polymer chemistry - uwi

Polymer ChemistryPolymer Chemistry--------

PolymersPolymers

What is a polymer? What is a polymer? Very Large molecules structures chain-like in Very Large molecules structures chain-like in

nature.nature.PolyPoly mermer

manymany repeat unitrepeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)HH

HHH H

Polypropylene (PP)

C C C C C CCH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

Ancient Polymer HistoryAncient Polymer History

Originally natural polymers were usedOriginally natural polymers were used WoodWood – Rubber– Rubber CottonCotton – Wool– Wool LeatherLeather – Silk– Silk

Polymer CompositionPolymer Composition

Most polymers are hydrocarbonsMost polymers are hydrocarbons – – i.e. made up of H and Ci.e. made up of H and C Saturated hydrocarbonsSaturated hydrocarbons

Each carbon bonded to four other atomsEach carbon bonded to four other atoms

CCnnHH2n+22n+2

C C

H

H HH

HH

Unsaturated HydrocarbonsUnsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bondsDouble & triple bonds relatively reactive – can form new bonds

Double bondDouble bond – ethylene or ethene - C – ethylene or ethene - CnnHH2n2n

4-bonds, but only 3 atoms bound to C’s4-bonds, but only 3 atoms bound to C’s

C CH

H

H

H

Unsaturated HydrocarbonsUnsaturated Hydrocarbons

Triple bondTriple bond – acetylene or ethyne - C – acetylene or ethyne - CnnHH2n-22n-2

C C HH


An An aromatic hydrocarbonaromatic hydrocarbon (abbreviated (abbreviated as AH) or as AH) or arenearene is a hydrocarbon, of is a hydrocarbon, of which the molecular structure incorporates which the molecular structure incorporates one or more planar sets of six carbon one or more planar sets of six carbon atoms that are connected by delocalised atoms that are connected by delocalised electrons numbering the same as if they electrons numbering the same as if they consisted of alternating single and double consisted of alternating single and double covalent bonds covalent bonds


Benzene, C6H6, is the simplest and first Benzene, C6H6, is the simplest and first recognized aromatic hydrocarbonrecognized aromatic hydrocarbon


What is actually found is that all of the What is actually found is that all of the bond lengths in the benzene rings are bond lengths in the benzene rings are 1.397 angstroms1.397 angstroms

This is roughly intermediate between the This is roughly intermediate between the typical lengths of single bonds (~1.5 typical lengths of single bonds (~1.5 angstroms) and double bonds (~1.3 angstroms) and double bonds (~1.3 angstroms) angstroms)

IsomerismIsomerism IsomerismIsomerism

two compounds with same chemical formula can have two compounds with same chemical formula can have quite different structures/atomic arrangementquite different structures/atomic arrangement

Ex: CEx: C88HH1818

n-octanen-octane

2-methyl-4-ethyl pentane (isooctane)2-methyl-4-ethyl pentane (isooctane)

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

Chemistry of PolymersChemistry of Polymers Free radical polymerizationFree radical polymerization

InitiatorInitiator: example - benzoyl peroxide: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HHmonomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

Chemistry of PolymersChemistry of PolymersAdapted from Fig. 14.1, Callister 7e.

Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity PolymersBulk or Commodity Polymers

Range of PolymersRange of Polymers

Traditionally, the industry has produced Traditionally, the industry has produced two main types of synthetic polymer – two main types of synthetic polymer – plastics and rubbers. plastics and rubbers.

Plastics are (generally) rigid materials at Plastics are (generally) rigid materials at service temperatures service temperatures

Rubbers are flexible, low modulus Rubbers are flexible, low modulus materials which exhibit long-range materials which exhibit long-range elasticity.elasticity.


Plastics are further subdivided into Plastics are further subdivided into thermoplastics and thermosetsthermoplastics and thermosets


Another way of classifying polymers is in Another way of classifying polymers is in terms of their form or functionterms of their form or function

Synthesis of Synthesis of PolymersPolymers

Synthesis of PolymersSynthesis of Polymers There are a number different methods There are a number different methods

of preparing polymers from suitable of preparing polymers from suitable monomers, these are monomers, these are step-growth (or condensation) step-growth (or condensation)

polymerisationpolymerisation addition polymerisationaddition polymerisation insertion polymerisation.insertion polymerisation.

Types of PolymerizationTypes of Polymerization

Chain-growth polymers, also known as addition polymers, are made by chain reactions

Types of PolymerizationTypes of Polymerization

Step-growth polymers, also called condensation polymers, are made by combining two molecules by removing a small molecule

Addition Vs. Condensation Addition Vs. Condensation PolymerizationPolymerization

Polymerisation reactions can generally be Polymerisation reactions can generally be written aswritten as

x-mer + y-mer x-mer + y-mer (x +y)-mer(x +y)-mer

In a reaction that leads to In a reaction that leads to condensation condensation polymerspolymers, x and y may assume any value, x and y may assume any value

i.e. chains of any size may react together i.e. chains of any size may react together as long as they are capped with the as long as they are capped with the correct functional groupcorrect functional group

Addition Vs. Condensation Addition Vs. Condensation PolymerizationPolymerization

In In addition polymerizationaddition polymerization although x although x may assume any value, y is confined to may assume any value, y is confined to unityunity

i.e. the growing chain can react only with a i.e. the growing chain can react only with a monomer molecule and continue its monomer molecule and continue its growthgrowth

ThermodynamicsThermodynamics

Thermodynamics of polymerization Thermodynamics of polymerization determines the position of the equilibrium determines the position of the equilibrium between polymer and monomer(s).between polymer and monomer(s).

The well known thermodynamic The well known thermodynamic expression:expression:

G = G = H - TH - TSSyields the basis for understanding yields the basis for understanding polymerization/depolymerization behavior.polymerization/depolymerization behavior.

ThermodynamicsThermodynamics For polymerization to occur (i.e., to be For polymerization to occur (i.e., to be

thermodynamically feasible), the Gibbs thermodynamically feasible), the Gibbs free energy of polymerization free energy of polymerization GGpp < 0 < 0. .

If If GGp p > 0> 0, then depolymerization will be , then depolymerization will be

favored.favored.

ThermodynamicsThermodynamics Standard enthalpy and entropy changes, Standard enthalpy and entropy changes,

HHoop and p and SSoop are reported for reactants p are reported for reactants and products in their appropriate standard and products in their appropriate standard states. Generally:states. Generally: Temperature = 25Temperature = 25ooC = 298KC = 298K Monomer – pure, bulk monomer or 1 M Monomer – pure, bulk monomer or 1 M

solutionsolution Polymer – solid amorphous or slightly Polymer – solid amorphous or slightly

crystallinecrystalline

ThermodynamicsThermodynamics Polymerization is an association reaction Polymerization is an association reaction

such that many monomers associate to such that many monomers associate to form the polymer form the polymer

Thus: Thus: Sp < 0 for nearly all polymerization Sp < 0 for nearly all polymerization processesprocesses


Since depolymerization is almost always Since depolymerization is almost always entropicallyentropically favored, the favored, the HHpp must then be must then be sufficiently sufficiently negativenegative to compensate for the to compensate for the unfavorable entropic term. unfavorable entropic term.

Only then will polymerization be Only then will polymerization be

thermodynamically favored by the thermodynamically favored by the resulting negative resulting negative Gp. Gp.


In practice:In practice: Polymerization is favored at low Polymerization is favored at low

temperatures: Ttemperatures: TSp is smallSp is small

Depolymerization is favored at high Depolymerization is favored at high temperatures: Ttemperatures: TSp is largeSp is large


Therefore, thermal instability of polymers Therefore, thermal instability of polymers results when results when TTSSpp overrides overrides HHpp and thus and thus GGpp > O > O; this causes the system to ; this causes the system to spontaneously depolymerize (spontaneously depolymerize (if kinetic if kinetic pathway existspathway exists).).


the activation energy for the the activation energy for the depropagation reaction is higher,depropagation reaction is higher,

Compared to the propagation reaction its Compared to the propagation reaction its rate increases more with increasing rate increases more with increasing temperature temperature

As shown below, this results in a ceiling As shown below, this results in a ceiling temperature.temperature.


ceiling temperature ceiling temperature the temperature at which the propagation and the temperature at which the propagation and

depropagation reaction rates are exactly depropagation reaction rates are exactly equal at a given monomer concentrationequal at a given monomer concentration

300 350 400 450 500 550 6000

1

2

3

4

5

6

Tc

kp[M] - kdp

kp[M]

kdp

k, s

ec-1

Temperature, oK


At long chain lengths, the chain At long chain lengths, the chain propagation reaction propagation reaction

is characterized by the following is characterized by the following equilibrium expression:equilibrium expression:

+ Mkp

kdp

Pn* *Pn+1

kk Mp

dp c

[ P ]

[ P ][M]n 1*

n*

1[ ]


The standard-state enthalpy and entropy The standard-state enthalpy and entropy of polymerization are related to the of polymerization are related to the standard-state monomer concentration, standard-state monomer concentration, [M][M]oo (usually neat liquid or 1 M solution) as (usually neat liquid or 1 M solution) as follows:follows:

G H T S RTo o ln [ ][ ]MM

o

ThermodynamicsThermodynamics At equilibrium, At equilibrium, G = 0, and T = TG = 0, and T = Tcc

(assuming that (assuming that HHppoo and and SSpp

oo are are independent of temperature). independent of temperature).

Or:Or:

H T S RT [M][M]

oc

oc

o

c

ln

T H

S Rln [M][M]

c

o

o c

o


Or:Or:ln [M]

[M]H

RTSR

c

o

o

c

o


At [M]At [M]cc = [M] = [M]oo, T, Tcc = = HHppoo//SSpp

oo Specific Examples of Monomer - Polymer Equilibrium

kcal/mol cal/mol-deg (H/S)

Monomer Hp Sp Tc(oC)

Ethylene -21.2 -24 610

Isobutylene -12.9 -28 175

Styrene -16.7 -25.0 395

-methyl styrene -8.4 -24 66

2,4,6-trimethyl styrene -16.7 --- ---

TFE -37 -26.8 1100

ThermodynamicsThermodynamics Notice the large variation in the -Notice the large variation in the -H H

values.values. ethylene > isobutylene - attributed to steric hinderance ethylene > isobutylene - attributed to steric hinderance

along the polymer chain, which decreases the along the polymer chain, which decreases the exothermicity of the polymerization reaction.exothermicity of the polymerization reaction.

ethylene > styrene > ethylene > styrene > -metylstyrene - also due to -metylstyrene - also due to increasing steric hinderance along the polymer chain.increasing steric hinderance along the polymer chain.

Note, however, that 2,4,6-trimethylstyrene has the Note, however, that 2,4,6-trimethylstyrene has the same -same -H value as styrene. Clearly, the major effect H value as styrene. Clearly, the major effect occurs for substituents directly attached to the polymer occurs for substituents directly attached to the polymer backbone.backbone.

Types of Addition Types of Addition PolymerizationPolymerization

Free RadicalFree Radical CationicCationic AnionicAnionic

Free Radical PolymerizationFree Radical Polymerization

Usually, many low molecular weight Usually, many low molecular weight alkenes undergo rapid polymerization alkenes undergo rapid polymerization reactions when treated with small amounts reactions when treated with small amounts of a radical initiator. of a radical initiator.

For example, the polymerization of For example, the polymerization of ethylene ethylene

Free Radical PolymerizationFree Radical Polymerization

Thermodynamic considerations for Thermodynamic considerations for the free radical polymerization the free radical polymerization


Chain growthChain growth Activation energy for chain growth much Activation energy for chain growth much

lower than for initiation.lower than for initiation. i.e. Growth velocity less temperature i.e. Growth velocity less temperature

dependent than initiation dependent than initiation

Macromonomer/Comonomer Macromonomer/Comonomer Copolymerization Kinetics : free radicalCopolymerization Kinetics : free radical

In such copolymerizations, owing to the large differences in molar mass between Macromonomer M and Comonomer A, the monomer concentration is always very small : consequently the classical instantaneous copolymerization equation

][]([r][][][]([

][d][d

M AMMMArA

MA a

Reduces to

][][

][d][d

MAr

MA a

As in an « ideal » copolymerization the reciprocal of the radical reactivity of the comonomer is a measure of the macromonomer to take part in the process

Controlled Free Radical Copolymerization

Ionic PolymerizationIonic Polymerization

Ionic polymerization is more complex than Ionic polymerization is more complex than free-radical polymerization free-radical polymerization


Whereas free radical polymerization is Whereas free radical polymerization is non-specific, the type of ionic non-specific, the type of ionic polymerization procedure and catalysts polymerization procedure and catalysts depend on the nature of the substituent depend on the nature of the substituent (R) on the vinyl (ethenyl) monomer.(R) on the vinyl (ethenyl) monomer.


Cationic initiation is therefore usually Cationic initiation is therefore usually limited to the polymerization of monomers limited to the polymerization of monomers where the R group is electron-donating where the R group is electron-donating

This helps stabilise the delocation of the This helps stabilise the delocation of the positive charge through the p orbitals of positive charge through the p orbitals of the double bondthe double bond


Anionic initiation, requires the R group to Anionic initiation, requires the R group to be electron withdrawing in order to be electron withdrawing in order to promote the formation of a stable promote the formation of a stable carbanion (ie, -M and -I effects help carbanion (ie, -M and -I effects help stabilise the negative charge).stabilise the negative charge).


M is a Monomer Unit. M is a Monomer Unit. As these ions are associated with a As these ions are associated with a

counter-ion or gegen-ion the solvent has counter-ion or gegen-ion the solvent has important effects on the polymerization important effects on the polymerization procedure.procedure.


(ii) Chain Propagation depends on :(ii) Chain Propagation depends on : Ion separationIon separation The nature of the SolventThe nature of the Solvent Nature of the counter IonNature of the counter Ion

Anionic PolymerizationAnionic Polymerization

Involves the polymerization of monomers Involves the polymerization of monomers that have strong electron-withdrawing that have strong electron-withdrawing groups, eg, acrylonitrile, vinyl chloride, groups, eg, acrylonitrile, vinyl chloride, methyl methacrylate, styrene etc. The methyl methacrylate, styrene etc. The reactions can be initiated by methods (b) reactions can be initiated by methods (b) and (c) as shown in the sheet on ionic and (c) as shown in the sheet on ionic polymerizationpolymerization


eg, for mechanism (b)eg, for mechanism (b)


The gegen-ion may be inorganic or The gegen-ion may be inorganic or organic and typical initiators include organic and typical initiators include KNH2, n-BuLi, and Grignard reagents KNH2, n-BuLi, and Grignard reagents such as alkyl magnesium bromidessuch as alkyl magnesium bromides


If the monomer has only a weak electron-If the monomer has only a weak electron-withdrawing group then a strong base withdrawing group then a strong base initiator is required, eg, butyllithium; for initiator is required, eg, butyllithium; for strong electron-withdrawing groups only a strong electron-withdrawing groups only a weak base initiator is required, eg, a weak base initiator is required, eg, a Grignard reagent. Grignard reagent.


Initiation mechanism (c) requires the direct Initiation mechanism (c) requires the direct transfer of an electron from the donor to transfer of an electron from the donor to the monomer in order to form a radical the monomer in order to form a radical anion. anion.

This can be achieved by using an alkali This can be achieved by using an alkali metal eg.,metal eg.,

Anionic Polymerization of StyreneAnionic Polymerization of Styrene

Anionic Polymerization of StyreneAnionic Polymerization of Styrene

The activation energy for transfer is larger thanfor propagation, and so the chain length decreases with increasing temperature.

Anionic KineticsAnionic Kinetics

A general description of the kinetics is A general description of the kinetics is complicated however some useful complicated however some useful approximations may be attained.approximations may be attained.

Anionic Kinetics Anionic Kinetics —— approximations approximations

1.1. The rate of polymerization will be proportional The rate of polymerization will be proportional to the product of the monomer concentration of to the product of the monomer concentration of growing chain ends.growing chain ends.

2.2. Under conditions of negligible association each Under conditions of negligible association each initiator molecule will start a growing chaininitiator molecule will start a growing chain

3.3. In the absence of terminating impurities the In the absence of terminating impurities the number of growing chain ends will always equal number of growing chain ends will always equal the number of initiator molecules addedthe number of initiator molecules added


1.1. If propagation is rate controlingIf propagation is rate controling (11-1)(11-1) 0IMk

dtMdr pp


2.2. In BuLi polymerization at high In BuLi polymerization at high concentrations in non polar solvents, the concentrations in non polar solvents, the chain ends are present almost exclusively chain ends are present almost exclusively as inactive dimmers, which dissociate as inactive dimmers, which dissociate slightly according to the equilibriumslightly according to the equilibrium

LiBuMLiBuM xk

x 22


Where K=Where K=3.3.The concentration of active chain ends is The concentration of active chain ends is

thenthen(11-3)(11-3)

Now it takes two initiator molecules to Now it takes two initiator molecules to make one inactive chain dimmer, somake one inactive chain dimmer, so(11-4)(11-4)

1/ 2

2 LiBuMLiBuM xx

2/1

22

1 LiBuMKLiBuM xx

220

2

IBuLiLiBuM x

Anionic KineticsAnionic Kinetics The rate of polymerisation then becomesThe rate of polymerisation then becomes

(11-5)(11-5)

The low value of K, reflecting the presence of most chain The low value of K, reflecting the presence of most chain ends in the inactive association state, gives rise to the ends in the inactive association state, gives rise to the low rates of polymerisation in nonpolar solvents. At very low rates of polymerisation in nonpolar solvents. At very high concentrations, association may be even greater high concentrations, association may be even greater and the rate essentially independent of [Iand the rate essentially independent of [I00]]

2/102/1

2

IKk

dtMdr pp

Cationic PolymerizationCationic Polymerization


(ii) PropagationChain growth takes place (ii) PropagationChain growth takes place through the repeated addition of a through the repeated addition of a monomer in a head-to-tail manner to the monomer in a head-to-tail manner to the ion with retention of the ionic character ion with retention of the ionic character throughoutthroughout


(iii) Termination(iii) TerminationTermination of cationic polymerization Termination of cationic polymerization reactions are less well-defined than in reactions are less well-defined than in free-radical processes. Two possibilities free-radical processes. Two possibilities exist as follows:exist as follows:


Hydrogen abstraction occurs from the Hydrogen abstraction occurs from the growing chain to regenerate the catalyst-growing chain to regenerate the catalyst-co-catalyst complex. co-catalyst complex.

Covalent combination of the active centre Covalent combination of the active centre with a catalyst-co-catalyst complex with a catalyst-co-catalyst complex fragment may occur giving two inactive fragment may occur giving two inactive species. species.


The kinetic chain is terminated and the The kinetic chain is terminated and the initiator complex is reduced - a more initiator complex is reduced - a more effective route to reaction termination.effective route to reaction termination.


The kinetics of these reactions is not well The kinetics of these reactions is not well understood, but they proceed very rapidly understood, but they proceed very rapidly at extremely low temperatures.at extremely low temperatures.

Polymerization Processes

TWO USEFUL DISTINCTIONS ; BETWEEN BATCH AND CONTINUOUS AND BETWEEN SINGLE - PHASE AND

MULTI -PHASE SINGLE - PHASE

Bulk or Melt Polymerization Solution Polymerization

Polymerization Processes

Bulk PolymerizationBulk Polymerization

The simplest techniqueThe simplest technique Gives the highest-purity polymerGives the highest-purity polymer Only monomer, a monomer soluble Only monomer, a monomer soluble

initiator and perhaps a chain transfer initiator and perhaps a chain transfer agent are usedagent are used

This process can be used for many free This process can be used for many free radical polymerizations and some step-radical polymerizations and some step-growth (condensation) polymerisation.growth (condensation) polymerisation.

Polymerization TechniquesPolymerization Techniques

These include:These include: Bulk PolymerizationBulk Polymerization Solution PolymerizationSolution Polymerization Suspension PolymerizationSuspension Polymerization Emulsion PolymerizationEmulsion Polymerization


Advantages:Advantages: High yield per reactor volume High yield per reactor volume Easy polymer recoveryEasy polymer recovery The option of casting the polymerisation The option of casting the polymerisation

mixture into final product formmixture into final product form


Limitations:Limitations: Difficulty in removing the last traces of Difficulty in removing the last traces of

monomermonomer The problem of dissipating heat produced The problem of dissipating heat produced

during the polymerizationduring the polymerization In practice, heat dissipated during bulk In practice, heat dissipated during bulk

polymerization can be improved by providing polymerization can be improved by providing special baffles special baffles

Solution PolymerizationSolution Polymerization

Definition:Definition: A polymerization process in A polymerization process in which the monomers and the which the monomers and the polymerization initiators are dissolved in a polymerization initiators are dissolved in a nonmonomeric liquid solvent at the nonmonomeric liquid solvent at the beginning of the polymerization reaction. beginning of the polymerization reaction. The liquid is usually also a solvent for the The liquid is usually also a solvent for the resulting polymer or copolymer. resulting polymer or copolymer.


Heat removed during polymerization can Heat removed during polymerization can be facilitated by conducting the be facilitated by conducting the polymerization in an organic solvent or polymerization in an organic solvent or waterwater


Solvent Requirements:Solvent Requirements: Both the initiator and the monomer be Both the initiator and the monomer be

soluble in it soluble in it The solvent have acceptable chain The solvent have acceptable chain

transfer characteristics and suitable transfer characteristics and suitable melting and boiling points for the melting and boiling points for the conditions of the polymerization and conditions of the polymerization and subsequent solvent-removal step.subsequent solvent-removal step.


Solvent choice may be influenced by other Solvent choice may be influenced by other factors such as flash point, cost and factors such as flash point, cost and toxicitytoxicity

Reactors are usually stainless steel or Reactors are usually stainless steel or glass linedglass lined


Disadvantages:Disadvantages: small yield per reactor volume small yield per reactor volume The requirements for a separate solvent The requirements for a separate solvent

recovery steprecovery step

Suspension PolymerizationSuspension Polymerization

Definition:Definition: A polymerization process in A polymerization process in which the monomer, or mixture of which the monomer, or mixture of monomers, is dispersed by mechanical monomers, is dispersed by mechanical agitation in a liquid phase, usually water, agitation in a liquid phase, usually water, in which the monomer droplets are in which the monomer droplets are polymerized while they are dispersed by polymerized while they are dispersed by continuous agitation. Used primarily for continuous agitation. Used primarily for PVC polymerizationPVC polymerization


If the monomer is insoluble in water, bulk If the monomer is insoluble in water, bulk polymerization can be carried out in polymerization can be carried out in suspended droplets, i.e., monomer is suspended droplets, i.e., monomer is mechanically dispersed. mechanically dispersed.

The water phase becomes the heat The water phase becomes the heat transfer medium. transfer medium.


So the heat transfer is very good. In this So the heat transfer is very good. In this system, the monomer must be either system, the monomer must be either 1) insoluble in water or 1) insoluble in water or 2) only slightly soluble in water, so that when 2) only slightly soluble in water, so that when

it polymerizes it becomes insoluble in water.it polymerizes it becomes insoluble in water.


The behavior inside the droplets is very The behavior inside the droplets is very much like the behavior of bulk much like the behavior of bulk polymerization polymerization

Since the droplets are only 10 to 1000 Since the droplets are only 10 to 1000 microns in diameter, more rapid reaction microns in diameter, more rapid reaction rates can be tolerated (than would be the rates can be tolerated (than would be the case for bulk polymerization) without case for bulk polymerization) without boiling the monomer. boiling the monomer.

Emulsion PolymerizationEmulsion Polymerization

Emulsion polymerizationEmulsion polymerization is a type of is a type of radical polymerization that usually starts radical polymerization that usually starts with an emulsion incorporating water, with an emulsion incorporating water, monomer, and surfactant. monomer, and surfactant.


The most common type of emulsion The most common type of emulsion polymerization is an oil-in-water emulsion, polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are in which droplets of monomer (the oil) are emulsified (with surfactants) in a emulsified (with surfactants) in a continuous phase of water. continuous phase of water.

Water-soluble polymers, such as certain Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as celluloses, can also be used to act as emulsifiers/stabilizers.emulsifiers/stabilizers.

Emulsion Polymerization – SchematicEmulsion Polymerization – Schematic


Advantages of emulsion polymerization include:Advantages of emulsion polymerization include: High molecular weight polymers can be made at High molecular weight polymers can be made at

fast polymerization rates. By contrast, in bulk and fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a solution free radical polymerization, there is a tradeoff between molecular weight and tradeoff between molecular weight and polymerization rate.polymerization rate.

The continuous water phase is an excellent The continuous water phase is an excellent conductor of heat and allows the heat to be conductor of heat and allows the heat to be removed from the system, allowing many removed from the system, allowing many reaction methods to increase their rate.reaction methods to increase their rate.


Advantages Continued:Advantages Continued: Since polymer molecules are contained Since polymer molecules are contained

within the particles, viscosity remains within the particles, viscosity remains close to that of water and is not dependent close to that of water and is not dependent on molecular weight.on molecular weight.

The final product can be used as is and The final product can be used as is and does not generally need to be altered or does not generally need to be altered or processed.processed.


Disadvantages of emulsion polymerization include:Disadvantages of emulsion polymerization include: For dry (isolated) polymers, water removal is an For dry (isolated) polymers, water removal is an

energy-intensive processenergy-intensive process Emulsion polymerizations are usually designed Emulsion polymerizations are usually designed

to operate at high conversion of monomer to to operate at high conversion of monomer to polymer. This can result in significant chain polymer. This can result in significant chain transfer to polymer.transfer to polymer.

Fabrication methodsFabrication methods

ExampleExample Suggest a polymer and fabrication process

suitable to produce the following items. Support your choice by contrasting it with other possible alternatives. Car bumper Carry bag Machine gear Shower curtain Tooth brush stand

SolutionSolution i) Car bumper Polyurethane is one of the suitable materials for car

bumpers. another suitable material is PP. Reaction injection molding process is suitable to produce polyurethane bumpers. Polyurethane is molded by mixing of highly reactive liquids (isocyanateandpolyol). Because the materials are very reactive liquids, Other molding processes such as injection molding and compression molding can not be used for this purpose. However, injection molding and compression molding methods can be used to make PP bumpers.

SolutionSolution ii) Carry bag Polyethylene (PE)is used widely for making

carry bags. Blown film extrusion methodis best suitable to produce carry bags. Calendering method also can be applied for the same purpose. However, considering the production rate and thickness range that can be produced, blown film extrusion method is ideal to produce carry bags.

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