNMR.bObtainedfromTHFSEC(PMMAstandards).

4.4.2 XPSanalysisofmodifiedcellulosesubstrates

ThemodifiedcellulosesubstrateswereanalyzedbyXPStoconfirmthesuccessfulreactions. InFigure11A, theC1sspectraofpureWhatmanno.1 filterpaper isdepicted,showingmainlythethreecharacteristicspeaks:theCHandCCbondsat285.0eV, theCObondofalcohol (COH)andether (COC)groupsat286.8eV,and theOCObondofacetals (and/orC=O)bondsat288.3eV.However,afourthpeak at 289.5 eV can oftenbedetected, corresponding to oxidation andimpurities of cellulose.112 Upon immobilization of the initiator CldiSBr theatomicpercentages ofCC andCHbonds increase, and thepeak at 289.4 eV,corresponding to the esterbonds137 (OC=O) that havebeen introduced to the

substrate, isalsoenhanced,seeFigure11B.Additionally, thesuccessfulgraftingofPMMAfromthecellulosesurfacescanalsobeconfirmed,as thecharacteristicpolymer peaks at 285.0 (CC and CH) and 289.4 (OC=O) eV significantlyincrease,whichisseeninFigure11Cand11DfortheCeldiSPMMA132BrandforCeldiSPMMA960Br,respectively.Comparatively,uponcleavageofthepolymergraft, the characteristic polymer peaks at 285.0 and 289.3 eV have clearlydecreased,confirmingthedetachmentofthegrafts,seeFigure11E.


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Figure 11. C 1s XPS spectra for the cellulose substratesmodified via the graftingfrom

approach: (A) Pure cellulose, (B) CeldiSBr, (C) CeldiSPMMA132Br, (D) CeldiS

PMMA132Br,and(E)CelSH132.Thespectrawerenormalizedtothepeakwiththehighest

intensity.115

XPSwasalsoperformedon thecellulose substratesmodifiedvia thegraftingto

approach.TheC1sspectrainFigure12showthatthepeakcorrespondingtoO

C=O is enhanced upon immobilization of the initiator ClRBr and the post

transformationofthebrominesintoazides(Figure12B),asfortheimmobilization

of the initiatorCldiSBr for thegraftingfromreaction (Figure11B).Figures12C

and12Ddepict the spectraafter theconjugation reactio

ARGET ATRP Una Herramienta Para La Modificacion de La Celulosa Tesis(1)

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