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Introduction To The PhysicalIntroduction To The Physical

Chemistry Of PolymerChemistry Of Polymer


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Lecture 1


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Introduction to polymersIntroduction to polymersPoly = many, mer = unit, many unitsPoly = many, mer = unit, many units

Polymer science is relatively a new branch of science . It deals with chemistryPolymer science is relatively a new branch of science . It deals with chemistry

physics and mechanical properties of macromolecule .physics and mechanical properties of macromolecule .

Macromolecule are involved in all human aspect ; the human body itself isMacromolecule are involved in all human aspect ; the human body itself is

made from proteins a polymer (made of poly amino acid ). Cellulose anmade from proteins a polymer (made of poly amino acid ). Cellulose an

Important natural material essential for the existence of man since the downImportant natural material essential for the existence of man since the down

of history, is the complicated polymer structure.of history, is the complicated polymer structure.

Beyond the many natural polymer , the man made polymers ore now forBeyond the many natural polymer , the man made polymers ore now for

human development . It is impossible to imagine modern life without all thehuman development . It is impossible to imagine modern life without all the

different types of synthetic textile materials (polyester , polyamide..)different types of synthetic textile materials (polyester , polyamide..)


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In this course we will discuss the followingIn this course we will discuss the following::

11-- Types of polymerTypes of polymer

22-- Step polymerizationStep polymerization33-- Addition free radical polymerizationAddition free radical polymerization

44-- Addition ionic polymerizationAddition ionic polymerization

55-- CopolymerizationCopolymerization

66-- Molecular weights of polymerMolecular weights of polymer

77-- Elucidation of the structure of polymerElucidation of the structure of polymer


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PolymerPolymeris a large molecule consisting of a number of repeatingis a large molecule consisting of a number of repeating

units with molecular weight typically several thousand orunits with molecular weight typically several thousand or

higherhigher

Repeating unitRepeating unit is the fundamental recurring unit of a polymeris the fundamental recurring unit of a polymer

MonomerMonomer -- is the smaller molecule(s) that are used to prepare ais the smaller molecule(s) that are used to prepare a

polymerpolymer

OligomerOligomeris a molecule consisting of reaction of several repeatis a molecule consisting of reaction of several repeat

units of a monomer but not large enough to be consider aunits of a monomer but not large enough to be consider a

polymer (dimer , trimer, tetramer, . . .)polymer (dimer , trimer, tetramer, . . .)

Degree of polymerizationDegree of polymerization -- number of repeating unitsnumber of repeating units

Definitions


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Nomenclature of polymerNomenclature of polymer11-- Based on monomer sourceBased on monomer sourceThe addition polymer is often named according to the monomer that wasThe addition polymer is often named according to the monomer that was

used to form itused to form itExample : poly( vinyl chloride ) PVC is made from vinyl chlorideExample : poly( vinyl chloride ) PVC is made from vinyl chloride

--CHCH22--CH(Cl)CH(Cl)--

If X is a single word the name of polymer is written outIf X is a single word the name of polymer is written out

directlydirectly

ex. polystyreneex. polystyrene --CHCH22--CH(Ph)CH(Ph)--

Poly XPoly X

If X consists of two or more words parentheses should beIf X consists of two or more words parentheses should be

usedused

ex , poly (vinyl acetateex , poly (vinyl acetate ) -CH2-CH(OCOCH

3)-

22-- Based on polymer structureBased on polymer structureThe most common method for condensation polymers since the polymerThe most common method for condensation polymers since the polymer

contains different functional groups than the monomercontains different functional groups than the monomer


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Classification schemes

Classification by Origin

Synthetic organic polymersSynthetic organic polymers

Biopolymers (proteins, polypeptides, polynucleotide,Biopolymers (proteins, polypeptides, polynucleotide,

polysaccharides, natural rubber)polysaccharides, natural rubber)

SemiSemi--synthetic polymers (chemically modified synthetic polymers)synthetic polymers (chemically modified synthetic polymers)

Inorganic polymers (siloxanes, silanes, phosphazenes)Inorganic polymers (siloxanes, silanes, phosphazenes)


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Classification by Monomer Composition

Homopolymer Copolymer

Block Graft Alternating Statistical

HomopolymerHomopolymerConsist of only one type of constitutional repeating unit (A)Consist of only one type of constitutional repeating unit (A)

AAAAAAAAAAAAAAA

copolymercopolymer

Consists of two or more constitutional repeating units (A.B )Consists of two or more constitutional repeating units (A.B )


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Several classes of copolymer arepossible

Statistical copolymer(Random)

ABAABABBBAABAABB

two or more different repeating unitare distributed randomly

Alternating copolymer

ABABABABABABABAB

are made of alternating sequences

of the different monomers Block copolymer

AAAAAAAAABBBBBBBBB

long sequences of a monomer arefollowed by long sequences of anothermonomer

Graft copolymerAAAAAAAAAAAAAAAAAA

B B B

B B B

Consist of a chain made from one type ofmonomers with branches of another

type (d)


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Classification by Chain structure (molecular architecture)

Linear chains :a polymer consisting of a single continuous chain of

repeat unitsBranched chains :a polymer that includes side chains of repeat unitsconnecting onto the main chain of repeat units

Hyper branched polymerconsist of a constitutional repeating unit

including a branching groups

Cross linked polymer:a polymer that includes interconnections

between chainsNet work polymer:a cross linked polymer that includes numerousinterconnections between chains

Linear Branched Cross-linked Network

Direction of increasing strength


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Classification by Chain Configuration and Conformation

Configuration or cisConfiguration or cis--trans isomerismtrans isomerismConfiguration :Configuration : Is defined by polymerization method. AIs defined by polymerization method. A

change in configuration require the rupture of covalentchange in configuration require the rupture of covalent

bonds .bonds .

Stereoisomerism or tacticityStereoisomerism or tacticity

IsotacticIsotactic

SyndiotacticSyndiotactic

AtacticAtactic

Conformation :Conformation : is defined by its sequence of bonds andis defined by its sequence of bonds and

torsion angles. The change in shape of a giventorsion angles. The change in shape of a givenmolecule due to torsion about single (molecule due to torsion about single ( ) bonds) bonds


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Geometric Isomerism

CH2 CH CH CH2


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isotactic

Microstructure - Tacticity

atactic syndiotactic

Side groups on alternating

sides of the backboneSide groups on the same

side of the backbone

Side groups on random

Sides of the backbone


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Polyolefins with side chains have stereocenters on every other carbon

C H3n

C H3 C H3 C H3 C H3 C H3 C H3C H3

With so many stereocenters, the stereochemistry can be complex. There

are three main stereochemical classifications for polymers.

Atactic: random orienta tion

Isotact ic: All stereocenters have same orientation

Syndiotact ic: Alternating stereochemistry


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1. Tacticity affects the physicalproperties

1. Atactic polymers will generally be amorphous, soft, flexiblematerials

2. Isotactic and syndiotactic polymers will be more crystalline, thusharder and less flexible

2. Polypropylene (PP) is a good example

1. Atactic PP is a low melting, gooeymaterial

2. Isoatactic PP is high melting (176), crystalline, tough materialthat is industrially useful

3. Syndiotactic PP has similarproperties, but is very clear. It is

harder to synthesize


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Classification by Thermal BehaviorThermoplastics - materials become fluid and processible uponheating, allowing them to be transformed into desired shapes that

are stabilized by cooling.

Thermosets - initial mixture of reactive, low molar mass compoundsreacts upon heating in the mold to form an insoluble, infusible

network

Classification by Application Plastics Fibers

Elastomers

Coatings

Adhesives

Classification Based on Kinetics or MechanismStep-growth

Chain-growth


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1. A number-average molecular weight Mn : divide chains into series of

size ranges and then determine the number fraction Ni of each sizerange

where Mi represents the mean molecular weight of the size range i, andN

iis the fraction of total numberof chains within the corresponding size

range

To create a solid with usefulmechanicalproperties the chain must be long !!One may describe chain length in terms ofpolymeraverage molecularweight, which can be defined in several ways:

Molecular weight averages

2. A weight average molecular weight Mw is based on the weightfraction wi within the size ranges:

Mn = MiNi/ Ni

Mw= MiWi / Wi


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Ameasure of the molecular-weight distribution is given by the ratios of molecular

-weight averages.

For this purpose, the most commonly used ratio is Mw/Mn, which is called the

polydispersity index orPDI.

PDI= Mw/Mn

Mw/Mn = 1 monodispersePolymer sample consisting of molecules all of which have the same

chain length

Mw/ Mn > 1 polydisperse

Polymer consisting of molecules with the variety of chain length


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Description of polymer physical propertiesDescription of polymer physical properties1-Primary bonds : the covalent bonds that connect the atoms of the mainchainchain

22-- Secondary bonds :Secondary bonds : nonnon covalent bonds that hold one polymer chain tocovalent bonds that hold one polymer chain toanother including hydrogen bond and other dipoleanother including hydrogen bond and other dipole dipole attractiondipole attraction33--Crystalline polymer :Crystalline polymer : solid polymers with a high degree of structural ordersolid polymers with a high degree of structural orderand rigidityand rigidity

44-- Amorphous polymers :Amorphous polymers :polymers with a low degree of structural orderpolymers with a low degree of structural order

55--SemiSemi crystalline polymer :crystalline polymer : most polymers actually consist of bothmost polymers actually consist of both

crystalline domains and amorphous domains with properties between thatcrystalline domains and amorphous domains with properties between thatexpected for a purely crystalline or purely amorphous polymerexpected for a purely crystalline or purely amorphous polymer

66--Glass :Glass : the solid form of an amorphous polymer characterized by rigiditythe solid form of an amorphous polymer characterized by rigidityand brittlenessand brittleness

Amorphous Crystalline


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77 Crystalline melting temperature (TCrystalline melting temperature (Tmm) :) : temperature at which crystallinetemperature at which crystallinePolymer converts to a liquid or crystalline domains of a semi crystallinePolymer converts to a liquid or crystalline domains of a semi crystalline

Polymer melt (increased molecular motion )Polymer melt (increased molecular motion )

88-- Glass transition temperature (TGlass transition temperature (Tgg) :) :temperature at which an amorphoustemperature at which an amorphous

polymer converts to a liquid or amorphous domains of a semi crystallinepolymer converts to a liquid or amorphous domains of a semi crystalline

polymer meltpolymer melt

99 Thermoplastics (plasticsThermoplastics (plastics )) ::polymers that undergo thermally reversiblepolymers that undergo thermally reversibleInterconversion between the solid state and the liquid stateInterconversion between the solid state and the liquid state

1010-- Thermosets :Thermosets :polymers that continue reacted at elevated temperaturespolymers that continue reacted at elevated temperatures

generating increasing number of crosslinks such polymers do not exhibitgenerating increasing number of crosslinks such polymers do not exhibitmelting or glass transitionmelting or glass transition

1111-- LiquidLiquid crystalline polymers :crystalline polymers :polymers with a fluid phase that retainspolymers with a fluid phase that retainssome ordersome order

1212-- Elastomers :Elastomers : rubbery , stretchy polymers the effect is caused by lightrubbery , stretchy polymers the effect is caused by lightcrosslinkingcrosslinking that pulls the chains back to their original statethat pulls the chains back to their original state


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Temperature

3

9

6

7

8

4

5

Glass phase (hard plastic)

Rubberphase (elastomer)

Liquid

Leatheryphase

Log (stiffness)Pa


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Polymerization mechanismsPolymerization mechanisms

Chain GrowthChain Growth

The only growth reaction isThe only growth reaction is

addition of monomer to aaddition of monomer to a

growing chain with agrowing chain with a

reactive monomerreactive monomer

The reaction mixture consistsThe reaction mixture consists

of high polymer andof high polymer and

unreacted monomers withunreacted monomers with

very few actively growingvery few actively growing

chainchainMonomer concentrationMonomer concentration

decreases steadily asdecreases steadily as

reaction time increasesreaction time increases

Step GrowthStep Growth

Reaction can occurReaction can occur

independently between anyindependently between any

pair of molecular speciespair of molecular species

The reaction mixture consists ofThe reaction mixture consists of

oligomers of many sizes in aoligomers of many sizes in a

statically calculablestatically calculable

distributiondistribution

Monomer disappear early in favorMonomer disappear early in favor

of low oligomerof low oligomer


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Chain growth

High polymer appearsHigh polymer appearsimmediately , averageimmediately , averagemolecular weight doesmolecular weight does

not change much asnot change much as

reaction proceedsreaction proceeds

Increased reaction timeIncreased reaction timeincreases overallincreases overall

product yield , but doesproduct yield , but does

not affect polymernot affect polymeraverage molecularaverage molecularweightweight

Step GrowthStep Growth

Oligomers steadilyOligomers steadilyincreases in size,increases in size,

polymer averagepolymer averagemolecular weightmolecular weight

increases as reactionincreases as reactionproceedsproceeds

Long reaction time areLong reaction time areessential to produceessential to produce

polymer with heightpolymer with heightaverage molecularaverage molecularweightweight

Polymerization mechanismsPolymerization mechanisms


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Lecture 2

Polymerization mechanisms


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- Step-growth polymerization


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Stepwise (Condensation) polymerization Reaction

Requirements forStep-Growth PolymerizationHigh monomer conversionHigh monomer conversion

High monomer purityHigh monomer purity

High reaction yieldHigh reaction yield

Stoichiometric equivalence of functional groupsStoichiometric equivalence of functional groups

The characteristic features of this type of polymerizationThe characteristic features of this type of polymerization

process as followprocess as follow.11--Growth occurs throughout the matrixGrowth occurs throughout the matrix

22--There is the rapid loss of the monomer speciesThere is the rapid loss of the monomer species

33--The molecular weight slowly increases throughout the reactionThe molecular weight slowly increases throughout the reaction44-- The same mechanism operate throughout the reactionThe same mechanism operate throughout the reaction

55--The polymerization rate decreases as the number of functionalThe polymerization rate decreases as the number of functional

group decreasesgroup decreases

66--No initiator is required to start the reactionNo initiator is required to start the reaction


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Stage 1

C onsu ptionof ono er

n n

Stage 2

C o bina tionof s a ll fra g e nts

Stage 3

Re action ofo ligo ers to givehigh olecular

e ight poly e r

Step-Growth Polymerization


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ExampleExampleformation of polyesternHOnHO--RR--OH + nHOOCOH + nHOOC--RR--COOH HCOOH H--(O(O--RR--OOCOOC--RR--COCO--))nnOH+(OH+(22nn--11)H)H22OO

Kinetics of condensation (stepKinetics of condensation (step Growth ) polymerizationGrowth ) polymerizationConsider the synthesis of polyester from a diol and a diacid. The firstConsider the synthesis of polyester from a diol and a diacid. The first

step is the reaction of the diol and the diacid monomers to formstep is the reaction of the diol and the diacid monomers to form

dimerdimer,,

HO-R-OH + HOOC-R"-COOH--> HO-R-OCO-R'-COOH + H2OThe dimer then forms trimer by the reaction with diol

monomer,HOHO--RR--OCOOCO--R'R'--COOH + HOCOOH + HO--RR--OHOH----> HO> HO--RR--OCOOCO--R'R'--COOCOO--RR--OH +HOH +H22OO

and also with diacid monomer,HOHO--RR--OCOOCO--R'R'--COOH + HOOCCOOH + HOOC--R'R'--COOHCOOH---->>

HOOCHOOC--R'R'--COOCOO--RR--OCOOCO--R'R'--COOH + HCOOH + H22OO


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Kinetics of Condensation (Step-Growth) Polymerization StepStep--Growth polymerization occurs by consecutive reactions in whichGrowth polymerization occurs by consecutive reactions in which

the degree of polymerization and average molecular weight of thethe degree of polymerization and average molecular weight of thepolymer increase as the reaction proceeds. Usually (although notpolymer increase as the reaction proceeds. Usually (although not

always), the reactions involve the elimination of a small molecule (e.g.,always), the reactions involve the elimination of a small molecule (e.g.,water). Condensation polymerization may be represented by thewater). Condensation polymerization may be represented by thefollowing reactions:following reactions:

Monomer + Monomer Dimer + H2O

Monomer + Dimer Trimer + H2O

Monomer + Trimer Tetramer + H2O

Dimer + Dimer Tetramer + H2ODimer + Trimer Pentamer + H2O

Trimer + Trimer Hexamer + H2O

Generally, the reactions are reversible, thus the eliminated water must beGenerally, the reactions are reversible, thus the eliminated water must beremoved if a high molecular weight polymer is to be formed.removed if a high molecular weight polymer is to be formed.

Based on the assumption that the polymerization kinetics areBased on the assumption that the polymerization kinetics are

independent of molecular size, the condensation reactions may all beindependent of molecular size, the condensation reactions may all besimplified to:simplified to:

~~~~COOH + HO~~~~~~~~COOH + HO~~~~ pp ~~~~COO~~~~ + H~~~~COO~~~~ + H22OO


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Kinetic analysisKinetic analysis~~~~COOH + HO~~~~ p ~~~~COO~~~~ + H2OMost step polymerization involve bimolecular reaction that are often catalyzed

~~~~A + B~~~~ + catalyst p ~~~~AB~~~~ + catalystThe rate is accelerated according to

-d [A]

By

integration

dtdt= k= k [A[A][B]][B][catalyst][catalyst]

--d [A]d [A]

dtdt

= k= k''[A[A][B]][B]

--d [A]d [A]

dtdt= k= k''[A[A]]22

11

[M][M]--

11= k 't= k 't

[M]o[M]o

OrOr

Where k =Where k = kk[catalyst][catalyst]

I f [AI f [A] = [B]] = [B]

****

By use the extent of the reaction P (fraction of A or B functional groupsBy use the extent of the reaction P (fraction of A or B functional groups

that has reacted at time t )that has reacted at time t )

P = extent of the reaction = the fraction of conversionP = extent of the reaction = the fraction of conversion


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The concentration at any time given by[M]

[M] =[M] = [M]o[M]o -- [M]o P = [M]o ([M]o P = [M]o (11-- P )P )

By substitution in (** )By substitution in (** )

11

((11--p)p)

= k= k''[A[A]o t +]o t + 11

Note that experimental data are usuallylinearonly beyond ca. 80% conversion.


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Polyesterification Without Acidic Catalyst

dt= k[A]2[B]

-d [A]

dt

= k[A]3

Or

I f [A] = [B]

1

[M]2-

1= 2k t

[M]o2**

The rate equation is given by

- d[A]

By

integration

[M

]=

[M

]o -[M]o P = [M]o (1- P )

By substitution in (** )

1

(1-p)2=2 k [A]2ot + 1


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Uncatalyzed Polyesterification

Note that experimental data for esterification reactions show that plots ofNote that experimental data for esterification reactions show that plots of

11/(/(11--p)p)22vs. time are linear only after ca.vs. time are linear only after ca. 8080% conversion% conversion..


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This behavior has been attributed to the reaction medium changingThis behavior has been attributed to the reaction medium changingfrom one of pure reactants to one in which the ester product is thefrom one of pure reactants to one in which the ester product is the

solvent.solvent.

Thus, the true rate constants for condensation polymerizationsThus, the true rate constants for condensation polymerizations

should only be obtained from the linear portions of the plots (i.e., theshould only be obtained from the linear portions of the plots (i.e., the

latter stages of polymerization).latter stages of polymerization).

For example, the kinetic plots for the polymerization of adipic acid andFor example, the kinetic plots for the polymerization of adipic acid and

11,,1010--decamethylene glycol show that atdecamethylene glycol show that at 202202ooC, the thirdC, the third--order rateorder rate

constant for the uncatalyzed polyesterification is k =constant for the uncatalyzed polyesterification is k = 11..7575 xx 1010--22

(kg/equiv)(kg/equiv)22minmin--11..

Polyesterification Without Acidic Catalyst (continued)


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The NumberAverage Molecular Weight in

Polycondensation

. The number. The number--average degree of polymerization Xaverage degree of polymerization Xnn is given as the totalis given as the totalnumber of monomer molecules initially present divided by the total numbernumber of monomer molecules initially present divided by the total number

of molecules present at time t,of molecules present at time t,

XXnn = N= Noo/N= [M ]/N= [M ]oo/[M ] [M ] = [M ]/[M ] [M ] = [M ]oo (( 11 P )P )

XXnn == 11 // 11 -- PP

This relationship is theThis relationship is the Carother's EquationCarother's Equation..

ExampleExampleIf monomer conversion is 99% what is X

n

?

Xn = 1 / 1 P = 1 / 1 - 0.99 = 100If P =99.5 % Xn = 1 / 1 - 0.995 = 200If P =99.6 % Xn = 1 / 1 - 0.996 = 250


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The numberThe number--average molecular weight Maverage molecular weight Mnn

, defined as, defined as

MMnn = M= MooXXnn + M+ Megeg = M= Moo// 11 P + MP + Megegwhere Mwhere Moo is the mean of the molecular weights of the structuralis the mean of the molecular weights of the structural

units, and Munits, and Megegis the molecular weight of the end groups. The latteris the molecular weight of the end groups. The latter

becomes negligible at even moderate molecular weightbecomes negligible at even moderate molecular weight

MMnn = M= MooXXnn + M+ Megeg = M= Moo// 11 PP1

(1-p)= k'[A]ot + 1

Xn= k'[A]ot + 1

X2n=2 k [A]2ot + 1

HH--(O(O--RR--OOCOOC--RR--COCO--))nnOHOH


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Mn as a Function of Conversion


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3- Another method of controlling the molecular weight is by addingAnother method of controlling the molecular weight is by adding

small amounts of monofunctional monomer. (Acetic acid )small amounts of monofunctional monomer. (Acetic acid )

Type (Type (22))

For the polymerization of bifunctional monomers AFor the polymerization of bifunctional monomers A--A and BA and B--B where BB where B--BB

is present in excess, the numbers of A and B F.gs. are given byNis present in excess, the numbers of A and B F.gs. are given byNAA andNandNBB. Notice thatN. Notice thatNAA andNandNBB are equal to twice the number of Aare equal to twice the number of A--A and BA and B--BB

molecules, respectively.molecules, respectively.

The stoichiometric imbalance r of the two f.gs. is given byThe stoichiometric imbalance r of the two f.gs. is given by

r = Nr = NAA/N/NBB. . 11The total number of monomer molecules is given byThe total number of monomer molecules is given by

(N(NAA+N+NBB)/)/22 orNorNAA((11++11/r)//r)/22..

, the total number of polymer molecules is one half the total number, the total number of polymer molecules is one half the total number

of chain ends orof chain ends or

[N[NAA((11--p)+Np)+NBB((11--rp]/rp]/22..

The numberThe number--average DP( Xaverage DP( Xnn)is the total number of A)is the total number of A--A and BA and B--B moleculesB molecules

initially present divided by the total number of polymer molecules:initially present divided by the total number of polymer molecules:

XXnn = N= NAA((11++11/r)//r)/22..

[N[NAA((11--p)+Np)+NBB((11--rp]/rp]/22..


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Xn

= 1 + r

1 + r 2rP

If r = 1

Xn = 1 / 1-p

If p = 1Xn = 1 + r / 1 - r

ExampleExampleWhat is XWhat is Xnn when P =when P = 11 but usebut use 00.. 98009800 moles of Amoles of A--A andA and 11.. 01000100

moles of Bmoles of B BB

r = Nr = NAA/N/NBB == 00..9898 xx 22 // 11..0101 xx 22 == 00..9797

XXnn == 11 + r /+ r / 11 r =r = 11..9797 // 00..0303 == 6666


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Type (Type (33))the molecular weight can also be controlled by adding smallthe molecular weight can also be controlled by adding small

amounts of monofunctional monomer.amounts of monofunctional monomer.

Moles of AMoles of A--A = NA = NAA// 22Moles of BMoles of B--B = NB = NBB// 22

Moles of mono functional B = NMoles of mono functional B = NBB

r = Nr = NAA/ N/ NBB + N+ NBB = N = NAA/N/NBB ++ 22NNBB

ExampleExampleFind Xn for 1 mole of A-A ,1mole of B-B and 0.01 mole of RB when P = 1r = 1/ 1 + 2x 0.01 = 0.99

Xn 1 + r / 1 r = 1 + 0.99 / 1 0.99 = 199

The poly dispersity indexThe poly dispersity indexXXww/ X/ Xnn == 11 + P+ P

XXnn == 11 //11--P XP Xww== 11 + p /+ p / 11 -- PP


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SummarySummary11((11--p)p)

= k= k''[A[A]o t +]o t + 11

1(1-p)2

=2 k [A]2ot + 1

XXnn = N= Noo/N= [M ]/N= [M ]oo/[M]/[M] XXnn == 11 // 11 -- PP

MMnn = M= MooXXnn + M+ Megeg = M= Moo// 11 PP

Xn= k'[A]ot + 1

X2n=2 k [A]2ot + 1

r = Nr = NAA/N/NBB. . 11

Xn = 1 + r1 + r 2rP

If r = 1Xn = 1 / 1-p

If p = 1Xn = 1 + r / 1 - r

r = Nr = NAA/ N/ NBB + N+ NBB = N = NAA/N/NBB ++ 22NNBB

The poly dispersity indexThe poly dispersity index

XXww/ X/ Xnn == 11 + P+ P

XXnn == 11 //11--P XP Xww== 11 + p /+ p / 11 -- PP


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Lecture 3



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- Chain-growth polymerization


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ChainpolymerizationChainpolymerization

The characteristic of chain polymerization are as followThe characteristic of chain polymerization are as follow::11-- Growth is by the addition of the monomer at the end of the chainGrowth is by the addition of the monomer at the end of the chain22--Even at long reaction time some monomer are remain in the reactionEven at long reaction time some monomer are remain in the reaction

mixturemixture

33--The molecular weight of the polymer are increase rapidlyThe molecular weight of the polymer are increase rapidly

44--Different mechanisms operates at different stages of the reactionDifferent mechanisms operates at different stages of the reaction

55--The polymerization rate initially increases and then become constantThe polymerization rate initially increases and then become constant66--An initiator is required to start the reactionAn initiator is required to start the reaction

Chain polymerization reaction consists of three stagesChain polymerization reaction consists of three stages1- Initiation

2- Propagation

3-Termination


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Polymerization depend on thermodynamicPolymerization depend on thermodynamicPolymerization is possible only if the free energy difference betweenPolymerization is possible only if the free energy difference between

monomer and polymer is negativemonomer and polymer is negative

((G =G = ((HH -- TT((SS00Must beMust be --ve forve for

Polymerization toPolymerization to

workwork In chainIn chain

polymerizationpolymerization

are exothermicare exothermic

AlwaysAlways

+ve+ve

AlwaysAlways ve in chainve in chain



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ChainpolymerizationChainpolymerization

Radical polym.Radical polym.The C=C is preferThe C=C is prefer

the Polym. by R.P.the Polym. by R.P.

and also can beand also can be

used in the stericused in the steric

hindrance of thehindrance of the

substituentsubstituent

Ionic polymIonic polym..

Anionic polym.Anionic polym. Cationic polym.Cationic polym.

X X X

radical cationic anionic

Electron with drawingElectron with drawing

substituent decreasingsubstituent decreasing

the electron density onthe electron density onthe double bond andthe double bond and

facilitate the attack offacilitate the attack of

anionic speciesanionic species

such as cyano andsuch as cyano and

carbonylcarbonyl++ --

CHCH22=CH Y=CH Y

Electron donatingElectron donating

substituent increasingsubstituent increasing

the electron density onthe electron density onthe double bond andthe double bond and

facilitate the attack offacilitate the attack of

cationic speciescationic species

such as alkoxy, alkyl,such as alkoxy, alkyl,

alkenyl, and phenylalkenyl, and phenyl-- ++

CHCH22=CH Y=CH Y

Vi l f dditi


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Unsubstituted (ethylene)

Works fine.

Monosubstituted

Works fine.

1,1-Disubstituted

Usually works.

1,2-DisubstitutedSeldom works.

TrisubstitutedAlmost never works.

Tetrasubstituted

Almost never works.

The only exceptions to the

unreactivity of tri- and tetra-

substituted vinyl monomers are

those with fluorine, liketetrafluoroethylene (CF2=CF2). The

main cause of this reactivity

pattern is the steric size of the

substituents.

Vinylmonomers for additionpolymerizations


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Free Radical Vinyl Chain PolymerizationFree Radical Vinyl Chain Polymerization

Rate of Radical Chain PolymerizationRate of Radical Chain Polymerization

Radical polymerization consists of three stepsRadical polymerization consists of three steps--initiation, propagation,initiation, propagation,and termination.and termination.

TheThe initiationinitiation step consists of two reactions.step consists of two reactions.

11--The production of the free radicalThe production of the free radical

kd

I ------> 2R22-- Addition of this radical to a monomer molecule to produce theAddition of this radical to a monomer molecule to produce thechain initiating species Mchain initiating species M11

kkiiR + MR + M11 ----------> M> M11

TheThepropagationpropagation consists of the growth of Mconsists of the growth of M11kkppMMnn ++ MM11

------------> M> Mn+n+11

(Rapid reaction )(Rapid reaction )


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. TerminationTermination with the annihilation of the radical centers occurs bywith the annihilation of the radical centers occurs by

bimolecular reaction between radicals either by combination or,,bimolecular reaction between radicals either by combination or,,

by disproportionationby disproportionation

kktctcMMnn + M + Mmm ----------> M> Mn+mn+m

kktdtdMMnn + M + Mmm ----------> M> Mnn+ M+ Mmm

The termination step can be represented byThe termination step can be represented by

kkttMMnn+ M+ Mmm

--------> dead polymer> dead polymer


53/172

Kinetic Rate ExpressionKinetic Rate Expression

The rate of monomer disappearance, = the rate of polymerization,The rate of monomer disappearance, = the rate of polymerization,

is given byis given by

pi RRdt

]M[d!

Since forthe productionofhighmolarmassmaterialRSince forthe productionofhighmolarmassmaterialRpp RRii thisequationcanbe rethisequationcanbe re--writtenas:writtenas:

]M][M[kRdt

]M[dpp !!

Fromthe beginningofthe polymerization:Fromthe beginningofthe polymerization: increasingnumberofradicalsdue todecompositionoftheincreasingnumberofradicalsdue todecompositionofthe

initiatorinitiator increasingterminationdue toincreasingradicalincreasingterminationdue toincreasingradical

concentration(Rconcentration(Rtt ww [M][M]22)) eventuallyasteadystate inradicalconcentration:eventuallyasteadystate inradicalconcentration:

****


54/172

This is equivalent to stating that the rate of initiation RThis is equivalent to stating that the rate of initiation Riiequals theequals the

rate of termination Rrate of termination Rtt

RRpp =k=kpp [M] ( R[M] ( Rii//22kktt) )

Ri = 2kt[M.

]2

[M ] = ( R[M ] = ( Rii//22kktt) )

and substitution in Eq.* * yields for the rate of polymerization.and substitution in Eq.* * yields for the rate of polymerization.


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Initiation free radical polymerizationInitiation free radical polymerization

ThermalThermal initiatorsinitiators

PhotochemicalPhotochemical

Redox initiatorsRedox initiators Ionizing radiationIonizing radiation

Thermal initiators:Thermal initiators:


56/172

Thermal initiators:Thermal initiators:

Most common kind of FR initiator.Most common kind of FR initiator.

Unimolecular decomposition.Unimolecular decomposition.

First order kinetics.First order kinetics.

Most common examples: peroxides (benzoyl peroxide)orMost common examples: peroxides (benzoyl peroxide)or

azo compounds(azo isobuteronitrile).azo compounds(azo isobuteronitrile).

Peroxides Azo compaunds

(I p 2R)

(Temperatures are for 10 hour half-lives.)


57/172

The thermal, homolytic dissociation of initiators is the most widelyThe thermal, homolytic dissociation of initiators is the most widely

used method for generating radicals to initiate polymerization.used method for generating radicals to initiate polymerization.

The compounds used as initiators are those with bond dissociationThe compounds used as initiators are those with bond dissociation

energies in the rangeenergies in the range 100100--170170 kJ/mole.kJ/mole.

The rate of producing primary radicals by thermal homolysis of anThe rate of producing primary radicals by thermal homolysis of an

initiator Rinitiator Rddis given byis given by

RRdd== 22fkfkdd[I][I]

where [I] is the concentration of the initiator and f is the initiatorwhere [I] is the concentration of the initiator and f is the initiatorefficiency.efficiency.

and the rate of initiation is given byand the rate of initiation is given by

RRii==22fkfkdd[I][I]

By substitute inBy substitute in RRpp =k=kpp [M] ( R[M] ( Rii//22kktt) )

RRpp =k=kpp [M] (fk[M] (fkdd[I] /k[I] /ktt) )

PhotochemicalPhotochemical initiatorsinitiators:


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PhotochemicalPhotochemical initiatorsinitiators:One or two component.One or two component.

Used for thin films.Used for thin films.

PeroxidesPeroxides

Azo compaundsAzo compaunds DisulfidesDisulfides

KetonesKetones

S S S

h

R

RedoxRedox initiatorsinitiators::

UsuallyUsually 22 component.component.Rarely used.Rarely used.

Ionizing radiation:

XX--ray, gammaray, gamma--ray.ray.Random destruction leads toRandom destruction leads to

radical formation.radical formation.

Used only in very specialUsed only in very special

casescases.

Fentons reagent


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Experimental Determination of RExperimental Determination of Rpp

RRpp can be experimentally determined by measuring the change in anycan be experimentally determined by measuring the change in any

property that differs for the monomer(s) and polymer, for example,property that differs for the monomer(s) and polymer, for example,solubility, density, refractive index, and spectral absorptionsolubility, density, refractive index, and spectral absorption

The polymerization can also be followed by separation and isolation ofThe polymerization can also be followed by separation and isolation of

the reaction products. Chemical analysis of the unreacted monomersthe reaction products. Chemical analysis of the unreacted monomers

as a function time is also used.as a function time is also used.

The disappearance of monomers or the appearance of polymer can beThe disappearance of monomers or the appearance of polymer can be

followed spectroscopically, i.r. or uv spectroscopyfollowed spectroscopically, i.r. or uv spectroscopy

DilatometryDilatometry

Dilatometry is the volume changes that occurs upon polymerization toDilatometry is the volume changes that occurs upon polymerization tofollow the conversion. It is the most accurate method for chainfollow the conversion. It is the most accurate method for chain

polymerization because of the large difference in density betweenpolymerization because of the large difference in density between

monomer and polymermonomer and polymer


60/172


61/172

Kinetic chain lengthKinetic chain length RR

i

p

R

R

speciesinitiating

unitsmonomerofnumber!!R

By substituteBy substitute

RRii==22f kf kdd[I] R[I] Rpp =k=kpp [M] (f k[M] (f kdd[I] /k[I] /kii) )

=R=Rpp/R/Rii=R=Rpp/R/Rtt= k= kpp [M] /[M] / 22 (f k(f kddkktt[I] )[I] )11//22

Kinetic chain length v is defined as the average number of monomerKinetic chain length v is defined as the average number of monomer

molecules polymerized per each radical, which initiate amolecules polymerized per each radical, which initiate a

polymer chain. In other words, v is the ratio between the propagationpolymer chain. In other words, v is the ratio between the propagation

rate to that of initiation, or termination.rate to that of initiation, or termination.


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The number average degree of polymerization XThe number average degree of polymerization Xnn of chainsof chains

formed at a certain moment is dependent on the terminationformed at a certain moment is dependent on the termination

mechanism:mechanism:** combination: Xcombination: Xnn == 22YY

** disproportionation: Xdisproportionation: Xnn == YY

chemistry:chemistry:

CH2 C

H

+ C

H

CH2 CH2 C

H

C

H

CH2

CH2 C

CH3

C O

OMe

+ C

CH3

C

CH2

O

OMe

CH2 C

CH3

C

H

O

OMe

+ C

CH2

C

CH2

O

OMe


63/172

Lecture 4


Polymerization

Monomer Polymer


64/172

Chain TransferChain TransferChain transfer is a chain breaking reaction; it is a premature terminationChain transfer is a chain breaking reaction; it is a premature termination

of polymer growing radical by the transfer of hydrogen or other atomof polymer growing radical by the transfer of hydrogen or other atom

or species to it from some compound present in the system .or species to it from some compound present in the system .

This leads to a decrease in the molecular weight than expected.This leads to a decrease in the molecular weight than expected.

MMnn + XY M+ XY Mnn--X +YX +Y

..

where XY may be monomer, solvent, initiator, or other moleculewhere XY may be monomer, solvent, initiator, or other molecule

and X is the atom or species transferred.and X is the atom or species transferred.

The rate of chain transfer reaction is given byThe rate of chain transfer reaction is given by

RRtrtr= K= Ktrtr[M[M..][XY]][XY]

where Kwhere Ktrtr is the chain transfer rate constant.is the chain transfer rate constant.

Chain transfer results in the production of a new radical Y which couldChain transfer results in the production of a new radical Y which could

induce polymerization. The effect of chain transfer on the polymerizationinduce polymerization. The effect of chain transfer on the polymerization

rate depends on whether the rate of reinitiation is comparable to therate depends on whether the rate of reinitiation is comparable to the

original rate of initiationoriginal rate of initiation

kktrtr


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Effect on XEffect on XnnEffect onEffect on

RRpp

Type ofeffectType ofeffectRelative rateconstantsRelative rateconstants

for Transfer,for Transfer,Propagation, andPropagation, and

ReinitiationReinitiation

CaseCase

DecreaseDecreaseNoneNoneNormalchainNormalchain

transfertransfer

KKpp>>k>>ktrtr kkaa~K~Kpp11

LargeLargedecreasedecrease

NoneNoneTelomerizationTelomerizationKKppktrtr kkaa


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The degree of polymerization now should be redefined as theThe degree of polymerization now should be redefined as the

polymerization rate divided by the sum of all the chain breakingpolymerization rate divided by the sum of all the chain breaking

reactions:reactions:

XXnn = R= Rpp(R(Rtt//22) + K) + KtrMtrM [M][M] + K[M][M] + Ktrstrs [M][S] + K[M][S] + KtrItrI[M][I][M][I]

C=chain transfer constant = KC=chain transfer constant = Ktrtr/ K/ Kpp

CCMM=K=KtrMtrM/K/Kpp CCSS=K=KtrStrS/K/Kpp CCII=K=KtrItrI/K/Kpp RRpp=K=Kpp [M][M][M][M]

11/X/Xnn=R=Rtt//22RRpp+ K+ KtrMtrM[M][M]/R[M][M]/Rpp+ K+ Ktrstrs [M][S]/R[M][S]/Rpp+K+KtrItrI[M][I]/R[M][I]/RppSubstitute by the value of RSubstitute by the value of Rpp11/X/Xnn=R=Rtt// 22RRpp+K+KtrMtrM/ K/ Kpp+ K+ Ktrstrs [S] / K[S] / Kpp[M] +K[M] +KtrItrI[I] / K[I] / Kpp [M][M]

Substitute by the value of CSubstitute by the value of C

11/X/Xnn=R=Rtt// 22RRpp+C+CMM+C+CSS[S] /[M] +C[S] /[M] +CII[I] /[M][I] /[M] MayoMayo walling equationwalling equation

11/X/Xnn== 11/(X/(Xnn))oo +C+CMM+C+CSS[S] /[M]+C[S] /[M]+CII[I] /[M][I] /[M]

F i t f i iti t [I] d [M] dF i t f i iti t [I] d [M] d


67/172

1/D

s

1

/D

[ ]/[ ]

Generic Mayo plotGeneric Mayo plot

For a given amount of initiator[I] and monomer[M] andFor a given amount of initiator[I] and monomer[M] and

In the presence of chain transfer agentIn the presence of chain transfer agent

11/X/Xnn == 11/(X/(Xnn))oo +C+CSS[S] /[M ][S] /[M ]

Energetic CharacteristicsEnergetic Characteristics


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Energetic CharacteristicsEnergetic Characteristics

Activation Energy and Frequency FactorActivation Energy and Frequency Factor

. Increasing the temperaure usually increase the rate and decrease theIncreasing the temperaure usually increase the rate and decrease the

molecular weight.molecular weight.The rate constants of initiation, propagation, and termination can beThe rate constants of initiation, propagation, and termination can be

expressed by an Arrheniusexpressed by an Arrhenius--type relationshiptype relationship

k = A ek = A e E / RTE / RT

orlnk = lnAlnk = lnA E / RTE / RT

where A is the collision frequency factor, and E the Arrheniuswhere A is the collision frequency factor, and E the Arrhenius

activation energy. A plot of ln k vsactivation energy. A plot of ln k vs 11/T allows the determination/T allows the determination

of both values.of both values.


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Rate of PolymerizationRate of Polymerization

For a polymerization initiated by the thermal decomposition of anFor a polymerization initiated by the thermal decomposition of an

initiator the polymerization rate depends on three rate constantsinitiator the polymerization rate depends on three rate constants

KKpp ( k( kdd/ k/ ktt))1/21/2

? A40)-(3

)2/()2/(lnln

2/12/1

RT

EEE

A

AA

k

kk tdP

t

dP

t

dP

-

!

-

The composite or overall activation energy for the rate of polymerizationThe composite or overall activation energy for the rate of polymerization

EERRis [Eis [Epp + (E+ (Edd//22))--(E(Et/t/22)]. can be written as)]. can be written as

? A 41)-(3RTE-][])[(lnlnln R2/1

2/1

MIfAAAR

t

d

-

!

RRpp =k=kpp [M] (fk[M] (fkdd[I] /k[I] /ktt) ) WhereWhere


70/172

Must beMust be --ve forve forPolymerization toPolymerization to

workwork In chainIn chain


are exothermicare exothermic

AlwaysAlways

+ve+ve

AlwaysAlways ve in chainve in chain


((G =G = ((HH -- TT((SS00

Thermodynamics of PolymerizationThermodynamics of Polymerization

Polymerization ofPolymerization of 11,,22--Disubstituted EthylenesDisubstituted Ethylenes

11,,22--Disubstituted ethylenes exhibit very little or no tendency toDisubstituted ethylenes exhibit very little or no tendency to

undergo polymerization. Steric inhibition is the cause of thisundergo polymerization. Steric inhibition is the cause of this

behaviorbehavior

RR RR substitutedsubstituted

PolymerizationPolymerization Depolymerization EquilibriaDepolymerization Equilibria


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PolymerizationPolymerization--Depolymerization EquilibriaDepolymerization Equilibria

Ceiling TemperatureCeiling Temperature

For most chain polymerization there is some temperature at which theFor most chain polymerization there is some temperature at which the

reaction becomes a reversible one, that is, the propagation step shouldreaction becomes a reversible one, that is, the propagation step shouldbe written as an equilibrium reactionbe written as an equilibrium reaction

Mn + Mkp

kdpMn+1 (3- 3)

where kwhere kdpdp is rate constant for the reverse reactionis rate constant for the reverse reaction--termedtermed

depolymerization or depropagationdepolymerization or depropagation

The reaction isothermThe reaction isotherm

G = Go + RT lnK

. For an equilibrium situationFor an equilibrium situation ((G=G=00 byby

(Go = (Ho - T(So = - RT ln K

equilibrium constant is defined by Kp/kdp or by

46)-(3][

1

]][[

][.

.

1

MMM

MK

n

n!!

48)-(3ln[M]

or

47)-(3][

1]ln[

cR

S

RT

H

MMRSHT

o

c

o

c

o

o

c

(

(!

!((!


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Ionic chain polymerizationIonic chain polymerization

The characteristic of ionic chain polymerization are as followThe characteristic of ionic chain polymerization are as follow

11--Ionic polymerization is limited because the ions are usually unstableIonic polymerization is limited because the ions are usually unstable

and require stabilization by solvation and lower temperature forand require stabilization by solvation and lower temperature for

polymerization to proceedpolymerization to proceed

22-- The ionic polymerization proceeds with very high rates and is veryThe ionic polymerization proceeds with very high rates and is very

sensitive to the presence of small amounts of impuritiessensitive to the presence of small amounts of impurities

33--Cationic and anionic polymerizations have very similar characteristics.Cationic and anionic polymerizations have very similar characteristics.both depend on the formation and propagation of ionic speciesboth depend on the formation and propagation of ionic species

44--solvents of high polarity cannot be used. The highly polar hydroxylicsolvents of high polarity cannot be used. The highly polar hydroxylic

solvents (water, alcohol) react and destroy most ionic initiators. Other polarsolvents (water, alcohol) react and destroy most ionic initiators. Other polar

solvents such as ketones form highly stable complexes with the initiatorssolvents such as ketones form highly stable complexes with the initiators

preventing thus the polymerization. Ionic polymerization,thus require solventpreventing thus the polymerization. Ionic polymerization,thus require solventof low or moderate polarity such as CHof low or moderate polarity such as CH33Cl,CHCl,CH22ClCl22, and pentane, and pentane..

55--Ionic polymerizations are characterized by a wide variety of modesIonic polymerizations are characterized by a wide variety of modes

of initiation and termination.of initiation and termination.

CATIONIC POLYMERIZATIONCATIONIC POLYMERIZATION


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CATIONIC POLYMERIZATIONCATIONIC POLYMERIZATION

InitiationInitiation

aa--Protonic AcidsProtonic Acids

Protonic acids can be used to some extent but the anion of the acidProtonic acids can be used to some extent but the anion of the acidshould not be highly nucleophilicshould not be highly nucleophilic

HA+ RR"C=CH2 RR"C+(A)

-

CH3

(4_1)

Halogen acids areHalogen acids are not usednot usedbecause of the highly nucleophilicbecause of the highly nucleophilic

character of the halide ioncharacter of the halide ionOther strong acids such as perchloric, sulfuric, phosphoric,Other strong acids such as perchloric, sulfuric, phosphoric,

chlorosulfonic, methansulfonic,etc,chlorosulfonic, methansulfonic,etc,usedusedfor cationicfor cationic

polymerization.polymerization.

mineral acids (initiators): H2SO4, H3PO4 (p provide H+)

The molecular weight obtained is low (few thousand).The molecular weight obtained is low (few thousand).


74/172

bb--Lewis AcidsLewis Acids

Lewis acids used to initiate cationic polymerization at lowLewis acids used to initiate cationic polymerization at low

temperatures, may yield high molecular weight polymerstemperatures, may yield high molecular weight polymers

Lewis acids (co-initiators): AlCl3, BF3, TiCl4, SnCl4(often require otherproton or cation source)

Forming (co-initiator / initiator) system

BF3 + H H BF3-H+ (7.2)

lCl3 + Cl AlCl4-R

+ (7.3)

Very active Lewis acids p can undergo auto-ionization

2AlBr3 AlBr4-

AlBr2(7.4)

The initiation process can be generalized asThe initiation process can be generalized as

I + ZYk Y

+(IZ)

-

Y

+

(IZ)

-+M

k YM+(IZ)

- (4-5 )


75/172

Propagation:Propagation:depending on the association degree between ions

The initiator ion pair (the carbonium ion and its counter ion)The initiator ion pair (the carbonium ion and its counter ion)produced in the initiation step proceeds to grow by theproduced in the initiation step proceeds to grow by the

successive addition of monomer moleculessuccessive addition of monomer molecules

This addition can be occuring by insertion of ( M ) between theThis addition can be occuring by insertion of ( M ) between the

carbonium ion and its counter ioncarbonium ion and its counter ion

HMHMnn++(IZ)(IZ) --+ M HM+ M HMnnMM

++(IZ)(IZ)--

TerminationTermination


76/172

11--Chain Transfer to MonomerChain Transfer to Monomer. This involves transfer of a proton to a monomer molecule withThis involves transfer of a proton to a monomer molecule with

the formation of terminal unsaturation in the polymer moleculethe formation of terminal unsaturation in the polymer molecule

HMnM+(IZ)- + M Mn+1 + HM

+(IZ)-

22--Spontaneous TerminationSpontaneous TerminationSpontaneous termination involves regeneration of the initiatorSpontaneous termination involves regeneration of the initiator--

coinitiator complex by expulsion from the propagating ion paircoinitiator complex by expulsion from the propagating ion pairwith the polymer molecule left with terminal unsaturation.with the polymer molecule left with terminal unsaturation.

HMnM+(IZ)- Mn+1 + H

+(IZ)-

33--Combination with counter ionCombination with counter ionTermination by combination of the propagating carbonium ionTermination by combination of the propagating carbonium ion

with its counter ion occurswith its counter ion occurs

HMnM+(IZ)- HMnMIZ

TerminationTermination

Ki tiKi ti


77/172

KineticsKineticsUnder steady state conditions (RUnder steady state conditions (Rii=R=Rtt) follows in a manner similar) follows in a manner similar

to that for radical polymerization. The rates of initiation, propagation,to that for radical polymerization. The rates of initiation, propagation,

and termination are given byand termination are given by

RRii= Kk= Kkii[I][ZY][M] R[I][ZY][M] Rpp = K= Kpp [YM[YM++(IZ)(IZ)--][M]][M] RRtt= k= ktt[YM[YM

++(IZ)(IZ)--]]

Where [YMWhere [YM++(IZ)(IZ)--] is the total concentration of all sized] is the total concentration of all sized

propagation centerspropagation centers

13)-(4]][][[

])([t

i

k

MZYIkIZYM !

14)-(4]][][[][ 2

t

Pi

t

PiP

k

MZYIkKk

k

MkRR !!

The numberThe number--average degree of polymerization is obtained as theaverage degree of polymerization is obtained as the

propagation rate over the termination ratepropagation rate over the termination rate

1 )-(4][

tt

nk

Mk

R

RX !!


78/172

When chain breaking involves chain transfer to monomer and/orWhen chain breaking involves chain transfer to monomer and/or

termination in addition to combination with gegenion, the degree oftermination in addition to combination with gegenion, the degree of

polymerization ispolymerization is

1 )-(4,Mtrt

nRR

RX

!

The rate of chain transfer to monomer is given byThe rate of chain transfer to monomer is given by

Rtr,M = ktr,M[YM+(IZ-)][M]

17a)-(4][

][

, Mkk

MkX

Mtrt

Pn !

ThenThen

OrOr

17b)-(4C

][

1M

!

Mk

k

X P

t

n

where Cwhere CMMis the chain transfer constant for monomer.is the chain transfer constant for monomer.


79/172

Effect of Reaction MediumEffect of Reaction Medium

Solvent EffectsSolvent Effects

Large increase in the rate and degree of polymerization are observedLarge increase in the rate and degree of polymerization are observed

if one increases the solvating power of the solvent.if one increases the solvating power of the solvent.

. The free ion concentration increases with increased solvating power,. The free ion concentration increases with increased solvating power,

this leads to an increase in Rthis leads to an increase in Rpp as the free ions propagate faster thanas the free ions propagate faster than

the ion pair.the ion pair.

Effect of GegenionEffect of Gegenion

The larger and less tightly bound the gegenion, the greater shouldThe larger and less tightly bound the gegenion, the greater should

be the reactivity of the ion pair toward propagationbe the reactivity of the ion pair toward propagation


80/172

EnergeticsEnergetics

Cationic polymerization is also exothermic, since the reaction involvesCationic polymerization is also exothermic, since the reaction involvesthe conversion of the conversion of --bond into bond into --bond.bond.

the activation energies for the rate and degree of polymerization arethe activation energies for the rate and degree of polymerization are

obtained asobtained as

EERR= E= Eii+E+Epp--EEtt


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Trommsdorff effect


82/172

Trommsdorff effect

In radical polymerization we speak about:1) lowconversion, i.e. polymerchainsare in dilute solution (no

contactamong chains)2) intermediate conversion, i.e. the area in between low andhigh conversion3) high conversion, i.e. chainsare getting highly entangled; kpdecreases.

Somewhere in the intermediate conversion regime:* polymerchains loose mobility.* Termination rate decreases* Radical concentration increases* Rate ofpolymerization increases* Molarmass increases

Thiseffect iscalled: gel effect, Trommsdorff effect,orauto-acceleration

In the polymerization of MMA thisoccursatrelatively low

conversion.


83/172

Molar mass

R!

R!

n

n

P

2P If termination takes place by combination)

If termination by takes place disproportionation)

However, a growing chain may transfer its activity to a new chain:

This reaction is then followed by re-initiation, the start of a new chain:

Mi + T Mi + Tktr

T + M M1ki'

in the ideal case:


84/172

Kinetics of free-radical chain polymerizationconsidering chain transfer reactions

RMn + S-Hp RMn-H+ S

Rtr=ktr[M][Transfer agent]


85/172

Chaintransfer

chaintransferto: monomer initiator solventorchaintransferagent polymer allylic transfer

monomer, initiatorand chaintransferagentare mathematically treated identically:

][X]M[k]M[k2]M[kdt

]polymer[dX,tr

2

td

2

tc !

As derived beforethis leads to:

]M[k

]I[k

]M[k

]S[k

k

k

]M[k

]M[k2

]M[k

]M[k

]M[k]X[k

]M[k]M[k2

]M[k]M[k1

p

I,tr

p

S,tr

p

M,tr

p

td

p

tc

p

X,tr

p

td

p

tc

n

!

!!


86/172

The rate ofpolymer formation is now defined as:

]T][M[k]M[k2]M[kdt

]polymer[dtr

2

td

2

tc !

The rate of polymerization as derived before:

]M][M[kdt

]M[d p!

From the definition of number average degree of polymerization it follows:

dt

]polymer[d

dt

]M[dPn !

]M[k

]T[k

]M[k

]M[k2

]M[k

]M[k

]M][M[k

]T][M[k]M[k2]M[k

P

1

p

tr

p

td

p

tc

p

tr2

td2

tc

n

!

!! thus:

]M[

]T[C

P1

1P

T0,n

n

!


87/172

Chain transfer to polymer


88/172

p y

Intermolecular chain transfer

Intramolecular chain transfer

Traditional approach: intermolecular, strong increase in

branching density towards high conversion.

Recent results:

Hardly conversion dependent

Dilution results in higher

degree of grafting

0 10 20 30 40 50

1

2

3

4

5

6

7

8

co rsio c . 25%

co rsio > 80%

m

ol

%

r

c

s

[BA]0

/ %( / )


89/172

Summary

tMkL p ][!

]][[ y! MMkR pp

Chain-length

Rate ofpolymerization

Initiator decomposition is the reaction stepmost stronglyinfluenced by temperature.

][

1

~ yMtTime of chain-growth


90/172

Thermodynamics of radical


91/172

Ea = 1/2Ed+(Ep-1/2Et)Overall activation energy of polymerization:

Ed } 125 170 kJ mol-1 (Ep-1/2Et) } 20 30 kJ mol-1 Thus, initiation is the rate determining step

Polymeriszation rate } exp(-Ea/RT) Thus, it will increase as the temperature is raised

y

polymerisation


92/172


93/172

Copolymerization


94/172

Copolymerizationffii:: fraction ofmonomerfraction ofmonomeriiin reaction mixturein reaction mixture

ff11 = [M= [M11] / ([M] / ([M11] [M] [M22])])

FFii:: fraction ofmonomerfraction ofmonomeriibuilt intopolymerbuilt intopolymer

FF11 = d[M= d[M11] / (d[M] / (d[M11] d[M] d[M22])])

22221

211

212

111

2 ffff

fffF

Long chain assumption (Long chain assumption (kkii,, kkdd ignored;ignored; kkpp,, kkttnot ~ chainnot ~ chainlength)length)

Reactivity ratios independent of environmental factorsReactivity ratios independent of environmental factors

22221111

22221

211

p

2

kfrkfr

frfffrk

!Average copolymerisation rate:Average copolymerisation rate:

Id l l i i


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0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1f1

F1

MMA / BA

Ethene / VAc

VDC / VC

Ideal copolymerisation

Composition driftComposition drift

IfIfff11 F F11

ff11 changeschanges FF11 changeschanges

What does compositionWhat does compositiondrift mean for the polymerdrift mean for the polymerthat is formed?that is formed?

Polymerization techniques


96/172


Kinetic / mechanistic factors related to chain

length, chain composition Technological factors e.g. heat removal,reaction rate, viscosityof the reaction mixture,morphologyof the product

Economic factors; production costs,enviromental aspects, purification steps etc.

Sometimes for one monomer several techniques of

polymerizing are available. Choice of a specifictechnique depends on a number of factors:



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Homogeneous systems Bulk polymerization Solution polymerization

Heterogeneous systems Suspension polymerization Emulsion polymerization Precipitation polymerization Polymerization in solid state Polymerization in the gas phase


Bulk polymerization


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Advantages: Disadvantages:

Simple, only the monomer and initiatorare present in the reaction mixture

High molecular weight

Exothermof the reaction might be hard tocontrol- molecular weights verydisperse

The polymer is solublein the monomer:

Thepolymeris not soluble

inthe monomer:

Viscosityof the reactionincreases markedly (geleffect)

Polymerprecipitates outwithout increase in solutionviscosity

Rp

Solution polymerization


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Solution polymerizationMonomer dissolved in solvent, formed polymer stays

dissolved. epending on concentration ofmonomer thesolution does not increase in viscosity.

Advantagesisadvantages* Product sometimes * Contamination with

directly usable solvent* Controlled heat * Chain transfer to

release solvent* Recycling solvent

ApplicationsAcrylic coating, fibrespinning, film casting


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Suspension polymerization


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p p y

Water insoluble monomers are dispersed in water.

Initiator dissolved in monomer. Stabilization of droplets/polymerparticles with non-micelle formingemulsifiers like polyvinylalcohol or Na-carboxymethylcellulose.

Equivalent to bulk polymerization,small droplets dispersed in water.

Product can easily be separated,particles 0.01-1mm.

Pore sizes can be controlled by adding a combination of solvent(swelling agent) and non-solvent.

Viscosity does not change much.


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103/172

Emulsion Polymerization


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y

A micelle forming emulsifier is used.

Initiator is water soluble. The formed latex particles are much smaller

than suspension particles (0.05-2 m). Kinetics differ considerable fromother techniques. Polymer is formed within the micelles

and not in the monomer droplets.

Emulsion Polymerization


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y

Advantages Disadvantages

* Low viscosity even * Contamination ofat high solid contents products with additives

* Independent control * More complicatedof rate and in case of watermolecular-weight soluble monomers

* Direct application ofcomplete reactor contents

Overview of Ionic Polymerization:


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Ionic polymerizations are more selective than radicalprocesses due to strict

requirements for stabilization of ionic propagating species.Cationic: limited tomonomers with electrondonating groups

y

Selectivity

limited tomonomers with electron withdrawing groupsAnionic:

Overview of Ionic Chain Polymerization:


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A counterion is present in both anionic and cationic polymerizations,yielding ion pairs, not free ions.

Cationic: ~~~C+(X-) Anionic: ~~~C-(M+)

There will be similar effects of counterion and solvent on the rate,

stereochemistry, and copolymerization for both cationic and anionicpolymerization.

Formation of relatively stable ions is necessary in order to have

reasonable lifetimes for propagation. This is accomplished by using low

temperatures (-100 to 50 C) to suppress termination and transfer and

mildly polar solvents (pentane, methyl chloride, ethylene dichloride).

y

Counterions

Overview of Ionic Polymerization


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There are four states of ion-pair binding:(I) ~~~BA ~~~B+A- (II)covalent bond tight or contact ion pair,

intimate ion pair

(III) ~~~B+||A- ~~~B+ + A- (IV)

solvent-separated, Free ion, very reactiveloose ion pair but low concentration

Most ionic polymerizations have equilibrium between ion pairs (II or III,

depending upon solvent) and free ion (IV).

Ion-pairBinding

Overview of Ionic Chain Polymerization:


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Reactions are fast but are extremely sensitive to small amounts ofimpurities. Highly polar solvents (water, alcohols, ketones) will reactwith and destroy or inactivate the initiator. Moreover, heterogeneous

initiators are used making the nature of the reaction medium unclear

and determination of the mechanism difficult.

Termination by neutralization of the carbo-cation (carbonium ion,carbenium ion) occurs by several processes for cationicpolymerization, but termination is absent for anionic polymerization.

y

Mechanistic Analysis

Initiation of Cationic Chain Polymerization:


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Initiation of Cationic Chain Polymerization:

ProtonicAcids HBr, HI

HA + (CH3)2C=CH2 (CH3)3C+(A-)

LewisAcids AlCl3

, BF3

, SnCl4

A co-initiator (water, protonic acids, alkyl halides) is needed to activatethe Lewis acid.

BF3 + H2O BF3-OH2

BF3-OH2 + (CH3)2=CH2 (CH3)2C+(BF3OH)

-

m

o

nCationic Chain Propagation:


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no

m

ers:

CH

2=CH

RM

ono

m

er

Substituents must be able to stabilize a formalpositive charge. Forolefins, tertiary secondary primary due to inductive effect.For styrenic monomers:

Monomer kp, liter/mole sec

R = Cl 0.0012R = H 0.0037R = CH3 0.095R = OCH3 6

Steric effects dominate forortho substitution in styrene,and all substituents reduce kp irrespective of inductive effects.

Cationic Chain Propagation:

MonomerStructure

Cationic initiators:


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Proton acids with unreactive counterions

Lewis acid + other reactive compound

R

S

O

OH

O O

H

R

S

O

OH

O O

H

R

S

O

OH

O O

HNOT

With Lewis acid initiator one must need a co-initiator,

a protogen:

a cationogen:

or

Common steps of cationic polymerization:

(i ii) initiation propagation


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(i, ii) initiation, propagation

The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.

Electron donating groups are needed as the R groups because these can stabilize the

propagating species by resonance. Examples:Propagation is usually very

fast. Therefore, cationic

vinyl polymerizations must

often be run at low

temperatures. Unfortunately,cooling large reactors is

difficult and expensive. Also,

the reaction can be inhibited

by water if present in more

than trace amounts, so

careful drying of ingredients

is necessary (another

expense).

Lewis acids form active catalyst-co-catalyst


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complexes with proton donors

Regiochemistry of propagation


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Markownikov addition form the most stable carbocation:

Electron-donating groups R stabilize a cation and affect regiochemistry by

directing the incoming groupE to an opposite side to the donating group.

RH

H H

E

RH

H H

E

RH

H H

ENOT

R = Alkyl,Aryl,

Halide,OR

Common steps of cationic polymerization: (iii)


116/172

termination by unimolecular rearrangment of the ion pair

A

B

Common steps of cationic polymerization:

(i ) h i t f t


117/172

(iv) chain transfer to monomer

Cationic vinyl polymerization canbe stopped also by numerousside reactions, most of whichlead to chain transfer. It isdifficult to achieve high MWbecause each initiator can give

rise to many separate chainsbecause of chain transfer. Theseside reactions can be minimizedbut not eliminated by running thereaction at low temperature.


118/172

Initiation

General kinetic scheme for cationic polymerisation


119/172

Initiation

Propagation

Termination

Ri =kic[M]

I+ A +Mp IM+ A

IM1+ A +Mp IM2

+A

IMn+ Ap IMn +H

+ A

Rt =kt[M+]

+

M

IM1+A

XX

I

X

I

X

IM2+A

XX

I

IMn+A

n-1

+

XX

R

IMn

n

A H+A

I+

+

M

IM+

X

I

X

A

A

A

IMn+A+Mp IMn+1

+A

Rp =kp[M+][M]

XX

R

n-

+

M

X XX

R

n

IMn+A IMn+1

+A

General kinetic scheme for cationic polymerisation

(continuation)


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(continuation)

Steady-state approximation:Ri =Rt

kic[M]=kt[M+]

[M+] =kic[M]/kt

Rp =kp[M+][M]= (kpki/kt)c[M]

2

Xn =vp/vt = (kp/kt) [M]


121/172

Anionic initiators:

For initiation to be successful, the free energy of the initiation step must be favorable.


122/172

, gy pTherefore, it is necessary to match the monomer with the appropriate strength ofinitiator so that the first addition is "downhill." If the propagating anion is not ver y

strongly stabilized, a powerful nucleophile is required as initiator. On the other hand,if the propagating anion is str ongly stabilized, a rather weak nucleophile will besuccessful as initiator. (Of course, more powerful ones would work, too, in the lattercase.)

But two EWGs are so effective in stabilizing anions that even water can initiatecyanoacrylate ("Super Glue"). Weak bases (such as those on the proteins in skin)

work even better.

Anionic initiators (continuation):

There is one other category of initiator known as electron transfer that works best


123/172

There is one other category of initiator, known as electron transfer, that works bestwith styrene and related monomers. The actual initiating species is derived from analkalai metal like sodium. An ar omatic compound is required to catalyze the

process by accepting an electron from sodium to form a radical anion salt with Nacounterion. A polar solvent is required to stabilize this complex salt. The electron issubsequently transferred to the monomer to create a new radical anion whichquickly dimerizes by free radical combination (similar to the termination reaction infree radical polymerization). The eventual result i s a dianion, with reactive groups ateither end. Propagation then occurs from the middle outwards. This system is

especially useful for producing ABA block copolymers, which have importanttechnological uses as thermoplastic elastomers.

Common steps of anionic polymerization:

(iii) chain transfer


124/172

(iii) chain transfer

Acrylates have problems in anionic propagation because of chain transfer topolymer. The hydrogen atoms adjacent to the ester groups are slightly acidic, andcan be pulled off by the propagating anion. The new anion thus created canreinitiate, leading to branched polymers. This side reaction is difficult to suppress.

Common steps of anionic polymerization:

(iv) termination (continuation)


125/172

(iv) termination (continuation)

When carried out under the appropriate conditions, termination reactions do notoccur in anionicpolymerization. One usually adds purposefully a compound suchas wateror alcohol to terminate the process. The new anionic species is too weakto reinitiate.

The "Dark Side:" Compounds such as water, alcohols, molecular oxygen, carbon dioxide,

etc. react very quickly with the carbanions at the chain ends, terminating the propagation.Therefore, one must scrupulously dry and deaerate the polymerization ingredients to be able

to get a truly living system. This is not easy to do, and adds to the potential costs of the

process.

Functionalization of the chain ends in anionic polymerization


126/172

The beauty of anionic polymerization lies in the lack of ter mination reactions when

carried out under the appropriate conditions (livingpolymerization). This means thatthe propagating species remains unchanged at the chain end when the monomer isconsumed, so subsequent chemical reactions can be carried out. (The chain end is acarbanion, and the organic chemistry of carbanions is diverse.) Here are a fewexamples among many possible:

Carboxylation of end groups:

Alcohol end groups viaethylene oxide:

Coupling agents:

Living anionic polymerization


127/172

Chains are initiated all at once (fastinitiation) Little or no termination (exceptpurposeful). Little or no depolymerization.All chains grow under identical conditions.

The usual circumstances:

The resultis thatthe monomers getdivided evenly among chains.

Narrow MW distribution (P approaches 1.0, typically 1.05 - 1.2). The MW is predictable (unlike otherpolymerizations).

For monofunctional initiators, the chain length is simply x = [monomer] /[initiator]. For difunctional initiators (electron transfer), the chain lengthis twice as large.


128/172

T f B I iti t


129/172

Types ofBase Initiators:

Base Initiators are often organometallic compounds or salt of astrong base, such as an alkali metal alkoxide.Examples: Potassium with liquid ammonia. Stable alkali metal complexes may be formed with aromaticcompounds (e.g. Na/naphthalene) in ether.

Sodiummetal in tetrahydrofuran.


130/172

Anionic Initiation:

T f f El t t I t di t


131/172

M +A: A: -M+

A:-M+ + CH2=CHR [CH2=/CHR]-M+ + A:

Monomer radical anion

Stable alkali metal complexes may be formed with aromatic compounds(e.g. Na/naphthalene) in ether.

Rapid dimerization often occurs due to high free radical concentration:

2[CH2=/CHR]-Na+ Na+-RHCCH2CH2CHR-+Na

Propagation from both ends! dianion

Transfer of an Electron to an Intermediate


132/172

Mechanism ofBase Initiation:

Relative Initiator Activity


133/172

Initiation could be instantaneous, of comparable rate, ormuch slower

than propagation. If termination is absent,

Termination

By impurities and transfer agents: Oxygen and carbon dioxide can react with propagating anions, andwater will terminate the chain byproton transfer. Thus, the reactionsmust be carried out under high vacuumor in an inert atmosphere.

By nucleophilic attack of initiator on polar monomer Polarmonomers such as methylmethacrylate, methyl vinyl ketone,and acrylonitrile have substituents that will react with nucleophiles.These side reactions broaden the molecular weight distribution. Tominimize the effect, use a less nucleophilic initiator, lower reactiontemperatures, and more polar solvents.

Relative InitiatorActivity

Effect of Reaction Medium: Solvent


134/172

Solvent ielectric Constant kp (liter/mole sec)

Benzene 2.2 2ioxane 2.2 5

Tetrahydrofuran 7.6 5501,2-Dimethoxyethane 5.5 3,800

As the dielectric constant increases, the solvating powerof the reactionmedium increases and there is an increased fraction of free ions (whichare highly reactive).

Effect of Reaction Medium: Counterion


135/172

The separation between the counterion and the carbanion end grouponthe polymer is the major factor determining the rate, equilibrium, andstereochemistry.

Counterion kp, liter/mole secin tetrahydrofuran in dioxane

Cation

sizeLi+ 160 0.94Na+ 80 3.4K+ 60-80 19.8Rb+ 50-80 21.5

Cs+ 22 24.5Free anion 65,000!

Tetrahydrofuran is a good solvating solvent ( = 7.4)Dioxane is a poor solvating solvent ( = 2.2)

Kinetics ofAnionic Polymerization:


136/172

Reaction Set

Initiation:GA G+ + A-

G+ + A- + M G+ + AM-

Note that the nature of the solvent will determine whether the propagatinganion behaves as a free ion, AM-, as a loose or tight ion pair, AM-G+, or

both. We will assume free ions for this treatment.

Propagation:AM- + M AMM-

AMM-+ M AM2M-

AMn-1M

-

+ M AMnM

-

Termination:There is no termination step in the absence of impurities.

Rate of Polymerization


137/172

[A-] = total concentration of anions of alllengths=[GA]o = concentration of initiator before dissociation

Integrate toobtain:


138/172

Solid-State Properties

POLYMERS IN THE SOLIDSTATE


139/172

Semi-crystallineAmorphous

Glassy Rubbery

Questions: Relationship to microstructureRelationship of structure to properties

Glass Transition Temperature


140/172

The glass transition, Tg, is temp. below which a

polymer OR glass is brittle or glass-like; above that

temperature the material is more plastic.

The Tg to a first approximation is a measure of thestrength of the secondary bonds between chains in a

polymer; the stronger the secondary bonds; the

higher the glass transition temperature.

Polyethylene Tg = 0C; Polystyrene = 97 C

PMMA (plexiglass) = 105 C.Since room temp. is < Tg for PMMA, it is brittle at

room temp.

For rubber bands; Tg = - 73C. So to make rubber

brittleyou need to cool it. Had the Challenger taken off on a

warm day the disaster may never have happened!

Determination of the glass transition temperature:


141/172

Dynamic Mechanical tests: E', E", TanH

Torsionpendulum Tensile

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