Polymers

15.14

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.

15.13 

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.

15.12

In Addition polymerisation, monomers generally join together to form saturated 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.

15.12

Elastic properties in elastomers :

Elastomers possess elastic properties because the intermolecular forces are the weakest. As a result, they can be readily stretched by applying small stress and regain their original shape when the stress is removed. The elasticity can be further increased by introducing some cross – links in the polymer chains.

Natural rubber is the most popular example of elastomers. A few more examples are buna-S, buna-N and neoprene.

15.11

It is polyvinylchloride. It is a homopolymer as it is made up of the same type of repeating monomeric units.

15.10

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.

15.9 

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).

15.8 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.

15.7 Based on source, polymers can be classified as below:

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

Semi Natural polymers: 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.

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

15.1

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.