POLYMERS notes
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June 30, 2015 |
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POLYMERS
Polymers are compounds of very high
molecular masses formed by the combination of a large number of
simple molecules called monomers. The process by which the
simple molecules are converted into polymers is called
polymerization.
nCH2═CH2
---------- >
-(CH2-CH2-) n
ethylene (monomer)
polymerisation polyethylene (polymer)
The process by which the simple
molecule converted into polymers is called polymerization.
Homopolymer and Copolymer
A polymer formed from one type of
monomers is called homo polymer. A polymer formed from two or more
different monomers is called co polymer or mixed polymer.
Polymers and macro molecules
All polymers are macro molecules but all
molecules are not polymers. Polymers always consists of thousands of
repeating monomer units but macro molecules is a giant molecule which
may or may not contain monomer molecules.
CLASSIFICATION OF POLYMER
A. Classification of Polymers on the
Basis of Source.
1. Natural
polymers— The polymers obtained from nature are called natural
polymers. For example, starch, cellulose, natural rubber, proteins
etc.
2. Synthetic
polymer—The polymer which are prepared in the laboratories are
called synthetic polymers. For example, polyethylene, PVC, nylon,
Teflon eyc.
B. Classification of Polymers on the
Basis of Structure.
1. Linear polymers—These
are the polymers in which
monomeric units are linked together to form linear chains. Example polyethylene, nylons, polyesters etc.
monomeric units are linked together to form linear chains. Example polyethylene, nylons, polyesters etc.
2. Branched
chain polymers—These are the polymers in which monomers are joined to form
long chain with side chains or branches of different lengths.
Example glycogen, starch etc..
3.Cross-linked
polymers—These are polymers in which monomer units are
cross-linked together to form a three-dimensional network. Examples
Bakelite, resin etc.
C. Classification of Polymers on the
Basis of Molecular Forces
1. Elastomers—The
polymers that have elastic character like rubber are called
elastomers. The polymers chains are held together by weak
intermolecular forces. Example natural rubber.
2. Fibers—These
are the polymers which have strong intermolecular forces between the
chains. These forces are either hydrogen bonds or dipole-dipole
interaction. Example nylon-66, Dacron, silk etc.
3.
Thermoplastics—These are the polymers which can be easily
softened repeatedly when headed and hardened when cooled with little
change in their properties. Examples polythene, polystyrene, Teflon
etc.
4. Thermosetting
polymers—These are the polymers which undergo permanent change
on heating. Examples Bakelite, melamine formaldehyde etc.
D. Classification of Polymers on the
Basis of mode of Synthesis
1.Addition
polymers—A polymer formed by direct addition of repeated
monomers without the elimination of by product molecules is called
addition polymers. Examples the addition polymers polythene or
polypropylene.
2.Condensation
polymers—A polymers formed by the condensation of two or more
monomers with the elimination of simple molecules like water,
ammonia, alcohol etc. is called condensation polymers. Example
nylon 66.
GENERAL METHOD OD POLYMERISATION
- Addition polymerization or chain growth polymerization.
This process
involves a series of reaction each of which consumes a reactive
particle and produced another similar particle resulting a chain
reaction. So this is called chain growth polymerization.
It consists of
three steps: chain initiation, chain propagation and chain
termination
In the initiation
step, the initiator (free radical, a cation, or anion) produces a
reactive species. This species combines with a molecules of a monomer
forming another reactive molecule (1) Initiator
I*, (2) I* + M (monomer) = IM*.
In chain
propagation this active molecule is added to another monomer forming
another intermediate. (3) IM* + M = IMM*, (4) IMM* + M = IMMM*.
In chain termination step
the reaction is terminated when the active end reacts
with a species which do not
formed reactive species. IMM* + T
I(M)nMT.
B. Condensation polymerization
or step growth polymerization.
In this type the monomers
contain two functional groups. In this process, no Initiator is needed and each
step is the same type of chemical reaction. The polymer is formed in a step wise manner, it is called step growth polymer and the process is called step growth
polymerization.
A + B (monomer)
A-B (dimmer)
A-B + A
A-B-A
Alternatively, A + B
A-B
A-B + A-B
A-B-A-B or (A-B)2
COPOLYMERS
Polymers obtained from two or more
different monomer units are called copolymers. Copolymerization of
monomer mixtures often leads to the formation of polymers which have
quite different properties than those corresponding homopolymer.
Copolymers are obtained depending upon the distribution of monomer
units in the chain. Two monomers can combine either regular fashion
or random fashion.
Two other types of copolymers that can
be prepared under certain conditions are –
- Block copolymers are those in which different blocks of identical monomer units alternate with each other. –(A-A-A-B-B-B-A-A-A-B-B-B-)n.
- Graft polymers are those in which homopolymer branches of one monomer units are grafted on the homopolymer chains of another unit as
- (A – A – A –
A – A – A – A – A – A – A -)n
B
B B
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NATURAL RUBBER
Natural rubber is a polymer of isoprene
(2-methyl buta-1,3-diene)
CH3
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CH2═C
– CH ═CH2
In natural rubber, about 11,000 to
20,000 isoprene units are linked together in a chain like
arrangement.
Vulcanisation of Rubber
The process of heating natural rubber
with sulphur to improve its properties is called vulcanisation.
During vulcanisation, sulphur cross links are formed which makes it
hard, tough and greater tensile strength.When the rubber is starched,
the chains straighten out to some extent. The chains cannot slip past
each other because of the polysulphide bridges. Thus, rubber can be
stretched only to a limited extent. When the tension is removed, the
chains try to coil up again and the rubber resumes its original
shape.
Synthetic Rubber
The synthetic rubber is obtained by
polymerizing certain organic compounds which may have properties
similar to rubber and some additional properties. Examples are
neoprene, styrene butadiene rubber, thiokol, silicones, polyurethane
rubber etc..
- Neoprene rubber
It is prepared by
polymerization of chloroprene (2-chlorobuta-1, 3-diene)
nCH2═C
- CH═CH2 --------- >[ -CH2 – C═CH –
CH2 ]n
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Cl
(cloroprene) Cl (Neoprene)
Neoprene is
superior to natural rubber in its stability to aerial oxidation and
its resistance to oils, gasoline and other solvents.
- Styrene Butadiene Rubber (SBR)
It is obtained by
the polymerization of buta-1,3-diene and styrene in the ratio of 3:1
in the presence of sodium.
It is also called
Buna-s, in which Bu stands for butadiene, Na for sodium and S stands
for styrene. It has slightly less tensile strength than natural
rubber.
- Thiokol
It is prepared by
condensation of 1,2 dichloroethene with sodium polysulphide Na2S4
in the process of Mg(OH)2.
nCl-CH2-CH2-Cl
+ Na2S4
(-CH2-CH2-S-S-S-S-)n
It is also called
polysulphide rubber. It is mechanically inferior to natural rubber
but has exceptional resistance to mineral oils, solvents, oxygen and
ozone.
CONDENSATION POLYMER
There are two common types of
condensation polymers:
poly amides and polyesters.
poly amides and polyesters.
Polyamides are formed by
reacting two monomers: one having two carboxylic groups and other
having two amino groups.
For examples nylon-66 is obtained by condensation of two monomers.
For examples nylon-66 is obtained by condensation of two monomers.
Hexamethylenediamine H2N – (CH2)6 –
NH2
Adipic acid HOOC – (CH2)4
– COOH
nH2N–(CH2)6-NH2
+ nHOOC – (CH2)4 – COOH
(-NH-(CH2)6-NH-CO-(CH2)4-CO-)n
In this polymerization reaction, -NH2
group of hexamethylenediamine combines with COOH group of adipic acid
forming –NH-CO- linkage with the elimination of H2O
molecules.
Polyesters are formed by the
condensation of a monomer containing two carboxylic acids and a
monomer containing two hydroxyl groups (-OH group).
Telyrene is a polyester prepared from:
Terephthalic acid HOOC-C6H6
COOH
Ethylene glycol HO-CH2-CH2-OH
It is obtained by the condensation of these two monomers as:
nHO-CH2-CH2-OH +
nHOOC-C6H6-COOH
HO-(CH2-CH2-O-CO-C6H6-)nCOOH
In this polymerization reaction –OH
group of one combines with –COOH group of other forming –O-CO-
linkage, which is repeated.
Molecular Mass of Polymers
A synthetic polymer contains a number
of species of varying chain lengths. Since each species has a
different molecular mass and a given sample of a polymer contains a
such species, therefore, the polymer as a whole has an average
molecular mass.
Types of average molecular mass-
- Number average molecular mass (Mn)-
If N1,
N2, N3…. are the number of macromolecules
with molecular masses M1, M2, M3…
respectively then the number average molecular mass of the polymer is
given by
Mn = N1M1
+ N2M2 + N3M3
+…. = ∑ NiMi
N1 + N2 + N3….
∑ Ni
Where Ni is the number
of macromolecules of ith type with molecular mass Mi.
- Weight average molecular mass(Mw)-
If m1, m2, m3….
are the masses of macromolecules with molecular masses M1, M2, M3…..
respectively, then the weight average molecular mass of the polymer
is given by
Mw = m1M1 +
m2M2 + m3M3 + …. = ∑ miMi
m1 + m2 + m3 ∑ mi
But mi = NiMi where Ni
is the number of macromolecules of ith type with
molecular mass Mi.
Mw = ∑ NiMi ×Mi
= ∑ NiMi2
∑NiMi
∑ NiMi
Poly Dispersed Index (PDI)—the
ratio of weight average molecular mass and number average molecular
mass is called poly dispersed index (PDI).
PDI = Mw .
Mn
PDI is used to determine the
homogeneity of a polymer. On the basis of PDI, polymer have been
classified in two categories.
- Monodisperse polymers- Polymers whose molecules have same or narrow range of molecular masses are called monodispersed polymers.
- Polydispersed polymers – polymers whose molecules have a wide range of molecular masses are called polydisperse polymers.
Biopolymers and Biodegradable
Polymers
The polyamides contain the –CO-NH-,
called peptide bond and the polymers are also called polypeptides.
Proteins are naturally occurring polypeptides. The structures of
polypeptides are made up of about 20 amino acids. A protein may
contain many or all of them. That amino acid differs in the R-group.
Biodegradable polymers are the polymers
which are degraded by micro-organisms within a suitable period so
that biodegradable. Polymers and their degraded products do not cause
any serious affects on the environment.
biodegradable-polymers
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reaction-kinetics
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enzyme-catalysis regulation
carboxylic-acids-and-their-derivatives
introduction-to-d-block-element
transition-metals
coordination-chemistry
solid-state-2
polymer
amino-acidspeptide-and-proteins
benzene-and-aromatic-compounds
solutions-questions
biomolecules
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