WEEK 4.pdf

INTRODUCTION TO POLYMER SCIENCE
PROF. DIBAKAR DHARA
DEPARTMENT OF CHEMISTRY, IIT KHARAGPUR
Module 03: Radical Chain Polymerization
Lecture 16: Process conditions, Emulsion Polymerization
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Content of Lecture 16
 Process conditions – Bulk, Solution, Suspension
 Emulsion Polymerization
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Polymerization Processes
Homogeneous and Multiphase/Heterogeneous
Some homogeneous systems may become heterogeneous as polymerization proceeds as a
result of insolubility of the polymer in the reaction media. This classification is usually based
on whether the initial reaction mixture is homogeneous or heterogeneous
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Homogeneous
 Melt polymerization: Eg., PET, PBT, melt PC
 Solution polymerization: Eg., Solution polyethylene
Multiphase/ heterogeneous
Multiphase
 Emulsion polymerization: Eg., SBR, ABS
 Suspension / bead / pearl polymerization: Eg., Crystal PS, PMMA
Heterogeneous
 Precipitation Polymerization: Eg., PAN
 Polymerizations with a heterogeneous catalyst: Eg., ZN‐PE, ZN‐PP
 Interfacial polymerization: Eg., BPA‐PC
 Solid state polymerization: Eg., Bottle grade PET / high MW PBT
Polymerization Processes
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Homogeneous Polymerization
Melt polymerization:
 The monomers as well as the resulting polymers are in the melt state (ie., above Tg
or Tm) under the conditions of polymerizations
Eg., PET, PBT, Polycarbonate
Solution polymerization:
 The monomers and the resulting polymers are in solution under the
conditions of polymerization
Eg., Solution polyethylene
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Multiphase Polymerization
Suspension polymerization:
 Also called Pearl or Bead polymerization, is used to produce small spherical beads
(5 to 100 microns in diameter).
 It can be employed with any water insoluble monomer and initiator.
 A suspending agent is added to stabilize the suspension of the monomer in water
in the form of droplets
Egs., Crystal polystyrene, “Foamable” polystyrene, Ion exchange resins (anionic
and cationic), polymethylmethacrylate
Precipitation Polymerization:
 The monomers are in solution but the resulting polymer precipitates
out of the solution
Eg., Acrylic fiber (polyacrylonitrile)
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Multiphase Polymerization
Polymerizations with a heterogeneous catalyst:
 Polymerization where a catalyst which is insoluble in either in the monomer or the
solvent used for polymerization
Eg., Polyethylene, polypropylene
Emulsion polymerization:
 Emulsion polymerizations are essentially carried out in water medium.
Of the three components – monomer, initiator, surfactant, only the
initiator is soluble in water, while the monomer and the polymer are
insoluble in water and are dispersed as very small spherical particles
Eg., Styrene – butadiene rubbers, acrylonitirile – butadiene
rubbers, acrylonitrile – butadiene ‐ styrene
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Heterogeneous Polymerization
Interfacial polymerization:
 Low temperature polymerization technique for the synthesis of step‐growth
polymers. Polymerization of the two reactants is carried out at the interface
between two liquid faces each containing one of the reactants.
Eg., BPA‐PC
Solid state polymerization:
 Process wherein the chain growth reaction occurs predominantly in the
solid state and the amorphous regions of the polymer. The reactive end
groups are concentrated in the amorphous regions and are excluded
from the crystallites
Eg., Bottle grade PET / high molecular weight PBT
High molecular weight polyamides
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 With only monomer, no solvent – Bulk Polymerization
 In a solvent – Solution Polymerization
 With monomer dispersed in aqueous phase – Suspension polymerization
 With produced polymers dispersed in organic solvents – Dispersion Polymerization
 In Emulsion ‐ Emulsion polymerization
Free‐radical Polymerization Processes
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Bulk Polymerization
 Simplest
 Only monomer and monomer‐soluble initiator
 Polymers are also soluble in monomers
 Free radical kinetics apply
 Monomer concentration is high – rate of polymerization and MW is high
 High purity, minimum contamination of product – applicable for optical
applications
Advantages
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Bulk Polymerization
Used for styrene, methylmethacrylate, vinyl chloride, etc.
Disadvantages
 Viscosity becomes high even at low concentration – stirring becomes difficult,
processing also becomes difficult
 High reaction rate – heat generation at high rate
 Rt decreases at high conversion due to the Trommsdorf effect, making the
reaction hard to control ‐ Gel‐effect
 Degradation, coloration, branching, cross‐linking, high dispersity values
Keep to low conversion, separate and recycle the unreacted monomers
Bulk Polymerization
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Solution Polymerization
 Monomer, solvent and soluble initiator
 Free radical kinetics apply
 Produced polymers are soluble in the solvent
 Solvent acts as a diluent and aids in removal of heat of polymerization
 Solvent reduces viscosity, making stirring easier
 Thermal control is easier than in the bulk.
Advantages
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Disadvantages
 Concentration of monomer is lower (compare to bulk), hence rate of polymerization is
slower and MW is lower
 Chain transfer to solvent may occur, leading to low molecular weights
 Difficult to remove solvent from final form, causing degradation of bulk properties
 Need of solvent recovery and recycling
 Environmental pollution due to solvent release
Used for ethylene, acrylonitrile, vinyl acetate, etc.
Solution Polymerization
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Heterogeneous Polymerization: Suspension
 Monomer, monomer‐soluble initiator, water, dispersant
 Similar to bulk polymerization in Kinetics
 Initiator is dissolved in monomer and then preheated dispersing medium is
added
 Vigorous stirring convert the monomer containing initiator into small drops
of size around 100 – 500 microns
 Dispersants present (~0.1 wt%) and stirring stabilize the suspension
Process
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Heterogeneous Polymerization: Suspension
Disadvantages
 Polymerization needs to be completed
 Dispersant may impart impurities
Used for methylmethacrylate and other acrylate esters, vinyl acetate, vinyl chloride,
TFE, etc.
 Aqueous medium acts as a diluent and aids in removal of heat of polymerization
 High polymerization rate and high MW
 Polymer produced as beads (bead or pearl polymerization) and can be taken out by
filtration
Advantages
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Heterogeneous Polymerization: Dispersion
• Monomer, soluble initiator, organic solvents, dispersant
• Polymers formed are insoluble in solvent
• Polymers remain dispersed because of presence of dispersants
• Polymers grow in size as the adsorbed monomer get polymerized
• Size of the polymer produced are about 1‐10 microns
Other heterogeneous polymerization:
Inverse suspension polymerization
Microsuspension polymerization
Emulsion polymerization – Will be taught in next lecture
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Emulsion Polymerization
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 Great industrial importance.
 Emulsion polymerization was first employed during World War II for
producing synthetic rubbers from 1,3‐butadiene and styrene. This was
the start of the synthetic rubber industry in the United States.
 Presently the predominant process for the commercial polymerizations
of vinyl acetate, chloroprene, various acrylate copolymerizations, and
copolymerizations of butadiene with styrene and acrylonitrile.
 It is also used for methacrylates, vinyl chloride, acrylamide, and some
fluorinated ethylenes.
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 Product, a colloidally‐stable dispersion of particulate polymer in water known as a
Latex – can be used directly in applications like paints, coating, finishes, floor
polishes.
 Size of the particles (0.05 to 1 microns) are about three orders of magnitude smaller
than suspension polymerization
 Similar advantages of suspension polymerization
 Mechanism and kinetics are different from normal radical chain polymerization
 MW can be increased without compromising polymerization rate
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Emulsion Polymerization: Ingredients, Conditions
Main Ingredients (parts by weight)
 Water (Water : Monomer = 7:3 to 6:4 by weight)
 Monomer: Water‐insoluble or slightly soluble in water
 Initiator: Water‐soluble (insoluble in the organic monomer ‐ oil‐insoluble initiators)
 Emulsifiers Surfactant: High conc. (0.1 to 3 wt%) >> CMC, 2‐10 nm, aggregation no.
50‐150
 Anionic surfactants: Salt of fatty acids, Sulphates or sulfonates
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Ingredients
Water
Butadiene
Styrene
Sodium dodecyl sulphate
Potassium persulfate
1‐Dodecanethiol
Conditions
Time
Temperature
Yield
190
70
30
5
0.3
0.5
12 h
50 oC
65%
parts by weight
Styrene‐Butadiene Copolymer
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Ingredients
Water
Ethyl acrylate
2‐Chloroethyl vinyl ether
p‐Divinylbenzene
Sodium dodecyl sulphate
Potassium persulfate
Sodium pyrophosphate
Conditions
Time
Temperature
Yield
133
93
5
2
3
1
0.7
8 h
60 oC
100%
parts by weight
Polyacrylate Latex
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log CB
Surface
tension,

CMC
Surfactants and Micelles
Micelles have ability to absorb considerable quantities of water‐
insoluble substances
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At the start of the Polymerization
 The aqueous phase
 Large droplets of monomer maintained in
suspension by adsorbed surfactant
molecules and agitation (>95%)
 Small monomer‐swollen micelles which are
far greater in number than the monomer
droplets but contain a relatively small
amount of the total monomer.
[Micelles] = 1019 – 1021 L‐1
[Monomer droplets] = 1012 – 1014 L‐1
Total surface area of the micelle
more than two orders of mag‐
nitude of total surface area of
the monomer droplets
M
M
M
M
M
M
M
M
M
M
I
I I
I
1 – 100 
I
I
I
M
M
M
M
M
M
M
⇋
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PROF. DIBAKAR DHARA
Lecture 17: Emulsion Polymerization (cont..), Common polymers by Radical Chain
Polymerization, RDRP
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 Emulsion Polymerization (cont..)
 Common polymers synthesized by radical chain polymerization
 Reversible‐Deactivation Radical Polymerizations (RDRP)
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Particle Nucleation (Interval I)
Emulsion Polymerization Intervals
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Particle Nucleation (Interval I)
 Oligomeric radical species
 Terminate in the aqueous phase to produce species that are surfactant‐like
 Continue to propagate until they reach a critical size at which they become surface active
 Grow still further until they reach another critical size at which they become insoluble in
the aqueous phase and precipitate from it
 No polymerization in monomer droplets
 The system consists of three types of particles: monomer droplets, inactive micelles, active
micelles / polymer particles
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Interval I (0 – 10‐15% conversion):
• Monomer in micelles (diameter ~ 10 nm)
• Monomer in droplets (diameter ~ 10 m)
• Monomer in polymer particles
• Growing number of polymer particles
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Interval II (15‐60%) and III (60%‐)
 Concentration [M]p of monomer within a
particle remains constant
 Number Np of particles per unit volume of
latex (typically 1016−1018 particles dm−3)
remains same
 Since Np is constant, the rate of
polymerization also is constant; this period
of the polymerization is known as Interval II
 Thereafter, in Interval III, [M]p and the rate
of polymerization decreases continuously as
the remaining monomer present in the
particles is polymerized
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Interval II (15 ‐ 60% conversion):
 No micellar surfactant
 Monomer in droplets
 Monomer in polymer particles
 Constant number of particles
Interval II and III
Interval III (60% ‐ conversion):
 No monomer droplets
 Monomer in polymer particles
 Constant number of particles
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Emulsion Polymerization Kinetics
p
]
M
[
]
M
[ 
 p
p
pp k
R
p
pp
p N
R
R 
)
/
(
]
M
[ A
p
p
pp N
n
k
R 
the number of particles per unit volume of aqueous phase
p
N
p
]
M
[ the monomer concentration in the
Micelles/Particles (M/P)
n the average number of radicals per particle
Rate of polymerization (Interval II onwards)
Rate of polymerization per particle (Interval II onwards)
pp
R
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)
/
(
]
M
[ A
p
p
pp N
n
k
R 
A
p
p
pp
N
k
R
2
]
M
[

n the average number of radicals per particle
 Only the simplest situation: Smith–Ewart Case 2 conditions
 Radical desorption from particles does not occur, cannot escape the particle
once captured
 Particles are so small that two radical species can not exist independently
 Termination occur immediately upon entry of a second radical species into a
particle that already contains one propagating chain radical
 The particle then remains dormant until entry of another radical initiates
the propagation of a new chain
 On average, each particle contains one propagating chain radical for half the
time of its existence and none for the remaining half
 Under these conditions, n = 1/2
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p
pp
p N
R
R 
4
.
0
6
.
0
)
/
(
)
(
53
.
0 u
R
w
a
N r
s
s
p 
]
I
[
2 d
A
r k
N
R  Rate at which a particle grows its volume (linearly)
with time once polymerization is initiated
the area occupied by unit weight of
surfactant
s
a
the weight concentration of surfactant
s
w
u
 The value of n can vary widely dependent upon the reaction formulation and
the conditions used
 During Interval I, Np increases, and during Interval III, [M]p is decreasing.
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4
.
0
6
.
0
)
/
]
I
[
2
(
)
(
53
.
0
2
]
M
[
u
k
N
w
a
N
k
N
R
R d
A
s
s
A
p
p
p
pp
p 

]
I
[
2
]
M
[
d
A
p
p
p
n
k
N
k
N
X 
 Both Rp and xn can be increased by increasing Np
 Propagating chain radicals are segregated into
separate particles (compartmentalization)
Rate of polymerization (Interval II onwards)
Np  1015 ‐ 1016 per cm3 of aqueous phase, Rr  1012 ‐ 1014 radicals cm‐3 s‐1,
asws  105 cm2/cm3 in aqueous phase, u  10‐20 cm3 s‐1
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Benefits and Applications of Emulsion Polymerization
 Excellent heat transfer
 Relatively low viscosity of the product latexes at high polymer concentrations, and
 Ability to control particle morphology (e.g. formation of core‐shell particle structures by
successive additions of different monomers)
 Water‐borne polymers (i.e. latexes) – no/low emission of volatile organic compounds
(VOCs) ‐ environmentally friendly alternatives to solvent‐borne polymers, especially for
coating application
Polymers prepared by emulsion polymerization are used either directly in latex
form (e.g. emulsion paints, water‐borne adhesives, paper coatings, binders for
non‐woven fabrics, foamed carpet‐backings) or
After isolation by coagulation or
Spray drying of the latex (e.g. synthetic rubbers and thermoplastics)
Contamination by inorganic salts and dispersion stabilizers
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Miniemulsion Polymerization
 Objective of controlling Np
 Starting the polymerization with a miniemulsion that comprises monomer
droplets which are sufficiently small (typically 50–300 nm diameter) and large
enough in number (1016−1018 dm−3) to capture efficiently all radicals
 The locus for particle nucleation is then the miniemulsion monomer droplets,
each of which (ideally) becomes a polymer particle, and so Np is defined by the
number of miniemulsion droplets present at the start of the polymerization
 Once polymers are produced within a miniemulsion droplet, it becomes a
monomer‐swollen particle and polymerization continues in much the same way
as during Interval III of an emulsion polymerization
 A two‐component stabilizer system is required: a surfactant plus a costabilizer
compound that is soluble in the monomer, but highly insoluble in water
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Microgels
 Microgels are crosslinked submicron (or multi‐micron) size particles prepared by emulsion
copolymerization of mono‐ and multi‐functional monomers that produce network
polymers, which are insoluble in water under the conditions of polymerization, but which
under other conditions are miscible with water
 Microgels swell but do not dissolve due to the crosslinks
 Microgel latexes ‐ particularly relevant to biomedical applications where advantage can
be taken of their swelling–deswelling behaviour (e.g. in diagnostics and controlled
release)
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 Batch: All reactants are added completely to the reaction vessel at the start of the
polymerization.
 Semi‐continuous batch (or semi‐batch): Part of the total formulation is introduced
at the beginning of the reaction, the remainder are added as per a predetermined
schedule during the course of the polymerization.
 Continuous: Reactants are added continuously to the reactor from which product is
removed continuously such that there is a balance between the input and output
streams
Strategies for Performing Polymerization Processes
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PROF. DIBAKAR DHARA
Lecture 18: Reversible‐Deactivation Radical Polymerizations (RDRP)
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 General criteria for Living polymerization
 SFRP/NMP
 ATRP
 RAFT
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Reversible‐Deactivation Radical Polymerizations
(RDRP)
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Radical Chain Polymerization
i
k
M1
R + M
t
k
dead polymer
+
Mm Mn
P
k
Mn+1
+ M
Mn
1. Living
2. Scope of generating functional groups at the ends or preparing
block copolymers
3. Control over molecular weight
If we can remove the termination step

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Reversible‐Deactivation Radical Polymerizations (RDRP)
By reversibly trapping and temporarily deactivating the chain radicals
 Activation–deactivation cycle is rapid – kinetics is fast
 Equilibrium is very much in favor of deactivated species
t
k
dead polymer
+
Mm Mn
Living radical polymerization (LRP)
Control radical polymerization (CRP)
Negligibly low
Negligibly low by decreasing the [M ] to very low value
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Two strategies
Reversible termination: Reversible end‐capping of the chain radical by a chain‐capping species
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Two strategies
Reversible termination: Reversible end‐capping of the chain radical by a chain‐capping
species
Stable free‐radical polymerization (SFRP) /
Nitroxide‐Mediated Radical Polymerization (NMP)
Rapid, reversible exchange between
the active and dormant states
Atom‐transfer radical
polymerization (ATRP)
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Reversible transfer: Highly‐efficient chain transfer reactions in which a free chain radical displaces
a trapped chain radical from an end‐capped species and in the process becomes end‐capped
 high efficiency of the exchange process
 much higher number of trapping agent
molecules present compared to the total
number of primary radicals produced from
the initiator
 Each chain then grows with approximately
equal probability in very short bursts of
activity
Radical Addition–Fragmentation Transfer (RAFT)
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 Irreversible chain transfer and chain termination reactions are minimized
 The rate of chain initiation is also much larger than the rate of chain propagation
 Chains grow at a more constant rate than seen in traditional chain polymerization
 Both yield and molecular weight of the polymers produced increase with reaction
time (conversion)
 Polymer chain lengths are nearly equal (very low dipersity)
 Predetermined molar mass
 Polymer can be synthesized in stages, each stage containing a different monomer
that is added to the reaction system, yielding a block copolymer
 Control over end groups
Reversible‐Deactivation Radical Polymerizations (RDRP): General Features
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Reversible‐Deactivation Radical Polymerizations (RDRP): General Features
 First Order Kinetics with Respect to Monomer
 Linear Increase of Degree of Polymerization
(Xn) with Conversion
 Narrow Molecular Weight Distribution
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Stable Free‐Radical Polymerization (SFRP) /
Nitroxide‐Mediated Radical Polymerization (NMP)
t
k
K p
SFRP
t ]
R
ONR
[
]
R
ONR
[R
]
M
[
]
M
[
ln 3
2
3
2
1
0


R1 ONR2R3
ka
kd
R1 ONR2R3
+
nM
R1Mn
ONR2R3
R1Mn ONR2R3
0
3
2
1
0
n
]
R
ONR
[R
]
M
[
c
X 
c : fractional
monomer
conversion
0
3
2
1
]
R
ONR
[R
Poly]
[ 
CH
H3C
O N
H3C CH3
CH3
H3C
CH
H3C
+ O N
H3C CH3
CH3
H3C
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]
[Cu
]
Br][Cu
‐
[R
]
M
[
]
M
[
ln 2
0


 p
ATRP
t
k
K
0
0
0
0
n
]
Br
R
[
]
M
[
]
Br
R
[
]
M
[
]
M
[





c
X
c : fractional
monomer
conversion
0
]
Br
‐
[R
Poly]
[ 
Atom‐Transfer Radical Polymerization (ATRP)
R Br + CuBr(L)
ka
kd
R CuBr2(L)
+
RMn
RMn Br + CuBr(L)
nM
CuBr2(L)
+
An organic halide undergoing a reversible redox
process catalyzed by a transition metal compound
such as cuprous halide
n
X
X
X
D
1
1
n
w



CH3 O
C
O
CH
X
CH3
CH
X
H3C
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H
SnBu3
OH
OH
Br
Bu3SnH
in situ
O
Br
O
H2N OH
NH
OH
NaN3, DMF
N3
PPh3
N PPh3
LiAlH4 NH2
H2O, THF
Br
Atom‐Transfer Radical Polymerization (ATRP): End Functional Groups
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Atom‐Transfer Radical Polymerization (ATRP): Block Copolymers
X
B
RA
X
RA
A m
n



X
B
‐
RA
A
‐
XB
X
RA
XA
A m
n
n
m
n
n


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Reversible Addition–Fragmentation Transfer (RAFT)
Polymerization is carried out with a conventional initiator such as a peroxide or
AIBN in the presence of the chain‐transfer agent (RAFT agent)
S
Z
S A
2 R
I
 Reactivities of the two radicals (R• and A•) must be
similar
 The Z group must activate the exchange process
 RAFT agent must be highly active in chain transfer
 Number of RAFT agent >> total number of primary radicals generated from the
initiator during the course of the whole polymerization
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Dithioesters RAFT agents
C
S
CH2 C
S S
C
CH3
CN
CH3
C
S S
C
CH3
CH3
Trithiocarbonate RAFT agents
CH2
S
C
S
S
CH2 HO2C
C
H3C
CH3
S
C
S
S
C
CH3
CH3
CO2H
Dithiocarbamate RAFT agents
N
C
S CH3
CN
CH3
S
N
C
S
CH2
S
Xanthate RAFT (MADIX) agents
H3CH2C
O
C
S
S
C
CH3
CN
CH3
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Mn
+
S
R
SR
/
MnS
R
SR
/
MnS
R
S
/
+ R
Mm
+
S
R
SMn
/
MmS
R
SMn
/
MmS
R
S
+ Mn
/
Initially, addition‐fragmentation chain‐transfer to the RAFT agent dominates, but as
the RAFT agent is consumed and the concentration of polymeric RAFT species grows,
the latter begin to contribute and eventually become the only dithioester species
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c/c’/c’’ : fractional monomer/RAFT agent/Initiator
conversion
)
]
I
[
'
'
(
]
[RAFT
(c'
c[M]
0
2
2
0
0
n
fc
X
a



RAFT agents are consumed with first few percent of monomer conversion, p’ = 1, due to large
chain‐transfer coefficient of RAFT agent
]
[RAFT
[M]
0
0
n
p
X 
0
2
2
0
0
0
]
I
[
'
'
]
[RAFT
and
]
I
[
]
[RAFT
fp
p' a



 Reaction rate expression is similar to conventional radical polymerization
 Can be used for synthesizing block, graft, star copolymers as shown for ATRP
N
P
T
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L

PROF. DIBAKAR DHARA
Module 04: Other Chain Polymerization Methods
Lecture 19: RAFT Polymerization (cont..), Ionic Polymerization
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 RAFT Polymerization (continuation of Module 3)
 Common polymers synthesized by radical chain polymerization (continuation of
Module 3)
 General characteristics of ionic chain polymerization and its comparison with
radical chain polymerization
 Practical considerations in ionic chain polymerization
 Anionic chain polymerization
 Cationic chain polymerization
N
P
T
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Mn
+
S
R
SR
/
MnS
R
SR
/
MnS
R
S
/
+ R
Mm
+
S
R
SMn
/
MmS
R
SMn
/
MmS
R
S
+ Mn
/
Initially, addition‐fragmentation chain‐transfer to the RAFT agent dominates, but as
the RAFT agent is consumed and the concentration of polymeric RAFT species grows,
the latter begin to contribute and eventually become the only dithioester species
N
P
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c/c’/c’’ : fractional monomer/RAFT agent/Initiator
conversion
)
]
I
[
'
'
(
]
[RAFT
(c'
c[M]
0
2
2
0
0
n
fc
X
a



RAFT agents are consumed with first few percent of monomer conversion, p’ = 1, due to large
chain‐transfer coefficient of RAFT agent
]
[RAFT
[M]
0
0
n
p
X 
0
2
2
0
0
0
]
I
[
'
'
]
[RAFT
and
]
I
[
]
[RAFT
fp
p' a



 Reaction rate expression is similar to conventional radical polymerization
 Can be used for synthesizing block, graft, star copolymers as shown for ATRP
N
P
T
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L

Some Common Polymers Synthesized using
Radical Chain Polymerization
N
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H2C CH2 H2C CH2 *
* n
 Synthesized at high pressure at a temperature above it’s melting temperature
 Continuous process
 Mostly LDPE are produced in this process – highly branched
 Low crystalline (40‐60%)
 Low density (0.91 – 0.93 g cm‐3 )
 MW 20,000 – 100,000; PDI 3‐10
 Tg ~ ‐120 C; Tm ~ 105 C ‐ 110 C
 Low density (0.91 – 0.93 g cm‐3 )
Polyethylene
N
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Polyethylene
 Good combination of strength, flexibility, impact resistance and melt flow
behavior
 Mostly used as films – e.g. packaging and household use (bags, pouches, wraps
for foods, clothes, etc); agricultural and constructional applications (green
houses, industrial tanks, etc.)
Trade names: Alathon, Fertene, Marlex, etc
H2C CH2 H2C CH2 *
* n
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Polystyrene
H2C CH H2C CH *
* n
 Generally synthesized by continuous solution process
 Some cases by suspension polymerization
 MW 50,000 – 150,000; PDI 2‐4
 Tg ~ 85 C;
 Rigid plastics, completely amorphous
 Good strength and dimensional stability
 Good resistance to aq. bases and acids
 Poor weatheribility, resistance to hydrocarbon solvents
 Often used with additives, and as blends or copolymers
N
P
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Applications as inj. molded articles:
 Office fixtures, tumbles, etc
 In medical – pipettes, petry dishes, containers, etc
 Expanded PS is formed by impregnated blowing agents – PS Foams
 Disposable drinking cups, cushioned packaging, thermal insulations, egg
cartoons, fast food trays, etc
 Cross‐linked PS beads – used in chromatographic columns
Trade names: Carinex, Cellofoam, Dytene, Styrofoam, etc
H2C CH H2C CH *
* n
Polystyrene
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 Generally synthesized by suspension polymerization batch process
 Low crystalline
 Tg ~ 81 C;
 HCl formation during processing at high temp.; heat stabilizers
like metal oxides, fatty acid salts are always added
 Tough, rigid plastics, find extensive application
 Often plasticized by adding additives – flexible PVC
H2C CH H2C CH *
* n
Cl Cl
Vinyl family: Poly(vinyl chloride)
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Applications:
• Pipes for home and other applications
• Vinyl sliding – window frames, rain gutters, etc
• Packaging – bottles, box‐lids, etc
• Flooring
• Wire and cable insulation
• Surgical and protective gloves
Trade names: Carina, Nipeon, etc
Vinyl family: Poly(vinyl chloride)
H2C CH H2C CH *
* n
Cl Cl
N
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Vinyl family: Other members
H2C CH H2C CH *
* n
OCOCH3 OCOCH3
H2C C H2C C *
* n
Cl Cl
Cl Cl
N
P
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Acrylic family: Poly(methyl methacrylate)
H2C C H2C C *
* n
COOCH3 COOCH3
CH3 CH3
 Synthesized by solution, suspension, and emulsion polymerization
 Amorphous
 Tg ~ 105 C;
 Low RI – exceptional optical clarity, weatheribility, strength, etc.
 Sheets, rods, etc
 Safety glasses, indoor and outdoor lighting, optical fibres for light
transmissions, eye glass lenses, etc.
Trade names: Plexiglass, Leucite, etc.
N
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Acrylic family: Other Members
H2C C H2C C *
* n
COOH COOH
CH3 CH3
H2C CH H2C CH *
* n
CONH2 CONH2
H2C CH H2C CH *
* n
COOH COOH
H2C CH H2C CH *
* n
CN CN
Other Monomers
N N
O
N
P
T
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Non‐linear Radical Polymerizations Involving Crosslinking Monomers
CH
H2C
HC
H2C
H2C
H
C
C
HN
CH2
HN
C
HC
O
H2C
O
CH3
C
C
O
CH2
HC
H2C
O
H2C
N
P
T
E
L

Ionic Chain Polymerization
N
P
T
E
L

 Cationic
 Anionic
Y
Y
*
I
1. No inherent termination: Living
2. Pre‐decide molecular weight
3. Scope of generating functional groups at
the ends
4. Fast reaction
CI
Ionic Polymerization
























MM
M
e
M
MM
M
e
M
I
M
I
M
I
M
I
M
N
P
T
E
L

Much more selective
Ionic Polymerization: General Characteristics
Y anionic cationic
OR
COR
COOR/CONR2
CN
Ph
CH=CHR
+
+
+
‐
+ ‐
Y
Y
*
I
I*
N
P
T
E
L

Effect of Polarity of the Solvent






 ‐/
‐
/
1
n
‐/
‐
/
n
X
M
M
X
M








 ‐/
‐
/
n
‐/
‐
/
n
‐/
‐
/
n
n
X
M
X
M
X
M
X
M
Low to moderate polarity solvents like THF, ether, mixed solvents of
THF‐toluene, etc
II
tight ion‐pair
contact ion‐pair
I
covalent
bonded
III
solvated ion‐pair
loose ion‐pair
IV
free ion‐pair
N
P
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Nature of Reaction
 Very fast : 104 – 106 times faster than radical chain polymerization
 Bimolecular termination reactions between propagating species are absent
 Very susceptible to impurities like O2, CO2, H2O, hydroxylated solvents, etc
 Large effect of cocatalysts
 Complex: Kinetic data are often not reproducible, especially for cationic
polymerization
N
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Comparison Between Radical‐Cationic‐Anionic Chain Polymerization
 Ionic polymerizations are very susceptible to O2, CO2, H2O, etc
CH2 C

H
+ H2O CH2 CH2

+ HO
N
P
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Comparison Between Radical‐Cationic‐Anionic Chain Polymerization
 Ionic polymerizations are very susceptible to O2, CO2, H2O, etc
 Ionic polymerizations are very sensitive to solvent polarity, especially anionic
polymerization; radical polymerizations are not that sensitive
 Ionic polymerization are generally carried out at ambient or lower temperatures;
radical polymerization > 50 C
 Eradical > Eionic (low, sometimes negative)
 Radical scavengers stops/inhibits radical polymerization; no effect on ionic
polymerization
 Ionic polymerizations can be easily employed to make copolymers
N
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Ionic Polymerization: Practical considerations
 Very susceptible to O2, CO2, H2O, etc
 All reagents must be extremely pure
 Glasswares should be cleaned rigorously – generally high vacuum techniques are used
 N2 / Ar (used for creating inert atmosphere) must be extremely dry and pure
Commercial applications of anionic chain polymerizations are limited to
making specialty copolymers and MW standards
N
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H2C C
CH3
CH3
H2C C
OR
H2C C
OR
H
OR
O
CH CH2
C CH2
H3C
CH2
N
HC
CH2
H2C CH
X
+
E G
Initiator
E C C
X
G
Cationic Polymerization
N
P
T
E
L

Cationic Polymerization
Initiator
Protonic acid H+
Lewis acid AlCl3, SnCl4, BF3
Cationogen RCl + Lewis acid (R2AlCl, BCl3)
R = CH2, t‐Bu
Solvent
Cholorinated (Good)
Hydrocarbon (Poor)
Ethers, Solvents with active hydrogen atom, non‐aromatic (Can not be used)
N
P
T
E
L

Cationic Polymerization BF3 + H2O
H BF3OH
H3C C
CH3
CH3
CH2C
CH3
CH3
CH2C
CH2
CH3
H3C C
CH3
CH3
CH2 C
CH3
CH3
CH2 C
CH3
CH3
BF3OH
H3C CH
CH3
CH3
H2C C
CH3
CH3
H2C C
CH3
CH3
n
Chain Transfer
Initiation
Propagation
BF3OH
N
P
T
E
L

Cationic Polymerization: Termination
N
P
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Cationic Polymerization:
Example: Synthesis of Butyl Rubber
 Polymerization isobutylene with about 5 mol% isoprene
 Produced commercially using AlCl3 as Lewis acid catalyst in methyl chloride at ‐100 C
 Low temperatures helps in minimizing chain transfer reactions and produce polymers
of predetermined MW
 Butyl rubbers are elastomers with outstanding weatherablity characteristics and have
lowest air permeability among all known elastomers
N
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Anionic Polymerization
 Initiation
 Propagation
 Termination
Initiation
Nu M + C C
X
Y
C C C
X
Y
Nu M
N
P
T
E
L

Initiation
N
P
T
E
L

Na +
THF
Na
CH CH2
+
HC CH2 Na HC CH2 Na HC CH2 CH2 CH Na
Na
HC CH2 CH2 CH Na
Na
CH CH2
CH CH2 CH2 CH PS
PS
N
P
T
E
L

Anionic Polymerization:
CH CH2
m
n BuLi cyclohexane
CH
H2C
Bu CH2 CH
m-1
Li
+
CH
H2C
Bu
m
CH2 CH CH CH2 CH2 CH CH2
CH
CH2
CH
CH
CH2
Li
n
o-1
CH
H2C
Bu
m
CH2 CH CH CH2 CH2 CH
CH
CH2
n
o
CH2 CH CH2 CH Li
p-1
quenching
SBS
Example: Synthesis of SBS thermoplastic Elastomer
N
P
T
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AAA CH2 CH2 OH
O
2. H+ , H2O
1.
AAA
2. H+ , H2O
1. CO2
AAA CO2H
Br CH2CH CH2
AAA CH2CH CH2
Anionic Polymerization: Generation of End‐functional Polymers
N
P
T
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L

Anionic Polymerization: Synthesis of Star Copolymer
N
P
T
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PROF. DIBAKAR DHARA
Lecture 20: Polymer Stereochemistry and Zeigler‐Natta Coordination Polymerization
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 Constitutional (structural) isomers
 Polymerization of conjugated dienes (geometric isomers)
 Tacticity
 Ziegler‐Natta Polymerization (in brief)
 Metallocene Polymerization (in brief)
 Commercial coordination polymers (in brief)
N
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Constitutional Isomerism (Structural Isomerism)
Poly(vinyl alcohol)
Molecules have the same overall chemical composition (i.e., same molecular formula)
but differ in connectivity
Polyacetaldehyde Poly(ethylene oxide)
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Stereoisomerism
Significant effect that stereoisomerism has on many polymer properties changes
that occur by rotations about single bonds
Stereoisomers have the same connectivity, but differ in their configurations
 Configuration is the relative orientation in space of the atoms of a stereoisomer, independent of spatial
changes that occur by rotations about single bonds
(Conformation refers to the different orientations of atoms and substituents in a molecule that result
from rotations around single bonds)
 Cis–trans (geometric) isomers arise from different configurations of substituents
on a double bond or on a cyclic structure.
 Enantiomers arise from different configurations of substitutents on a sp3
(tetrahedral) carbon or other atom
 Various tacticity arises from regularity in the configurations of successive
stereocenters in polymer backbone ‐ isotactic and syndiotactic, atactic ‐
stereoregular polymers
N
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Chirality in Polymers
 Chiral is the term used to describe objects which are non‐superimposable on their mirror image
 Simplest chiral molecules have an sp3‐hybridized carbon atom to which four different groups are
attached – asymmetric carbon – show optical activity
 Although polymers with general structure contain many asymmetric carbon,
no significant optical activity because the two polymer chain residues attached to the
asymmetric carbon atom are almost identical
H
C
H
C
* *
Y
H
N
P
T
E
L

Polymerization of 1,3‐Butadiene and Substituted 1,3‐Butadienes
Gutta percha and balata ‐ predominantly trans‐1,4‐polyisoprene ‐ hard, rigid materials
Natural rubber ‐ cis‐
1,4‐polyisoprene ‐
an amorphous
rubbery material
CH3
CH3 CH3 CH3
CH3
CH3
CH3
CH3
N
P
T
E
L

 In free‐radical polymerization, for substituted 1,3‐butadienes, a high proportion of trans‐1,4 repeat
units specially as the reaction temperature is reduced
*
R R
*
R
*
R
*
 Anionic polymerization in a non‐polar solvent using Li+ as the counter‐ion leads to formation
of polymers with high proportions of cis‐1,4 repeat units
R
‐
Li+
R
R
Li
R
R
‐
R
Li+
 Anionic polymerization in non‐polar solvents using counter‐ions other than Li+ or in
polar solvents (regardless of the counter‐ion), stereochemical control is lost
N
P
T
E
L

Microstructure (Mole Fractions)
cis‐1,4 trans‐1,4 1,2‐ 3,4‐
Monomer Polymerization
Conditions
Butadiene Free radical at −20 °C 0.06 0.77 0.17 —
Butadiene Free radical at 100 °C 0.28 0.51 0.21 —
Isoprene Free radical at −20 °C 0.01 0.90 0.05 0.04
Butadiene Anionic in hexane with
Li+ counter‐ion at 20 °C 0.68 0.28 0.04 —
Butadiene Anionic in diethyl ether
with Li+ counter‐ion at 0 °C 0.08 0.17 0.75 —
Isoprene Anionic in cyclohexane
with Li+ counter‐ion at 30 °C 0.94 0.01 0.00 0.05
N
P
T
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Tacticity in Polymers Synthesized by Radical/Ionic Chain Polymerization
 The tacticity of a polymer is controlled by the stereochemistry of propagation
C
Y
X
C
C
C
C
H
Y
H
Y
H
X
H
X C
Y
X
C
C
C
C
H
X
H
Y
H
Y
H
X
H2C
X
Y
H2C
X
Y
C
C
C
Y Y
H
X
H
X C
C
C
Y X
H
X
H
Y
Rotation of terminal bond
 The terminal active centres of propagating chains in free‐radical, cationic and anionic polymerizations can
be considered to be sp2 hybridized, the remaining p‐orbital containing one, none and two electrons,
respectively ‐ a planar arrangement of the groups about the terminal active carbon at
N
P
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Zeigler‐Natta Polymerization
(Coordination Polymerizations)
N
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Ziegler’s Discovery
 Karl Ziegler’s discovery ( in Germany‐1950s)
at low temperatures (50–100 C) and pressures
Ethylene High‐MW polyethylene
Transition metal compound (Ti, V, Cr)
Organometallic compound (AlR3)+
Ethylene Oligomers, highest MW 5000
Trialkylaluminum (AlR3)
high temperatures and pressures
much less branched
property enhancements
compared to PE by
radical polymerizaton
N
P
T
E
L

Propene
 ‐ olefins (1‐alkenes)
Stereoselective polymerizations
(both isoselective and syndioselective)
Catalysts of the type
described by Ziegler
(Ziegler & Natta ‐1963 Nobel Prize)
Natta’s Discovery
This was a huge achievement since ‐olefins cannot be polymerized to high‐
molecular‐weight polymer by radical or other ionic initiators
 Giulio Natta’s discovery (in Italy, 1965)
N
P
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 HDPE
Examples of Commercial Polymers by Z‐N Polymerization
‐ Low branching
‐ High crystallinity
‐ High density
‐ Films: packaging
‐ Sheets
‐ Tubing
‐ Wires and Cables
 HMW‐HDPE ‐ MW 0.25M to 1.25M
 UHMW‐HDPE ‐ MW > 1.5M
 LDPE ‐ Copolymerized by slight amount of 1‐olefin
 PP
‐ High abrasion resistance and impact strength
‐ Very high strength
‐ High Tm
‐ Very low density
denser, tougher, higher
melting because the more
regular structure allows
closer chain packing
 Polymers from 1,3‐dienes
N
P
T
E
L

Heterogeneous Ziegler‐Natta Polymerization
Heterogeneous Catalysts (traditional Ziegler–Natta initiators)
(1) transition metal compound
(an element from groups IV to VIII)
‐ catalyst
‐ halides or oxyhalides of Ti, V, Cr, Mo, Zr
(2) organometallic compound
(a metal from groups I to III)
‐ cocatalyst
‐ hydrides, alkyls, or aryls of metals
(such as Al, Li, Zn, Sn, Cd, Be, Mg)
combination of
Most important from the commercial standpoint
TiCl3
TiCl4
modify and activate
the transition metal
compound for initiation
+ R3Al
N
P
T
E
L

Heterogeneous Ziegler‐Natta Polymerization
Generally carried out in a hydrocarbon solvent such as n‐hexane
 Catalyst preparation
 mixing the components in a dry
 inert solvent in the absence of oxygen
 usually at a low temperature
 Character of Catalysts
 having high reactivity toward many nonpolar monomers
 high degree of stereoregularity.
N
P
T
E
L

Heterogeneous Ziegler‐Natta Polymerization: Mechanism
Most Ziegler–Natta components participate in a complex set of reactions involving alkylation
and reduction of the transition‐metal component by the group I–III component –
for TiCl4 and AlR3:
 TiCl4‐AlR3 (R = alkyl) system – initially exchange reactions
AlR3 + TiCl4
AlR2Cl + TiRCl3
AlR2Cl + TiCl4
AlRCl2 + TiRCl3
AlR3 + TiRCl3 AlR2Cl + TiR2Cl2
 Reduction via homolytic bond cleavage
TiRCl3 TiCl3+ R
TiR2Cl2 TiRCl2 +R
 radicals formed in these reactions disappears
by combination, disproportionation, or
reaction with solvent.
N
P
T
E
L

Mechanism and Reactivity in Heterogeneous Polymerization
Two mechanism
a) Monometallic mechanism
b) Bimetallic mechanism
Ti
R
Cl
Al
H2C CHX
n+1
Ti
H2C
Cl
Al
CHX CH2 CHX R
n
Ti
R
Cl
H2C CHX
n+1
Ti
CH2
Cl
CHX CH2 CHX R
n
N
P
T
E
L

Two mechanism
a) monometallic mechanism
 Propagation occurs by insertion of monomer
at transition metal–carbon bonds, the active
sites are surface Ti atoms which have been
alkylated by reaction with AlR3
 Monomer is complexed at the empty d‐
orbitals of Ti
 Insertion reaction
 Shifting the vacant octahedral position
 Migration of the chain occurs to reestablish
the vacant site on the surface
Ti
CH2
CH X
P
H2C CHX
Ti
CH2
CH X
P
CHX
CH2
Ti
H2C
CH X
P
C
CH2
X
Ti
Cl
C
H2
CH X
H2C
CH
P
X
Ti Cl
H2C
CH X
H2C
CH
P
X
* CH2 CHX R
n
P =
N
P
T
E
L

b) Bimetallic mechanism
 Active site is an electron‐deficient bridge
complex formed by reaction between a
surface Ti atom and AlR3
 Propagation by insertion of monomer at
groups I–III metal–carbon bonds after initial
polarization of the monomer by coordination
to the transition metal
Ti
H2C
Cl
Al
CH P
X
H2C CHX
Ti
CH2
Cl
Al
CH P
X
H2C CH
X
H2C CH
X






Ti
Cl Al
CH2
CH
X
P
Ti
Cl
Al
CH CH2
X
H2C
CH P
X
Ti
Cl
Al
CH CH2
X
H2C
CH P
X
* CH2 CHX R
n
p =
N
P
T
E
L

Ziegler‐Natta Polymerization: Termination
 transfer to monomer
 internal hydride transfer
 transfer to cocatalyst or to an added alkylmetal compound
 transfer to added hydrogen
CH2CH
R
CH2 CHR
CHR
CH2
AlR'3
H2
CH2CH2R
H
R'
H
CH CH
+ CH2 C
R
+ CH3CH
R
+ CH2 C
R
+ R'2Al CH2CH
R
+ CH3CH
R
N
P
T
E
L

 Ziegler‐Natta – nonpolar monomers
monomer activity – decreases with increasing steric hindrance about the double bond.
Ziegler‐Natta Polymerization: Monomer Reactivity
H2C CH2 H2C CH CH3 H2C CH CH2CH3 H2C CH CH2CH(CH3)2
H2C CH CH(CH3)2 H2C CH CH(CH2CH3)2 H2C CH C(CH3)3
> > > >
> >
N
P
T
E
L

Homogeneous Ziegler‐Natta Polymerization
Metallocene Catalysts
Cp2TiCl2 R2AlCl
 The earliest metallocene catalysts
 MAO – used in conjunction with metallocene catalysts
the most important metallocene
catalysts are based upon
zirconocene dichloride derivatives
which are activated by reaction
with methylaluminoxane (MAO)
Al
O
Al
O
Al
O
CH3
CH3 CH3
H3C H3C H3C
Al Al Al
O O O
Al O Al
CH3
Al
H3C
CH3
CH3
CH3
N
P
T
E
L

Cyclopentadienyl Indenyl Fluorenyl
LL`MtX2
L or L`
Mt: gen. a Gr. IV transition metal, mainly Zr
X : mainly Cl, sometimes CH3
Metallocene Catalysts
R
R
Z
R
R
X
M
X
M : Zr, Ti, Hf
X : Cl, alkyl
Z : C(CH3)2, Si(CH3)2, CH2CH2
R : H, alkyl
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Character of polymers prepared with metallocene catalysts
 Narrower molecular weight distributions than those prepared with heterogeneous catalysts.
 Better mechanical properties.
 Dispersities (Mw/Mn) range from 2 to 2.5 for the former, compared with 5 to 6 for the latter.
 The molecular weight of the metallocene‐based polymers decreases with increasing
polymerization temperature, increasing catalyst concentration, and addition of hydrogen to
the monomer feed.
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 Ziegler‐Natta (Heterogeneous)
 High density polyethylene (HDPE)
 Linear low density polyethylene (LLDPE)
 Isotactic polypropylene
 Ethylene propylene diene monomer rubber (EPDM rubber)
 Metallocenes
 Linear low density polyethylene (LLDPE)
 Isotactic polypropylene
 Syndiotactic polypropylene
 Syndiotactic polystyrene
 Cyclic olefin copolymers (COC)
 Ethylene propylene diene monomer rubber (EPDM rubber)
Commercial Polymers by Ziegler‐Natta Polymerization
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PROF. DIBAKAR DHARA
Lecture 21: Ring Opening Polymerization, Copolymers
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Ring‐Opening Polymerization (ROP)
 Types of monomers
 Mechanism and kinetics
 Cationic ROP
 Anionic ROP
Chain Copolymerization
• Types of copolymers
• Importance of copolymerization
• Copolymer composition equation: Terminal Model
• Types of copolymerization: Copolymer microstructure
• Composition drift
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Polymers with Backbone Containing Heteroatom
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Polymer Type Polymer Repeating
Group
Monomer
Structure
Monomer Type
Polyalkene Cycloalkene
Polyether Trioxane
Polyether Cyclic ether
Polyester Lactone
* CH CH(CH2)x *
* CH2O *
* (CH2)xO *
* (CH2)x C
O
O *
* CH2 CH2 NH *
CH2
CH2
(CH2)x
O O
O
(CH2)x O
(CH2)x
C
O
O
H
N
H2C CH2
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Polymer Type Polymer Repeating
Group
Monomer Structure Monomer Type
Polyamide Lactam
Polysiloxane Cyclic siloxane
Polyphosph‐
azene
Hexachloro‐
cyclotriphosphazene
Polyamine Aziridene
* (CH2)x C
O
H
N *
* Si O
CH3
CH3
*
* P N
Cl
Cl
*
* CH2 CH2 NH *
(CH2)x
C
O
NH
Si (CH3)2
X
N
P
N
P
N
P
Cl Cl
Cl
Cl
Cl
Cl
H
N
H2C CH2
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 Cyclic monomers such as cyclic ethers, esters, amides, carbonates and siloxanes undergo
polymerization by ring opening mechanism
 Cyclic monomers should be able to polymerize provided a suitable mechanism for ring
opening is available (kinetic feasibility)
 More important consideration is the thermodynamic feasibility of polymerization
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-120
-100
-80
-60
-40
-20
0
20
0 2 4 6 8 10
Ring Size
Energy
(kJ/mol)
ΔG (kJ/mol)
ΔH (kJ/mol)
TΔS (kJ/mol)
Thermodynamic factor with ring size
 The most important factor which determines whether a cyclic monomer can be converted to linear polymer is the
thermodynamic factor, i.e, the relative stabilities ring and linear structures
Thermodynamic Parameters for
Polymerization of Cycloalkanes
at 25 C [Sawada (1976)]
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 The most reactive monomers are those containing 3‐ or 4‐membered rings.
For cyclic ethers: 3 > 4 > 8 > 7 > 5 > 6.
 Thermodynamic feasibility does not always guarantee realization in practice. There
should also be a kinetic pathway for the ring to open, facilitating polymerization
 Small changes in the physical conditions and chemical structure can have a large
effect on the polymerizability of a cyclic monomer:
THF vs. ‐butyrolactone
6‐membered cyclic ethers vs. cyclic esters
THF vs 2‐methyl tetrahydrofuran
 Ceiling temperatures are often quite low in ROP as compared to vinyl
polymerizations, particularly for 5‐ or 6‐membered ring
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 Does not result in the loss of small molecules or loss of unsaturation
 Initiation by same types of ionic initiators (including coordinate ionic) as in the case of ionic
polymerization of C‐C double bond (few cases by molecular species, e.g., water)
 Exhibit most of the characteristics of cationic and anionic polymerizations of vinyl monomers ‐
effects of solvent and counterion, participation of different species (covalent, ion pairs, and free
ions) in propagation
 Chain polymerization
 Many cases living
 Polymerization‐depolymerization equilibrium is very important
ROP : Mechanism and Kinetics
 Propagation rate constants are similar to the rate constants of step polymerizations ‐
several order of magnitude lower than normal chain polymerization
 MW increases slowly, depends on conversion and monomer:initiator ratio
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Cationic ROP
O
+ R+A‐
O
R
A‐
O
O
A‐
(CH2)4
RO
O
O
A‐
(CH2)4
O
(CH2)4
RO
Polymer
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Cationic ROP
O
+ R+A‐ R O (CH2)3CH2 A‐
O
R O (CH2)4 O (CH2)3 CH2 A‐
Polymer
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Anionic ROP
Z
+
Z Z
O
H2C CH2
+ M+A‐ A CH2 CH2 O‐
M+
A CH2 CH2 O‐
M+
O
H2C CH2
+ A CH2 CH2 O CH2 CH2 O M+
A CH2 CH2 O CH2 CH2 O M+
n
O
H2C CH2
+ A CH2 CH2 O CH2 CH2 O M+
n+1
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Two monomers can copolymerize to form either of the following four structures
Random: Nitrile rubber (butadiene‐acrylonitrile), SAN (styrene‐acrylonitrile)
Alternating: SMA (styrene‐maleic anhydride)
Block: SBS (stryrene‐butadiene styrene)
Graft : ABS (styrene‐acrylonitrile grafted on to polybutadiene)
Chain copolymerization
For homopolymers – limited options, copolymers ‐ unlimited
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Some Examples of Commercial Copolymers
High styrene (50‐70%) – used as latex paints; little bit of unsat. carboxylic acid
used as cross‐linker
 SBR : Styrene (~25%) ‐ co – 1,3‐butadiene (~75%)
Elastomeric, synthesized by emulsion or anionic polymerization
 EVA : Ethylene‐vinyl acetate
 Unsaturated polyesters
 SAN : Styrene‐co‐acrylonitrile (10‐40%)
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* CH2 CH CH CH2 *
n
AIBN
* CH2 CH CH CH *
n
H2C CH
CN
* CH2 CH CH CH *
n
CH2
CH
CH2
CH
CH2
CH
CH2
NC
CH
NC
Polybutadiene
Styrene‐acrylonitrile
ABS (styrene‐
acrylonitrile grafted
on to polybutadiene)
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Main Chain
Side Chain
+
A B
Poly(A‐graft‐B)
"Grafting through"
Graft Copolymers
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Main Chain
Side Chain
R
+
R
X
B
A X X
R'
"Grafting from"
Graft Copolymers
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Step‐growth Copolymerization
 Can be obtained by using monomers of different structures
 For effective copolymerization, the monomers must have similar chemical reactivity, generally
random copolymers are produced
 Block copolymerization can ideally be synthesized by reacting two prepolymers of defined MW
and maintaining the stoichiometry for necessary end groups at both ends, but in reality is
difficult to achieve since many linkages in step growth polymers undergo exchange (trans)
reactions that results in randomization of structure
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OH
HO
+
C
C
O O
Cl Cl
+
C
C
O
O
Cl
Cl
O O C
O
*
C
O
*
n
n Bu4N
Br
CH2Cl2 / NaOH
30 ‐ 35 oC
45 min
pH < 10
Example: Copolyester
Step‐growth Copolymerization
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Chain‐growth Copolymerization
Copolymer composition
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Chain‐growth Copolymerization: Steps
Initiation
Propagation
Termination
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Chain‐growth Copolymerization: Kinetics
Propagation
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PROF. DIBAKAR DHARA
Lecture 22: Copolymerzation (cont..)
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• Types of copolymers
• Importance of copolymerization
• Copolymer composition equation: Terminal Model
• Types of copolymerization: Copolymer microstructure
• Composition drift
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Chain‐growth Copolymerization Composition
1
1
]
[M
*]
[M
]
[M
*]
[M
]
[M
*]
[M
]
[M
*]
[M
dt
]
d[M
dt
]
d[M
1
2
2
2
1
1
2
2
22
2
1
12
1
2
21
1
1
11
2
1
2
1





















f
f
r
f
f
r
k
k
k
k
F
F
21
22
2
12
11
1 and
k
k
r
k
k
r 

2
2
2
2
1
2
1
1
2
1
2
1
1
1
2 f
r
f
f
f
r
f
f
f
r
F



 1
2 1 F
F 

1
2 1 f
f 

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Types of Copolymerization Behavior
 r1 × r2 = 1 : Ideal copolymerization. Two types of propagating species M1* and M2* show the
same preference for adding one or the other monomer
r1 = r2 = 1 : Bernoullian copolymerization. Two monomers show equal reactivity towards both
propagating species. Copolymer composition = feed composition
2
1
1
1
1
1
f
f
r
f
r
F


22
21
12
11
2
1
1
and
k
k
k
k
r
r 

1
1 f
F 
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 r1 × r2 = 1 : Ideal copolymerization. Two types of propagating species M1* and M2* show the
same preference for adding one or the other monomer
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 r1 × r2 = 0 :
r1 = r2 = 0 : Alternating copolymer. Two monomers enter into the copolymer in equimolar amounts
in a non‐random alternating arrangement
21
22
12
11
0
k
k
k
k


5
.
0
2
1 
 F
F
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 r1 × r2  0  1
 r1 > 1 and r2 < 1: (common)
 r1 > 1 and r2 > 1, hence, r1 × r2 > 1 :
tendency to form block copolymer (rare)
 r1 >> r2 : Both types of propagating
species preferentially add to M1. M1 will
tend to homopolymerize until it is
consumed: M2 will subsequently
homopolymerize (not common)
 r1 < 1 and r2 < 1, hence, r1 × r2 < 1 :
tendency to form alternate copolymer
(common)
Similar to boiling
point diagrams of
few liquid−liquid
mixtures
azeotropic
composition
(f1)azeo
2
1
2
1
2
1
)
(
r
r
r
f azeo




azeotropic copolymerization
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 The overall monomer conversion is limited (usually to ≤5%)
 Feeding of additional monomer that are consumed preferentially to the
reaction vessel at a controlled rate during the copolymerization
Starve‐feeding of the comonomer mixture, feeding at rate a lower than the
potential rate of polymerization, very high instantaneous conversions (close
to 100%) are acheived
Types of Copolymerization Behavior: Composition Drift
1
1 2
1 and 
 r
r 1
1 f
F 
For example,
Strategies to control composition drift
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Types of Copolymerization Behavior: Initiation Mechanism Dependence
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Free‐radical Copolymerization
Many commercially important copolymers are prepared by free‐radical copolymerization of ethylenic
monomers
1
1 2
1 and 
 r
r 1
1 2
1 and 
 r
r
H2C CH2 C R
O
C N C O R
O
R Cl C R
O
O
OR
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Free‐radical Copolymerization
M1 M2 r1 r2 r1r2
Styrene Butadiene 0.78 1.39 1.08
Styrene MMA 0.52 0.46 0.24
Styrene Acrylonitrile 0.40 0.04 0.02
Styrene Maleic anhydride 0.02 0.00 0.00
Styrene Vinyl chloride 17.0 0.02 0.34
Vinyl acetate Vinyl chloride 0.23 1.68 0.39
Vinyl acetate Acrylonitrile 0.06 4.05 0.24
Vinyl acetate Styrene 0.01 55.0 0.55
MMA MA 1.69 0.34 0.57
MMA N‐Butyl acrylate 1.80 0.37 0.67
MMA Vinyl acetate 20.0 0.015 0.30
trans‐Stilbene Maleic anhydride 0.03 0.03 0.001
H2C CH2 C R
O
C N C O R
O
R Cl C R
O
O
OR
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Ionic copolymerization : Characteristics
 Much more selective no. of monomer pairs that undergo copolymerization are limited
 Cationic ‐ EDG; anionic – EWG
 General tendency towards ideal type behavior : r1r2~ 1
 Value of r changes with initiator, medium polarity, and temperature
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 Poly(ethylene oxide) and poly(propylene oxide) by anionic ROP of ethylene oxide
and propylene oxide
 Polytetrahydrofuran by cationic ROP of THF
 Polyacetals by cationic ROP of trioxane
 Aliphatic polyesters by cationic ROP of lactones
 Polyamides by anionic ROP of lactams
 Linear polysiloxanes by anionic or cationic ROP of cyclic siloxanes
Examples of commercial ring opening polymers
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Powder
Pellets
Emulsi‐
ons
Raw materials
(monomers)
Natural
sources
In solution
or in bulk
Intermediate
product
(Resins:
pellet,
granules or
flakes)
Final product:
sheets, films,
etc.
Petroleum
sources
OR
Application of
polymers in
formulations
Polymerization
Processing,
compounding
Renewable
sources
Final
Product
Processing,
design
fabrication,
finishing,
assembly
Waste
Waste
management
reuse / recycle
/ biodegrade
POLYMERS
additives
additives
“The Big Picture”: Life Stages and Transformation
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