Polythene, polyvinyl and polystyrene soften on heating and harden on cooling. Such polymers are called thermoplastic polymers. These possess intermolecular forces of attraction whose strength lies between strength of intermolecular forces of elastomers and. fibers. These polymers are linear or slightly.

High-density polythene is obtained by coordination polymerization while low-density polythene is obtained by free radical polymerization. High-density polyethene has linear structure while low-density polythene has branched chain structure. High-density polythene has greater toughness, hardness and tensile strength than low-density polyethene. Therefore, high-density polyethene is used for making buckets, tubs, containers, household pipes, etc. while low-density polythene is mainly used as a packaging material (in form of thin films and bags), as insulation for electrical wires and cables and also for the manufacture of flexible pipes and squeeze bottles

The repeating structural unit is obtained by condensing at both ends of both the bifunctional molecules,
i.e., succinic acid (HOOCCH₂CH₂COOH) and ethylenediamine with the elimination of water molecules,
i.e., -OCCH₂CH₂CO-NHCH₂CH₂NH^-

Those molecules which are formed from large number of small molecules (monomers) under specific conditions of temperature and pressure are called polymers.

Addition polymers :  Polyvinyl chloride, Polythene.

It is because even the traces of certain impurities, which can act as chain transfer agent (or inhibitor can interfere with the original polymerisation chain reaction. Hence, monomers should be free from all impurities, particularly inhibitors (or chain tranfer agents) like CC _4,CBr _4, amines, phenols, quinones etc _.

Biodegradable polymers: Those polymers which are biodegradable, i.e. decomposed by microorganisms and do not create water pollution, e.g. PHBV.
Poly (Glycolic acid) and Poly (Lactic acid) are biodegradable polymers such as sutures.Dextron was the first bioabsorbable suture made from biodegradable polyester for post operative stiches

(i) Natural polymers are biodegradable and therefore do not accumulate in the environment. Hence, the use of natural polymers in our daily life is safe from pollution point of view. 1
(ii) Silicates are natural inorganic polymers and are quite abundant in nature. 
(iii) Cotton, wool and natural silk are abundantly used in textile industry. 
(iv) Natural rubber is a gummy material with poor elasticity and has no cross links. Therefore, it gets permanently.-Aeformed on being stretched. To improve it vulcanization process is followed. For most of the purpose, it is used in the vulcanized form.

(i) From the given passage, we can conclude that the students show awareness about the environment.
(ii) A polymer that can be decomposed by microorganisms within a definite period of time so that the polymer or its degraded product does not cause any harm to the environment, is called a bio-degradable polymer. For example, poly- \(\beta\)- hydroxybutyrate-co-\(\beta\)-hydroxy valerate.
(iii) Polythene is an addition polymer that is formed by the addition of ethene molecules.