Chapter 15 Polymers

 Polymers or Macromolecules are very high molar mass compounds consisting of several structural units inter-connected by covalent bonds. Polymer is obtained from Greek words POLY means MANY and MERS means parts or units. The structural units which combine to form polymers are called MONOMERS. For example: Polymers

Polymers are classified on the basis of structure as follows:

1. Linear polymers: These polymers are formed of long straight chains. They can be depicted as:

For e.g., high density polythene (HDP), polyvinyl chloride, etc.

2. Branched chain polymers: These polymers are basically linear chain polymers with some branches. These polymers are represented as:

For e.g., low density polythene (LDP), amylopectin, etc.

3. Cross-linked or Network polymers: These polymers have many cross-linking bonds that give rise to a network-like structure. These polymers contain bi-functional and tri-functional monomers and strong covalent bonds between various linear polymer chains. Examples of such polymers include bakelite and melmac.

(i) Hexamethylenediamine

[H₂N  -  (CH₂)₆  -  NH₂]

and adipic acid

[HOOC  -  (CH2)4  -  COOH]

(ii)

(iii) Tetrafluroethene

(CF₂  =  CF₂)

Addition polymers:

A polymer formed by direct addition of repeated monomers without the elimination of by product molecules is called addition polymers. Polyvinyl chloride, polythene.

Condensation polymers:

A polymer formed by the condensation of 2 or more than 2 monomers with the elimination of simple molecules like water, ammonia is called condensation polymer. Terylene, bakelite.

Buna – N

Buna - N is a copolymer of 1,3−butadiene and acrylonitrile. It is also called Nitrile rubber (NBR). It is obtained by polymerization of 1,3-butadiene and acrylonitrile in the presence of sodium .It is used for making oil seels, manufacturing of tank linings, protective gloves etc.

Buna − S

Buna - S is a copolymer of 1,3−butadiene and styrene. It is obtained by polymerization of 1,3-Butadiene and styrene in the ratio 3:1 in the presence of sodium.

In Buna - S, Bu stands for butadiene, Na for sodium and S for styrene. It is also called SBR (Styrene Butadiene Rubber).It has slightly less tensile strength than natural rubber. It is used for making automobile tyres, rubber soles, belts, hoses etc.

Polymers can be classified on the basis of molecular forces like vander waals forces, hydrogen bonds and dipole- dipole interactions. They are classified as:

1 Elastomers- They have elastic character and have weak intermolecular forces. Example - natural rubber.

2 Fibres- They have strong hydrogen bond or dipole- dipole interactions between polymers chain. Example - nylon 66.

3 Thermoplastics- They have intermolecular forces intermediate between elastomers and fibres. Example – polythene.

4 Thermosetting polymers- They undergoes permanent change on heating. Example – bakelite.

(i) Buna − S < polythene < Nylon 6, 6

(ii) Neoprene < polyvinyl chloride < Nylon 6

Addition polymers:

Polyvinyl chloride, polythene

Condensation polymers:

Terylene, bakelite

Polymers are high molecular mass macromolecules composed of repeating structural units derived from monomers. Polymers have a high molecular mass (103 - 107 u). In a polymer, various monomer units are joined by strong covalent bonds. Polymers can be natural as well as synthetic. Polythene, rubber, and nylon 6, 6 are examples of polymers.

Monomers are simple, reactive molecules that combine with each other in large numbers through covalent bonds to give rise to polymers. For example: ethene, propene, styrene, vinyl chloride.

Natural polymers are polymers that are found in nature. They are formed by plants and animals. Examples include protein, cellulose, starch, etc.

Synthetic polymers are polymers made by human beings. Examples include plastic (polythene), synthetic fibres (nylon 6, 6), synthetic rubbers (Buna - S).

The functionality of a monomer is the number of binding sites that is/are present in that monomer.

For example, the functionality of monomers such as ethene and propene is one and that of 1, 3-butadiene and adipic acid is two.

Polymerization is the process of forming high molecular mass (10³ - 10⁷ u) macromolecules, which consist of repeating structural units derived from monomers. In a polymer, various monomer units are joined by strong covalent bonds.

( NH — CHR — CO )_n is a homopolymer because it is obtained from a single monomer unit, NH₂ - CHR - COOH.

On the basis of magnitude of intermolecular forces present in polymers, they are classified into the following groups:

(i) Elastomers

(ii) Fibres

(iii) Thermoplastic polymers

(iv) Thermosetting polymers

Addition polymerization is the process of repeated addition of monomers, possessing double or triple bonds to form polymers. For example, polythene is formed by addition polymerization of ethene.

Condensation polymerization is the process of formation of polymers by repeated condensation reactions between two different bi-functional or tri-functional monomers. A small molecule such as water or hydrochloric acid is eliminated in each condensation. For example, nylon 6, 6 is formed by condensation polymerization of hexamethylenediamine and adipic acid.

The process of forming polymers from two or more different monomeric units is called copolymerization. Multiple units of each monomer are present in a copolymer. The process of forming polymer Buna-S from 1, 3-butadiene and styrene is an example of copolymerization.

Nylon 6, 6 is also a copolymer formed by hexamethylenediamine and adipic acid.

Thermoplastic polymers are linear (slightly branched) long chain polymers, which can be repeatedly softened and hardened on heating. Hence, they can be modified again and again. Examples include polythene, polystyrene.

Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.

1) Polyvinyl chloride (PVC) - The monomeric unit present is vinyl chloride (CH₂ =CH –Cl). PVC is used in the manufacture of rain coats, curtain cloths, hand bags, toys, artificial flooring, gramophone records and as a good insulating material in wires and other electrical goods.

2) Teflon (PTFE) - The monomeric unit present is tetrafluoroethene (n F₂C = CF₂). Teflon is used as a material resistant to heat and chemical attack. It is also used for making gaskets, pump packings, valves, oil seals, non -lubricated bearings.

3) Bakelite - The monomeric unit present is formaldehyde (HCHO) and phenol (C₆H₅OH). Bakelite is used for making glue for binding laminated wooden planks and in varnishes, combs, fountain pens, electrical switches.

For polymerization reaction, an initiator is required. The initiator may be an cation, anion or free radical. So the type of polymerization in which free radical is used as an initiator is known as free radical addition polymerization. The common free radical initiator used is benzoyl chloride.

The structure of benzoyl chloride is as follows:

Natural rubber is a linear cis-1,4- polyisoprene in which double bonds are present between C₂ and C₃ of isoprene units.

The cis configuration about double bonds does not allows the chains to come closer for effective intermolecular attraction due to weak intermolecular attractions. Therefore, the natural rubber has coiled structure and shows elasticity and is non crystalline.

On the other hand, all trans configurations occur in gutta-percha (synthetic rubber) which is trans-polyisoprene. These zig-zag chains pack more closely in gutta-percha and hence it is highly crystalline, non elastic, more hard and brittle than rubber.

Natural rubber though useful has some problems associated with its use. These limitations are discussed below:

1. Natural rubber is quite soft and sticky at room temperature. At elevated temperatures (> 335 K), it becomes even softer. At low temperatures (< 283 K), it becomes brittle. Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of 283 K-335 K.

2. It has the capacity to absorb large amounts of water.

3. It has low tensile strength and low resistance to abrasion.

4. It is soluble in non-polar solvents.

5. It is easily attacked by oxidizing agents.

Vulcanization of natural rubber is done to improve 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 proess. 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 and organic solvents.

The monomeric repeating unit of nylon 6 is [NH – (CH₂)₅ – CO], which is derived from Caprolactam.

The monomeric repeating unit of nylon 6, 6 is [NH – (CH₂)₆ - NH  – CO – (CH₂)₄ – CO], which is derived from hexamethylene diamine and adipic acid.

1) Buna-S (styrene butadiene rubber) - 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 used for making automobile tyres and footwear.

 2) Buna-N (Nitrile rubber)- It is obtained by polymerization of 1,3-butadiene and acrylonitrile in the presence of a peroxide catalyst. It is used for making oil seals, manufacture of hoses and tank linings.

3) Dacron – It is obtained by the polymerization of ethylene glycol and terphthalic acid.

4) Neoprene –It is prepared by the free radical polymerization of chloroprene. It is used as insulator, for making belts, gaskets, hoses etc.

(i) The monomers of the given polymeric structure are decanoic acid [HOOC –(CH₂)₈ – COOH] and hexamethylene diamine [H₂N(CH₂)₆ NH₂ ].

(ii) The given structure is the polymerization product of N2-methyl-1,3,5-triazine-2,4,6-triamine & formaldehyde (HCHO)

Dacron (terylene) is obtained by the polymerization of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst.

The reaction is as follows:

It is used in blending with cotton or wool fibres, as glass reinforcing materials in safety helmets, for making magnetic recording tapes.

A polymer that can be decomposed by bacteria is called a biodegradable polymer.

The biodegradable polymer are the polymers which are degraded by the micro-organism within a suitable period so that biodegradable polymers and their degraded products do not cause any serious effects on the environment. They degrade by enzymatic hydrolysis and oxidation. The decomposition reactions involves hydrolysis (either enzymatically induced or by non –enzymatic mechanism) to non- toxic small molecules which can be metabolized or excreted from the body.

The common examples of aliphatic biodegradable polymers are polyglycolic acid(PGA), Polyhydroxy butyrate (PHB), Polyhydroxy butyrates-co-beta hydroxyl valerate( PHBV), Polycaprolactone(pcl), Nylon-2-nylon-6.

These polymers are used mainly for medical goods such as surgical sutures, tissues in growth materials, for controlled drug release, plasma substitutes etc. They are also used in agriculture materials, such as films, seed coatings, fast food wrappers, personal hygiene products etc.

Poly-β-hydroxybutyrate-CO-β-hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.