1. Engineering Chemistry-I
S.Dhanasekar

2. Chapter I
Polymer Chemistry

3. INTRODUCTION
The word polymer is derived from Greek words “poly” means many and “mer” means unit or part.
Polymers are generally macromolecules formed by the repeated linking of large number of small molecules.
Polymers are widely used in automobiles, defence, electrical goods and computer components etc.

4. Polymers
Polymers are macro molecules (giant molecules of higher molecular weight) formed by the repeated linking of large number of small molecules called monomers.
Polyethylene is a polymer formed by the repeated linking of large number of ethylene molecule.

5. Monomer
Monomer is a micro molecule (small molecule) which combines with each other to form a polymer.
Some monomers and repeating unit of the polymers are given below.

12. POLYMERISATION
Polymerisation is a process in which large number of small molecules (called monomers) combine to give a big molecule (called a polymer) with or without elimination of small molecules like water.

13. Degree of Polymerization (DP)
The number of repeating units (n) in a polymer chain is known as degree of polymerisation. It is represented by the following relationship.
molecular weight of the polymeric network
Degree of polymerization (n)
molecular weight of the repeating unit

14. In this example, five repeating units are present in the polymer chain
In this example, five repeating units are present in the polymer chain. So, the degree of polymerization is 5.

15. Oligomers
Polymers with low degree of polymerisation are known as Oligomers, their molecular weight ranges from
High Polymers
Polymers with high degree of polymerisation are known as High polymers, their molecular weight ranges from 10, ,00,000.

16. FUNCTIONALITY AND ITS SIGNIFICANCE
The number of bonding sites or reactive sites or functional groups, present in a monomer, is known as its functionality.

18. Significance 1. Bifunctional Monomers
Bifunctional monomers (i.e., functionality of the monomer is 2) mainly form linear (or) straight chain polymer.
Each monomeric unit in the linear chain is linked by strong covalent bonds (primary bonds), but the different chains are held together by weak vander waal’s forces of attraction (secondary bonds).
Therefore, there is no restriction to movement of one chain over another.
This type of polymers are soft and flexible, and possess less strength, low heat resistance.
These are soluble in organic solvents.

20. 2. Trifunctional Monomers
When a trifunctional monomer (i.e., functionality of the monomer is 3) is mixed in small amounts with a bifunctional monomer, they form branched chain polymer.
The movement of polymer chain in branched polymer is more restricted than that of straight chain polymers.

22. 3. Polyfunctional Monomers
Poly functional monomers form cross-linked polymer(three-dimensional network polymer).
All the monomers in the polymer are connected to each other by strong covalent bonds.
Therefore the movement of polymer chain is totally restricted.
This type of polymers are hard and brittle and possess very high strength and heat resistance.
They are insoluble in almost all organic solvents.

24. CLASSIFICATION OF POLYMERS
Based on the source, polymers are broadly classified into two types.
1. Natural polymers.
2. Synthetic polymers.

26. I. Natural Polymers
The polymers obtained from nature (plants and animals) are called natural polymers.

27. It is a polymer of glucose. It is a chief food reserve of plants.
EXAMPLES OF NATURAL POLYMERS
1. Starch
It is a polymer of glucose. It is a chief food reserve of plants.
2. Cellulose
It is also a polymer of glucose. It is a chief structural material of the plants. Both starch and cellulose are produced by plants during photosynthesis.

28. These are polymers of  amino acids.
3. Proteins
These are polymers of  amino acids.
They have generally 20 to 100  amino acids joined together in a highly organized arrangement.
These are building block of animals.

29. These are polymers of various nucleotides. RNA and
4. Nucleic Acids
These are polymers of various nucleotides. RNA and
DNA are common nucleotides.
5. Natural Rubber
It is a polymer of unsaturated hydrocarbon, 2-methyl-1, 3 butadiene, called isoprene. It is obtained from latex of rubber trees.

31. II. Synthetic Polymers
The polymers which are prepared in the laboratories, are called synthetic polymers. These are also called man-made polymers.
PVC, polyethylene, nylon, teflon, bakelite, terylene, etc.,

32. Types of Synthetic Polymers
1. Organic Polymers
These are polymers containing hydrogen, oxygen, nitrogen, sulfur and halogen atoms apart from carbon atoms.
Polyethylene, Polyvinyl alcohol, PVC,
Epoxy polymers, Polyurethane.

33. 2. Elemento-organic (or) Hetero-organic Polymers
These are polymers composed of carbon atoms and hetero-atoms (like N, S & O).
The main chain consists of carbon atoms and whose side groups contain hetero atoms linked directly to the C atoms in the chain.
Polysiloxanes, Polytitoxanes

34. 3. Inorganic Polymers
These are polymers containing no carbon atoms.
The chains of these polymers are composed of different atoms joined by chemical bonds.

36. PLASTICS
Plastics are high molecular weight organic materials, that can be moulded into any desired shape by the application of heat and pressure in the presence of a catalyst.

37. Originally, plastics were discovered and then
Originally, plastics were discovered and then developed based on trial and error method.
The present time is sometime referred as plastic age, because the use of polymeric material is perculated in large variety of applications.
This rapid growth has taken place only in the last 50 years.
It was man’s desire to develop plastics for engineering applications to replace, glass, metal, ceramic, wood and other materials of constructions.
Since plastics possess the following advantages, they have wider applications.

38. Advantages of Plastics Over other Materials
They are light in weight.
They possess low melting point.
They can be easily moulded and have excellent finishing.
They possess very good strength and toughness.
They possess good shock absorption capacity.
They are corrosion resistant and chemically inert.
They have low co-efficient of thermal expansion and possess good thermal and electrical insulating property.
They are very good water-resistant and possess good adhesiveness.

39. Disadvantage of Plastics
Softness.
Embrittlement at low temperature.
Deformation under load.
Low heat-resistant and poor ductility.
Combustibility.
Degradation upon exposure to heat and UV- radiation.
Non bio-degradable.

40. Classification of Plastics
Based on the structure and type of resin used for the manufacture of plastics, plastics are classified into two main types.
1. Thermoplastics.
2. Thermosetting plastics.
A resin is a basic binding material, present in plastics, which undergoes polymerization reaction during moulding.

41. 1.Thermoplastic Resins

42. Thermoplastics are prepared by addition polymerisation.
They are straight chain (or) slightly branched polymers and various chains are held together by weak vanderwaal’s forces of attraction.
Thermoplastics can be softened on heating and hardened on cooling. They are generally soluble in organic solvents.

43. Polyethylene, Polyvinyl chloride.

44. Thermosetting plastics are prepared by condensation polymerisation.
2.Thermosetting Resins (or) Thermosets
Thermosetting plastics are prepared by condensation polymerisation.
Various polymer chains are held together by strong covalent bonds (called crosslinks).
Thermosetting plastics get harden on heating and once harden, they cannot be softened again.
They are almost insoluble in organic solvents.

45. Bakelite, Polyester.

46. Difference between thermoplastic and thermosetting resins

48. TYPES OF POLYMERISATION
Polymerisation reactions may be carried out by any one of the following methods.
1. Addition (or) chain growth polymerisation
2. Condensation (or) step wise polymerisation
3. Copolymerisation

49. 1. Addition (or) Chain Growth Polymerisation
It is a reaction that yields a polymer, which is an exact multiple of the original monomeric molecule.
The original monomeric molecule, usually, contains one or more double bonds.
In this addition polymerisation there is no elimination of any molecule.

53. 2. Condensation (or) step wise polymerisation
It is a reaction between simple polar groups containing monomers with the formation of polymer and elimination of small molecules like H2O, HCl, etc.,

56. 3. Co-polymerisation
It is the joint polymerisation in which two (or) more different monomers combine to give a polymer.
High molecular weight polymers, obtained by co-polymerisation, are called copolymers Copolymerisation is mainly carried out to vary the properties of polymers such as hardness, strength, rigidity, heat resistance etc.

59. Differences between addition (chain) polymerisation and condensation (step) polymerisation

61. Mechanism of Addition Polymerisation
The mechanism of addition polymerisation can be explained by any one of the following types.
1. Free radical mechanism
2. Ionic mechanism
All the above mechanisms occur in three major steps namely,
i. Initiation
ii. Propagation and
iii. Termination

62. Free Radical Mechanism
1. Initiation
It is considered to involve two reactions.
(a) First reaction; It involves production of free radicals by homolytic dissociation of an initiator (or catalyst) to yield a pair of free radicals R.

63. Examples of some commonly used thermal initiators
Thermal initiator is a substance used to produce free radicals by homolytic dissociation at high temperature.

65. (b). Second reaction; It involves addition of this free radical to the
(b). Second reaction; It involves addition of this free radical to the first monomer to produce chain initiating species.

66. 2. Propagation
It involves the growth of chain initiating species by successive addition of large number of monomers.
The growing chain of the polymer is known as living polymer.

67. 3.Termination
Termination of the growing chain of polymer may occur either by coupling reaction or disproportionation.
(a) Coupling (or) Combination
It involves coupling of free radical of one chain end to another free radical forming a macromolecule

68. The product of addition polymerisation is known as Dead polymer.
(b) Disproportionation
It involves transfer of a hydrogen atom of one radical centre to another radical centre, forming two macromolecules, one saturated and another unsaturated.
The product of addition polymerisation is known as Dead polymer.

69. IONIC MECHANISM (or) IONIC POLYMERISATION
Ionic polymerisation is faster than the free radical polymerisation. Ionic polymerisation is initiated by a positive
ion (cation) or a negative ion (anion). Depending upon the nature of ions (initiators) the ionic polymerisation takes place in two ways.
1. Cationic polymerisation (or) Carbonium ion mechanism.
2. Anionic polymerisation (or) Carbanion mechanism.

70. 1.Cationic polymerisation
This type of polymerisation takes place when electron donating groups like CH3, C6H5 are present in a monomer.
These groups stabilise the carbonium ion formation.
Styrene, Isobutylene, Isoprene.
The catalysts (initiators) used to initiate the reaction are Lewis acids like AlCl3, BF3 with a co-catalyst water.

71. (i) Initiation
It involves the formation of chain initiating species

72. (ii) Propagation
It involves the growth of chain initiating species by successive addition of large number of monomers and the positive charge simultaneously shifts to the newly added monomer.

73. (iii) Termination

74. 2.Anionic Polymerization
This type of polymerization takes place when electron withdrawing groups like Cl-, CN- are present in a monomer. These groups stabilise the carbanion formation.
Vinyl chloride, Styrene, Acrylonitrile.
The catalysts (initiators) used to initiate the reaction are
Lewis bases like NaNH2, KNH2, LiNH2 etc.,

75. It involves the formation of chain initiating species.
(i) Initiation
It involves the formation of chain initiating species.

76. (ii) Propagation
It involves the growth of chain initiating species by successive addition of large number of monomers and the negative charge simultaneously shifts to the newly added monomer.

77. Termination of the growing chain occurs by adding ammonia.
(iii) Termination
Termination of the growing chain occurs by adding
ammonia.

78. Properties of Polymers
1. Glass transition temperature (Tg)
Glass transition temperature (Tg) is the temperature at which the amorphous solid state is transformed to the melt state.
Below the glass transition temperature (Tg) the polymer is hard and above which it is soft.
The hard brittle state is called as glassy state and the soft flexible state is called as rubbery or viscoelastic state.
Thus the glass transition temperature is an important property of a polymer and it decides whether the polymer behave like glass or rubber.

80. Below Tg the polymer is hard and brittle (like glass).
Factors influencing Tg
The value of Tg depends on (a) chain-length (b) extent of cross-linking (c) the barrier which hinders the internal rotation of the chain links.
The value of Tg of a given polymer varies with the rate of heating or cooling.
Below Tg the polymer is hard and brittle (like glass).
4. Tg of a liner polymer is sharp, because of the free movement of polymer chains (chains are held by weak vanderWaals forces).
5. A cross-linked polymer does not possess any Tg
because polymer chains will not move.
Thus the polymers with a symmetrical structure are crystalline, while the polymers with irregular chain back bone are amorphous.

81. 2. Stereospecific Polymer (or) Tacticity
The orientation of monomeric units or functional groups in a polymer molecule can take place in an orderly (or) disorderly manner with respect to the main chain is known as Tacticity.
Tacticity do affect the physical properties of the polymer.
This orientation results in three types of stereo- regular polymers.

82. a. Isotactic Polymer
If the functional groups are arranged on the same side of the main chain, the polymer is called Isotactic polymer.

83. b. Syndiotactic Polymer
If the functional groups are arranged in an alternating fashion, the polymer is called Syndiotactic polymer.

84. c. Atactic Polymer
If the functional groups are arranged randomly, the
polymer is called Atactic polymer.

85. Molecular - Mass of Polymer
solution, softening temperature, tensile and impact strength and heat resistance are influenced by molecular-mass.
Generally low molecular weight polymers are soft and gum-like resins.
The high molecular weight polymers are tougher and more heat resistant.
Thus, control of molecular weight of polymers is very important for industrial polymerization.
Therefore, determination of molecular weight is of greater importance.

86. The molecular weight of polymers depends on the type
of monomers involved in polymerization reactions.
The final product will contain the macromolecules of different masses.
Most of the polymers are polydisperse i.e.., the molecules of polymers do not have identical molecular weight (M).
So it is necessary to take average molecular mass.

92. Weight average molecular mass not only
Weight average molecular mass not only depends on the number of particles, but also on the molecular size.
Hence, in the averaging process, molecular weight of each individual species in multiplied by the mass and not by the number. i.e.,

95. Determination
Weight - average molecular mass is determined by light - scattering techniques and ultra-centrifugation techniques.
Determination

96. POLYDISPERSITY INDEX
When a graph is ploted between molecular mass of the polymer (M) to the weight fraction of the polymer (w) the following graph is obtained.

99. Determination of Molecular Masses
The molecular masses of a polymer can be determined by the following methods.
1. Measuring colligative properties such as osmometry, ebullioscopy and cryoscopy
2. Light scattering measurements.
3. Measuring viscosities of polymer solutions.
4. By end group chemical analysis.
5. By ultracentrifuge.

100. TECHNIQUES OF POLYMERIZATON
The following methods are generally used for the polymerisation reaction
1. Bulk polymerisation.
2. Solution polymerisation.
3. Suspension polymerisation.
4. Emulsion polymerisation

101. 1. Bulk Polymerisation
Bulk polymerisation is the simplest method of polymerisation. The monomer is taken in a flask as a liquid form and the initiator, chain transfer agents are dissolved in it.
The flask is placed in a thermostat under constant agitation and heated.

102. The reaction is slow but becomes fast as the temperature rises.
After a known period of time, the whole content is poured into a methanol (non-solvent) and the polymer is precipitated out.
Polystyrene, PVC, PMA are prepared
by this method

103. Advantages 1. It is quite simple and requires simple equipments.
2. Polymers are of high-purity obtained.
3. As the monomer is solvent, excess monomer can
be removed by evaporation.
4. The polymer has high optical clarity.

104. Disadvantages
1. During polymerisation, viscosity of the medium increases hence mixing and control of heat is difficult.
2. Polymerisation is highly exothermic

105. Applications
1. The polymers obtained by this method are used in casting formulations.
2. Low molecular weight polymers, obtained by this method, are used as adhesives, plasticizers and lubricant additives

106. 2. Solution Polymerisation
In solution polymerisation, the monomer, initiator and the chain transfer agents are taken in a flask and dissolved in an inert solvent.
The whole mixture is kept under constant agitation.
After required time, the polymer produced is precipitated by pouring it in a suitable non- solvent.

108. The solvent helps to control heat and reduces viscosity built up.
Polyacrylic acid, polyisobutylene and polyacrylonitrile are prepared by this method.

109. Advantages
1. Heat control is easy.
2. Viscosity built up is negligible.
3. The mixture can be agitated easily

110. Disadvantages
1. The removal of last traces of solvent is difficult.
2. This polymerization requires solvent recovery and recycling.
3. It is difficult to get very high molecular weight polymer.
4. The polymer formed must be isolated from the solution either by evaporation of the solvent or by precipitation in a non-solvent.

111. Applications
As the polymer is in solution form, it can be directly used as adhesives and coatings.

112. 3. Suspension Polymerisation
Suspension polymerisation is used only for water insoluble monomers.
This polymerisation reaction is carried out in heterogeneous systems.
At the end of polymerisation, polymer is separated out as spherical beads or pearls.
This method is also called pearl polymerisation.

113. The water insoluble monomer is suspended in water
The water insoluble monomer is suspended in water tiny droplet and a initiator is dissolved in it by continuous agitation.
The suspension (droplets) is prevented from coagulation by using suspending agents like PVA, gelatin, methyl cellulose. Each droplet of the monomer contains dissolved initiator.
The whole content is taken in a flask and heated at constant temperature with vigorous agitation in a thermostat with nitrogen atmosphere. After the end of 8 hrs, pearl-like polymers are obtained, which is filtered and washed by water.

114. Polystyrene, Polystyrene-divinyl benzene

115. Advantages
1. Since water is used as a solvent, this method is more economical.
2. Products obtained is highly pure.
3. Isolation of product is very easy.
4. Efficient heat control.
5. Viscosity build up of polymer is negligible.

116. Disadvantages
1. This method is applicable only for water insoluble monomers.
2. Control of particle size is difficult.

117. Applications 1. Polystyrene beads are used as ion exchangers.
2. This technique is used in heterogeneous system.

118. 4. Emulsion Polymerisation

120. Advantages
1. The rate of polymerisation is high.
2. Heat can be easily controlled and hence viscosity built up is low.
3. High molecular weight polymer can be obtained

121. Disadvantages
1. Polymer needs purification.
2. It is very difficult to remove entrapped emulsifier and de-emulsifiers.
3. It requires rapid agitation.

122. Applications
1. Emulsion polymerisation is used in large- scale production like water-based paints, adhesives, plastics, etc.,
2. This method is also suitable for manufacturing tacky polymers like butadiene and chloroprene.

123. IMPORTANT POLYMERS
I. Polyamide (Nylon)

125. Nylon-6:6 is a less soft and stiff material when compared to nylon 6.
Properties of Nylon-6:6
Nylon-6:6 is a less soft and stiff material when compared to nylon 6.
Its melting point is 2640C.

127. Properties of Nylon-6
(i) Nylon-6 is a light weight, soft and less stiff material.
Its melting point is 2250C.

128. General Properties of Nylons
(i) They behave as plastic as well as fiber.
(ii) They are translucent, white, horny and high melting polymers.
(iii) They are insoluble in common organic solvents but soluble in phenol and formic acid.
(iv) They possess good mechanical properties and fairly resistant to moisture.
(v) They are characterised by combination of high strength, elasticity, toughness and abrasion resistance. `

129. Uses of Nylons
(i) Nylon-6:6 is used for fibers, which are used in making socks, ladies shoes, dresses, carpets, etc,.
(ii) Nylon-6 is mainly used for moulding purposes for gears, bearings, electrical mountings, etc,. Nylon bearings work without any lubrication.
(iii) Nylons are used for making filaments for ropes, bristles for tooth-brushes, films and tyre-cords

130. II. Epoxy Resins (or) Epoxide Polymers
These are cross-linked thermosetting resins.
They are polyethers because the monomeric units in the polymer have an ether type of structure i.e., R - O - R.

131. Preparation
Epoxy polymer (or) Epoxy resins are prepared by condensing epichlorohydrin with bisphenol.
The reactive epoxide and hydroxyl groups give a three dimensional cross-linked structure.
The value of n ranges from 1 to 20.

133. Properties
. Due to the presence of stable ether linkage, epoxy resin possess high chemical- resistance to water, acids, alkalis, various solvents and other chemicals.
. They are flexible, tough and possess very good
heat resisting property.
. Because of the polar nature of the molecules, they possess excellent adhesion quality.

134. Uses
Epoxy resins are used as surface coatings, adhesives like araldite, glass-fibre- reinforced plastics.
These are applied over cotton, rayon and bleached fabrics to impart crease-resistance and shrinkage control.
These are also used as laminating materials in electrical equipments.
Moulds made from epoxy resins are employed for the production of components for aircrafts and automobiles.