Unit 6 6 hrs
Organic polymers
Introduction and general characteristics of polymers. Polymerization reactions: step-growth
(condensation) and chain growth (addition), free radical co-polymerization, emulsion
polymerization, anionic polymerization, cationic polymerization, ring opening
polymerization, co-ordination polymerization.
Ziegler-Natta polymerisation of alkenes; Preparation and applications of plastics –
thermosetting (phenol-formaldehyde, polyurethanes) and thermosoftening (PVC, polythene);
Fibers – natural and synthetic (acrylic, polyamido, polyester), conducting, semiconducting
and biodegradable polymers.
Amphiphilic polymers; Block (di-block, tri-block) & Graft co-polymers.
ORGANIC POLYMERS
A polymer is a large molecule made by linking together repeating units of small
molecules called monomers.
Polymers can be divided into two broad groups- Synthetic polymers and biopolymers
(natural polymers). Synthetic polymers are synthesized by scientists, whereas
biopolymers are synthesized by organisms and essential for our very existence.
Synthetic polymers can be divided into two major classes depending upon the basis on
which monomers undergo chemical reactions to form polymers, polymers can be further
subdivided.
A. Chain-growth polymers or addition polymers
B. Step-growth polymers or condensation polymers
A. Chain-growth polymers or addition polymers
Chain-growth polymers, also known as addition polymers are obtained by repeated addition
of large number of monomer units containing double bonds, generally without
elimination of any by-products. They are formed by chain growth polymerization
mechanism. e.g. Polyvinyl chloride, polyethene, polystyrene, polyacrylonitrile, etc.
1
,Chain-growth polymerization proceed by one of three mechanisms:
1. Radical polymerization
2. Ionic Polymer which is subdivided into
(a) cationic polymerization
(b) anionic polymerization
3. Co-ordination polymerization
1. Radical polymerization
The process involves successive addition of monomers to the chain via free radical
mechanism. The monomer unit is converted into an active species with the help of a small
amount of initiator like benzoyl peroxide, potassium persulphate, lauryl sulphate, etc., which
produces free radicals. The initiator breaks homolytically into radicals and each radical
adds to an alkene monomer, converting it into a radical. This radical reacts with another
monomer, adding a new subunit that propagates the chain. The radical site is now at the end
of the most recent unit added to the end of the chain. This is called the propagating site.
The process involves three stages:
Step 1- Initiation: (Formation of free radicals):
Step 2-Propagation:
Step 3-Termination:
The propagation process is repeated over and over. Hundreds or even thousands of alkene
monomers can add, one at a time, to the growing chain. Eventually the chain reaction stops
because the propagating sites are destroyed.
Propagating sites are destroyed (a) when two chain combines at their propagating sites;
(b) when two chains undergo disproportionation, with one chain being oxidised to an
2
, alkene and the other being reduced to alkane; (c) when a chain reacts with an impurity
that consumes radical.
Chain combination
Disproportination
Reaction with an impurity
2. Ionic polymerization
The ionic polymerization is divided into two categories:
(i) Cationic polymerization
(ii) Anionic polymerization
(i) Cationic polymerization
In the cationic polymerization reaction, the initiator is an electrophile that adds to the alkene
(monomer), causing it become a cation. The initiator most often used in cationic
polymerization is a Lewis acid, such as AlCl3, BF3, and SnCl4. The cation form in the
initiation step enters the chain of a self propagating polymerization process.
Step 1- Initiation: (Formation of cation)
3
Organic polymers
Introduction and general characteristics of polymers. Polymerization reactions: step-growth
(condensation) and chain growth (addition), free radical co-polymerization, emulsion
polymerization, anionic polymerization, cationic polymerization, ring opening
polymerization, co-ordination polymerization.
Ziegler-Natta polymerisation of alkenes; Preparation and applications of plastics –
thermosetting (phenol-formaldehyde, polyurethanes) and thermosoftening (PVC, polythene);
Fibers – natural and synthetic (acrylic, polyamido, polyester), conducting, semiconducting
and biodegradable polymers.
Amphiphilic polymers; Block (di-block, tri-block) & Graft co-polymers.
ORGANIC POLYMERS
A polymer is a large molecule made by linking together repeating units of small
molecules called monomers.
Polymers can be divided into two broad groups- Synthetic polymers and biopolymers
(natural polymers). Synthetic polymers are synthesized by scientists, whereas
biopolymers are synthesized by organisms and essential for our very existence.
Synthetic polymers can be divided into two major classes depending upon the basis on
which monomers undergo chemical reactions to form polymers, polymers can be further
subdivided.
A. Chain-growth polymers or addition polymers
B. Step-growth polymers or condensation polymers
A. Chain-growth polymers or addition polymers
Chain-growth polymers, also known as addition polymers are obtained by repeated addition
of large number of monomer units containing double bonds, generally without
elimination of any by-products. They are formed by chain growth polymerization
mechanism. e.g. Polyvinyl chloride, polyethene, polystyrene, polyacrylonitrile, etc.
1
,Chain-growth polymerization proceed by one of three mechanisms:
1. Radical polymerization
2. Ionic Polymer which is subdivided into
(a) cationic polymerization
(b) anionic polymerization
3. Co-ordination polymerization
1. Radical polymerization
The process involves successive addition of monomers to the chain via free radical
mechanism. The monomer unit is converted into an active species with the help of a small
amount of initiator like benzoyl peroxide, potassium persulphate, lauryl sulphate, etc., which
produces free radicals. The initiator breaks homolytically into radicals and each radical
adds to an alkene monomer, converting it into a radical. This radical reacts with another
monomer, adding a new subunit that propagates the chain. The radical site is now at the end
of the most recent unit added to the end of the chain. This is called the propagating site.
The process involves three stages:
Step 1- Initiation: (Formation of free radicals):
Step 2-Propagation:
Step 3-Termination:
The propagation process is repeated over and over. Hundreds or even thousands of alkene
monomers can add, one at a time, to the growing chain. Eventually the chain reaction stops
because the propagating sites are destroyed.
Propagating sites are destroyed (a) when two chain combines at their propagating sites;
(b) when two chains undergo disproportionation, with one chain being oxidised to an
2
, alkene and the other being reduced to alkane; (c) when a chain reacts with an impurity
that consumes radical.
Chain combination
Disproportination
Reaction with an impurity
2. Ionic polymerization
The ionic polymerization is divided into two categories:
(i) Cationic polymerization
(ii) Anionic polymerization
(i) Cationic polymerization
In the cationic polymerization reaction, the initiator is an electrophile that adds to the alkene
(monomer), causing it become a cation. The initiator most often used in cationic
polymerization is a Lewis acid, such as AlCl3, BF3, and SnCl4. The cation form in the
initiation step enters the chain of a self propagating polymerization process.
Step 1- Initiation: (Formation of cation)
3
