polymer_science_and_fundamental.pdf

CORPORATE TRAINING AND PLANNING
Introduction To Plastics Materials
Introduction To Plastics Materials

Classiﬁcation of Polymers

POLYMER
“Polymers having long chain macromolecules, which are built up by
the linking together of a large number of small molecules, called
monomer”.
OR
“The polymer (poly- many; mer-unit or parts) is a high molecular
weight compound, formed by the combination of small
molecules of low molecular weight”.

Basic Terms of Polymers
•Polymerization: “The process by which, monomer combine to form
polymers is known as polymerization”.
Degree of Polymerization (DP): “The numbers of repeating unit
present in it call degree of polymerization (DP)”.
Addition Polymerization: : ‘When molecules just add on to form the
polymer, the process is called ‘addition polymerization’
In ‘addition polymerisation’ the molecular weight of the polymer is
roughly equal, to that of all the molecules, which combine to form
the polymer.
Ex; Polyethylene, polypropylene

•
•
Condensation Polymerization: When, however, molecules do not
just add on but also undergo some reaction in forming the
polymer, the process is called ‘condensation polymerisation’
The molecular weight of polymer is lesser by the weight of
simple molecules eliminated during the condensation process
The condensation takes place between the two reactive
functional groups, like the carbonyl group (of an acid) and
hydroxyl group (of an alcohol) to form polyesters.
Ex. Nylon, PET

Polymer Material Properties Depends on
1.
2.
3.
4.
5.
6.
Degree of Polymerization
Molecular Weight of the Polymer
Molecular Weight Distribution
Glass Transition Temperature
Percentage of Crystallinity
Structure and Distribution of Chain Branching

Types of Polymers
A polymer consist of identical monomers or monomers of
different chemical structure and accordingly they are called
homopolymer and copolymers respectively. If the main chain is
made up of same species of atoms, the polymer is called
‘homochain polymer’ Graft copolymer are branched structures in
which the monomer segments on the branches and backbone
differ

The monomeric unit in a polymer may be present in a linear,
branched or cross-linked (three dimensional) structure

Tacticity
1. The head to tail conf i
guration in which the functional groups
are all on the same side of the chain, is called ‘isotactic polymers’.
2. If the arrangements of functional groups are at random around
the main, it is called ‘atactic polymers’ e.g. polypropylene.
3. If the arrangements of side groups is in alternating fashion, it is
called ‘synditactic polymers’
Where R = Alkyl Group

Functionality

Thermoplastics Polymers
Thermoplastics are resins that repeatedly soften when heated
and harden when cooled
Thermosetting Polymers
1.
2.
3.
Thermosets are resins that undergo reaction during processing
to become permanently insoluble and infusible due to they
formed three-dimensional cross linked network structure when
heat is applied.
Characteristics of thermosetting resins:
During the hardening the cross-links are formed between
adjacent molecules, resulting in a complex, interconnected
network that can be related to its viscosity and performance
These cross-links prevent slippage of individual chains, thus
preventing plastic ﬂow under addition of heat
If excessive heat is added after cross links, degradation rather
than melting will occur.
Ex: Phenolic Resin, Epoxy Resin, Polyester resin

Thermoplastics and Thermosetting Polymers
Structure of Thermoplastics and Thermosetting Polymers

Difference Between Thermoplastics and Thermosetting Polymers

Molecular weight of Polymers
“Molecular weight of a polymer is def i
ned as sum of the atomic
weight of each of the atoms in the molecules, which is present in
the polymer”.
This distribution of molecular weights is caused by the statistical
nature of the polymerization process e.g. methane (CH4 )
molecules have the same molecular weight (16), but all
polyethylene do not have the same molecular weight because the
statistical distribution of molecular weight may be different for the
different grade of the polyethylene and the degree of
polymerization may also be different.

Generalization of Concept

Number-average molecular weight
niM i w i
M n= =
ni w i/M i
 
 
Weight-average molecular weight
2
niM i w iM i
M w = =
niM i w i
 
 
z-Average molecular weight
3 2
2
niM i w iM i
M z= =
niM i w iM i
 
 
Where,
n = Moles of molecules (n1 + n2 + n3 + ----------ni) i.e. weight
(w)/molecular weight (M)
w = Weight of individual molecules (w1 + w2 + w3 + ---------wi)
M = Molecular weight of each molecules

Number Average Weight (Mn)
The number average molecular weight is not too dif f
icult to
understand. It is just the total weight of all the polymer molecules
in a sample, divided by the total number of polymer molecules in a
sample.
Consider a polymer, which contains four molecular weight
polymers in different numbers and weight and these

Total number of polymer in containing each entity of poly-1, poly-2,
poly-3 and poly-4 is = 15
Number of Poly-1 present in the polymer = 2
Number of fraction of poly-1 = 2/15
Similarly, Number of fraction of poly-2 = 4/15, Number of fraction of
poly-3 = 6/15, Number of fraction of poly-4 = 3/15
Contribution made by poly-1 towards the average weight of polymer
= number of fraction of each polymer x weight of each poly entity
Therefore, each poly contribution is
(2/25) x 10 =1.33g, (4/15) x 20 = 5.33g, (6/25) x 100 =40g, (3/15) x
250 = 50g
Summing up the contribution to get Number Average Molecular
Weight=
1.33 + 5.33 + 40 + 50 = 96.66g

Total number of molecules (n) id given by
Number fraction of each molecule is =
Number average weight contribution of each entity is =
Number average weight molecular weight is
1 2 3 4
........... i
n n n n n n
      
i
i
n
n

i i
i
n M
n

3 3
1 2 2 2 4 4
......
i i
n
i i i i i
n M
n M
n M n M n M
M
n n n n n
    

    

Total weight of each poly present in the polymer =1450g
Weight of poly-1 present in polymer = 20g
Weight fraction of poly-1 = 20/1450, Weight fraction of poly-2 =
80/1450, Weight fraction of poly-3 = 600/1450, Weight fraction of
poly-4 = 750/1450
Contribution made by each poly towards average weight of
polymer = weight fraction of poly-1x weight of each unit
For poly-1 (20/1450) x 10 = 0.14g
For poly-2 (80/1450) x 20 = 1.10g
For poly-3 (600/1450) x 100 = 41.38g
For poly-4 (750/1450) x 250 = 129.31g
Summing up the contribution made by each poly to get weight
average molecular weight is
0.14 + 1.10 + 41.38 + 129.31 = 171.93g
Weight Average Molecular Weight (Mw )

Total number of molecules (n) id given by
Total weight of the polymer is = W
Weight fraction of each molecule is =
Weight average weight contribution of each entity is =
Number average weight molecular weight is
For synthetic polymers Mw is greater than the Mn. If they are equal
than they will consider as perfectly homogeneous. (Each molecule
has same molecular weight).
1 2 3 4
........... i
n n n n n n
      
i i
N M

1 1 1 1
1 1
n M n M
W n M


2
1 1 1 i i
i i i i
n M
n M M
n M n M
 
2
2
2 2 2
3 3
1 1 2 2 4 4
......
i i
w
i i i i i i i i i i
n M
n M
n M n M n M
M
n M n M n M n M n M
    

    

Molecular weight and degree of polymerisation
Number of repeating unit in a polymer called as degree of
polymerisation (DP). DP provides the indirect method of
expressing the molecular weight and the relation is as follows;
M = DP x m
Where, M is the molecular weight of polymer, DP is the degree of
polymerisation and m is the molecular weight of the monomer
Each of these averages can be related to the corresponding
molecular weight average by the following two equations;
Mn = (DP)n.m
Mw = (DP)w.m
2
i i i i
n w
i i i
n (D P) n (D P)
(D P) = and( D P ) =
n n (D P)
 
 

Influence of Molecular weight of Polymers
•
•
•
•
•
•
The influence of molecular weight on the bulk properties of
polyolefin's, an increase in the molecular weight leads to
Increase in:
Melt viscosity
Impact strength
Lowers in:
Hardness
Stiffness
Softening point
Brittle point
High molecular weight polymer does not crystallize so
easily as lower molecular weight material crystallizes due to chain
entanglement and that ref lect in bulk properties of the high
molecular weight polymer.

•
•
•
A high molecular weight polymer increases the mechanical
properties. Higher molecular weight implies longer polymer
chains and a longer polymer chain implies more entanglement
thereby they resist sliding over each other.
Increasing the molecular weight and the chain length of the
polymer increases impact strength.
Thermal properties can also improved by increasing the
molecular weight.

Polydispersity Index for Molecular weight of Polymers
Polydispersity is a very important parameter and it gives an idea
of lowest and the highest molecular weight species as well as the
distribution pattern of the intermediate molecular weight species.
Plastics processing are affected by the molecular weight
distribution.

Polydispersity Index for Molecular weight of Polymers

To bring into sharper focus the effect of molecular weight on
physical properties, a more generalized form of representation is
given in ﬁgure, mechanical strength is plotted against 'DP .
The useful range of DP is from 200 to 2000, which corresponds to
its molecular weight 20000 to 200000

Determination of Molecular Weight

1.
Determination of Molecular Weight
Number-average Molecular Weight
a) End-group Analysis:
If functional groups present in a given weight of the
sample and this is expressed as a functional group
equivalent/100 g. From knowledge of the functional group
equivalent and the functionality, The molecular weight is
calculated using the equation:
Then the functionality of the polymer sample can be given by the
equation
Functionality = molecular weight X functional equivalent
F unctionality
M n
F unctionalgr oupequivalent


b) Measurement of Colligative Properties
1.
2.
Depression in freezing point (cryoscopy) and elevation in boiling
point (ebulliometry). As these two properties are Colligative (i.e.
depend only on the number of moles of a solute present in a
liquid and not on their nature).
Colligative properties are those that depend on the number of
species present rather than on their kind. From thermodynamic
arguments it may be shown that for very dilute ideal solutions
Where, 1 is the activity of the solvent in a dilute ideal solution
and X2 is the mole fraction of solute. From this relationship the
solute molecular weight may be calculated if the weight
fraction W2 is known.
1 2
ln X
  
2 2 1
2
1 2 1
n w M
X
n M w
 

Lowering of Vapour Pressure
The partial vapor pressure PI of solvent 1 over a solution is
lower than the vapor pressure over the pure solvent p1
0. This is
expressed by Raoult's law:
0
1 1 1
p X p

Where X1 is the mole fraction of the solvent.
For a binary solution containing a mole fraction X2 of solute then,
The suitability of methods based on Colligative properties Vs Molecular Weight

0
1 1 1
2 0 0
1 1
p p p
X
p p
 
 
For a dilute solution,
2 2 1
2
1 2 1
n w M
X
n M w
 
Combining above two equation,
0
1 1
2 2
1 1
M p
M w
w p


Assuming ideal solution behavior, the unknown molecular weight
is calculated from
2
2
s s
n
n
w M V
M
V w


i) Ebulliometry
Ebulliometry is another technique for determining the depression
of the solvent activity by the solute. In this case the elevation of
the boiling point is determined. The boiling-point elevation Tb is
measured with sensitive thermocouples or matched thermostats
in a Wheatstone bridge. The molecular weight M" is calculated
from
b
n
b
K c
M
T


Where c is the concentration of solute in g/1000g of solvent and
2
1000
b
b
v
R T M
K
H


is the molal ebullioscopic constant. M is the molecular weight of
the solvent and Tb its boiling point; Hv is the molar latent heat of
vaporization of the solvent.

ii) Cryoscopy
The freezing point of a solution is depressed below that of the pure
solvent by an amount proportional to the mole fraction of solute.
The value for Mn is obtained from
f
n
K c
M
T


Where c is the concentration of solute in g/1000 g of solvent and
2
1000
m
f
fus
R T M
K
H


is the molal cryoscopy .constant; M is the molecular weight of
the solvent and Tm its melting point; Hfus is the molar latent
heat of fusion of the solvent.

c) Osmometry
I) Vapor-pressure Osmometry
The number-average molecular weight of the unknown sample
may then be calculated from this equation,
R bm
bc
M n
R


  

 
Where c is the concentration of solute in g/1000 g of solvent is the
straight line equation and the plot intercept of which would provide
Mn.

ii) Membrane Osmometry
The membrane osmometer apparatus basically measures
osmotic pressure of polymer solutions of known concentration,
say 1 g dl-1
As already mentioned the van't Hoff law forms the basis for the
determination of number average molecular weight, Mn Following
equation gives the relation ship between osmotic pressure,  of
polymer solution to Mn of the polymer
2 3
1 2 3 4
/ ......
C R T A A C A C A C
  
    
 
Where, A1, A2 and A3 are the f i
rst, second and third virial coef f
i
cients,
C2 is the concentration of polymer solution and R and T are the gas
constant and temperature respectively. For very dilute solutions
(typically less than 1 g dl-I), the concentration terms containing
higher order powers can be neglected and hence it can be written
that
 
2
/ 1 /
C R T M n A C
  

Working Principle

2. Viscosity-Average Molecular Weight
The molecular weight is related to the intrinsic viscosity by the
Mark-Houwink relationship.
  M

 

Where k and  are empirical constants and are characteristic
for a polymer solvent pair at a given temperature.

   
2
/ '
sp
C C
   
 
   
2
(ln ) / "
r el
C C
   

Huggins Equation
Kraemer Equation

Weight-average Molecular Weight
Light Scattering
When a beam of light is passed through a colloidal solution, it is
scattered. This is well-known Tyndall effect, which results from
the scattering of a part of the beam of light by the colloidal
particles in all directions. Since polymer solutions can be
considered as colloidal (lyophilic) solutions and as the intensity
of light scattered depends on the size of colloidal particles (or
polymer molecules)

90
0
0
/ / 1 / 2
1
2
1
w
K C R H C M B C
K C
B C
R M w
K C
R M w
 
 



  
 
 
 
 
 

 
 
Where B is the second virial coef f
i
cient, C is the concentration
of the solution, and R90 is the Rayleigh ratio at 90° observation
angle. This ratio in a generalized case is represented as R i.e.
the Rayleigh ratio is determined at an observation angle of 90°,
R = R90'

Ultracentrifuge
Polymer solutions are lyophilic colloids arid these can be made to
settle down on the application of centrifugal force. The
ultracentrifuge is used to determine molecular weights. The rate of
the settling (sedimentation rate) of polymer molecules depends on
the size of the molecules:
(1 )
sp
SR T
M olecular W eight
D dV

Where Rand T are the gas constant and temperature in Kelvin
scale. The above equation is known as Svedberg equation. For a
polydisperse solute the correct averages for Sand D are combined
in the Svedberg equation

Gel Permeation Chromatography (GPC)
•
•
•
The basic principle underlying the separation of different'
fractions of a polydispersed sample is based on the size of
individual polymer molecules that explore the pore system of
the column material.
Large molecules are excluded from small pores and can only
diffuse into a restricted part of the pore system within the
beads while smaller ones would enter into the pores of the
bead.
Thus large molecules would have less residence time and
would emerge ﬁrst.

Working Principle of GPC

GPC Curve for standard polystyrene sample showing elution
volumes corresponding to different molecular weight

*Relative methods and need calibration from standard polymers samples
**Indirect method and needs values of k and a for particular polymer - solvent
system

Glass Transition Temperature (Tg)
The temperature below, which a polymer is hard and above which
it is soft, is called the glass transition temperature (Tg).
Or
The molecular mobility is just starts above that temperature or
below which mobility arrested called Tg

Tg is unique to amorphous thermoplastics. It occurs at a specif i
c
temperature that depends on pressure and specif i
c volume and is
lower than melting point.

Tg
•
•
•
The glass transition temperature is a measure of evaluating the
f l
exibility of polymer molecules and the type of response the
polymeric material would exhibit to mechanical stress.
The glass transition temperature depends on the structure and
polarity of a polymer, which affects both chain f lexibility and
intermolecular interaction.
Since non-polar polymers have lower potential energy barrier for
rotation, their chain remains f l
exible and they have low Tg. Chains
based on aliphatic C-C bond and C-O bond are quite ﬂexible, on the
other hand introduction of ring structure into main chain has a
marked stiffening effect which results in higher Tg of such polymer.

•
•
•
Rotation about a single C-C bond is also impended by the
substitutions of attached hydrogen atoms by methyl or other
attached hydrogen atoms by methyl or other attached to the main
chain carbon atom can inﬂuence the Tg.
Inclusion of double bonds will stiffen the chain at the point of
inclusion but at the same time decrease the f l
exibility of adjacent
bond. The net effect may therefore be to reduce the glass
transition temperature.
The presence of polar groups or atoms will result in high glass
transition temperature. Hydrogen bonding has a similar effect to
that of a polar group.

Glass Transition Temperature and Molecular Weight
Where, Tg is the glass transition temperature at inf i
nite
molecular weight and K and A are arbitrary constants.
K
Tg Tg
M n

 
1 1 A
Tg Tg M n

 
For a given weight of the polymer, a low molecular weight
sample will have more chain end segments than a high
molecular weight sample.

Glass Transition Temperature and Plasticizer
1.
2.
The plasticizer substantially reduces the brittleness of many
amorphous polymers because its addition even in small
quantities markedly reduces the Tg of the polymer.
The effectiveness of plasticizer in lowering the Tg of a
polymeric composition is directly proportional to its Tg and
volume fraction of plasticizer
1 1 2 2
1 2
1 2
1
Tg V Tg V Tg
w w
Tg Tg Tg
 
 
The extent of reduction in the glass transition values is called
as’plasticizer ef f
i
ciency'. The extent of reduction is depends on
factors such as solubility parameter, polarity and density

Effect of Co-polymerization on Glass Transition Temperature
( ) ( ) ( )
1 A B
A B A B
w w
Tg Tg Tg
 
Where Tg(A), Tg(B) and Tg(AB) are the glass transition
temperature of homo polymers A and B and the copolymer AB
respectively. that the glass transition temperature of a
copolymer will be in between those· of the. respective homo
polymers

Glass Transition Temperature and Melting Point
1
2
Tg Tm
 For symmetrical polymers
2
3
Tg Tm
 For unsymmetrical polymers
A combined version of these two equations, in which the effect
of molecular symmetry is not taken into account, is also in
vogue, as now shown:
1 2
2 3
Tg
Tm
 
Since this relation gives a range for Tg/Tm (rather than a
sharp value), it may more realistically ref l
ect the thermal
behavior of polymers.

Crystallinity in Polymers
A small region of a macromolecules materials in which
portions of large molecules are arranged in regular way, is
called crystalline.
1. Crystallization imparts a denser packing of molecules, thereby
increasing the intermolecular forces of attraction.
2. This accounts for a higher and sharper softening point, greater
rigidity and strength and greater density. A completely
crystalline polymer tends to acquire brittleness.
An ‘amorphous polymers’ is characterized by completely random
arrangement of molecules of polymer chain.
While a crystalline region (called crystallites) embedded in
amorphous random matrix. e.g. Polystyrene, polyvinyl acetate and
polymethyl methacrylate (all are having bulky side groups
attached at random to the main carbon chain) are typically
amorphous.
Amorphous Polymers

Crystalline and Amorphous Polymers
•
•
•
•
Relatively short chains organize themselves into crystalline
structures more readily than longer molecules. Therefore, the
degree of polymerization (DP) is an important factor in
determining the crystallinity of a polymer.
Polymers with a high DP have dif f
i
culty organizing into layers
because they tend to become tangled.
As symmetrical molecules approaches within a critical distance,
crystal begins to form in the areas where they are most densely
packed.
A crystallized area is stiffer and stronger. A non-crystalline
(amorphous) are more ﬂexible.

Crystalline and Amorphous Polymers
•
•
•
•
•
An increasing crystalline in polypropylene increased in creep,
heat and stress cracking as well as increased in mould shrinkage.
Crystallization tendency decreases by co-polymerization, because
it lowers the structural symmetry, so by controlling the extent of
co-polymerization, the relative extent of crystallize.
The amorphous region can be adjusted to get required hardens,
rigidity, heat resistance and ﬂexibility.
A crystallize or any amorphous polymer can also be made
ﬂexible and plastics by adding plasticizer.
The plasticizer neutralization of intermolecular forces of
attraction between macromolecules.

Performance of Crystalline and Amorphous Polymers
•
•
•
•
Crystalline types of plastics are more dif f
i
cult to process; requiring
more precise control during fabrication, have higher melting
temperature and melt viscosities, and tends to shrink and warp
more than amorphous types.
They have a relatively sharp melting point.
That is they do not soften gradually with increasing temperature
but remain hard until a given quantity of heat has been absorbed.
Heating and sudden quenching of crystalline materials will lead to
produce amorphous material. Its properties can be signif i
cantly
different than if it is cooled properly (slowly) and allowed to re-
crystallize.

Effect of Crystallization on the properties of polymers
The properties of polymer such as density, modulus, hardness,
permeability and heat capacity will be largely affected by its
crystallinity. Semi crystalline polymer, its crystalline and
amorphous region exhibit different properties even than both
the region are chemically same. E.g. density of crystalline
region will be higher than the density of amorphous region.
Most of the properties are dependent on the percentage of the
crystalline region present in the polymer.

Effect of Crystallization on Processability
•
•
strong crystalline forces may make the melting
temperature of crystalline polymer higher than its
thermal decomposition temperature, and thus
prevent thermal decomposition temperature.
Fast and complete crystallization must be
occurring in the mold so that the molded piece can
be ejected rapidly without later distortion a n d
shrinkage

Effect of Crystallization on Mechanical Properties
•
•
•
•
Packing of polymer molecules into crystalline lattice generally
restricts their mobility and stiffness their reversible mechanical
properties.
Elongation in polymer that is near or above the glass transition
temperature generally decreases as crystallinity increases.
Creep and compliance creep of polymer, above the glass
transition temperature, are greatly reduced by crystallinity.
Crystallinity often decreases impact strength and large
crystallites particularly produce brittleness as explained below

Effect of Crystallization on Mechanical Properties

Effect of Crystallization on Thermal Properties

Effect of Crystallization on Electrical Properties
•
•
•
Crystallinity restricts the mobility of polar groups in the polymer
molecules and thus affects dielectric properties of the polymer.
In the molten state thermal randomization of polar groups is
greater than orienting effects of an electric f i
eld and dielectric
constant rather low.
With decreasing the temperature thermal randomization
decreases, the orienting and polarization effects of the electric
ﬁeld remains and thus dielectric constant increases.

Effect of Crystallization on Electrical Properties

Effect of Crystallization on Optical Properties
•
•
•
Crystallinity increases the density of the polymer, which decreases
the speed of light passing through it and thus increases the
refractive index
When crystals are larger than the wavelength of visible light about
400-700mµ lights passes through many successive crystalline and
amorphous area is scattered and clarity of the polymer decreased.
Large single crystal scatters light at wide angels and thus causes
haze.

Effect of Crystallization on Chemical Properties
•
•
•
•
The tigtly packed regular structure of crystalline polymers is so
stable that it is impossible to break the lattice using chemicals
Liner amorphous polymers dissolve easily in a range of organic
solvents
Crystalline polymers are usually insoluble and can be dissolved
only under limited condition in few speciﬁc solvents
Crystalline polymers can be dissolve above their melting point
where suf f
icient thermal energy has already been supplied to
separate the polymer molecules from the tight crystalline structure.

Effect of Molecular Weight on Crystallinity

Chemistry of Polymerisations

Chain Growth Polymerisation
1.
2.
3.
4.
5.
6.
7.
8.
The monomers contain at least a double bond that
participates in the polymerisation reaction.
The initiators for chain polymerisation may be different
depending upon the nature of initiation i.e. free radical, cation
and anion.
The carbon-carbon double bond in vinyl monomers and the
carbon-oxygen double bond in aldehydes and ketones are the
two main types of linkages that undergo chain polymerisation.
Important characteristics of chain polymerisation
Once the initiation occurs, the polymer chains form very
quickly i.e. in the time scale of 10-1 to 10-6 s,
The catalyst concentration needed is very low and that
means during the course of polymerisation only monomers
and polymer are present,
The process is exothermic
High polymers with molecular weights of 10,000 to 10 million
can be obtained,)

Free radical polymerisation
1. The decomposition of peroxide type initiators in aqueous
systems is generally accelerated in the presence of a
reducing agent.
Initiation:
Proportion:
Termination:
Or Disproportion

Radical Molecule Reaction (Chain Transfer)
Although the three steps of initiation, propagation, and
termination are both necessary and suf f
icient for chain
polymerisation, other steps 'also takes places during the
polymerisation. As these often involve the reaction between a
radical and a molecule, they are conveniently so classif i
ed Chain
Transfer .
The reaction involves the transfer of an atom between the
radical and the molecule. If the molecule is saturated, like a
solvent or other additive, the atom must be transferred to the
radical.

The major effect of chain transfer to a saturated small molecule
(solvent, initiator, or deliberately added chain-transfer agent) is
the formation of additional polymer molecules for each radical
chain initiated.
Transfer to polymer and transfer to monomer with subsequent
polymerization of the double bond lead to the formation of
branched molecules.
The latter reaction has a pronounced effect on molecular weight
distribution and is important in the production of graft
copolymers.
The ef f
i
ciency of compounds as chain transfer agents varies
widely with molecular structure. Aromatic hydrocarbons are
rather unreactive unless they have benzylic hydrogens. Aliphatic
hydrocarbons become more reactive when substituted with
halogens.
Carbon tetrachloride and carbon tetrabromide are quite reactive.
The reactivity of various polymer radicals to transfer varies widely.

Inhibition and Retardation
A retarder is def i
ned as a substance that can react with a radical
to form products incapable of adding monomer.
It both reduces the concentration of radicals and shortens their
average lifetime and thus the length the polymer chain.
In the simplest case, the retarder may be a free radical, such as
triphenylmethyl or diphenylpicrylhydrazyl, which is too unreactive
to initiate a polymer chain.
The mechanism of retardation is simply the combination or
disproportionation of radicals.
Inhibitors are useful in determining initiation rates; since their
reaction with radicals is so rapid that the decomposition of
inhibitor is independent of its concentration but gives directly the
rate of generation of radicals.
As a result, the length of the induction period before
polymerization starts is directly proportional to the number of
inhibitor molecules initially present. to one radical.

Cationic Polymerisation
The ionic mechanism of chain polymerisation also involves an attack
on the  electron pair of the monomer.
chain reaction initiated and propagated. Strong Lewis acids
such as -BF3 (generally called the 'catalysts' in ionic
polymerisation), in the presence of small amounts of water
or methanol (known as the 'co-catalysts'), form hydrates,
which exist as ion pairs:

The initiators used for this purpose, in addition to BF3, AlCl3,
SnCI4 and TiCI4. Examples of monomers that can undergo
cationic polymerisation are isobutylene, styrene, methyl styrene
and many vinyl ethers.

It involves further two steps:
(i) Donation of a proton to the counter-ion (which is, in
fact, the reversal of the initiation step), resulting in the formation of
a double bond at the end of the growing polymer molecule and the
resultant arrest of the chain growth:
This process of donation of a proton and the re-formation of the
BF3 hydrate is called 'ion-pair precipitation'.
Formation of a covalent bond between the carbonium ion and
the counter-ion when the termination occurs by simple ‘coupling’

Anionic Polymerisation
The attack on the  electron pair of the monomer is done by a
negatively charged ion (an anion). Such a system has extra
electrons and the resultant negative charge attacks the  electron
pair away from end of the molecule.

Co-ordination Polymerisation
Polymerisation reactions, especially of olef ins and Dienes,
catalysed by Organo-metallic compounds, fall under the category
of coordination polymerisation.
Where Mt represents transition metals such as Ti, Mo, Cr, V, Ni or Rh.
The coordinated metal-carbon bond formed in the monomer
catalyst complex acts as the active center from where
propagation starts.

coordination polymerisation is also known as insertion
polymerisation. The well-known Ziegler Natta Catalyst
belongs to this category for the polymerisation.

Ziegler-Natta Catalysts
The catalyst component consists of halides of IV-VIII group
elements having transition valence and the co-catalysts are
organo-metallic compounds such as alkyls, aryls and hydrides of
group I-IV metals.
Natta Monometallic Mechanism:

Cossee Mechanism

Step Polymerization
•
•
•
The polymer build-up proceeds through a reaction between
functional groups of the monomers.
The reaction takes place in a step-wise manner (i.e., one after
another), and the polymer build-up is, therefore, slow (unlike in
chain polymerisation where the chain growth is very rapid).
Although many known reactions with organic functional groups
can be made use of in step polymerisation, condensation,
addition, ring opening, amidation and ester-interchange
reactions are the most commonly used ones. In step
polymerisation reactions are accompanied by the elimination of
small molecules.

Polycondensation
Polycondensation is brought about by monomers containing two
or more reactive functional groups (such as hydroxyl, carboxyl
and amino) condensing with each other

•
•
•
•
For polycondensation the monomer should have:
1. Those m onom e r s should have t wo re act ive
functional groups for polymerisation to proceed.
2. That polymerisation proceeds by stop-wise reaction
between reactive functional groups.
3. That only one type of reaction (i.e., condensation
reaction, in this case) between two functional groups
is involved in polymer formation.
4. That the polymer formed still contains both the
reactive functional groups at its chain ends (as end
group) and, hence, is 'active' and not 'dead', as in
chain polymerization.

Polyaddition Polymerisation

Ring-opening Polymerisation

Miscellaneous Polymerization Reactions
Electrochemical Polymerisation
Once the initiating species is formed at the electrode, the
initiation and propagation are similar to those taking place under
the free radical, anionic or cationic polymerisation.

Metathetical Polymerisation
Olef i
ne metathesis is basically an exchange reaction wherein the
alkyIidene moieties of the reactant molecules are redistributed
through the cleavage and re-forming of olef i
nic double bonds, to
form new product molecule

Group Transfer Polymerisation
This process used for an organo-silicon initiator and a bi f l
uoride
catalyst, ,  unsaturated esters, ketones, nitrile and carboxamides
can be polymerized to provide excellent yields of polymers of narrow
molecular weight distribution.

Polymerisation Techniques
Bulk Polymerisation
a. The monomer is taken in the liquid state and the initiator is
dissolved in the monomer. The chain transfer agent, whenever
used to control the molecular weight, is also dissolved in the
monomer itself.
b. The whole system is, therefore, in a homogeneous phase.
The reaction mass is heated or exposed to a radiation source for
initiating the polymerisation and is kept under agitation for proper
mass and heat transfers.
c. As the polymerisation proceeds, the viscosity of the medium
increases and mixing becomes progressively dif f
i
cult, leading to
products with very broad molecular weight distribution.

The major Disadvantages are:
1. The medium gets viscous, the diffusibility of the growing
polymer chains becomes restricted, the probability of chain
collision becomes less, termination becomes dif f
icult, active
radical sites accumulate and the rate of polymerisation increases
enormously.
2. This whole phenomenon is called 'auto acceleration' and,
sometimes, the uncontrolled exothermic reaction can lead to an
explosion.
Some advantages:
3. Bulk polymerisation, however, is quite simple and the product
obtained has a high purity since, except the initiator and the chain
transfer agent, no other additive that could contaminate the
product is used. The polymer obtained can also be used as such
since no isolation from other components is involved.
4. Ex: vinyl chloride to get PVC resin

Solution Polymerisation
a. In solution polymerisation, the monomer is dissolved in a
suitable inert solvent along with the chain transfer agent.
b. The free-radical initiator is also dissolved in the solvent
medium, while the ionic and coordination catalysts, can either be
dissolved or suspended.
c. The presence of the inert solvent medium helps to control
viscosity increase and promote a proper heat transfer.
The major disadvantage of the solution polymerisation technique is:
1. The selected solvent cannot be completely ruled out and
hence, it is diﬃcult to get very high molecular weight products.

2. The polymer formed will also have to be isolated from the
solution either by evaporation of the solvent or by precipitation in
a non-solvent, and removal of their f inal traces is always
extremely diﬃcult.
3. Solution polymerisation techniques, nevertheless, can
advantageously be used where the polymer is used in its solution
form, as in the case of certain adhesives and coating
compositions, or in systems where the polymer formed is
insoluble in its monomer or solvent and precipitates out as a
slurry and is amenable for easy isolation.
4. Polyacrylonitrile by free-radical polymerisation and that of
polyisobutylene by cationic polymerisation, use the solution
technique. Block co-polymers are also made exclusively by this
technique

Suspension Polymerisation
 Only water-insoluble monomers can be polymerised by this
technique. The initiators are monomer soluble.
 The monomer is suspended in water, in the form of f i
ne
droplets, which are stabilized and prevented from coalescing by
using suitable water-soluble protective colloids, surface active
agents and by stirring.
 The size of the monomer droplets formed depends on the
monomer-to-water ratio, the type and concentration of the
stabilizing agents and also on the type and speed of agitation
employed.
 Since each monomer droplet is isolated and independent
of the other droplets, it can be visualized to act as an
independent bulk polymerisation nucleus.
 The continuous aqueous phase separating the monomer
droplets acts as an ef f
i
cient heat transfer medium and, hence,
the exothermic is well controlled.

Advantages
1. Since water is used as the heat transfer medium, the
process is also economical as compared to solution polymerisation.
2. As the entire bulk of the monomer is divided into innumerable any
droplets control on the kinetic chain length of the polymer formed is
also quite 'good, resulting in a fairly narrow 'molecular weight
distribution of the product.
3. Polymerisation proceeds to 100% conversion and the product
is obtained as spherical beads or pearls. (For this reason, this
technique is also known as bead or pearl polymerisation).
4. Isolation of the product becomes easy as this involves only
f i
ltration of the beads and removal of the surface-active agents and
protective colloids by water washing.
5. The water washed and dried product can be used as such for
moulding purposes or can be dissolved in a suitable medium for use
as adhesives and coatings.

Emulsion Polymerisation
1. As in the case of suspension polymerisation, in emulsion
polymerisation too, the monomer is dispersed in the aqueous
phase not as discrete droplets, but as a uniform emulsion.
2. The emulsion is stabilized by surface-active agents
(surfactants), protective colloids and also by certain buffers.
The surfactants can be anionic (alkali salts of fatty acids and of
aryl and alkyl sulfonic acids), cationic (alkyl amine
hydrochlorides and alkyl ammonium halides) or non-ionic (alkyl
glycosides and saccharose esters of fatty acids).
3. Surfactants serve the purpose of lowering the surface
tension at the monomer water interface and facilitate
emulsiﬁcation of the monomer in water.

•
•
•
1. Owing to their low solubility, surfactants get fully dissolved or
molecularly dispersed only at low concentrations. Beyond a
particular concentration, the excess quantity does not get
molecularly dispersed, out forms molecular aggregates known as
'micelles', and an equilibrium is set up between the dissolved
surfactant molecules and the aggregated ones.
2. The highest concentration, wherein all the molecules are in a
dispersed state, or the concentration beyond which only micelle
formation is possible, is known as the 'critical micelle
concentration' (CMC).
3. Emulsif i
er molecules are made of two parts: a long non-polar
hydrocarbon' chain to which is attached a polar group such as -
COONa, -S03Na, -NH2HCl or -NBr.
4. In micelle formation, the emulsif i
er molecules aggregate in
such a way that the polar end of the molecules align themselves
outward and. the hydrocarbon ends come close to each other at
the interior.

•
•
5. Due to the close proximity of the hydrocarbon ends of all
emulsif ier molecules, the interior of the micelle acts as a
hydrocarbon phase where the monomer can be soIubilize. When
the monomer is added and agitated, emulsiﬁcation takes place.
6. The resultant emulsion is a complex system a molecular
solution of the emulsif ier in water is the continuous phase
wherein the monomer droplets and micelles (having the
soIubilize monomer at their interior) are uniformly dispersed.
7. If the monomer is slightly soluble in water, then the aqueous
emulsiﬁer solution phase will also dissolved in it.

Melt Polycondensation
1. This technique is used for the polymerisation of monomers,
which have at least one solid component, and do not decompose
around their melting points.
2. The temperatures involved in melt polycondensation are
rather high and, hence, the reaction has to be carried out in an inert
atmosphere of N2 or CO2 to avoid such side reactions as can lead to
oxidation, decarboxylation, degradation etc.
3. The reaction is carried out under reduced pressure to facilitate
removal of the byproduct, which becomes essential if a high
molecular weight product is aimed at.
4. Removal of the byproduct becomes extremely dif f
i
cult at later
stages of the reaction, as there is considerable increase in the
viscosity of the medium, like in bulk polymerisation.
5. Ex: polyethylene terephthalate, nylon 6, 6

Solution Polycondensation
1. The reactants are taken as a solution in a suitable inert
solvent. The reaction can be carried out at comparatively lower
temperatures during which heat and mass transfer processes are
easier than in the melt technique.
2. The solvent can also serve as an entrapping agent for the
byproduct formed and hence, the removal of the byproduct
becomes easy. Owing to the presence of the solvent phase,
however, the kinetic probability of chain growth is low and this leads
to a reduced rate and a low degree of polymerisation.
Many of the liquid polyester resins based on glycols and
unsaturated dicarboxylic acid are prepared by this technique using
high boiling aromatic hydrocarbons as solvent with which the water
(byproduct) forms an azeotrope and, hence, can be removed easily.

Interfacial Condensation
1. Polymerisation is allowed to proceed at the interface
between an aqueous and an organic medium. Reactants having
highly reactive functional groups, which can readily react at
ambient temperatures to form condensation products, are suited
to this technique.
2. The polymer formation at the interface is a diffusion
controlled process, very high molecular weight products can be
achieved by this technique.
3. The two solutions are very thoroughly agitated so as to form
an emulsion wherein the interface surface/volume ratio is
increased tremendously and, hence, both the rate and degree of
polymerisation become very high
4. Preparation of fully aromatic polyarnides from terephtholoyl
chloride and paraphenylene diamine is a typical example:

1. The diamine is dissolved in water and the acid chloride in
chloroform or carbon tetrachloride. When the two solutions are
brought in contact with each other, at the interface, the diamine
molecules diffuse into the organic phase and react with the acid
chloride, resulting in the formation of a polymer, which
precipitates out immediately.
2. The byproduct formed (HCI) diffuses back into the aqueous
phase and gets dissolved. The precipitate is formed at the
interface in the form of a thin f i
lm and, when removed from the
system, exposes a fresh surface of the acid chloride to the organic
phase resulting in the formation of the fresh quantity of the
polymer.
3. As the polymer is formed, it precipitates out and, under the
inf l
uence of high-speed agitation, forms slurry. The polymer is
isolated from the slurry and washed free of adherent reactants.

Solid and Gas Phase Polymerisation
Solid phase polymerisation is mostly restricted to chain·
polymerisation. Also, since such polymerisation is restricted to
low temperatures, thermal activation of the reaction is quite
dif f
i
cult and, hence, the photo or radiation-activation technique
is resorted to.

Gas phase polymerisation is known in the case of very few
olef i
nic monomers. The disadvantage of the system is that it has
a very poor heat transfer. The two methods used in the gas phase
polymerisation are: (i) spraying the catalyst (usually Zeigler-Natta
type) into the gaseous monomer and (ii) feeding the gaseous
monomer into a ﬂuidised bed made up of the catalyst particles. In
both the cases, the polymer is obtained as a free-f l
owing powder.
Polymerisation of ethylene and p-xylene can be cited as
examples for gas phase polymerisation.

Thank You

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