FUNDAMENTALS OFSecond EditionRevised and ExpandedAnilKumarIndianInstitute of TechnologyKanpur,IndiaRakesh K. G uptaWestVirginiaUniversityMorgantown,WestVirginia,U.S.A.MAR CELM A R C E LD E K K E R , I N C . N E W Y O R K B A S E LCopyright 2003 Marcel Dekker, Inc.LibraryofCongressCataloging-in-PublicationDataAcatalogrecordforthisbookisavailablefromtheLibraryofCongress.ISBN:0-8247-0867-9ThersteditionwaspublishedasFundamentalsofPolymersbyMcGraw-Hill, 1997.Thisbookisprintedonacid-freepaper.HeadquartersMarcelDekker, Inc.270MadisonAvenue,NewYork, NY10016tel:212-696-9000;fax:212-685-4540EasternHemisphereDistributionMarcelDekkerAGHutgasse4, Postfach812,CH-4001Basel, Switzerlandtel:41-61-260-6300;fax:41-61-260-6333WorldWideWebhttp:==www.dekker.comThepublisheroffersdiscountsonthisbookwhen orderedinbulkquantities.Formoreinformation, write to Special Sales=ProfessionalMarketing at the headquarters addressabove.Copyright # 2003byMarcelDekker, Inc. AllRightsReserved.Neitherthis book nor any part may be reproduced or transmitted in any form or by anymeans, electronic or mechanical, including photocopying, microlming, and recording,or by any information storage and retrieval system, without permission in writing fromthepublisher.Currentprinting(lastdigit):10987654321PRINTEDINTHEUNITEDSTATESOF AMERICACopyright 2003 Marcel Dekker, Inc.PLASTICS ENGINEER INGFounding EditorDonald E. HudginProfessorClemson UniversityClemson, S outh Carolina1. Plastics Wa s t e R e cove ry ofEconomicVa lu e , Jacob Letdner2Polye s t e rMoldingComp ou nds ,Robert Burns3Ca rbonBlack-PolymerComp os it e s Th e Ph ys ics ofEle ct rica llyCondu ctingComp os it e s ,edited by Enid KeilSichel4Th e Strength a ndSt iffne s s ofPolyme rs , edited byAnagnostisZachanadesand RogerSPorter5Se le ctingTh e rmop la s t ics forEngineeringAp p lica t ions ,CharlesPMac-Dermott6Engineeringwit h R igidPVC Proce s s a bih t ya nd Ap p lica t ions , edited by ILuisGomez7Comp u t e r-Aide dDe s ignofPolyme rs a ndComp os it e s , DHKaelble8Engine e ringTh e rmop la s t ics Prop e rt ie s a ndAp p lica t ions ,editedbyJamesMMargolis9St ru ct u ra lFoa mAPu rchasinga ndDesignG u ide,Bruce CWendle10Pla s t ics inArch it e ct u re A G u ide toAcrylica ndPolyca rbona t e ,RalphMontella11Metal-FilledPolymers Properties andApplications,editedbySwapanKBhattacharya12Plastics Te ch nologyHa ndbook , Manas Chanda andSalil KRoy13R e a ctionInjectionMoldingMachinerya ndProce s s e s , FMelvin Sweeney14Pra ctica l Th e rmoformingPrincip le s a nd Ap p lica t ions , John Flonan15Injectiona ndComp re s s ionMoldingFu nda me nt a ls ,editedbyAvraamIIsayev16PolymerMixinga ndExtru sionTe ch nology, Nicholas PCheremismoff17High Modu lu s Polyme rs Ap p roa ch e s t oDe s igna ndDe ve lop me nt , edited byAnagnostis EZachanades and Roger SPorter18Corros ion-R e s is t a nt PlasticComp os it e s inChemicalPla nt De s ign,JohnHMallinson19Handbook ofEla s t ome rs Ne wDevelop ments a ndTe ch nology,edited by AnilKBhowmick and Howard LStephens20R u bbe rComp ou ndingPrincip les,Ma te ria ls,a ndTechniqu es,FredWBarlow21Th e rmop la s t icPolyme rAddit ive s Theorya ndPra ct ice ,editedbyJohnTLutz, Jr22Emu lsionPolymerTe ch nology,Robert DAthey, Jr23MixinginPolymerProce s s ing, edited by Chns Rauwendaal24Ha ndbook ofPolymerSynt h e s is ,Parts A a ndB,editedbyHansRKncheldorfCopyright 2003 Marcel Dekker, Inc.25. ComputationalModelingofPolymers, edited by JozefBicerano26. Plastics TechnologyHandbook:SecondEdition,RevisedandExpanded,Manas Chanda andSalil K. Roy27.PredictionofPolymerProp erties, JozefBicerano28. FerroelectricPolymers:Chemistry,Physics,andAp p lications,editedbyHariSingh Nalwa29. Degradable Polymers,Recycling,andPlastics Waste Management,editedby Ann-ChristineAlbertsson and Samuel J. Huang30. Polymer Toughening, edited by Charles B. Arends31. Handbook ofAppliedPolymerProcessingTechnology,edited by Nicholas P.Cheremisinoffand Paul N.Cheremisinoff32. DiffusioninPolymers, edited by P. Neogi33. Polymer Devolatilization,edited by Ramon J. Albalak34. AnionicPolymerization:Principles andPractical Ap p lications,Henry L. Hsiehand Roderic P. Quirk35. CationicPolymerizations:Me ch a nisms,Synt h e s is ,and Ap p lications,editedby KrzysztofMatyjaszewski36. Polyimides:Fundamentals a ndApplications,editedbyMalayK.Ghosh andK. L. Mittal37. Thermop lasticMelt Rheologya ndProcessing, A.V. Shenoy and D. R. Saini38. PredictionofPolymerProp erties:SecondEdition,RevisedandExpanded,JozefBicerano39. PracticalThermoforming:Principles andAp p lications,SecondEdition,R e vise da ndExpanded, JohnFlorian40. MacromolecularDesignofPolymericMaterials,editedbyKoichiHatada,Tatsuki Kitayama,andOttoVogl41. Handbook of Thermop lastics, edited by Olagoke Olabisi42. SelectingThermop lastics forEngineeringAp p lications:SecondEdition,Reviseda ndExp anded, Charles P. MacDermott and AroonV. Shenoy43. MetallizedPlastics:Fundamentals a nd Ap p lications, edited by K. LMittal44. OligomerTechnologya ndAp p lications,ConstantinV. Uglea45. ElectricalandOpticalPolymerSystems:Fu ndamentals,Methods,andAp p lications,editedbyDonaldL.Wise,GaryE.Wnek,DebraJ.Trantolo,ThomasM. Cooper, and Joseph D. Gresser46. Stru ctu re andProp erties ofMu ltip hase PolymericMaterials,editedbyTakeoAraki,Qui Tran-Cong, and MitsuhiroShibayama47. Plastics TechnologyHandbook:ThirdEdition,R evisedandExp anded,ManasChanda andSalil K. Roy48. Handbook ofRadicalVinylPolymerization,MunmayaK.MishraandYusufYagci49. PhotonicPolymerSystems:Fu ndamentals,Methods,andAp p lications,editedbyDonaldLWise,GaryE.Wnek,DebraJ.Trantolo,ThomasM.Cooper, and JosephD. Gresser50. HandbookofPolymer Testing:PhysicalMethods, edited by RogerBrown51. Handbook ofPolyp rop yle ne a ndPolyp rop ylene Comp osites, editedby Har-utunG.Karian52. PolymerBlends a ndAlloys , editedbyGabrielO. ShonaikeandGeorgeP.Simon53. Stara ndHyp erbranchedPolymers, editedby MunmayaK. MishraandShi-roKobayashi54. PracticalExtru sionBlowMolding,editedby SamuelL.BelcherCopyright 2003 Marcel Dekker, Inc.55Polyme rVis coe la s t icit yStre ss a ndSt ra ininPractice,EvaristoRiande,RicardoDiaz-Calleja,MargaritaGProlongo,RosaMMasegosa, and Cat-almaSalom56Ha ndbook ofPolyca rbona t e Scie nce a ndTe ch nology,edited by Donald GLeGrand and JohnTBendler57Ha ndbook ofPolye th yle ne Stru ctu res,Prop e rt ie s ,a ndAp p lica t ions , AndrewJPeacock58PolymerandComp os it e R h e ologySecondEdition,R e vise dandExp a nde d,Rakesh KGupta59Ha ndbook ofPolyole fms Se condEditionR e vis e da ndExp a nde d,editedbyCorneliaVasile60Polyme rModifica t ionPrincip le s ,Te ch niqu e s ,a ndAp p lica t ions ,editedbyJohn JMeister61Ha ndbook ofEla s t ome rs SecondEdit ion,R e vis e da ndExp a nde d,editedby Anil KBhowmickand Howard LStephens62PolymerModifiers a ndAddit ive s ,editedby JohnTLutz, Jr,and Richard FGrossman63Pra ct ica lInje ct ionMolding,Bernie AOlmstea and Martin EDavis64Th e rmos e t t ingPolyme rs ,Jean-PierrePascault,HenrySautereau,JacquesVerdu,andRoberto JJWilliams65PredictionofPolymerProp e rt ie s ThirdEdit ion,R e vis e da ndExp a nde d, JozefBicerano66Fu ndamentals ofPolymerEngineeringSecondEdition,R e vis e dandExp a nde d, Anil Kumarand RakeshKGuptaA d d itio n a l Vo l u m e s in Pr e p a r a t i o nHa ndbook ofPla s t ics Ana lys is , edited by Hubert Lobo and JoseBonillaMe t a lloce ne Ca t a lys t s inPla s t icsTe ch nology, AnandKumar KulshreshthaCopyright 2003 Marcel Dekker, Inc.Tothememoryofmyfather.AnilKumarTothememoryofmyfather.RakeshGuptaCopyright 2003 Marcel Dekker, Inc.Preface to the Second EditionThe objectives and organization of the second edition remain essentiallyunchanged. The major difference fromthe rst editionis the inclusionofnewmaterial on topics such as dendrimers, polymer recycling, Hansensolubilityparameters, nanocomposites, creepinglassypolymers, andtwin-screwextrusion. Newexampleshavebeenintroducedthroughout thebook,additional problemsappearat theendofeachchapter, andreferencestotheliterature have been updated. Additional text and gures have also been added.Therst editionhasbeensuccessfullyusedinuniversitiesaroundtheworld, and we have received many encouraging comments. We hope thesecond edition will also nd favor with our colleagues, and be useful to futuregenerationsofstudentsofpolymerscienceandengineering.Anil KumarRakeshK. GuptavCopyright 2003 Marcel Dekker, Inc.Preface to the First EditionSyntheticpolymershaveconsiderablecommercialimportanceandareknownby several common names, such as plastics, macromolecules, and resins.Thesematerialshavebecomesuchanintegralpartofourdailyexistencethatanintroductorypolymer courseis nowincludedinthecurriculumof moststudentsof scienceandengineering. Wehavewrittenthisbookasthemaintextforanintroductorycourseonpolymersforadvancedundergraduatesandgraduate students. The intent is to provide a systematic coverage of theessentialsofpolymers.Afteranintroductiontopolymersasmaterialsinthersttwochapters,themechanismsofpolymerizationandtheireffectontheengineeringdesignof reactors areelucidated. Thesucceedingchapters consider polymer char-acterization, polymer thermodynamics, and the behavior of polymers asmelts, solutions, and solids both above and below the glass transitiontemperature. Also examined are crystallization, diffusion of and throughpolymers, andpolymer processing. Eachchapter can, for themost part, beviiCopyright 2003 Marcel Dekker, Inc.readindependently of the others, and this should allow an instructorto designthe course to his or her own liking. Note that the problems given at the end ofeach chapter alsoserve to complement the main text. Some of these problemscite references to the literature where alternative viewpoints are introduced. Wehave been teaching polymer science for a long time, and we have changed thecoursecontent fromyeartoyearbyadoptingandexpandingonideasofthekindembodiedintheseproblems.Sincepolymerscienceisanextremelyvastarea,thedecisiontoincludeorexcludeagivensubject matterinthetexthasbeenadifcult one. Inthisendeavor,althoughourownbiaseswillshowinplaces,wehavebeenguidedbyhowindispensableaparticulartopicistoproperunderstanding. Wehaveattemptedtokeepthetreatment simplewithout losingtheessential features;for depth of coverage, the reader is referred to the pertinent technical literature.Keepingthestudent inmind, wehaveprovidedintermediatesteps inmostderivations. For the instructor, lecturing becomes easy since all that iscontainedinthebookcanbeput ontheboard. Thefuturewill tell towhatextentwehavesucceededinourchosenobjectives.We have benetedfrom thecommentsof several friendsandcolleagueswhoreaddifferent partsofthebookindraft form. Ourspecial thanksgotoAshokKhanna, Raj Chhabra, DeepakDoraiswamy, Hota V. S. GangaRao,DaveKofke, MikeRyan, andJoeShaeiwitz. ProfessorKhannahasusedtheproblemsetsoftherstsevenchaptersinhisclassforseveralyears.After nishing my Ph.D. fromCarnegie-Mellon University, I (AnilKumar) joined the Department of Chemical Engineering at the Indian InstituteofTechnology,Kanpur,India, in1972. Myexperienceatthisplacehasbeenrichandcomplete, andI decidedtostayherefor therest of mylife. I amfortunatetohaveagoodsetofstudentsfromyeartoyearwithwhomIhavebeenabletoexperiment inteachingvarious facets of polymer scienceandmodifyportionsofthisbookcontinuously.Rakesh Gupta would liketo thankProfessorSantoshGuptaforintrodu-cingpolymer science tohimwhenhewas anundergraduate student. Thisinterest inpolymers was nurturedbyProfessor Art Metzner andDr. K. F.Wissbrun, who were his Ph.D. thesis advisors. Rakesh learned even more fromthemanygraduatestudentswhochosetoworkwithhim,andtheircontribu-tionstothisbookareobvious.KurtWissbrunreviewedtheentiremanuscriptandprovidedinvaluablehelpandencouragement duringthenal phasesofwriting. Progressonthebookwasalsoaidedbytheenthusiasticsupport ofGeneCilento, theDepartmentChairmanatWestVirginiaUniversity.Rakeshadds that these efforts would have come to nought without the determined helpofhiswife,Gunjan,who guardedhissparetimeandallowedhimtodevoteitviii Preface to the First EditionCopyright 2003 Marcel Dekker, Inc.entirelytothisproject. AccordingtoRakesh, ShebelievedmewhenItoldheritwouldtaketwoyears;sevenyearslatershestillbelievesme!I doubt that this bookwouldever have beencompletedwithout theconstantsupportofmywife, Renu. Duringthistimetherehavebeenseveralanxiousmoments, primarilybecauseourchildren, ChetnaandPushkar, weretrying to choose their careersand settle down. In taking care of them, my rolewasmerelyhelpingher, andsheallowedmetodividemyattentionbetweenhomeandwork. Thankyou,Renu.AnilKumarRakeshGuptaPreface to the First Edition ixCopyright 2003 Marcel Dekker, Inc.ContentsPrefacetotheSecondEdition vPrefacetotheFirst Edition vii1. Introduction 11.1 DeningPolymers 11.2 ClassicationofPolymersandSomeFundamentalConcepts 41.3 ChemicalClassicationofPolymersBasedonPolymerizationMechanisms 161.4 Molecular-WeightDistributions 191.5 CongurationsandCrystallinityofPolymericMaterials 221.6 ConformationofPolymerMolecules 271.7 PolymericSupportsinOrganicSynthesis 291.8 Conclusion 38xiCopyright 2003 Marcel Dekker, Inc.References 39Problems 392. EffectofChemical StructureonPolymerProperties 452.1 Introduction 452.2 Effect ofTemperatureonPolymers 452.3 AdditivesforPlastics 502.4 Rubbers 612.5 CellulosePlastics 662.6 CopolymersandBlends 682.7 Cross-LinkingReactions 722.8 Ion-ExchangeResins 802.9 Conclusion 89References 90Problems 913. Step-GrowthPolymerization 1033.1 Introduction 1033.2 EstericationofHomologousSeriesandtheEqualReactivityHypothesis 1053.3 KineticsofARBPolymerizationUsingEqualReactivityHypothesis 1073.4 AverageMolecularWeightinStep-GrowthPolymerizationofARBMonomers 1113.5 EquilibriumStep-GrowthPolymerization 1163.6 Molecular-WeightDistributioninStep-GrowthPolymerization 1183.7 ExperimentalResults 1253.8 Conclusion 140Appendix3.1:TheSolutionofMWDThroughtheGeneratingFunctionTechniqueinStep-GrowthPolymerization 140References 143Problems 1454. ReactionEngineeringofStep-GrowthPolymerization 1534.1 Introduction 153xii ContentsCopyright 2003 Marcel Dekker, Inc.4.2 AnalysisofSemibatchReactors 1564.3 MWDofARBPolymerizationinHomogeneousContinuous-FlowStirred-TankReactors 1664.4 AdvancedStageofPolymerization 1694.5 Conclusion 174Appendix4.1:SimilaritySolutionofStep-GrowthPolymerizationinFilmswithFiniteMassTransfer 175References 181Problems 1815. Chain-GrowthPolymerization 1885.1 Introduction 1885.2 RadicalPolymerization 1925.3 KineticModelofRadicalPolymerization 1975.4 AverageMolecularWeightinRadicalPolymerization 1995.5 VericationoftheKineticModelandtheGelEffectinRadicalPolymerization 2015.6 EquilibriumofRadicalPolymerization 2105.7 TemperatureEffectsinRadicalPolymerization 2155.8 IonicPolymerization 2165.9 AnionicPolymerization 2225.10 Ziegler-NattaCatalystsinStereoregularPolymerization 2265.11 KineticMechanisminHeterogeneousStereoregularPolymerization 2305.12 StereoregulationbyZiegler-NattaCatalyst 2325.13 RatesofZiegler-NattaPolymerization 2335.14 AverageChainLengthofthePolymerinStereoregularPolymerization 2385.15 DiffusionalEffectinZiegler-NattaPolymerization 2405.16 NewerMetalloceneCatalystsforOlenPolymerization 2425.17 Conclusion 244References 244Problems 2486. ReactionEngineeringofChain-GrowthPolymerization 2556.1 Introduction 2556.2 DesignofTubularReactors 2566.3 Copolymerization 273Contents xiiiCopyright 2003 Marcel Dekker, Inc.6.4 RecyclingandDegradationofPolymers 2856.5 Conclusion 287Appendix6.1:SolutionofEquationsDescribingIsothermalRadicalPolymerization 287References 293Problems 2947. EmulsionPolymerization 2997.1 Introduction 2997.2 AqueousEmulsierSolutions 3007.3 SmithandEwartTheoryforStateIIofEmulsionPolymerization 3047.4 EstimationoftheTotalNumberofParticles, Nt3137.5 MonomerConcentrationinPolymerParticles, [M] 3157.6 DeterminationofMolecularWeightinEmulsionPolymerization 3197.7 EmulsionPolymerizationinHomogeneousContinuous-FlowStirred-TankReactors 3247.8 Time-DependentEmulsionPolymerization 3267.9 Conclusions 334References 335Problems 3368. MeasurementofMolecularWeightandItsDistribution 3408.1 Introduction 3408.2 End-GroupAnalysis 3428.3 ColligativeProperties 3438.4 LightScattering 3508.5 Ultracentrifugation 3548.6 IntrinsicViscosity 3588.7 GelPermeationChromatography 3648.8 Conclusion 369References 369Problems 3719. ThermodynamicsofPolymerMixtures 3749.1 Introduction 374xiv ContentsCopyright 2003 Marcel Dekker, Inc.9.2 CriteriaforPolymerSolubility 3769.3 TheFlory-HugginsTheory 3799.4 Free-VolumeTheories 3969.5 TheSolubilityParameter 3989.6 PolymerBlends 4019.7 Conclusion 403References 403Problems 40510. TheoryofRubberElasticity 40710.1 Introduction 40710.2 ProbabilityDistributionfortheFreelyJointedChain 40810.3 ElasticForceBetweenChainEnds 41510.4 Stress-StrainBehavior 41810.5 TheStressTensor(Matrix) 42010.6 MeasuresofFiniteStrain 42310.7 TheStressConstitutiveEquation 42710.8 VulcanizationofRubberandSwellingEquilibrium 42910.9 Conclusion 432References 433Problems 43411. PolymerCrystallization 43711.1 Introduction 43711.2 EnergeticsofPhaseChange 44311.3 OverallCrystallizationRate 44711.4 EmpiricalRateExpressions:TheAvramiEquation 45011.5 PolymerCrystallizationinBlendsandComposites 45611.6 MeltingofCrystals 45911.7 InuenceofPolymerChainExtensionandOrientation 46211.8 PolymerswithLiquid-CrystallineOrder 46411.9 StructureDetermination 46711.10 WorkingwithSemicrystallinePolymers 47911.11 Conclusion 480References 481Problems 484Contents xvCopyright 2003 Marcel Dekker, Inc.12. MechanicalProperties 48712.1 Introduction 48712.2 Stress-StrainBehavior 48812.3 TheGlassTransitionTemperature 49712.4 DynamicMechanicalExperiments 50112.5 Time-TemperatureSuperposition 50412.6 PolymerFracture 50812.7 CrazingandShearYielding 51112.8 FatigueFailure 51612.9 ImprovingMechanicalProperties 518References 520Problems 52313. PolymerDiffusion 52613.1 Introduction 52613.2 FundamentalsofMassTransfer 52713.3 DiffusionCoefcientMeasurement 53113.4 DiffusivityofSpheresatInniteDilution 54213.5 DiffusionCoefcientforNon-ThetaSolutions 54613.6 Free-VolumeTheoryofDiffusioninRubberyPolymers 54713.7 GasDiffusioninGlassyPolymers 55213.8 OrganicVaporDiffusioninGlassyPolymers:CaseIIDiffusion 55713.9 Polymer-PolymerDiffusion 56013.10 Conclusion 564References 565Problems 56914. FlowBehaviorofPolymericFluids 57314.1 Introduction 57314.2 ViscometricFlows 57614.3 Cone-and-PlateViscometer 57814.4 TheCapillaryViscometer 58414.5 ExtensionalViscometers 58914.6 BoltzmannSuperpositionPrinciple 59214.7 DynamicMechanicalProperties 59514.8 TheoriesofShearViscosity 598xvi ContentsCopyright 2003 Marcel Dekker, Inc.14.9 ConstitutiveBehaviorofDilutePolymerSolutions 60514.10 ConstitutiveBehaviorofConcentratedSolutionsandMelts 61514.11 Conclusion 622References 622Problems 62615. PolymerProcessing 63015.1 Introduction 63015.2 Extrusion 63115.3 InjectionMolding 65115.4 FiberSpinning 66715.5 Conclusion 680References 680Problems 684Contents xviiCopyright 2003 Marcel Dekker, Inc.1Introduction1.1 DEFINING POLYMERSPolymers arematerials of veryhighmolecular weight that arefoundtohavemultifariousapplicationsinourmodernsociety. Theyusuallyconsist ofseveralstructural unitsboundtogetherbycovalent bonds[1,2]. Forexample, polyethy-leneisalong-chainpolymerandisrepresentedbyCH2CH2CH2 or CH2CH2n1:1:1wherethestructural(orrepeat)unitis CH2CH2andnrepresentsthechainlengthofthepolymer.Polymers areobtainedthroughthechemical reactionof small molecularcompounds called monomers. For example, polyethylene in Eq. (1.1.1) is formedfromthemonomerethylene. Inorder toformpolymers, monomerseither havereactive functional groups or double (or triple) bonds whose reaction provides thenecessarylinkagesbetweenrepeat units. Polymericmaterialsusuallyhavehighstrength, possess a glass transition temperature, exhibit rubber elasticity, and havehighviscosityasmeltsandsolutions.In fact, exploitation of many of these unique properties has made polymersextremely useful to mankind. They are used extensively in food packaging,clothing, homefurnishing, transportation, medicaldevices,informationtechnol-ogy,andsoforth.Naturalberssuchassilk,wool,andcottonarepolymersand1Copyright 2003 Marcel Dekker, Inc.TABLE 1.1 SomeCommonPolymersCommoditythermoplasticsPolyethylenePolystyrenePolypropylenePolyvinylchloridePolymersinelectronicapplicationsPolyacetylenePoly(p-phenylenevinylene)PolythiophenePolyphenylenesuldePolyanilinesBiomedicalapplicationsPolycarbonate(diphenylcarbonate)PolymethylmethacrylateSiliconepolymers2 Chapter 1Copyright 2003 Marcel Dekker, Inc.have been used for thousands of years. Within this century, theyhave beensupplementedand,insomeinstances,replacedbysyntheticberssuchasrayon,nylon, and acrylics. Indeed, rayon itselfis a modication of a naturally occurringpolymer, cellulose, which in other modied forms have served for years ascommercial plastics and lms. Synthetic polymers (some common ones are listedinTable1.1)suchaspolyolens, polyesters, acrylics, nylons, andepoxyresinsnd extensive applications as plastics, lms, adhesives, and protective coatings. Itmaybeaddedthat biological materialssuchasproteins, deoxyribonucleicacid(DNA), andmucopolysaccharidesarealsopolymers. Polymersareworthstudy-ingbecausetheirbehaviorasmaterialsisdifferentfromthatofmetalsandotherlow-molecular-weight materials. Asaresult, alargepercentageofchemistsandengineersareengagedinworkinvolvingpolymers, whichnecessitatesaformalcourseinpolymerscience.Biomaterials[3]aredenedasmaterialsusedwithinhumanbodieseitheras articial organs, bone cements, dental cements, ligaments, pacemakers, orcontact lenses. Thehumanbodyconsistsofbiological tissues(e.g., blood, cell,proteins, etc.)andtheyhavetheabilitytoreject materialswhichareincompa-tibleeitherwiththebloodorwiththetissues.Forsuchapplications,polymericmaterials, whicharederivedfromanimalsor plants, arenatural candidatesandsome of these are cellulosics, chitin (or chitosan), dextran, agarose, and collagen.Amongsynthetic materials, polysiloxane, polyurethane, polymethyl methacry-SpecialtypolymersPolyvinylidenechloridePolyindenePolyvinylpyrrolidoneCoumaronepolymerIntroduction 3Copyright 2003 Marcel Dekker, Inc.late, polyacrylamide, polyester, and polyethylene oxides are commonly employedbecausetheyareinert withinthebody. Sometimes, duetotherequirementsofmechanical strength, selectivepermeation, adhesion, and=or degradation, evennoncompatiblepolymericmaterials havebeenput touse, but before theyareutilized, theyare surface modiedbybiological molecules (suchas, heparin,biological receptors, enzymes, andsoforth). Some of these concepts will bedevelopedinthisandsubsequentchapters.This chapter will mainlyfocus ontheclassicationof polymers; subse-quent chaptersdeal withengineeringproblemsof manufacturing, characteriza-tion, andthebehaviorofpolymersolutions, melts, andsolids.1.2 CLASSIFICATION OF POLYMERS AND SOMEFUNDAMENTAL CONCEPTSOne of the oldest ways of classifying polymers is based on their response to heat.In this system, there are two types of polymers: thermoplastics and thermosets. Inthe former, polymers melt on heating and solidify on cooling. The heating andcooling cycles can be applied several times without affecting the properties.Thermosetpolymers,ontheotherhand,meltonlythersttimetheyareheated.Duringtheinitialheating,thepolymeriscured;thereafter,itdoesnotmeltonreheating, butdegrades.A more important classication of polymers is based on molecularstructure. Accordingtothissystem, thepolymercouldbeoneofthefollowing:1. Linear-chainpolymer2. Branched-chainpolymer3. NetworkorgelpolymerIt hasalreadybeenobservedthat, inorder toformpolymers, monomersmusthave reactive functionalgroups,ordoubleortriplebonds.Thefunctionality ofagivenmonomerisdenedtobethenumberofthesefunctional groups; doublebonds are regarded as equivalent to a functionality of 2, whereas a triple bond hasafunctionalityof4. Inordertoformapolymer, themonomermust beat leastbifunctional; whenit isbifunctional, thepolymerchainsarealwayslinear. It ispointed out that all thermoplastic polymers are essentially linear molecules,whichcanbeunderstoodasfollows.Inlinearchains, therepeat unitsareheldbystrongcovalent bonds, whiledifferentmoleculesareheldtogetherbyweakersecondaryforces.Whenthermalenergy is supplied to the polymer, it increases the randommotion of themolecules, whichtriestoovercomethesecondaryforces. Whenall forces areovercome, themoleculesbecomefreetomovearoundandthepolymer melts,whichexplainsthethermoplasticnatureofpolymers.4 Chapter 1Copyright 2003 Marcel Dekker, Inc.Branched polymers contain molecules having a linear backbone withbranches emanatingrandomlyfromit. Inorder toformthis class of material,themonomer must haveacapabilityof growinginmorethantwodirections,which implies that the starting monomer must have a functionality greater than 2.For example, consider thepolymerizationof phthalicanhydridewithglycerol,wherethelatteristri-functional:COCOOCHOHCH2OH+CH CH2OOHCOCOOCH2COCOOCH CH2(1.2.1)CH2OHCH2OHThebranchedchainsshownareformedonlyforlowconversionsofmonomers.This implies that the polymer formed in Eq. (1.2.1) is denitely of low molecularweight. Inordertoformbranchedpolymersofhighmolecularweight, wemustusespecial techniques, whichwill bediscussedlater. If allowedtoreact uptolarge conversions in Eq. (1.2.1), the polymer becomes a three-dimensionalnetworkcalledagel,asfollows:OCOCOCH2CH CH2OOOC OCOOOCOCOCH2CH CH2O OCH2CH CH2O COCOCH2CH CH2O COCO(1.2.2)OC OCOOIntroduction 5Copyright 2003 Marcel Dekker, Inc.In fact, whenever a multifunctional monomer is polymerized, the polymer evolvesthroughacollectionof linear chainstoacollectionof branchedchains, whichultimatelyformsanetwork(oragel)polymer. Evidently, thegel polymerdoesnot dissolve in any solvent, but it swells by incorporating molecules of the solventintoitsownmatrix.Generally, anychemical processcanbesubdividedintothreestages[viz.chemical reaction, separation(or purication) and identication]. Among thethreestages, themost difcult intermsoftimeandresourcesisseparation. Wewill discuss in Section 1.7 that polymer gels have gained considerable importanceinheterogeneouscatalysisbecauseit doesnot dissolveinanymediumandtheseparation step reduces to the simple removal of various reacting uids. In recenttimes, a newphase called the uorous phase, has been discoveredwhichisimmiscibletobothorganicandaqueousphases[4,5]. However, duetothehighcosts of their synthesis, theyare, at present, onlyalaboratorycuriosity. Thisapproachisconceptuallysimilar tosolid-phaseseparation, except that uorousmaterialsareinliquidstate.In dendrimer separation, the substrates are chemically attached to thebranchesofthehyperbranchedpolymer(calleddendrimers).Inthesepolymers,(A)CH2CHCO2Me(B)NH2CH2CH2NH2(Excess)Repeat steps (A) and (B)NH2NNNNH2NH2N NH2NH2H2N(Generation = 1.0) (etc)(Generation = 0)NNNNNNN NN NH2NH2NNH2NH2NH2NH2NH2NH2H2NH2NH2NH2NTerminalgroupsInitiatorcoreDendrimersGenerationsDendrimerrepeating units012(1.2.3a)NNHCCNHCNHOOONH2H2NNH2H2NNNH2NH2NH36 Chapter 1Copyright 2003 Marcel Dekker, Inc.theextentofbranchingiscontrolledtomakethembarelysolubleinthereactionmedium. Dendrimers[6]possessaglobularstructurecharacterizedbyacentralcore, branching units, and terminal units. They are prepared by repetitive reactionstepsfromacentral initiatorcore, witheachsubsequent growthcreatinganewgeneration of polymers. Synthesis of polyamidoamine (PAMAM) dendrimers aredone by reacting acrylamide with core ammonia in the presence of excessethylenediamine.Dendrimers have a hollow interior and densely packed surfaces. They havea high degree of molecular uniformity and shape. These have been used asmembrane materials and as lters for calibrating analytical instruments, andnewer paintsbasedonit givebetter bondingcapacityandwear resistance. Itsstickingnaturehas givenrisetonewer adhesivesandtheyhavebeenusedascatalystsforrateenhancement.Environmentalpollutioncontrolistheothereldin which dendrimers have found utility. A new class of chemical sensors based onthese molecules have been developed for detection of a variety of volatile organicpollutants.Inall cases, whenthepolymer is examinedat themolecular level, it isfound to consist of covalently bonded chains made up of one or more repeat units.The name given to any polymer species usually depends on the chemical structureoftherepeatinggroupsanddoesnot reect thedetailsof structure(i.e., linearmolecule, gel, etc.). For example, polystyrene is formed from chains of the repeatunit:CH CH2(1.2.3b)Sucha polymer derives it name fromthe monomer fromwhichit is usuallymanufactured. An idealized sample of polymer would consist of chains all havingidentical molecularweight. Suchsystemsarecalledmonodispersepolymers. Inpractice, however, all polymers are made up of molecules with molecular weightsthat varyoverarangeofvalues(i.e., haveadistributionofmolecularweights)and are said to be polydisperse. Whether monodisperse or polydisperse, thechemicalformulaofthepolymerremainsthesame.Forexample,ifthepolymerispolystyrene, itwouldcontinuetoberepresentedbyCH2CHnCH CH2X CH2CH Y(1.2.4)Foramonodispersesample,nhasasingle valueforallmoleculesinthesystem,whereasforapolydispersesample, nwouldbecharacterizedbydistributionofIntroduction 7Copyright 2003 Marcel Dekker, Inc.values. Theendchemical groupsXandYcouldbethesameordifferent, andwhat they are depends on the chemical reactions initiating the polymer formation.Uptothispoint,ithasbeenassumedthatalloftherepeatunitsthatmakeup the body of the polymer (linear, branched, or completely cross-linked networkmolecules)areallthesame.However,iftwoormoredifferentrepeatunitsmakeup this chainlikestructure, it isknown as a copolymer. If the various repeat unitsoccur randomlyalongthechainlikestructure, thepolymer is calledarandomcopolymer.Whenrepeatunitsofeachkindappearinblocks,itiscalledablockcopolymer. For example, if linear chainsare synthesized from repeatunits A andB,apolymerinwhichAandBarearrangedasiscalledanABblockcopolymer, andoneofthetypeis called an ABA block copolymer. This type of notation is used regardless of themolecular-weightdistributionoftheAandBblocks[7].Thesynthesisofblockcopolymerscanbeeasilycarriedout iffunctionalgroupssuchasacidchloride( COCl), amines( NH2), oralcohols( OH)arepresent at chainends. This way, a polymer of one kind(say, polystyrene orpolybutadiene) withdicarboxylicacidchloride(ClCO COCl) terminal groupscanreact withahydroxy-terminatedpolymer(OH OH)oftheotherkind(say,polybutadiene or polystyrene), resulting in an ABtype block copolymer, asfollows:ClC CCl+ OH OHO OC CO OO OnH Cl (1.2.7)In Chapter 2, we will discuss in more detail the different techniques of producingfunctional groups. Another commonwayof preparingblockcopolymers is toutilizeorganolithiuminitiators. Asanexample, sec-butyl chloridewithlithiumgivesrisetothebutyllithiumcomplex,CH3CHCH3CH2Cl+Li CH3CHCH3CH2Li+ . . .Cl(1.2.8)8 Chapter 1Copyright 2003 Marcel Dekker, Inc.which reacts quickly with a suitable monomer (say, styrene) to give the followingpolystyrylanion:. . .Cl+ n1CH2CH3CHCH3CH2Li+CH2. . .ClLi+CH CH2 1CHCH2CH3(1.2.9)CH3 nThisisrelativelystableandmaintainsitsactivitythroughoutthepolymerization.Because of this activity, the polystyryl anion is sometimes called a living anion; itwillpolymerize withanothermonomer(say,butadiene)afterallofthestyreneisexhausted:CH3CHCH3CH2CH. . .ClLi+CH CHCH3CH3(1.2.10)Cl+ n2CH2Li+CH2. . .CH CH CH2CH2CH2CH2HC CH CH1 n1 n 2 nIn this way, we can conveniently forman AB-type copolymer. In fact, thistechniqueofpolymerizingwithalivinganionlaysthefoundationformodifyingmolecularstructure.Graftcopolymersareformedwhenchainsofonekindareattachedtothebackboneof adifferent polymer. Agraft copolymer hasthefollowinggeneralstructure:(1.2.11)A A A A A A AB BB BB B. . . . . .......Introduction 9Copyright 2003 Marcel Dekker, Inc.Here (A)nconstitutes the backbone molecule, whereas polymer (B)nisrandomlydistributedonit. Graft copolymers are normallynamedpoly(A)-g-poly(B), and the properties of the resultant material are normally extremelydifferent fromthose of the constituent polymers. Graft copolymers can begenerallysynthesizedbyoneofthefollowingschemes[1]:The grafting-from technique. In this scheme, a polymer carrying activesitesisusedtoinitiatethepolymerizationofasecondmonomer. Dependingonthenatureoftheinitiator, thesitescreatedonthebackbonecanbefree-radical,anion, orZieglerNattatype. Themethodofgrafting-fromreliesheavilyonthefact that thebackboneismaderst andthegraftsarecreatedonit inasecondpolymerizationstep, asfollows:CH+ nCH2RCH2CH CH2CHR R(1.2.12)This process is efcient, but it has the disadvantage that it is usuallynotpossible to predict the molecular structure of the graft copolymer and the numberof grafts formed. Inaddition, thelengthof thegraft mayvary, andthegraftcopolymeroftencarriesafairamountofhomopolymer.The graft-onto scheme. In this scheme, the polymer backbone carried arandomly distributed reactive functional group X. This reacts with anotherpolymermoleculecarryingfunctional groupsY, locatedselectivelyat thechainends, asfollows:CH2CH CH2RX+Y CH2CHRCH2CHR(1.2.13)In this case, grafting does not involve a chain reaction and is best carried out ina commonsolvent homogeneously. Anadvantage of this technique is that itallows structural characterization of the graft copolymer formed because thebackboneandthependantgraftarebothsynthesizedseparately.Ifthemolecularweight of eachof thesechainsandtheir overall compositionsareknown, it ispossibletodeterminethenumber of graftsper chainandtheaveragedistancebetweentwosuccessivegraftsonthebackbone.Thegrafting-throughscheme. Inthisscheme, polymerizationwithamacromer is involved. Amacromer is a low-molecular-weight polymer chainwithunsaturationonatleastoneend. Theformationofmacromershasrecentlybeenreviewedandthe techniques for the maximizationof macromer amount10 Chapter 1Copyright 2003 Marcel Dekker, Inc.discussed therein [4]. A growing polymer chain can react with such anunsaturatedsite, resultinginthegraftcopolymerinthefollowingway:Thistypeofgraftingcanintroducelinkagesbetweenindividual moleculesifthegrowingsiteshappentoreact withanunsaturatedsitebelongingtotwoormore different backbones. As a result, cross-linked structures are also likely to beformed, andmeasuresmustbetakentoavoidgelformation.Thereareseveralindustrialapplications(e.g.,paints)thatrequireustoprepare colloidal dispersions of a polymer [5]. These dispersions are inaparticlesizerangefrom0.01to10 mm;otherwise,theyarenotstableand,over aperiodoftime,theysediment.Ifthepolymertobedispersedisalreadyavailablein bulk, one of the means of dispersion is to grind it in a suitable organic uid. Inpractice, however, the mechanical energyrequiredtoreduce the particle sizebelow10 mmisverylarge, andtheheat evolvedduringgrindingmay, at times,meltthepolymeronitssurface.Themoltensurfaceoftheseparticlesmaycauseagglomeration, andtheparticlesincolloidal suspensionsmaygrowandsubse-quently precipitate this way, leading to colloidal instability. As a variation of this,itisalsopossibletosuspendthemonomerintheorganicmediumandcarryoutthe polymerization. We will discuss these methods in considerable detail inChapter 7 (emulsion and dispersion polymerization), and we will show that theproblemofagglomerationofparticlesexistseveninthesetechniques.Polymer colloids arebasicallyof twotypes: lyophobicandlyophilic. Inlyophiliccolloids, polymer particlesinteract withthecontinuousuidandwithotherparticlesinsucha waythattheforcesofinteractionbetweentwoparticleslead to their aggregation and, ultimately, their settling. Such emulsions areunstableinnature. Now, supposethereexistsathermodynamicorstericbarrierbetweentwopolymer particles, inwhichcasetheywouldnot beabletocomeclose toeachother andwouldnot be abletoagglomerate. Suchcolloids arelyophobicin natureandcanbestableforlongperiodsoftime.Inthetechnologyofpolymercolloids,weusespecialmaterialsthatproducethesebarrierstogivethe stabilization of the colloid; these materials are called stabilizers. If we wantedto prepare colloids in water instead of an organic solvent, then we could use soap(commonly used for over a century) as astabilizer. Theactivity of soap isdue toitslyophobicandlyophilicends,whichgiverisetothenecessarybarrierfortheformationofstablecolloids.Introduction 11Copyright 2003 Marcel Dekker, Inc.Inseveral recent applications, it has beendesiredtopreparecolloids inmedia otherthan water. There is a constant need to synthesize new stabilizers foraspecicpolymerandorganicliquidsystem.Recentworkshaveshownthattheblockandgraftcopolymers[inEqs.(1.2.5)and(1.2.11)]giverisetotheneededstability. It is assumedthat theAblockis compatiblewiththepolymer tobesuspendedanddoesnot dissolveintheorganicmedium, whereastheBblockdissolvesintheorganicmediumandrepulsespolymerparticlesasinFigure1.1.Because of the compatibility, the section of the chain consisting of A-repeat unitsgetsadsorbedonthepolymer particle, whereasthesectionofthechainhavingB-repeatunitsprojectsoutward, thusresistingcoalescence.Example1.1: Micellarorampliphilicpolymers(havinghydrophobicaswellashydrophilicfragmentsinwater)havethepropertyofself-organization.Whataretheseandhowaretheysynthesized?Solution: Micellarpolymers have properties similar to surfactant molecules, andbecause of their attractive properties, they are used as protective colloids,emulsiers, wettingagents, lubricants, viscositymodiers, antifoamingagents,pharmaceuticalandcosmeticformulatingingredients,catalysts,andsoforth[8].FIGURE 1.1 Stabilizingeffectofgraftandblockcopolymers.12 Chapter 1Copyright 2003 Marcel Dekker, Inc.Micellar polymers can have six types of molecular architecture, and in thefollowing, hydrophobicandhydrophilicportions areshownbyachainandacircle, respectively, exactly as it is done for ordinary surfactant (i.e., tail and headportions).(a) BlockcopolymerOH (CH2CH2O) (CH CH2CH3O)n m(b) StarcopolymerCOOPCOOP POOCwhere(c) GraftpolymerCH2CH CH CH2CHnCH CHCH2NH CH2CH2mxIntroduction 13Copyright 2003 Marcel Dekker, Inc.(d) Dendrimer(e) SegmentedblockcopolymerN+(CH2)16BrCH3 H3Cn(f) PolysoapCH2CHCH2(CH2)17COONa+nExample 1.2: Describe polymers as dental restorative materials and theirrequirements.14 Chapter 1Copyright 2003 Marcel Dekker, Inc.Solution: Thedental restorativepolymersmust benontoxicandexhibit long-termstability in the presence of water, enzymes, and various oral uids. Inaddition, it should withstand thermal and load cycles and the materials should beeasy to work with at the time of application. The rst polyacrylolyte material usedfor dental restoration was zinc polycarboxylate. To form this, one uses zinc oxidepowderwhichismixedwithasolutionofpolyacrylicacid.Thezincionscross-linkthepolyacidchainsandthecross-linkedchainsformthecement.Another compositionusedfor dental restorationisglassionomer cement(GIC). The glass used is uoroaluminosilicate glass, which has a typicalcomposition of 2525 mol%SiO2, 1420 wt%Al2O3, 1335 wt%CaF2, 46 wt%AlF3, 1025%AlPO4, and520%Na3AlF6. Inthereactionwithpoly-acrylic acid, thelatter degrades theglass, causingthereleaseof calciumandaluminumions which cross-link thepolyacidchains. Thecementsetsaroundtheunreacted glass particles to forma reaction-bonded composite. The uorinepresentintheglassdisruptstheglassnetworkforbetteraciddegradation.Completelypolymericmaterial usedfordentalrestorationisapolymerofmethyl methacrylate (MMA), bisphenol-A, glycidyl methacrylate (bis GMA),andtriethylene glycoldimethacrylate(TEGDMA).Thenetworkthusformedhasbothhydrophilicas wellashydrophobicgroupsandcanreactwithteethas well,givingagoodadhesion. Inordertofurtherimprovetheadhesionbyinterpene-tration and entanglements into dental surfaces, sometimes additives like 4-META(4-methoxyethyl trimellitic anhydride), phenyl-P(2-methacryloxyethyl phenylhydrogenphosphate), orphenyl-Pderivativesareadded.Example1.3: Anticancercompoundsusedinchemotherapyarelow-molecular-weight compounds, andonitsingestion, it isnot site-specictothecanceroustissuesleadingtoconsiderabletoxicity. Howcanpolymerhelpreducetoxicity?Howdoesthishappen?Giveafewexamples.Solution: Macromolecules are usedas carriers, onwhosebackbone boththeanticancerous compounds as well as the targeting moieties are chemically bound.Asaresultofthis,thedrugtendstoconcentratenearthecanceroustissues.Thetargeting moieties are invariably complementary to cell surface receptors orantigens, andas aresult of this, thecarrier macromoleculecanrecognize(orbiorecognize) cancerous tissues. The polymer-mediated drug now has a consider-ably altered rate of uptake by body cells as well as distribution of the drug withinthebody.SomeofthesyntheticpolymersusedasdrugcarriersareHPMA(poly2-hydroxypropyl methacrylamide), PGA(poly L-glutamic acid), poly(L-lysine),andBlock(polyethyleneglycol coasparticacid). UsingHPMA, thefollowingdrugshavebeensynthesized[9]:Introduction 15Copyright 2003 Marcel Dekker, Inc.Drug TargetingmoietyAbriamycin GalactosamineDuanomycin Anti-IakantibodiesChlorine6anti-Thy1.2antibodyBy putting the targeting moiety to the polymer, one has created an ability inthe polymer to differentiate between different biological cells and recognizetumourcells[10]. Thispropertyissometimescalledmolecular recognitionandthis technique canalsobeusedfor separatingnondesirablecomponents fromfoodsoruids(particularlybiologicalones).Thegeneraltechniqueofcreatingmolecularrecognition(havingantibody-likeactivity)iscalledmolecularimprinting. Templatesaredenedasbiologicalmacromolecules, micro-organisms, or whole crystals. Whenfunctional mono-mersarebroughtincontactwiththetemplates,theyadheretoitlargelybecauseof noncovalentbonding. These couldnow becross-linkedusinga suitable cross-linkingagent. If the templates are destroyed, the resultingcross-linkpolymercouldhaveamirror-imagecavityof thetemplate, functioningexactlylikeanantibody.1.3 CHEMICAL CLASSIFICATION OF POLYMERSBASED ON POLYMERIZATION MECHANISMSIn olderliterature, it was suggestedthatall polymers couldbeassignedto oneofthetwofollowingclasses, dependingonthereactionmechanismbywhichtheyaresynthesized.1.3.1 Addition PolymersThese polymers are formed by sequential addition of one bifunctional orpolyfunctional monomer to growing polymer chains (say, Pn) without theelimination of any part of the monomer molecule. With the subscript nrepresentingthe chainlength, the polymerizationcanbe schematicallyrepre-sentedasfollows:Pn+M Pn + 1(1.3.1)Mrepresentsamonomermolecule; thischaingrowthstepisusuallyveryfast.16 Chapter 1Copyright 2003 Marcel Dekker, Inc.Theclassicexampleofadditionpolymerizationisthepreparationofvinylpolymers. Vinyl monomers are unsaturated organic compounds having thefollowingstructure:CH2CHR(1.3.2)whereRisanyofa widevarietyoforganicgroups:aphenyl,amethyl,ahalidegroup, andsoforth. For example, thepolymerizationof vinyl chloridetogivepoly(vinylchloride)canbewritteninthesimpliedformCH nCH2ClCH CH2Cln(1.3.3)Ring-openingreactions, suchas the polymerizationof ethylene oxidetogivepoly(ethylene oxide), offer another example of the formation of additionpolymers:CH2nCH2OCH2CH2On(1.3.4)The correct method of naming an addition polymer is to write poly( ), where thename of the monomer goes into the parentheses. If R in compound (1.3.2) is analiphatichydrocarbon, themonomer isanolenaswell asavinyl compound;these polymers are classied as polyolens. In the case of ethylene and propylene,theparentheses inthenames aredispensedwithandthepolymers arecalledpolyethyleneandpolypropylene.1.3.2 Condensation PolymersThesepolymersareformedfrombifunctionalorpolyfunctionalmonomerswiththe elimination of a small molecular species. This reaction can occur between anytwogrowingpolymermoleculesandcanberepresentedbyPm+PnPm + n+W (1.3.5)wherePmandPnarepolymerchainsandWisthecondensationproduct.Introduction 17Copyright 2003 Marcel Dekker, Inc.Polyesterication is a good example of condensation polymerization. In thesynthesisofpoly(ethyleneterephthalate),ethylene glycolreactswithterephthalicacidaccordingtothefollowingscheme:COOH OH+COOH CH2CH2OHOC C OCH2CH2O OOn+H2O(1.3.6)As indicated by the double arrow, polyesterication is a reversible reaction.Polyamides (sometimes callednylons) areanimportant class of condensationpolymersthatareformedbyreactionbetweenamineandacidgroups, asinNH2(CH2)6NH2+COOH (CH2)4COOHHexamethylenediamineAdipicacidNH (CH2)6NHCO (CH2)6+H2ONylon 66(1.3.7a)NH2(CH2)5COOH+H2O-Aminocaproic acid(CH2)5Nylon 6CO NHn(1.3.7b)Bothof these polymers are classiedas polyamides because the repeat unitscontainthe [CONH]amidegroup.Naming of condensation polymers is done as follows. The polymerobtainedfromreaction(1.3.6)iscalledpoly(ethyleneterephthalate)becausetherepeat unit is theester of ethyleneglycol andterephthalicacid. Similarly, thepolymerinEq.(1.3.7b)iscalledpoly(o-aminocaproicacid).TheproductinEq.(1.3.7a) iscalledpoly(hexamethyleneadipamide), inwhichthehexamethylenepart of the name is associated with the diamine reactant, and the adipamide part isassociatedwiththeamideunitinthebackbone.As researchers learnedmore about polymerizationchemistry, it becameapparent that thenotionof classifyingpolymersthiswaywassomehowincon-sistent. Certain polymer molecules could be prepared by more than onemechanism. Forexample, polyethylenecanbesynthesizedbyeitherofthetwomechanisms:CH2CH2CH2CH2(1.3.8a)nBr+ 2mH2CH2CH2(1.3.8b)5mCH210mBr18 Chapter 1Copyright 2003 Marcel Dekker, Inc.The latter is neither addition nor condensation polymerization. Likewise, thefollowingreaction, whichis atypical additionpolymerization, givesthesamepolyamideasreaction(1.3.7b):(CH2)4CH2CONH-Caprolactam(CH2)5CO NHnNylon 6(1.3.9)Similarly, thepolymerizationofpolyurethanedoesnot involvetheevolutionofa condensationproduct, eventhoughits kinetics canbe describedbythat ofcondensation polymerization. Clearly, it is not correct to classify polymersaccordingtotheschemediscussedearlier. It is nowestablishedthat therearetwoclassesofpolymerizationmechanisms:1. Chain-growth polymerization: an alternative, but more chemicallyconsistentnameforadditionpolymerization.2. Step-growth polymerization: mechanisms that have kinetics of this typeexhibited by condensation polymerization but include reactions such asthatin(1.3.9), inwhichnosmallmolecularspeciesareeliminated.Thisterminologyfordiscussingpolymerizationwillbeusedinthistextbook.Inchain-growthpolymerization,itisfoundthatindividualmoleculesstartgrowing, growrapidly, and then suddenly stop. At any time, therefore, thereaction mass consists of mainly monomer molecules, nongrowing polymermolecules, andonlyasmall numberofrapidlygrowingpolymer molecules. Instep-growth polymerization, on the other hand, the monomer molecules react witheach other at the beginning to formlow-molecular-weight polymer, and themonomer is exhaustedveryquickly. Theyinitiallyformlow-molecular-weightpolymer molecules thencontinuetoreact witheachother toformcontinuallygrowing chains. The polymers formed fromthese distinct mechanisms haveentirelydifferent propertiesduetodifferencesinmolecular-weight distribution,whichisdiscussedinthefollowingsection.1.4 MOLECULAR-WEIGHT DISTRIBUTIONSAll commercial polymers have a molecular-weight distribution (MWD). InChapters37, wewillshowthat thisiscompletelygovernedbythemechanismof polymerizationandreactor design. InChapter 8, we give some importantexperimental techniquestodeterminethemolecular-weight distributionanditsaverages,andin view oftheimportanceofthistopic,wegivesomeofthebasicconcepts here. The chain length n represents the number of repeat units in a givenpolymer molecule, including units at chain ends and at branch points (eventhoughtheseunitshaveasomewhatdifferentchemicalstructurethantherestofIntroduction 19Copyright 2003 Marcel Dekker, Inc.therepeatunits).Forchainmoleculeswithmolecularweightshighenoughtobeclassiedastruepolymer molecules, thereareat least oneorder of magnitudemore repeat units thanunits at chainends andbranchpoints. It is thereforepossibletowrite(withnegligibleerror)Mn nM01:4:1whereMnisthemolecular weight of apolymer moleculeandM0isthat of asinglerepeatunit.In reality, the average chain length of all polymermolecules in the reactionmassmustbeequaltosomewholenumber.Theproductofagivenpolymeriza-tion reaction can be thought of as having a distribution of the degrees ofpolymerization(DPs), whichisgivenbyahistogram, asshowninFigure1.2.In this representation, Wn* is the weight of a species of degree of polymerization nsuchthatWt Total weight of polymer P1n1Wn*1:4:2Bydenition, theweight-averagemolecularweight, Mw,isgivenbyMw P1n1Wn*MnWt1:4:3whereMnisthemolecular weight of aspeciesof chainlengthequal ton. Forsufcientlyhighmolecularweight, Mnis, forallpracticalpurposes, identicaltoMnofEq.(1.4.1).Forlower-molecular-weightspecies,themolecularweightsofendunits andbranchpoints wouldhavetobeconsideredindeterminingMn.Because polymers of high molecular weight are usually ofinterest, this complex-ityisnormallyignoredintheanalysis.Although Eqs. (1.4.1)(1.4.3) serve as the starting point for this discussion,itismoreusefultodenea weightdistributionofdegreesofpolymerizationWnbytheequationWn Wn*Wt*1:4:4Alternatively, Wncanbeinterpretedasthefractionofthemassofthepolymer,with the degreee of polimerization (DP) equal to n or a molecular weight of nM0.Theweight-averagechainlength, mw, isnowdenedbymw MwM0 Ptn1nWn1:4:520 Chapter 1Copyright 2003 Marcel Dekker, Inc.It isthusseenthatmwisjust therst moment oftheweight distributionofthedegreeofpolymerization.There isanalternative but equivalent method ofdescribingdistributions ofmolecular weight. If Nn*is the total number of moles of a polymer of chain lengthequaltoninagivensample, onecanwriteNtn Wn*Mn1:4:6Thetotalnumberofmolesofpolymer, Nt,canthenbewrittenasNt P1n1Nn* 1:4:7Bydenition, thenumber-averagemolecularweight, mn,isgivenbymn P1n1MnNn*Nt1:4:8It is convenient, however, to dene a number distribution of the degree ofpolymerization(DP)NnasNn Nn*Nt1:4:9FIGURE 1.2 Atypicalhistogramofthedegreeofpolymerization.Introduction 21Copyright 2003 Marcel Dekker, Inc.suchthatP1n1Nn 1 1:4:10BecauseNnisalsothefractionofthemoleculesofpolymerofDP equaltonormolecularweightofnM0, Eq. (1.4.8)thenbecomesMn P1n1MnNn1:4:11whichgivesthenumber-averagechainlength, mn, asmn MnM0 P1n1nNn1:4:12and,as before,wesee that mnisjusttherstmomentofthedistributionfunctionNn.Thehighermomentsofthemole fraction distributionNncanbedenedaslk P1n1nkNnk 0; 1; 2; . . . 1:4:13where lkrepresents the kth moment. The zeroth moment (l0) is, according to Eq.(1.4.10), unity. Therst moment (l1) is thesame asmninEq. (1.4.12). Thesecondmoment(l2)isrelatedto mwbymw l2l11:4:14The polydispersity index Q of the polymer is dened as the ratio of mwand mnbythefollowingrelation:Q mwmn l2l0l211:4:15The polydispersity index is a measure of the breadth of mole fraction (ormolecular weight) distribution. For a monodisperse polymer, Q is unity;commercial polymers may have a value of Q lying anywhere between 2and20.1.5 CONFIGURATIONS AND CRYSTALLINITY OFPOLYMERIC MATERIALSSofar, wehave examinedthebroaderaspects ofmoleculararchitecturein chain-likemolecules,alongwiththerelationshipbetweenthepolymerizationmechan-22 Chapter 1Copyright 2003 Marcel Dekker, Inc.ism and the repeat units making up the chain. We have introduced the concept ofdistributionofmolecularweightsandmolecular-weightaverages.Asexpected, thearchitectural features(branching, extentofcross-linking,nature of the copolymer) and the distribution of molecular weight play animportant roleindeterminingthephysical propertiesof polymers. Inaddition,thegeometricdetailsof howeachrepeat unit addstothegrowingchainisanimportant factor indeterminingthe properties of a polymer. Thesegeometricfeaturesassociatedwiththeplacement ofsuccessiverepeat unitsintothechainare called the congurational features of the molecules, or, simply, chainconguration. Let usconsider thechainpolymerizationof vinyl monomersasan example. Inprinciple, thisreactioncan be regarded as thesuccessive additionofrepeat unitsofthetypeCH2CHR(1.5.1)wherethedoublebondinthevinylcompoundhasbeenopenedduringreactionwiththe previouslyaddedrepeat unit. There are clearlythree ways that twocontiguousrepeatunitscanbecoupled.The head of the vinyl molecule is dened as the end bearing the organic group R.All three linkages might appear in a single molecule, and, indeed, the distributionof occurrence of the three types of linkage would be one way of characterizing themolecular structure. In the polymerization of vinyl monomers, head-to-tailplacementisfavored, andthisstructuralfeaturenormallydominates.Amoresubtlestructuralfeatureofpolymerchains,calledstereoregularity,playsanimportantfactorindeterminingpolymerpropertiesandisexplainedasfollows. Inapolymer molecule, thereis usuallyabackboneof carbonatomslinkedbycovalentbonds.A certainamountofrotationispossiblearoundany ofthesebackbonecovalent bonds and, as aresult, apolymer moleculecantakeseveral shapes. Figure 1.3a shows three possible arrangements of the substituentsIntroduction 23Copyright 2003 Marcel Dekker, Inc.ofanyonecarbonatomwithrespect tothoseofanadjacent onewhenviewedend-on, suchthat thetwoconsecutivecarbonatoms CnandCn1appear onebehind the other. The potential energy associated with the rotation of theCnCn1bond is shown in Figure 1.3b and is found to have three angularpositionsof minimumenergy. Thesethreepositionsareknownasthegauche-FIGURE 1.3 Different conformations inpolymer chains andpotential energies asso-ciatedwiththem.24 Chapter 1Copyright 2003 Marcel Dekker, Inc.positive(g),trans(t),andgauche-negative(g)conformationsofthebond;thetrans state is the most probable one byvirtue of havingthe lowest potentialenergy.Substituted polymers, such as polypropylene, constitute a very specialsituation. Becausethepolymer is substituted, theconformationof eachof thebackbone bonds is distinguishable. Each of the CC backbone bonds can take upany one of the three (g, t, and g) positions. Because the polymer is a sequenceof individual CCbonds, the entire molecule can be described in terms ofindividual bond conformations. Among the various conformations that arepossiblefor theentirechain, thereisoneinwhichall thebackboneatomsareinthetrans(t)state. FromFigure1.3c, itcanbeobservedthat ifbondsCnCnand Cn1Cn2are in the trans conformation, carbon atoms Cn1, Cn, Cn1, andCn2alllieinthesameplane. Byextendingthisargument,itcanbeconcludedthat theentirebackboneofthepolymer moleculewouldlieinthesameplane,provided all bonds are in the trans conformation. The molecule is then in a planarzigzagform,asshowninFigure1.4.IfalloftheRgroupsnowlieonthesamesideof thezigzagplane, themoleculeis saidtobeisotactic. If theRgroupsalternatearoundtheplane,themoleculeissaidtobesyndiotactic.Ifthereisnoregularityinthe placement of the Rgroups oneither side of the plane, themolecule is saidtobe atactic, or completelylackinginorder. Agivenvinylpolymer is never 100%tactic. Nonetheless, polymers canbesynthesizedwithhighlevels of stereoregularity, whichimplies that the molecules have a longblockof repeat units withcompletelytacticplacement (isotactic, syndiotactic,etc.), separatedbyshort blocksof repeat unitswithatacticplacement. Indeed,onemethodof characterizingapolymer isbyitsextent of stereoregularity, ortacticity.FIGURE 1.4 Spatialarrangementof[C2CHR]nwhen it isinaplanarzigzagconforma-tion: actactic when R is randomly distributed, isotactic when R is either above or below theplane, andsyndiotacticwhenRalternatesaroundtheplane.Introduction 25Copyright 2003 Marcel Dekker, Inc.Further, when a diene is polymerized, it can react in the two following waysbytheuseoftheappropriatecatalyst:The1,2polymerizationleadstotheformationofsubstitutedpolymersandgivesrisetostereoregularity, asdiscussedearlier (Fig. 1.5). The1,4polymerization,however, yields double bonds on the polymer backbone. Because rotation aroundadoublebondisnot possible, polymerizationgivesrisetoaninexiblechainbackboneand the g, t, and gconformations aroundsuch a bondcannotoccur.Therefore, if asubstituteddiene[e.g., isoprene(CH2CHC(CH3)CH2)] ispolymerized, thestereoregularityinmoleculesarisesinthefollowingway. It isknown that the double-bond formation occurs through sp hybridization ofmolecular orbitals, whichimplies that inFigure 1.4, carbonatoms Cn1, Cn,Cn1, andCn2, as well as HandRgroups, all lie onthe same plane. Twocongurationsarepossible, dependingonwhether HandCH3lieonthesamesideor onoppositesidesofthedoublebond. If theylieonthesameside, thepolymer has cis conguration; if they lie on opposite sides, the polymer has transconguration. Once again, it is not necessary that all double bonds have the sameconguration;ifa variety ofcongurationscanbefoundinapolymermolecule,itissaidtohavemixedconguration.Thenecessaryconditionforchainlikemoleculestotintoacrystallatticeis that they demonstrate an exactly repeating molecular structure along the chain.For vinyl polymers, this prerequisite is met only if they have predominantly head-to-tail placement and are highly tactic. When these conditions are satised,polymerscan, indeed, formhighlycrystallinedomainsinthesolidstateandinconcentrated solution. There is even evidence of the formation of microcrystallineFIGURE 1.5 Spatialarrangementofdienepolymers.26 Chapter 1Copyright 2003 Marcel Dekker, Inc.regionsinmoderatelydilutesolutionsofahighlytacticpolymer. Formationofhighly crystalline domains in a solid polymer has a profound effect on thepolymers mechanical properties. As a consequence, newsynthesis routes areconstantlybeingexploredtoformpolymersofdesiredcrystallinity.Aqualitative notion of the nature of crystallinity in polymers can beacquiredbyconsideringthecrystallizationprocess itself. It is assumedthat apolymer in bulk is at a temperature above its melting point, Tm. As the polymer iscooled, collectionsof highlytacticrepeat unitsthat arepositionedfavorablytomoveeasilyintoacrystallattice willdoso,formingthenucleiofamultitudeofcrystallinedomains. Asthecrystallinedomainsgrow,thechainmoleculesmustreorient themselvestotintothelattice.Ultimately, thesegrowingdomainsbegintointerferewiththeirneighborsandcompete withthemforrepeatunitstotintotheirrespectivelattices.Whenthisbeginstohappen, thecrystallizationprocessstops,leavingafractionofthechainsegmentsinamorphousdomains. Howeffectivelythegrowingcrystallitesacquire newrepeat units during the crystallization process depends on theirtacticity.Furthermore, chains of lowtacticityformdefectivecrystallinedomains.Indeed,aftercrystallizationhasceased,theremayberegionsoforderedarrange-ments intermediate between that associated with a perfect crystal and thatassociatedwithacompletelyamorphouspolymer. Theextent andperfectionofcrystallization even depends on the rate of cooling of the molten polymer. In fact,there are examples of polymers that can be cooled sufciently rapidly thatessentially no crystallization takes place. On the other hand, annealing just belowthemeltingpoint,followedbyslowcooling,willdevelopthemaximumamountof crystallinity (discussed in greater detail in Chapter 11). Similarly, severalpolymers that have been cooled far too rapidly for crystallization to take place canbecrystallizedbymechanicalstretchingofthesamples.1.6 CONFORMATION OF POLYMER MOLECULESOnce a polymer molecule has been formed, its conguration is xed. However, itcantakeonaninnitenumberofshapesbyrotationaboutthebackbonebonds.The nal shape that the molecule takes depends on the intramolecular andintermolecular forces, which, inturn, dependonthe state of the system. Forexample,polymermoleculesindilutesolution,meltphase,orsolidphase wouldeachexperiencedifferentforces.Theconformationoftheentiremoleculeisrstconsideredforsemicrystallinesolidpolymers. Probablythesimplestexampleisthe conformation assumed by polyethylene chains in their crystalline lattice(planar zigzag), as illustrated in Fig. 1.4. A polymer molecule cannot be expectedtobefullyextended, andit actuallyassumes achain-foldedconformation, asIntroduction 27Copyright 2003 Marcel Dekker, Inc.describedindetail inChapter 11. Themost commonconformationfor amor-phous bulkpolymers andmost polymers insolutionis the random-ight (orrandom-coil)conformation, whichisdiscussedindetaillater.In principle, it is possible for a completely stereoregular polymer in a dilutesolutiontoassumeaplanar zigzagor helical conformationwhichever repre-sentstheminimuminenergy. Theconformationofthelatter typeisshownbybiological polymers such as proteins and synthetic polypeptides. Figure 1.6showsasectionofatypicalhelix, whichhasrepeatunitsofthefollowingtype.Thebest knownexampleisdeoxyribonucleicacid(DNA), whichhasaweight-averagemolecularweightof67million.Eveninaqueoussolution,itislockedFIGURE 1.6 Thehelicalconformationofapolypeptidepolymerchain.28 Chapter 1Copyright 2003 Marcel Dekker, Inc.into its helical conformation by intramolecular hydrogen bonds. Rather thanbehaving as a rigid rod in solution, the helix is disrupted at several points: It couldbedescribedasahingedrodinsolution.Thehelicalconformationisdestroyed,however, if thesolutionis madeeither tooacidicor toobasic, andtheDNArevertstotherandom-coil conformation. ThetransformationtakesplacerathersharplywithchangingpHandisknownasthehelixcoiltransition.Sometimes,theenergyrequiredfor completehelical transformationisnot enough. Inthatcase,thechainbackboneassumesshortblocksofhelices,mixedwithblocksofrandom-ightunits.Thenetresultisahighlyextendedconformationwithmostofthecharacteristicsoftherandom-ightconformation.1.7 POLYMERIC SUPPORTS IN ORGANICSYNTHESIS [11^13]In conventional organic synthesis, organic compounds (say, A and B) are reacted.Becausethereactionseldomproceedsupto100%conversion,thenalreactionmass consists of the desired product (say, C) along with unreacted reactants A andB. TheisolationofCisnormallydonethroughstandardseparationtechniquessuchasextraction,precipitation,distillation,sublimation,andvariouschromato-graphicmethods. Theseseparationtechniquesrequireaconsiderableeffort andare time consuming. Signicant advancements have been made by binding one ofthereactants(AorB)throughsuitablefunctional groupstoapolymersupportthat is insolubleinthereactionmass. Tothis, the other reactant (Bor A) isintroducedandthesynthesisreactioniscarriedout. Theformedchemical Cisboundtothepolymer,whichcanbeeasilyseparated.The polymer support used in these reactions should have a reasonably highdegree of substitution of reactive sites. In addition, it should be easy to handle andmust not undergo mechanical degradation. There are several polymers in use, butthemostcommononeisthestyrenedivinylbenzenecopolymer.Introduction 29Copyright 2003 Marcel Dekker, Inc.Because of the tetrafunctionalityof divinyl benzene, the polymer shownis athree-dimensionalnetworkthatwouldswellinsteadofdissolvinginanysolvent.Thesepolymerscanbeeasilyfunctionalizedbychloromethylation, hydrogena-tion, and metalation. For example, in the following scheme, an organotin reagentisincorporated:Ti(OAc)31.5H2OPPBrn-BuLiin THFPLiMgBr2EtheratePMgBrnC4H9SnCl3PSnClClC4H9LiAlH4in THFPSnClC4H9(1.7.2)Because the cross-linkedpolymer molecule inEq. (1.7.1) has several phenylrings, the reaction in Eq. (1.7.2) would lead to several organotin groupsdistributedrandomlyonthenetworkpolymermolecule.Sometimes, ion-exchanginggroups are introduced ontothe resins andthesearesynthesizedbyrst preparingthestyrenedivinyl benzenecopolymer[as in Eq. (1.7.1)] in the form of beads, and then the chloromethylation is carriedout. Chloromethylation is a FriedelCrafts reaction catalyzed by anhydrousaluminum, zinc, orstannouschloride; thepolymerbeadsmust befullyswollenin dry chloromethyl methyl ether before adding the catalyst, ZnCl2. Normally, theresinhasverysmallinternalsurfaceareaandthereactiondependheavilyonthedegreeof swelling.Thisisa solidliquidreaction and the formedproduct canbeshowntobePClCH2OCH3ZnCl2PCH2Cl (1.7.3)Thisreactionisfastandcanleadtodisubstitutionandtrisubstitutiononagivenphenyl ring, but monosubstitutionhas beenfoundtogive better results. The30 Chapter 1Copyright 2003 Marcel Dekker, Inc.chloromethylated resin in Eq. (1.7.3) is quaternized using alkyl amines orammonia. Thisreactionissmoothandformsacross-linkedresinhavinganionsgroupswithinthematrix:PCH2Cl+NH3CH2NH4+ClP(1.7.4)whichisacommercialanion-exchangeresin.It is also possible to prepare anion-exchange resins by using otherpolymeric bases. For example beads of cross-linked polyacrylonitrile are preparedbyusingasuitablecross-linkingagent(say,divinylbenzene).Thepolymerbeadcanthenberepresentedas CN, wherethecyanidegroupis availableforchemicalreaction, exactlyasthephenyl groupinEq. (1.7.3)participatedinthequaternizationreaction. Thecyanidegroupisrst hydrogenatedusingaRaneynickelcatalyst, whichisfurtherreactedtoanalkylhalide, asfollows:CN PNiHCH2P NH2C2H5BrCH2N+BrPC2H5C2H5(1.7.5)Instead of introducing active groups into an already cross-linked resin, it ispossible to polymerize monomeric bases with unsaturated groups or salts of suchbases.Forexample,werstcopolymerizep-dimethylaminostyrenewithdivinylbenzenetoformapolymernetworkasinEq. (1.7.1):Theresultingnetworkpolymer intheformof beads is reactedwithdimethylsulfonatetogiveaquaternarygroup, whichisresponsiblefortheion-exchangeabilityoftheresin:Introduction 31Copyright 2003 Marcel Dekker, Inc.Sometimes,we wanttoprepareaquaternarysaltofthe vinylmonomerandthencopolymerize this with divinyl benzene to form the network polymer resin showninthefollowingdiagram:Evidently, this polymer resin has a greater number of sites because the quaternarygroupispresentateveryalternatecovalentbondonthebackbone.Theothersupport materialsthat arecommonlyusedareTentaGel resinswhichareobtainedbygraftingthestyrenedivinyl benzenecopolymer [of Eq.(1.7.1)] with polyethylene glycol (PEG). Due to the grafts of PEG, the support ispolar innature andit easilyswells inwater, methanol, acetonitrile, dimethylformamide, anddichloroethane. Crowns=pins(CP)areanother kindof supportwhich consists of radiation grafted polyethylene or polypropylene materials.Polymer formed frommonomer polyethylene glycol dimethacrylamide is anetworkbecauseofthetwoacrylamidemoleculesarechemicallyboundtothetwo ends of polyethylene glycoland issometimesabbreviated as PEGAsupport.This is highlypolar, swellingextensivelyinwater, havingextremelyexibleinterior, and suitable forreactions in which it is desired for large macromoleculeslikeenzymestoenterintothematrixofthesupport.The synthesis using polymer supports can be of the following two types. Inthe rst one, the catalyst metal is covalently linked to the support and this32 Chapter 1Copyright 2003 Marcel Dekker, Inc.covalentlyboundmetalservesasacatalystinagivenreaction.Inthefollowingexample, thesupportedmetalisutilizedasahydrogenationcatalyst:In the second type (called organic synthesis on solid support), one of the reactants(say, X) is rst reacted to the support (say, step a) and then the excess reagent X isremoved(say, stepb). Theresultant resinisthenreactedtothesecondreactant(called step c; in this way, X and Y chemically bonding to the support), and afterthis, the resultant support is reduced (called step d). This reduction process shouldbe such that the product of the surface-reacted X and Y cleave efciently from thesupportwithoutaffectingthesupport.Suchsupportsareregenerableandcanbeutilizedinseveral cyclesof chemical reactionbetweenXandY. For practicalreasons,suchsupportshavespecicfunctionalgroups(calledlinkers)whicharechemically stable during the synthesis of the product XY. In addition, the linkergroup is spaced from the surface of the support and it could be represented by spacer-linkers, whereas the chemical reaction between X and Y can be written asPrincipally, thepurposeof thespacer istoalter theswellingpropertiesof theresin,inthis wayimpartingtheresinabettersolventcompatibility.Forexample,in Tenta Gel resin, the graft polyethylene glycol serves as a spacer and makes thestyrenedivinylbenzenecopolymerswellinpresenceofwater, whichotherwisewouldnotdo.The organic synthesis on solid support was rst carried out by Merrield in1963 for synthesis of Peptide with a well-dened sequence of amino acids. As anexample, the support usedfor the synthesis was the styrenedivinyl benzenecopolymerhavingthefollowingstructure:whereC6H3(NO2)CH(CH3)Brservesastheintegral linker withnospacer. Inorder to load the resin with the rst amino acid NH2CH(R1)COOH, the aminoIntroduction 33Copyright 2003 Marcel Dekker, Inc.groupofthelatterisrst blockedwiththebenzyloxycarbonylgroup(Cbz)andthenreactedtotheresinasfollows:For resin1toreact withanotheraminoacidmolecule, NH2CH(R2)COOH,theCbzNHgroupoftheformer must bedeprotected(usingHBr inglacialaceticacidandthenneutralizing) andtheaminegroupof thelatter shouldbeprotectedusingCbzasfollows:Anotherveryactiveareaofresearchwherepolymersupportsareutilizedisthecombinatorial synthesismethods, appliedtothesynthesisofbiologicallyactivecompounds. Thesourcesof thelatter hasalwaysbeenthenatureitself andallnatural productsaremixtureof several compounds. Aconsiderableamount ofwork isrequiredto identify and isolate the active component which serves as thetargetmolecule.Becausethisisinsmallamount,invariablehavinganextremelycomplex structure, it cannot be easily synthesized, and therefore, as such, itcannot be adoptedfor commercial application. Inviewof this, a newactivesubstance,basedonthestudyofthetargetmolecule,isfoundbytrialanderror;this has a comparable biological activity but simpler molecular structure so that itcould be manufactured commercially. The identication of the newactivesubstance (having simpler molecular structure) evidently requires extensiveorganicsynthesisfollowedbypuricationof thecompoundsformedandtheiridentication.Afterthesearesynthesized,theyarethentestedforthebiological34 Chapter 1Copyright 2003 Marcel Dekker, Inc.activityandwewishtondthat newactivesubstancewhichhas thehighestbiological activity. Evidently, in order to achieve this extensive organic synthesis,traditional proceduresoforganicsynthesisreachesthelimit oftimeandeffort.Thespeedof synthesisisanewdimensioninwhichmanystructurallydiversesubstances are synthesized and are subjected to high characterization andscreeningthroughout.The goal of the combinatorial approach is to produce many differentproducts with dened structures and bind themchemically with a polymersupport through their linkers. The set of supports storing these chemicals isknownasalibrary,totallyinanalogywithalibrary ofbooks.Supposethatthereis an unknown molecule (assuming that it is available in pure form) whosemolecularstructureistobedetermined. Onedetermineseitheritshighpressureliquidchromatography(HPLC) or its mass spectra. Onecouldcomparethesewithvarious knowncompounds fromthelibraryas follows. Onereleases theboundcompoundbybreakingthebondwiththelinkersofthesupportandthencomparesthespectraoftheunknowncompoundwiththespectraofthis.Inthisway,onecoulddeterminethemolecularweightaswellasthechemicalstructureof the unknown compound. This is also not a simple task, but using the followingscheme (called combinatorial scheme), this task can be considerably simplied asfollows.Supposetheunknownproduct is knowntobeanamideformedbythereaction of an acyl chloride with an amine. Let us also say that there are 10 typesofacylchloride(A1toA10)and10typesofamine(B1toB10)andtheproductsformedarerepresentedbyA1to A10COCl B1to B10NH2 ! A1to A10CNHB1to B101:7:14In the combinatorial scheme, there are 100 products and they can be carried out in10reactionstepsasfollows. WeprepareamixtureofB1NH2toB10NH2inequal proportion and this mixture is reacted in 10 broths, each containingA1COCltoA10COCl. Inthisway,wegenerated10mixtureswhichcontainA1B1 A1B2 A1B3 A1B101:7:15aA2B1 A2B2 A2B3 A2B101:7:15b......A10B1 A10B2 A10B3 A10B101:7:15cThese 10 mixtures are then stored (by chemically binding) on 10 differentsupports. Inthis case, thelibraryconsists of 10supports andthecompoundsreleasedfromthelinkersisamixtureof 10amides. However, thereisneveraconfusion, simply because the peak positions of the 10 amides in HPLCIntroduction 35Copyright 2003 Marcel Dekker, Inc.experimentsareuniqueandareknownapriori.Similarlyinitsmassspectra,themolecularweightsofeachcomponentsandtheirfragmentsarepreciselyknownbeforehand. Itisthusseenthat theunknowncompoundcanbeeasilyidentiedpreciselywith10HPLC(ormassspectral)experimentsalone.Thesuccessofthecombinatorial schemediscussedaboveliesinthefactthatthemixturecouldbeeasilyboundtothepolymersupportcovalentlyas wellas they should easily be cleaved from the linker completely. In addition to this, theattachment pointsofthelinker(orspacer)withthepolymer support shouldbechemicallystableduringthebindingandcleavingofthemixture. Inpast years,severallinkershavebeendevelopedallowingmanymultisteporganicsynthesisandcleavageefciently.Theconditionsofthereactionsarefoundtodependnotonlyuponthelinker andspacer but alsouponthetypeof resin, its extent ofloading and the nature of compound. In light of this, in the present context, this isarapidlygrowingareaofresearch[11,12].Example1.4: Discussdifferent methodsoffunctionalizingthestyrenedivinylbenzenepolymer.Solution: A widevarietyofvinyl-derivitizedmonomersareavailablecommer-cially and some of these are 4-vinylimidazole, vinyl pyridines, and acryloylmorpholine [13]. This can be terpolymerized with styrene and divinyl benzene toobtainpolymergelhavingthefollowingstructures:Once the gel is formed, they can be functionalized only through chemicalmodication, and normally this is done via chloromethylation using chloromethylmethyletherandlithiationbylithiumbromineexchangeasfollows:36 Chapter 1Copyright 2003 Marcel Dekker, Inc.Chloromethylation is a versatile and reliable reaction, even through chloromethylmethyletherishighlycarcinogaricreaction.Example1.5: Denephotoconductivityandhowitachievedinpolymers.Solution: Photoconductivityisdenedasasignicant increaseinconductivitycaused by illumination and is attributed to increase in charge carriers (electrons orholes due to it). For polymers to be photoconductive, mobile charge carriers mustbe generated with light. The resulting charge pairmay then separate, and either apositive charge, an electron, or both may migrate in a polarizing electrical eld asfollows:D A !hnDA0

!electric fieldD APolymers are normally insulators with negligible conduction and the latter can beachievedby(1)additionofasmallmoleculardopantsor(2)chemicalmodica-tionofthepolymer.Thedopants (e.g., dyeslikerosebengal, methyl violet, methylaneblue,etc.) re charge-transfer agents. The dyes have the abilitytoabsorblight andsensitize the polymer by the addition of electron transfer. The technique ofchemical modicationimprovesthespectral response, givingahigherspeedofmovementofholes.Example1.6: Describethevariousstepsofphotocopying.Solution: Therearefoursteps:1. Thesurfaceofametal drumiscoatedwithphotoconductivematerial[seleniumorpolyvinylcarbazole(PVK)]. Thisischargedinthedarkbysprayingionsundercoronadischarge.Thisgivesauniformchargeonthesurface.2. The image to be copied is projected on the drum, and by this, differentareasofthedrumaredischarged inthelight signal.Charge isretainedin areas not illuminated, and in this way, an electrical pattern isgeneratedonthedrum.3. The developer consists of two components: carrier and toner. Thecarrierismetal beadsandthetonerisapolymerwithblackdye. Onshaking,thetonerbecomespositivelycharged,which,onexposuretothedrum, accumulatesarounddarkareas. Thepaper isexposedtoahigh voltage to make it negatively charged and the toner shifts from thedrumsurfacetothepaper.4. Theimagetransferredtothepaperisxedbyheatingthepaper, andthepolymerparticlesoftonerarethensintered.Introduction 37Copyright 2003 Marcel Dekker, Inc.Example1.7: After separatinggas, gasoline, naphtha, andgasoil fromcrudepetroleum, the residue is depolymerized and then further distilled for thesematerials. Discuss the depolymerization of long-chain hydrocarbon (calledVisbreakingstep). Sometimes, acatalyticcrackingprocessisused. Discussthedifferencebetweenthetwo.Solution: Under depolymerization, aCCbondbreaks homolyticallytogivetworadicals:R1CH2CH2R2 ! R1CH?2 R2CH?2Theyproduceethyleneunder b-scission:R1CH2CH2CH?2 ! RCH?2 CH2CH2Theradical neednot necessarilybeat thechainendalone. For productionofpropylene, we needtohave the radical at the thirdcarbonandthenthere isb-scissionreaction:R1CH2CH2CH2CH3 ?CH3 ! RCH2CH?CH2CH3 CH4R2CH2CH?CH2CH3 ! RCH?2 CH2CHCH3Catalytic cracking in the petroleum industry is a very important process in whichheavyoilsareconvertedintogasolineandlighterproducts.Thisoccursthroughcarboneumionformationinthepresenceof zeolitecatalyst (Lewisacid, L) asfollows:RH L ! LH R?In the case of Risomerization occurs easily and a shift of the double bond and ashiftofthemethylgrouparecommonlyseen.Thearomaticformationonzeoliteoccursasfollows:1.8 CONCLUSIONIn this chapter, various methods for classifying polymers have been discussed andbasicconceptsregardingmolecular-weight distribution, mechanismofpolymer-38 Chapter 1Copyright 2003 Marcel Dekker, Inc.ization, molecular conformations, congurations, and crystallinity have beenpresented. Theseconceptswill beampliedintherest of thebook. It maybementionedthat syntheticpolymersaremainlyemphasizedinthistextbook, butthe concepts developed here can be easily extended to naturally occurringpolymers such as proteins, nucleic acids, cotton, silk, wool, and paper. Nosingle textbook can do justice to so many fascinating areas of research. This is theonlyjusticationfortheexclusionofnaturalpolymersfromthisbook.REFERENCES1. Allen, G., andJ. C. Bevington, ComprehensivePolymer Science, Pergamon, NewYork,1989,Vols.17.2. Kumar, A., and S. K. Gupta, Fundamentals of Polymer Science and Engineering, TataMcGraw-Hill,NewDelhi,1978.3. Tsuruta, T., ContemporaryTopicsinPolymericMaterialsfor Biomedical Applica-tions,Adv.Polym.Sci.,126,154,1996.4. Curran, D. P., StrategyLevel SeparationinOrganicSynthesis: FromPlanningtoPractice,Angew.Chem.Int.Ed.Eng.,37.11741196,1998.5. Barrett,K.E.J.,DispersioninOrganicMedia,Wiley,NewYork,1982.6. Fischer, M., andF. Vogtle, Dendrimers: FromDesigntoApplicationsAProgressReport,Angew.Chem.Int.Ed.Eng.,38,885905,1999.7. Hazer, B., Synthesis andCharacterizationof BlockCopolymers, inHandbookofPolymer Science and Technology, N. Cheremisnoff (ed.), Marcel Dekker, New York,1989,Vol.1.8. Loschewsky, A., Molecular Concept,Self Organization and Properties of Polysoaps,Adv.Polym.Sci.,124,186,1995.9. Putnam,D.,andJ.K.Kopacek,PolymerConjugateswithAnticancerActivity,Adv.Polym.Sci.,122,55124,1995.10. Sellergren, B., ImprintedPolymerswithMemoryfor Small Molecules, ProteinorCrystals,Angew.Chem.Int.Ed.Eng.,39,10311037,2000.11. Laszlo, P., PreparativeChemistryUsingSupportedReagents, AcademicPress, SanDiego,CA,1987.12. Guillier, F., D. Orain, and M. Bradley, Linkers and Cleavage Strategies in Solid PhaseOrganic Synthesis and Combinatorial Chemistry, Chem. Rev. 100, 20912157, 2000.13. Hodge, P., and D. C. Sherrington, Polymer Supported Reactions in Organic Synthesis,Wiley,Chichester,1980.PROBLEMS1.1. The molecular functionality of real systems is usually expressed by afractioninviewof the possible occurrence of side processes involvingcyclization, cross-linking, andsoforth. WhenmorethanonemonomerisIntroduction 39Copyright 2003 Marcel Dekker, Inc.involved, wetalkofaveragefunctionality

ff denedas

f f PfiNiPNiwhere fiis the functionality of the ith monomer whose Nimoles are presentin the reaction mixture. Calculate

ff for a mixture of glycerolphthalicanhydrideintheratioof2 : 3.Forbranchedandnetworkpolymer,

ff mustbegreaterthan2.1.2. In the polymerization of phthalic anhydride with glycerol, one addsethyleneglycol also. Why?Assumingtheirconcentrationsintheratioof2 : 3 : 1, nd their average functionality. What is the main differencebetweenthispolymerandtheoneinEq. (1.22)?1.3. Methylmethacrylate[CH2C(CH3)COOCH3]israndomlycopolymerizedwithmaleicanhydrideintherst stage, andtheresultingcopolymer isreactedwithpolyvinyl alcohol [CH2CH(OH)]ninthe secondstage.Write down all of the chemical reactions involved and the molecularstructureoftheresultingpolymersinbothstages.1.4. 12-Hydroxystearic acid [CH3(CH2)5CH(OH)(CH2)10COOH] ispolymerized through the step-growth mechanism(ARBmonomer typereaction)instage1. Instage2, thisisreactedwithglycidylmethacrylateCH2CCH3COOCH2CHCH2; GMA;n =Owhich produces the end-capping of the polymer formed at the end of stage1. In this reaction, the epoxy group of the GMA reacts, keeping the doublebond safe.Thisiscopolymerizedwithmethyl methacrylate in stage3,andin this stage, the double bond of the GMA reacts. Write down the structureofthepolymerattheendofeachstage.1.5. A small amount of grafting ofpolyethylene(PE) changesthepropertiesofthepolymer. Thepolymer inthemoltenstageis mixedwithasuitablemonomer (e.g., acrylic acid CH2CHCOOH, fumaric acidCOOHCH2CHCOOH, or maleic anhydrideandaninitiator [e.g., dicumylperoxide; C6H5C(CH3)2OOC(CH3)2C6H6].Explainwhythepolymerformedwouldbeamostlygraftedoneandwhatitsstructurewouldbe. Discussotherpossibleproducts.1.6. Polyvinyl alcohol (PVA) is normally atactic and has a high concentration ofhead-to-headdefects. However, PVAinthecrystallinestateisintheall-trans stage and gives tough bers. PVA bers can also absorb water and, inthisway, losestrength. Toavoidthis, oneusuallytreatsthepolymerwithformaldehydeandthelatterreacts(thereactioniscalledketalization)withthe two adjacent hydroxyl groups of the polymer. What changes areproducedinthepolymerthroughthistreatmentandwhy?40 Chapter 1Copyright 2003 Marcel Dekker, Inc.1.7. Polyvinyl amine [CH2CH(NH2)]ncan be easily prepared throughHoffmann degradation of polyacrylamide [CH2CH(CONH2)]n. Writedownthebasicreactionsusingsodiumhydrochloride(NaCl)inmethanolmedium. The modied polymer now is an ammonium chloride salt and hastoundergoanionexchangetoobtainthedesiredpolymer.1.8. Linearpolyacetylene is known to be an intrinsically electrically conductivepolymer. The conguration could be cis, trans, or mixed. Do you think thatthecongurationwouldhaveanyinuenceontheelectricalconductivity?1.9. Polyphenylene sulde is an important electronic polymer and can besynthesized using p-dichlorobenzene with Na2S in n-pyrolidone (Campbellmethod). Itsinitiationstepisasfollows:InitiationPropagationTheterminationof thepolymer couldoccur withA(bycombination) aswellasB(bytransferreaction)andwritethesereactions.1.10. Thefollowingpolymerpoly(glutamicacid)isusedasanantitumoragent:whereIgis aproteinimmunoglobin. Inthis polymer molecule, identifyvarious groups serving as the solubilizer, pharmacon, and the homingdevice.Introduction 41Copyright 2003 Marcel Dekker, Inc.1.11. Poly(2-vinylpyridine-1-oxide) (PVNO) is a polymer that dissolves in waterandservesasamedicinaldrug:The polymerprevents the cytotoxic action of quartz dust, and its activity isfound for a molecular weight above 30,000. Assuming that the monomer isavailable,howwouldyouformtherequiredpolymer?1.12. Penicillinhasthefollowingchemicalformula:Onbindingit toapolymer, theactivityofthedrugisfoundtoincrease.Suggestasuitablepolymeronwhichthiscanbebound.1.13. Heparin is highly acidic dextrorotatory copolymer of glucosamine andglucuronicacidhavingthefollowingstructure:It isnaturallypresent inbloodandinhibit itsclotting. It isdesiredtousePVC bags for storing blood and this can be done only ifits inner surface ispassivatedwithheparin. Suggestamethodbywhichitcanbedone.1.14. In diabetes (where there is poor insulin delivery by the pancreas in responseto glucose) or Parkinsons disease (where there is poor release of dopamineinresponsetopotassium), it hasbeenrecommendedtotransplant encap-sulated animal cells in the human body to supplement the existingdeciency. Thelimitationof thiscell transplant istheimmune-mediatedrejection, and to overcome this, the cells are microencapsulated withacrylates and methacrylates, whichcannot be penetrated bylarge anti-bodies but insulinor dopaminecandiffuseout easily. Readilyavailablemethacrylates[CH2C(CH3)C(O)OR]areMAA(methacrylicacidwithRasH), MMA(methyl methacrylatewithRasCH3), HEMA(2-hydro-xyethyl methacrylate with R as CH2CH2OH), HPMA (2-hydroxy methylmethacrylatewithRas CH2CH(OH)CH3), andDMAEMA[dimethyl-42 Chapter 1Copyright 2003 Marcel Dekker, Inc.aminoethyl methacrylatewithRas CH2CH2N(CH3)2]. Findout whichmonomerwouldgiveacidic, basic, orneutrallm.1.15. Whenacidicandbasicpolymericpolyelectrolytesaremixed, theyformacomplexwhichprecipitates. Basedonthis, mammaliancells arecoatedwith sodium alginate (natural polymer) solution (in water) and then put in asuitablewater-solubleacrylatesolution. Thisformsahardencapsulationaroundthemammaliancell; this is impervious toantibodies. Suggest asuitableacrylate(asmethacrylate)fortheencapsulation.1.16. Oneofthesyntheticbloodplasmasispoly[N-(2-hydroxypropyl)]metha-crylamide:Suggestsome(atleasttwo)plausiblewaysofmakingthismaterial.1.17. In following multifunctional initiators, the azo (at 50

C) and peroxidegroups (at 90

C) decompose at separate temperatures and the monomer M1ispolymerizedrst:Consideringthat polymer radicals due tomonomer M1terminates (say,PM1)byrecombinationaloneandthatduetoM2(say,PM2),terminatesbyrecombinationalone, whatkindsofpolymerthat wouldbeformed. Whatwould be the nature of the polymer if the polymer radicals due to monomerM2terminatebydisproportionationonly.1.18. SupposeinProblem1.17that thepolymer radical duetomonomer M1terminate by disproportionation and that due to M2 terminates by combina-tion, what isthenatureofthenal polymer?SupposePM2terminatesbydisproportionationonly;whatwouldbethenatureofthepolymerthen?1.19. Iodinetransfer polymerizationrequiresaperoxide(ROOR) andanalkyl iodide(RFI). The primaryradicals aregeneratedandthepolymerradicals(RF M?n)undergotransferreactionasfollows:ROOR!heatRO?RO? RF !Product R?FRFM?n RFI !RFMnI R?FAssuming termination to occur by combination, give the complete mechan-ismofpolymerization.Introduction 43Copyright 2003 Marcel Dekker, Inc.1.20. Synthesis gas is a mixture of carbon monoxide and hydrogen. In theFischerTropschprocess,theformationofalkane,alkene,alcoholoflargechain-lengthoccurs,dependingonthecatalystused,asfollows:1. Formationofalkaneandalkeneoflargerchainlength:n 1H2 2nCO ! CnH2n2 nCO2nH2 2nCO ! CnH2n nCO2Hydrocarbons up to C20 (e.g., gasoline, diesel, or aviation fuel) areproducedusinga(Co, Fe, Ru)catalyst.2. Alcoholformation:2nH2 nCO ! CnH2n1OH n 1H2OEthanolusingaCocatalyst,higheralcoholsusinga(Fe,Co,Ni)catalyst,andethyleneglycolusingaRhcatalystareproduced.IntheFischerTropschprocess, carbonmonoxideisabsorbedonametal(M)asoneof itsligands, asfollows:CO M!MCOMCO M!OMC MOItistheseMCligandsthatgivethegrowthofchains:MC!H2MCH2 !H2MCH3Now writedown the propagation andtermination reactions in the FischerTropschprocess.1.21. Liquidhydrocarbons have been prepared usingthe ZSM-5 zeolite catalyst.At600

C, thefollowinginitiationreactionoccurs:With the help of your understanding of Example 1.7, explain the formationofC3H8,parafn, parafnisomers, andaromatics.44 Chapter 1Copyright 2003 Marcel Dekker, Inc.2Eect of Chemical Structure onPolymer Properties2.1 INTRODUCTIONInthepreviouschapter,wediscusseddifferentwaysofclassifyingpolymersandobservedthat their molecular structureplaysamajor roleindeterminingtheirphysicalproperties.Wheneverwewishtomanufactureanobject,wechoosethematerial of construction so that it can meet design requirements. The latterinclude temperature of operation, material rigidity, toughness, creep behavior, andrecovery of deformation. We have already seen in Chapter 1 that a given polymercanrange all thewayfromaviscous liquid(for linear low-molecular-weightchains)toaninsolublehardgel (fornetworkchains), dependingonhowit wassynthesized. Therefore, polymerscanbeseentobeversatilematerialsthatofferimmensescopetopolymerscientistsandengineerswhoareonthelookout fornew materials with improved properties. In this chapter, we rst highlight some oftheimportantpropertiesofpolymersandthendiscussthemanyapplications.2.2 EFFECT OF TEMPERATURE ON POLYMERS[1^4]Wehaveobservedearlierthatsolidpolymerstendtoformorderedregions,suchasspherulites(seeChapter11forcompletedetails);thesearetermedcrystallinepolymers. Polymers that havenocrystals at all arecalledamorphous. Areal45Copyright 2003 Marcel Dekker, Inc.polymer is never completely crystalline, and the extent of crystallization ischaracterizedbythepercentageofcrystallinity.Atypical amorphous polymer, suchas polystyreneor polymethyl meth-acrylate, canexist inseveral states, dependingonitsmolecularweight andthetemperature. InFigure2.1, wehaveshowntheinterplayofthesetwovariablesand compared the resulting behavior with that of a material with moderatecrystallinity. An amorphous polymer at low temperatures is a hard glassy materialwhich, when heated, melts into a viscous liquid. However, before melting, it goesthrough a rubbery state. The temperature at which a hard glassy polymer becomesFIGURE 2.1 Inuenceof molecular weight andtemperatureonthephysical stateofpolymers.46 Chapter 2Copyright 2003 Marcel Dekker, Inc.arubberymaterialiscalledtheglasstransitiontemperature,Tg(seeChapter12for thedenitionof Tgintermsofchangesinthermodynamicandmechanicalproperties;there exists a sufciently sharp transition,as seenin Fig. 2.1a).Thereisadiffusetransitionzonebetweentherubberyandliquidstatesforcrystallinepolymers; the temperature at which this occurs is called the ow temperature, Tf.Asthemolecularweight ofthepolymerincreases, weobservefromFigure2.1that bothTgandTfincrease. Finally, thediffusetransitionoftherubber totheliquid state is specic to polymeric systems and is not observed for low-molecular-weight species suchas water, ethanol, andsoforth, for whichwehaveasharpmeltingpointbetweensolidandliquidstates.Inthissection,onlytheeffectofchainstructureonTgisexaminedotherfactorswill bediscussedinChapters1012. Inordertounderstandthevarioustransitions for polymericsystems, weobservethat amoleculecanhaveall orsomeofthefollowingfourcategoriesofmotion:1. Translationalmotionoftheentiremolecule2. Long cooperative wriggling motion of 4050 CCbonds of themolecule, permittingexinganduncoiling3. Shortcooperativemotiono