Engineering Chemistry-I S.Dhanasekar

Chapter I Polymer Chemistry

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.

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.

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.

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.

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

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.

Oligomers Polymers with low degree of polymerisation are known as Oligomers, their molecular weight ranges from 500 - 5000. High Polymers Polymers with high degree of polymerisation are known as High polymers, their molecular weight ranges from 10,000 - 2,00,000.

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

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.

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.

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.

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

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

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.

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.

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.

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.,

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.

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

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

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.

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.

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.

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.

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.

1.Thermoplastic Resins

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.

Polyethylene, Polyvinyl chloride.

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.

Bakelite, Polyester.

Difference between thermoplastic and thermosetting resins

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

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.

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.,

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.

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

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

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.

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

(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.

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.

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

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.

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.

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.

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

(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.

(iii) Termination

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.,

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

(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.

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

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.

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.

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.

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

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

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

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.

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.

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.,

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

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.

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.

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

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.

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

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.

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

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

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.

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

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

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.

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

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.

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.

Polystyrene, Polystyrene-divinyl benzene

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.

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

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

4. Emulsion Polymerisation

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

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

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.

IMPORTANT POLYMERS I. Polyamide (Nylon)

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.

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

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. `

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

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.

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.

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.

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.