Polymerisation Mechanism ( chain growth )
This slides contain brief description of polymerisation mechanism, specially chain growth mechanism.
Published on: Mar 4, 2016
Transcripts - Polymerisation Mechanism ( chain growth )
The process by which the monomer molecules are linked
to form a big polymer molecule is called Polymerisation.
Polymerisation is a process of bonding monomer, or
“single units” together through a variety of reaction
mechanisms to form longer chains named Polymer
As important as polymers are, they exist with monomers,
which are small, single molecules such as hydrocarbons
and amino acids.
Polymerisation Mechanism 152
Functionality of Monomers
1. The number of reactive functional Group Represent in a monomer.
CH3 – COOH ; Functionality = 1
HO – CH2 -- CH2 – OH ; Functionality = 2
1. The number of Hydrogenation that can be added to a monomer.
CH2 = CH2 ----------------> CH3 – CH3 ; Functionality = 2
CH ≡ CH -------> CH2 = CH2 ------> CH3 – CH3 ; Functionality = 4
1. The number of replaceable atom present in a monomer.
Examplesof functionality of monomer 153,
156Functionality effects on line structure of
- In order to form a polymer, the monomer must be at least bi functional.
When it is bi functional monomer, polymer chain is always linear.
- Branched polymer contains molecules having a linear backbone with
branches emanating randomly from it. In order to form this class of
material, the monomer must have a capability of growing in more than
two directions, which implies that the starting monomer must have a
functionality greater than 2 .
Consider the polymerisation of phthalic anhydride with glycerol where
functionality of monomer is 3 .
FUNCTIONALITY of MONOMER = 2 LINEAR CHAIN POLYMER
FUNCTIONALITY of MONOMER > 2 BRANCHED CHAIN POLYMER
Characteristics of chain-growth
Growth by addition of monomer only at one end or both ends of
Some monomer remains even at long reaction times
Different steps operate at different stages of mechanism (i.e.
Initiation, propagation, termination, and chain transfer)
Molar mass of backbone chain increases rapidly at early stage and
remains approximately the same throughout the polymerisation
Chains not active after termination
Examplesof monomers, capableof undergoing chain polymerisation
This Polymerisation is divided into 3 parts.
The first step in chain polymerization, Initiation involves the formation of a free
This free radicals are produced by the decomposition of initiators.
Initiator: A relatively unstablemoleculethat decomposesinto afree
radical. Used to initiateapolymer growth reaction
Freeradicals: An especially reactiveatom or group of atomsthat has
oneor moreunpaired electrons
List of free-radical initiators:
It is the middle step in chain polymerization where successive monomers are
attached to the growing chain. In the propagation stage, the process of electron
transfer and consequent motion of the active center down the chain proceeds.
In following reaction(chain), refers to a chain of connected monomers, and X refers
to a substituent group (a molecular fragment) specific to the monomer. For
example, if X were a methyl group, the monomer would be propylene and the
1. Head to Tail
2. Head to Head
3. Tail to Head
4. Tail to Tail
Termination of reaction is nothing but stop the further propagation of chain.
In theory, the propagation reaction could continue until the supply of monomers is
exhausted. Most often the growth of a polymer chain is halted by the termination
reaction. Termination typically occurs in two ways:
1. Combination: It occurs when the polymer's growth is stopped by free electrons
from two growing chains that join and form a single chain. The following diagram
depicts combination, with the symbol (R) representing the rest of the chain
2. Disproportionation: It halts the propagation
reaction when a free radical strips a hydrogen atom from
an active chain. A carbon-carbon double bond takes the
place of the missing hydrogen.
- Disproportionation can also occur when the radical reacts with an
impurity. This is why it is so important that polymerization be carried
out under very clean conditions.
Chain Transfer Reaction
It is another method of termination, which takes place by transfer reaction.
In termination reaction, products form dead of non reactive due to coupling or
disproportionation. However in chain transfer reaction, one polymer chain is stopped
forming a dead polymer, there is a simultaneous generation of a new free radical
capable of initiating a fresh polymer chain growth.
A solvent of high chain transfer ability and having a chemical structure similar to
that of a monomer to form polymer is known as telomerisation. The solvent used in
such cases is known as telogen.
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