Polymers are long chains of repeating molecular units called monomers. There are several types of polymers including polyethene, polypropene, nylons, polyurethanes, and polyesters. Polymers are classified based on their structure and production method. Properties depend on factors like chain length and structure. Common applications include plastic containers, clothing, pipes, sports equipment, and medical devices.

1. POLYMERS and their PROPERTIES Introduction Basic Topics: Types of Polymers Classification of Polymers Characteristics of Polymers Properties of Polymers Strength of Polymers Applications of Polymers

2. Introduction Polymers are long chain giant organic molecules are assembled from many smaller molecules called monomers. Polymers consist of many repeating monomer units in long chains. A polymer is analogous to a necklace made from many small beads (monomers). Another common name for many synthetic polymers is plastic which comes from the Greek word "plastikos", suitable for molding or shaping. Many objects in daily use from packing, wrapping, and building materials include half of all polymers synthesized.

3. Basic Topics: Types of Polymers Classification of Polymers Characteristics of Polymers Properties of Polymers Strength of Polymers

4. Types of Polymers Polythene The first commercially produced polymer is also the simplest and most common: polythene. Its systematic name is poly(ethene) meaning it is a polymer made from the monomer, ethene. Ethene is a small molecule containing two carbon atoms linked by a double bond and four hydrogen atoms, two bonded to each carbon. Free Electrons When ethene is subjected to high temperature and pressure, or reacted in the presence of a catalyst, one of the bonds in the double bond is broken. Each of the carbon atoms then has a free electron which can form a covalent bond by pairing with another free electron. If other ethene molecules are present, the double bond in one of them can break, and the free electron on one of the carbons can combine with another on the original molecule. As this continues, a long chain of carbon atoms, bonded to one another by single covalent bonds forms. Each carbon atom has two hydrogen atoms bonded to it.

5. Poly(propene) Other polymers can be made in this way. Poly(propene) is very similar to poly(ethene). It is made from propene which has three carbon atoms, two of which are joined by a double bond. When it reacts to become a polymer (polymerises), the long chain is similar to poly(ethene) except that every other carbon atom has a methyl (CH3-) group attached to it. Varied Uses The properties of this type of polymer depend on the regularity of the arrangement of the chains. If they are lined up in a regular way, they are strong, hard materials. If they are more irregular, or there are more side-chains on the molecules, they are more flexible.

6. Amide Linkages One of the most common molecules in biochemistry is a type of polymer called protein. These are made up from monomers known as amino acids and they are joined by an amide linkage. These linkages are made by a carboxylic acid group (-COOH) reacting with an amine group (-NH2) forming the linkage (-NHCO-) and eliminating water. This linkage is the basis of another type of synthetic polymers, the Nylons. Nylon Whereas proteins use monomers with both a carboxylic acid and an amine in the same molecule. Nylons are made using two types of monomer. These are dicarboxylic acids and diamines. The first nylon synthesised used one monomer with a chain of four carbons with a carboxylic acid group on each end and another monomer with a six carbon chain with an amine group on each end. This produced a polymer with repeating units of six carbons joined with amide linkages, but alternately reversed. This polymer is Nylon-6.6. Nylons are mainly used as fibres for clothing and also other hard parts in light engineering.

7. Polyurethane A further important group of polymers are polyurethanes. These are very similar to nylons, but are formed by reacting alcohols with isocyanates and have an amide linkage with an additional oxygen atom in the chain. These polymers are softer and more elastic than nylons and are used as a substitute for rubber and in elastic and Lycra. Polyesters The final types of polymer we will deal with in this article are the polyesters. The ester linkage is a carboxylic acid group where the hydrogen has been replaced by the carbon of another organic group. Polyesters are widely used as fibres for clothes and also for many drinks bottles. They are also used to make thin films for applications such as video tape.

8. Classification of Polymers Homopolymers - consist of chains with identical bonding linkages to each monomer unit. This usually implies that the polymer is made from all identical monomer molecules. These may be represented as : -[A-A-A-A-A-A]- Copolymers - consist of chains with two or more linkages usually implying two or more different types of monomer units. These may be represented as : -[A-B-A-B-A-B]-

9. Polymers are further classified by the reaction mode of polymerization, these include: Addition Polymers - the monomer molecules bond to each other without the loss of any other atoms. Alkene monomers are the biggest groups of polymers in this class. Condensation Polymers - usually two different monomer combine with the loss of a small molecule, usually water. Polyesters and polyamides (nylon) are in this class of polymers. Polyurethane Foam in graphic.

10. Classification based upon the physical property related to heating: Thermoplastics - plastics that soften when heated and become firm again when cooled. This is the more popular type of plastic because the heating and cooling may be repeated. Thermosets - plastics that soften when heated and can be molded, but harden permanently. They will decompose when reheated. An example is Bakelite, which is used in toasters, handles for pots and pans, dishes, electrical outlets and billiard balls.

11. Characteristics of Polymers Low Density. Low coefficient of friction. Good corrosion resistance. Good mould ability. Excellent surface finish can be obtained. Can be produced with close dimensional tolerances. Economical. Poor tensile strength. Low mechanical properties. Poor temperature resistance. Can be produced transparent or in different colours.

12. Properties of Polymers The physical properties of a polymer, such as its strength and flexibility depend on: Chain length - in general, the longer the chains the stronger the polymer; Side groups - polar side groups give stronger attraction between polymer chains, making the polymer stronger; Branching - straight, un branched chains can pack together more closely than highly branched chains, giving polymers that are more crystalline and therefore stronger; Cross-linking - if polymer chains are linked together extensively by covalent bonds, the polymer is harder and more difficult to melt.

13. Strength of Polymers In general, the longer the polymer chain, the stronger the polymer. There are two reasons for this: longer chains are more tangled there are more intermolecular forces between the chains because there are more points of contact. These forces, however, are quite weak for polyethene. Areas in a polymer where the chains are closely packed in a regular way are said to be crystalline . The percentage of crystallinity in a polymer is very important in determining its properties. The more crystalline the polymer, the stronger and less flexible it becomes.

14. When a polymer is stretched (cold-drawn), a neck forms. In the neck the polymer chains line up producing a more crystalline region. Cold-drawing leads to an increase in strength. The first polyethene which was made contained many chains which were branched. This resulted in a relatively disorganised structure of low strength and density. This was called low density polyethene (ldpe) . In the crystalline form, the methyl groups all have the same orientation along the chain. This is called the isotactic form. In the amorphous form, the methyl groups are randomly orientated. This is called the atactic form. Polymers with a regular structure are said to be stereoregular .

15. Applications of Polymers: Polymeric materials are used in and on soil to improve aeration, provide mulch, and promote plant growth and health. Medicine Many biomaterials, especially heart valve replacements and blood vessels, are made of polymers like Dacron, Teflon and polyurethane. Consumer Science Plastic containers of all shapes and sizes are light weight and economically less expensive than the more traditional containers. Clothing, floor coverings, garbage disposal bags, and packaging are other polymer applications.

16. Industry Automobile parts, windshields for fighter planes, pipes, tanks, packing materials, insulation, wood substitutes, adhesives, matrix for composites, and elastomers are all polymer applications used in the industrial market. Sports Playground equipment, various balls, golf clubs, swimming pools, and protective helmets are often produced from polymers.