Polymer chemistry

Polymer ChemistryPolymer Chemistry
--------

PolymersPolymers
What is a polymer?What is a polymer?
Very Large molecules structures chain-like inVery 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 C
HHHHHH
HHHHHH
Polyethylene (PE)
ClCl Cl
C C C C C C
HHH
HHHHHH
Polyvinyl chloride (PVC)
HH
HHH H
Polypropylene (PP)
C C C C C C
CH3
HH
CH3CH3 H
repeat
unit
repeat
unit
repeat
unit

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 H
H
H
H

Unsaturated HydrocarbonsUnsaturated Hydrocarbons
 Double & triple bonds relatively reactive – can formDouble & triple bonds relatively reactive – can form
new bondsnew bonds
 Double bondDouble bond – ethylene or ethene - C– ethylene or ethene - CnnHH2n2n
C C
H
H
H
H

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

 AnAn aromatic hydrocarbonaromatic hydrocarbon (abbreviated(abbreviated
as AH) oras AH) or arenearene is a hydrocarbon, ofis a hydrocarbon, of
which the molecular structure incorporateswhich the molecular structure incorporates
one or more planar sets of six carbonone or more planar sets of six carbon
atoms that are connected by delocalisedatoms that are connected by delocalised
electrons numbering the same as if theyelectrons numbering the same as if they
consisted of alternating single and doubleconsisted of alternating single and double
covalent bondscovalent bonds

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

 What is actually found is that all of theWhat is actually found is that all of the
bond lengths in the benzene rings arebond lengths in the benzene rings are
1.397 angstroms1.397 angstroms
 This is roughly intermediate between theThis is roughly intermediate between the
typical lengths of single bonds (~1.5typical lengths of single bonds (~1.5
angstroms) and double bonds (~1.3angstroms) and double bonds (~1.3
angstroms)angstroms)

IsomerismIsomerism
 IsomerismIsomerism
 two compounds with same chemical formula cantwo compounds with same chemical formula can
have quite different structures/atomic arrangementhave quite 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
HH
monomer
(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 Polymers
Adapted 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 producedTraditionally, 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 atPlastics are (generally) rigid materials at
service temperaturesservice temperatures
 Rubbers are flexible, low modulusRubbers are flexible, low modulus
materials which exhibit long-rangematerials which exhibit long-range
elasticity.elasticity.

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

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

Synthesis ofSynthesis of
PolymersPolymers

Synthesis of PolymersSynthesis of Polymers
 There are a number different methodsThere are a number different methods
of preparing polymers from suitableof preparing polymers from suitable
monomers, these aremonomers, 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. CondensationAddition Vs. Condensation
PolymerizationPolymerization
 Polymerisation reactions can generally bePolymerisation reactions can generally be
written aswritten as
x-mer + y-merx-mer + y-mer (x +y)-mer(x +y)-mer
 In a reaction that leads toIn a reaction that leads to condensationcondensation
polymerspolymers, x and y may assume any value, x and y may assume any value
 i.e. chains of any size may react togetheri.e. chains of any size may react together
as long as they are capped with theas long as they are capped with the
correct functional groupcorrect functional group

Addition Vs. CondensationAddition Vs. Condensation
 InIn addition polymerizationaddition polymerization although xalthough x
may assume any value, y is confined tomay assume any value, y is confined to
unityunity
 i.e. the growing chain can react only with ai.e. the growing chain can react only with a
monomer molecule and continue itsmonomer molecule and continue its
growthgrowth

ThermodynamicsThermodynamics
 Thermodynamics of polymerizationThermodynamics of polymerization
determines the position of the equilibriumdetermines the position of the equilibrium
between polymer and monomer(s).between polymer and monomer(s).
 The well known thermodynamicThe well known thermodynamic
expression:expression:
∆∆G =G = ∆∆H - TH - T∆∆SS
yields the basis for understandingyields the basis for understanding
polymerization/depolymerization behavior.polymerization/depolymerization behavior.

 For polymerization to occur (i.e., to beFor polymerization to occur (i.e., to be
thermodynamically feasible), the Gibbsthermodynamically feasible), the Gibbs
free energy of polymerizationfree energy of polymerization ∆∆GGpp < 0< 0..
 IfIf ∆∆GGpp > 0> 0, then depolymerization will be, then depolymerization will be
favored.favored.

 Standard enthalpy and entropy changes,Standard enthalpy and entropy changes,
∆∆HHoo
p andp and ∆∆SSoo
p are reported for reactantsp are reported for reactants
and products in their appropriate standardand products in their appropriate standard
states. Generally:states. Generally:
 Temperature = 25Temperature = 25oo
C = 298KC = 298K
 Monomer – pure, bulk monomer or 1 MMonomer – pure, bulk monomer or 1 M
solutionsolution
 Polymer – solid amorphous or slightlyPolymer – solid amorphous or slightly
crystallinecrystalline

 Polymerization is an association reactionPolymerization is an association reaction
such that many monomers associate tosuch that many monomers associate to
form the polymerform the polymer
 Thus:Thus: ∆∆Sp < 0 for nearly all polymerizationSp < 0 for nearly all polymerization
processesprocesses

 Since depolymerization is almost alwaysSince depolymerization is almost always
entropicallyentropically favored, thefavored, the ∆∆HHpp must then bemust then be
sufficientlysufficiently negativenegative to compensate for theto compensate for the
unfavorable entropic term.unfavorable entropic term.
 Only then will polymerization beOnly then will polymerization be
thermodynamically favored by thethermodynamically favored by the
resulting negativeresulting negative ∆∆Gp.Gp.

In practice:In practice:
 Polymerization is favored at lowPolymerization is favored at low
temperatures: Ttemperatures: T∆∆Sp is smallSp is small
 Depolymerization is favored at highDepolymerization is favored at high
temperatures: Ttemperatures: T∆∆Sp is largeSp is large

 Therefore, thermal instability of polymersTherefore, thermal instability of polymers
results whenresults when TT∆∆SSpp overridesoverrides ∆∆HHpp and thusand thus
∆∆GGpp > O> O; this causes the system to; this causes the system to
spontaneously depolymerize (spontaneously depolymerize (if kineticif kinetic
pathway existspathway exists).).

 the activation energy for thethe activation energy for the
depropagation reaction is higher,depropagation reaction is higher,
 Compared to the propagation reaction itsCompared to the propagation reaction its
rate increases more with increasingrate increases more with increasing
temperaturetemperature
 As shown below, this results in a ceilingAs shown below, this results in a ceiling
temperature.temperature.

 ceiling temperatureceiling temperature
 the temperature at which the propagation andthe temperature at which the propagation and
depropagation reaction rates are exactlydepropagation reaction rates are exactly
equal at a given monomer concentrationequal at a given monomer concentration
300 350 400 450 500 550 600
0
1
2
3
4
5
6
Tc
kp
[M] - kdp
kp
[M]
kdp
k,sec
-1
Temperature,
o
K

 At long chain lengths, the chainAt long chain lengths, the chain
propagation reactionpropagation reaction
 is characterized by the followingis characterized by the following
equilibrium expression:equilibrium expression:
+ M
kp
kdp
Pn
* *Pn+1
k
k M
p
dp c
=
−
−
≅+[ P ]
[ P ][M]
n 1
*
n
*
1
[ ]

 The standard-state enthalpy and entropyThe standard-state enthalpy and entropy
of polymerization are related to theof 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
[ ]
[ ]
M
M
o

 At equilibrium,At equilibrium, ∆∆G = 0, and T = TG = 0, and T = Tcc
(assuming that(assuming that ∆∆HHpp
oo
andand ∆∆SSpp
oo
areare
independent of temperature).independent of temperature).
 Or:Or:
∆ ∆H T S RT
[M]
[M]
o
c
o
c
o
c
− = − ln
T
H
S Rln
[M]
[M]
c
o
o c
o
=
+
∆
∆

 Or:Or:
ln
[M]
[M]
H
RT
S
R
c
o
o
c
o
= −
∆ ∆

 At [M]At [M]cc = [M]= [M]oo, T, Tcc == ∆∆HHpp
oo
//∆∆SSpp
oo
Specific Examples of Monomer - Polymer Equilibrium
kcal/mol cal/mol-deg (∆H/∆S)
Monomer ∆Hp ∆Sp Tc(o
C)
Ethylene -21.2 -24 610
Isobutylene -12.9 -28 175
Styrene -16.7 -25.0 395
α-methyl styrene -8.4 -24 66
2,4,6-trimethyl styrene -16.7 --- ---
TFE -37 -26.8 1100

 Notice the large variation in the -Notice the large variation in the -∆∆HH
values.values.
 ethylene > isobutylene - attributed to stericethylene > isobutylene - attributed to steric
hinderance along the polymer chain, which decreaseshinderance along the polymer chain, which decreases
the exothermicity of the polymerization reaction.the exothermicity of the polymerization reaction.
 ethylene > styrene >ethylene > styrene > αα-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 theNote, however, that 2,4,6-trimethylstyrene has the
same -same -∆∆H value as styrene. Clearly, the major effectH value as styrene. Clearly, the major effect
occurs for substituents directly attached to theoccurs for substituents directly attached to the
polymer backbone.polymer backbone.

Types of AdditionTypes of Addition
 Free RadicalFree Radical
 CationicCationic
 AnionicAnionic

Free Radical PolymerizationFree Radical Polymerization
 Usually, many low molecular weightUsually, many low molecular weight
alkenes undergo rapid polymerizationalkenes undergo rapid polymerization
reactions when treated with small amountsreactions when treated with small amounts
of a radical initiator.of a radical initiator.
 For example, the polymerization ofFor example, the polymerization of
ethyleneethylene

Free Radical PolymerizationFree Radical Polymerization

Thermodynamic considerations forThermodynamic considerations for
the free radical polymerizationthe free radical polymerization

Thermodynamic considerations forThermodynamic considerations for
the free radical polymerizationthe free radical polymerization
Chain growthChain growth
 Activation energy for chain growth muchActivation energy for chain growth much
lower than for initiation.lower than for initiation.
 i.e. Growth velocity less temperaturei.e. Growth velocity less temperature
dependent than initiationdependent than initiation

Macromonomer/ComonomerMacromonomer/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 AMM
MArA
M
A a
+
+
=
Reduces to
][
][
][d
][d
M
Ar
M
A 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 thanIonic polymerization is more complex than
free-radical polymerizationfree-radical polymerization

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

 Cationic initiation is therefore usuallyCationic initiation is therefore usually
limited to the polymerization of monomerslimited to the polymerization of monomers
where the R group is electron-donatingwhere the R group is electron-donating
 This helps stabilise the delocation of theThis helps stabilise the delocation of the
positive charge through the p orbitals ofpositive charge through the p orbitals of
the double bondthe double bond

 Anionic initiation, requires the R group toAnionic initiation, requires the R group to
be electron withdrawing in order tobe electron withdrawing in order to
promote the formation of a stablepromote the formation of a stable
carbanion (ie, -M and -I effects helpcarbanion (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 aAs these ions are associated with a
counter-ion or gegen-ion the solvent hascounter-ion or gegen-ion the solvent has
important effects on the polymerizationimportant 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 monomersInvolves the polymerization of monomers
that have strong electron-withdrawingthat have strong electron-withdrawing
groups, eg, acrylonitrile, vinyl chloride,groups, eg, acrylonitrile, vinyl chloride,
methyl methacrylate, styrene etc. Themethyl 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 ionicand (c) as shown in the sheet on ionic
polymerizationpolymerization

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

 The gegen-ion may be inorganic orThe gegen-ion may be inorganic or
organic and typical initiators includeorganic and typical initiators include
KNH2, n-BuLi, and Grignard reagentsKNH2, 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 basewithdrawing group then a strong base
initiator is required, eg, butyllithium; forinitiator is required, eg, butyllithium; for
strong electron-withdrawing groups only astrong electron-withdrawing groups only a
weak base initiator is required, eg, aweak base initiator is required, eg, a
Grignard reagent.Grignard reagent.

 Initiation mechanism (c) requires the directInitiation mechanism (c) requires the direct
transfer of an electron from the donor totransfer of an electron from the donor to
the monomer in order to form a radicalthe monomer in order to form a radical
anion.anion.
 This can be achieved by using an alkaliThis 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 than
for propagation, and so the chain length
decreases with increasing temperature.

Anionic KineticsAnionic Kinetics
 A general description of the kinetics isA general description of the kinetics is
complicated however some usefulcomplicated however some useful
approximations may be attained.approximations may be attained.

Anionic KineticsAnionic Kinetics —— approximationsapproximations
1.1. The rate of polymerization will be proportionalThe rate of polymerization will be proportional
to the product of the monomer concentration ofto the product of the monomer concentration of
growing chain ends.growing chain ends.
2.2. Under conditions of negligible association eachUnder 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 theIn the absence of terminating impurities the
number of growing chain ends will always equalnumber 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
dt
Md
r pp =
−
=

2.2. In BuLi polymerization at highIn BuLi polymerization at high
concentrations in non polar solvents, theconcentrations in non polar solvents, the
chain ends are present almostchain ends are present almost
exclusively as inactive dimmers, whichexclusively as inactive dimmers, which
dissociate slightly according to thedissociate slightly according to the
equilibriumequilibrium( ) +−+−
→← LiBuMLiBuM x
k
x 22

 Where K=Where K=
3.3.The concentration of active chain ends isThe concentration of active chain ends is
thenthen
(11-3)(11-3)
 Now it takes two initiator molecules toNow 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
2
2
1
+−+−
= LiBuMKLiBuM xx
( )[ ] [ ] [ ]
22
0
2
IBuLi
LiBuM x ==+−

 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 chainThe low value of K, reflecting the presence of most chain
ends in the inactive association state, gives rise to theends in the inactive association state, gives rise to the
low rates of polymerisation in nonpolar solvents. At verylow rates of polymerisation in nonpolar solvents. At very
high concentrations, association may be even greaterhigh concentrations, association may be even greater
and the rate essentially independent of [Iand the rate essentially independent of [I00]]
[ ] [ ] 2/1
02/1
2






=
−
=
I
Kk
dt
Md
r pp

Cationic PolymerizationCationic Polymerization

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

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

 Hydrogen abstraction occurs from theHydrogen 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 centreCovalent combination of the active centre
with a catalyst-co-catalyst complexwith a catalyst-co-catalyst complex
fragment may occur giving two inactivefragment may occur giving two inactive
species.species.

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

 The kinetics of these reactions is not wellThe kinetics of these reactions is not well
understood, but they proceed very rapidlyunderstood, 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

Bulk PolymerizationBulk Polymerization
 The simplest techniqueThe simplest technique
 Gives the highest-purity polymerGives the highest-purity polymer
 Only monomer, a monomer solubleOnly monomer, a monomer soluble
initiator and perhaps a chain transferinitiator and perhaps a chain transfer
agent are usedagent are used
 This process can be used for many freeThis 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 volumeHigh yield per reactor volume
 Easy polymer recoveryEasy polymer recovery
 The option of casting the polymerisationThe option of casting the polymerisation
mixture into final product formmixture into final product form

Limitations:Limitations:
 Difficulty in removing the last traces ofDifficulty in removing the last traces of
monomermonomer
 The problem of dissipating heat producedThe problem of dissipating heat produced
during the polymerizationduring the polymerization
 In practice, heat dissipated during bulkIn practice, heat dissipated during bulk
polymerization can be improved by providingpolymerization can be improved by providing
special bafflesspecial baffles

Solution PolymerizationSolution Polymerization
 Definition:Definition: A polymerization process inA polymerization process in
which the monomers and thewhich the monomers and the
polymerization initiators are dissolved in apolymerization initiators are dissolved in a
nonmonomeric liquid solvent at thenonmonomeric liquid solvent at the
beginning of the polymerization reaction.beginning of the polymerization reaction.
The liquid is usually also a solvent for theThe liquid is usually also a solvent for the
resulting polymer or copolymer.resulting polymer or copolymer.

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

 Solvent Requirements:Solvent Requirements:
 Both the initiator and the monomer beBoth the initiator and the monomer be
soluble in itsoluble in it
 The solvent have acceptable chainThe solvent have acceptable chain
transfer characteristics and suitabletransfer characteristics and suitable
melting and boiling points for themelting and boiling points for the
conditions of the polymerization andconditions of the polymerization and
subsequent solvent-removal step.subsequent solvent-removal step.

 Solvent choice may be influenced by otherSolvent choice may be influenced by other
factors such as flash point, cost andfactors such as flash point, cost and
toxicitytoxicity
 Reactors are usually stainless steel orReactors are usually stainless steel or
glass linedglass lined

Disadvantages:Disadvantages:
 small yield per reactor volumesmall yield per reactor volume
 The requirements for a separate solventThe requirements for a separate solvent
recovery steprecovery step

Suspension PolymerizationSuspension Polymerization
 Definition:Definition: A polymerization process inA polymerization process in
which the monomer, or mixture ofwhich the monomer, or mixture of
monomers, is dispersed by mechanicalmonomers, is dispersed by mechanical
agitation in a liquid phase, usually water,agitation in a liquid phase, usually water,
in which the monomer droplets arein which the monomer droplets are
polymerized while they are dispersed bypolymerized while they are dispersed by
continuous agitation. Used primarily forcontinuous agitation. Used primarily for
PVC polymerizationPVC polymerization

 If the monomer is insoluble in water, bulkIf the monomer is insoluble in water, bulk
polymerization can be carried out inpolymerization can be carried out in
suspended droplets, i.e., monomer issuspended droplets, i.e., monomer is
mechanically dispersed.mechanically dispersed.
 The water phase becomes the heatThe water phase becomes the heat
transfer medium.transfer medium.

 So the heat transfer is very good. In thisSo the heat transfer is very good. In this
system, the monomer must be eithersystem, the monomer must be either
 1) insoluble in water or1) insoluble in water or
 2) only slightly soluble in water, so that when2) 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 veryThe behavior inside the droplets is very
much like the behavior of bulkmuch like the behavior of bulk
polymerizationpolymerization
 Since the droplets are only 10 to 1000Since the droplets are only 10 to 1000
microns in diameter, more rapid reactionmicrons in diameter, more rapid reaction
rates can be tolerated (than would be therates can be tolerated (than would be the
case for bulk polymerization) withoutcase for bulk polymerization) without
boiling the monomer.boiling the monomer.

Emulsion PolymerizationEmulsion Polymerization
 Emulsion polymerizationEmulsion polymerization is a type ofis a type of
radical polymerization that usually startsradical 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 emulsionThe 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) arein which droplets of monomer (the oil) are
emulsified (with surfactants) in aemulsified (with surfactants) in a
continuous phase of water.continuous phase of water.
 Water-soluble polymers, such as certainWater-soluble polymers, such as certain
polyvinyl alcohols or hydroxyethylpolyvinyl alcohols or hydroxyethyl
celluloses, can also be used to act ascelluloses, 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 atHigh molecular weight polymers can be made at
fast polymerization rates. By contrast, in bulkfast polymerization rates. By contrast, in bulk
and solution free radical polymerization, there isand solution free radical polymerization, there is
a tradeoff between molecular weight anda tradeoff between molecular weight and
polymerization rate.polymerization rate.
 The continuous water phase is an excellentThe continuous water phase is an excellent
conductor of heat and allows the heat to beconductor of heat and allows the heat to be
removed from the system, allowing manyremoved 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 containedSince polymer molecules are contained
within the particles, viscosity remainswithin the particles, viscosity remains
close to that of water and is not dependentclose to that of water and is not dependent
on molecular weight.on molecular weight.
 The final product can be used as is andThe final product can be used as is and
does not generally need to be altered ordoes 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 anFor dry (isolated) polymers, water removal is an
energy-intensive processenergy-intensive process
 Emulsion polymerizations are usually designedEmulsion polymerizations are usually designed
to operate at high conversion of monomer toto operate at high conversion of monomer to
polymer. This can result in significant chainpolymer. 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.

Polymer chemistry

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