Polymers

Polymer=Poly + Mer

Poly means “many” and “Mer” means unit or part. A polymer is a large molecule which is formed by linking repeating structural units. The structural units are generally simple molecules and they are linked by a covalent bond to form a polymer.

Example:

Polymers are classified based on structure, into 3 types:

i) Linear Polymer: They have a long and straight chain of molecules. Ex: high-density Polythene, Polyvinyl chloride

ii) Branched-chain Polymers: They have linear molecular chains along with some branches. Ex: less density polythene.

iii) Cross-linked or network Polymers: In these polymers, strong covalent bonds are between the linear chains. Generally, they contain 2 or 3 types of functional groups. Ex: Bakelite, melamine

i) Two monomers Hexamethylenediamine [H₂N-(CH₂)₆-NH₂] and adipic acid [HOOC-(CH₂)₄-COOH] combined to form the polymer and a water molecule is removed in this process. Hydrogen(H) is removed from both sides of Hexamethylenediamine and OH group is removed from both sides of adipic acid to give a water molecule.

ii) The monomer is OC-(CH₂)₅-NH known as Caprolactam. The cyclic structure of the monomer changes to linear to form the polymer, as shown below:

(iii) The monomer is Tetrafluroethene (CF₂= CF₂), the double bond breaks to form the polymeric chain.

1.ADDITION POLYMERS:

The repeated addition of monomers with double or triple bonds leads to the formation of addition polymers. The double or triple bonds break to form a single bond and the molecular chain is formed. Thus, Polyvinyl chloride, Polythene are addition polymers.

2.CONDENSATION POLYMERS:

The repeated condensation of 2 different bi-functional or tri-functional groups leading to the elimination of simple molecules like water, hydrogen chloride leads to the formation of condensation polymers. Thus, Terylene, Bakelite are condensation polymers.

Based on the intermolecular forces, the polymers are classified into elastomers, fibres, thermoplastic polymers, thermosetting polymers.

1. ELASTOMERS- They have the weakest intermolecular forces. Eg: Buna-S, Buna- N.

2. FIBRES- They have the strongest intermolecular forces like hydrogen bonding. Eg: Nylon(6,6)

3. THERMOPLASTIC POLYMERS- They have the intermolecular forces in between the elastomers and the fibres. Eg: Polystyrene

The structure of these three is shown as:

Thus, we can arrange them as follows:

(i) Buna-S < Polythene < Nylon(6,6):

(ii) Neoprene<Polyvinyl chloride<Nylon 6

The word polymer comes from poly- (many) and -mer (part). Polymers are generally high molecular mass substances that have repeating units of smaller molecules. They may be a natural or synthetic macromolecule. The single molecular units of which a large chains are made is known as ‘monomer’. These monomers generally have high molecular mass. (10³-10⁷u). Some examples of polymers are polythene, Bakelite, rubber, Buna-N and many more.

Based on source, polymers can be classified as below:

(i) Natural polymers: Those polymers obtained from plants and animals such as proteins, cellulose, starch and other resins.

(ii) Semi Naturalpolymers: Those polymers that are derivatives of natural polymers fall under this category. Cellulose Nitrate used as a propellant and in guncotton and Cellulose Acetate used in photography are prepared from Cellulose.

(iii) Synthetic polymers: Those polymers which are prepared in the laboratory are called synthetic polymers. Bakelite, Polythene, synthetic fibers and rubbers are some of its examples.

Copolymers are those polymers that consist of more than one monomeric repeating unit. Some of the copolymers are Saran, Butadiene, Nitrile Rubber, Butyl Rubber, Viton and many more.

Homopolymers are those in which there is only one monomeric unit. Some of the examples of homopolymers are polypropylene, Polythene and many more.

The functionality of a monomer is defined as the number of bonds that a monomer's repeating unit forms in a polymer with other monomers. A linear polymer is formed by polymerizing if the functionality of monomer is two and is bifunctional (a thermoplastic).

In chemical reactions when monomer units form chains or three–dimensional networks, it is known as polymerization. Various methods of polymerization are Step-growth polymerization, Chain-growth polymerization and Condensation polymerization.

It is polyvinylchloride. It is a homopolymer as it is made up of the same type of repeating monomeric units, i.e. NH_2-CHR-COOH.

Polymers are classified in four classes on basis of molecular forces. These classes are

(i) Elastomers: in these polymers, weak intermolecular forces mainly hold chains. These weak links permit polymers to be stretched.

(ii) Fibres: they possess high tensile strength and high modulus. They have strong intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains and thus impart crystalline nature. The examples are polyamides, polyesters, etc.

(iii) Thermoplastic Polymers: They are linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. They possess intermolecular forces of attraction intermediate between elastomers and fibers. Some common thermoplastics are polythene, polystyrene, polyvinyl, etc.

(iv) Thermosetting Polymers: They are cross-linked or heavily branched molecules, which on heating undergo extensive cross-linking in molds and again become infusible. Some common examples are Bakelite, urea-formaldehyde resins, etc.

In Addition polymerisation, monomers generally join together to form saturated polymers. There are three steps to chain reaction to form addition polymer, they are initiation, propagation and termination. In condensation polymerisation, functional groups of two monomers react together to release a small molecule to form a polymer. Generally, small molecules like water or HCl are released as by-products.

Copolymers are those polymers that consist of more than one monomeric repeating unit. Copolymerisation is a process of reacting a mixture of more than one monomeric species and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. For example, a mixture of 1, 3 – butadiene and styrene forms Buna-S and 1-3 – butadiene and acrylonitrile form Buna-N.

Chain initiation

The chain is initiated by free radicals, Ra•, produced by the reaction between some of the ethene and the oxygen initiator.

Chain propagation

Each time a free radical hits an ethene molecule a new longer free radical is formed.

Ra• + CH₂ = CH₂→ RaCH₂CH₂•

RaCH₂CH₂• + CH₂ = CH₂→ RaCH₂CH₂CH₂CH₂ •

Chain termination

Eventually two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed.

Ra(CH₂)_m• + •(CH₂)_n Ra → Ra(CH₂)_m(CH₂)_nRa

Because chain termination is a random process, polyethene will be made up of chains of all sorts of different lengths.

Thermoplastic Polymers: They are linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. They possess intermolecular forces of attraction intermediate between elastomers and fibers. Some common thermoplastics are polythene, polystyrene, polyvinyl, etc.

Thermosetting Polymers: They are cross-linked or heavily branched molecules, which on heating undergo extensive cross-linking in moulds and again become infusible. Some common examples are Bakelite, urea-formaldehyde resins, etc.

(i) Ethylene dichloride

(ii) Tetrafluoroethene

(iii) Phenol and formaldehyde

One of the common initiators used in free radical addition polymerization is Benzoyl peroxide. Its structure is

Natural rubber is a linear cis-1, 4-polyisoprene in which double bonds are present between C2& C3. It increases the elasticity of the rubber, as the chains are held together by weak van der waals forces and have coiled structure. Therefore, the natural rubber has coiled structure & shows elasticity & is non-crystalline.

Vulcanization of natural rubber is done to improve upon all these properties. In this process, a mixture of raw rubber with sulphur and appropriate additive is heated at a temperature range between 373 K and 415 K.

This is a slow process; therefore, some additives like zinc oxide etc. are used to accelerate the process. During this process, sulphur cross links are formed which makes rubber hard, tough with greater tensile strength .The vulcanized rubber has excellent elasticity, low water absorption, resistance to oxidation & organic solvents.

Monomeric units of nylon-6 is Caprolactam and for Nylon-6,6 is adipoyl chloride and hexamethylene diamine.

1,3-Butadiene and Styrene

1,3 Butadiene and Acrylonitrile

1,2-Ethanediol and Terephthalic Acid

Chloroprene

Adipoyl chloride and Hexamethylene diamine

Melamine and Formaldehyde

Dacron is a polyester batting that should be added to any foam surface so that it will not be exposed directly to the fabric. Dacron has many indispensable qualities like batting reduces the friction foam has, and thus reduces wear to fabric and because polyester batting remains springy, it is ever ready to put some light pressure against fabric. This means that even as the fabric stretches with age (and always happens) batting will push against the fabric and keep wear-worn waves from developing.

Biodegradable polymers are designed to degrade into simpler components like water, CO₂, Nitrogen etc. upon disposal by the action of living organisms. Extraordinary progress has been made in the development of practical processes and products from polymers such as starch, cellulose, and lactic acid. The need to create alternative biodegradable water-soluble polymers for down-the-drain products such as detergents and cosmetics has taken on increasing importance. Aliphatic polyesters are one of the important classes of biodegradable polymer. Example. Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV)