O’zbekiston respublikasi oliy va o’rta maxsus ta’lim vazirligi mirzo ulug’bek nomidagi o’zbekiston milliy universiteti

XIII. Answer the following questions after reading the text given below

1. 
   What do the properties of polymers depend upon?
   
2. 
   What are the main uses of plastics in industry and everyday life?
   
3. 
   What is the difference between thermoplastic and thermosetting plastic?
   
4. 
   How many types of polymers are there from a chemical standpoint?
   

HIGH ROLYMERS
Viewed from the standpoint of organic chemistry we are living in an "atomic age".

In the realm or organic chemistry it may be said that we are living in a "plastic age". So many articles formerly constructed of metal, wood, rubber, leather, or ceramics have been replaced by plastics that plastic articles are becoming commonplace- The use of plastic material in toys, swimming pools, home construction, automobiles, boats, airplanes, kitchen ware and articles of clothing has increased tremendously in the past few years. The superior properties of many plastics have resulted in the increased application of plastics in the electrical, radio, television, furniture, petroleum and food industries.

Plastics belong to a class of compounds called polymers by the chemist. A polymer is an organic compound with a large molecular weight made up of recurring units, The units are called monomers and the properties of the polymer depend to a large extent on the chemical nature and mode of linkage of the monomers. The two most important naturally occurring polymers are cellulose and rubber. In general, plastics are synthetic polymers produced in the laboratory by catalyzed reactions of various monomers. From a chemical standpoint, polymers are divided into two main types, addition polymers and condensation polymers.

Addition polymers are produced by the combination of unsaturated monomers at a double bond. This results in a polymer that is a multiple of its monomer units. Examples of such polymers are rubber, various synthetic rubbers, and the vinyl type polymers. Co-polymerization also produces a type of addition polymer by the combination of 2 or more different monomers.

Condensation polymers are formed by a chemical reaction between monomers that contain at least* two reactive groups. The monomer units can thus react with other units to produce an unbroken chain of high molecular weight. Water or other small molecules are usually split off in this reaction. Examples of condensation polymers are cellulose, nylon, and the phenolformaldehyde type of plastics.

Another classification of polymers is based on their behaviour toward heat. If the plastic consists essentially of a long chain of monomers units, it can be softened by heat and molded to any desired shape. These polymers are known as thermoplastics. When plastics are composed of cross-linked chains of monomer units, they will not soften when heated but will retain their original shape. This type is called a thermosetting plastic.