POLYMERS

POLYMERS

What is polymerization? Give examples to different kinds of polymerization. Polymer is a material which is built up by linking together a large number of small recurring molecules called monomers.  

Ans:-The process of linking several monomer molecules together to form a big molecule is called Polymerisation.

Polymerisation is two types, addition polymers and condensation polymerisation.

Addition polymers: These are formed by monomers containing double bonds. Addition polymers are formed by the simple addition of monomers at the double bonds. Addition polymers contain same number of atoms that present in all the monomer units from which the polymer is formed.

Eg : polyethylene, PVC, Rubber etc.      .

Condensation polymers: These are formed by monomers containing polyfunctional groups. These are formed by the condensation between functional groups of different monomer molecules with elimination of small molecules like H20, HCl, NH3 etc. Condensation polymers contain fewer atoms than the number of atoms in all the monomer units from which the polymer is formed.

E.g. Nylon – 66, Bakelite, Terylene.

 

Write a note on   a) condensation polymerization,    b) addition polymerization. Condensation polymerisation :

Ans:- The polymerisation due to combining of monomer molecules containing poly functional groups by the elimination of small molecules like H2O, HCl, NH3 etc. is called condensation polymerisation. The condensation polymer contains fewer atoms than the number of atoms in all the monomer units from which the polymer is formed.

Eg: Terylene is a condensation polymer of ethylene glycol and terepthalic acid. Water molecules are eliminated in the process.

Addition polymerisation : The polymerisation due to simple addition” of the monomer molecules containing double bonds. The addition polymer contains same number of atoms that present in all the monomer units from which the polymer is formed. The molecular weight of the polymer unit is simple integer multiple of the molecular weights of monomer units from which the polymer is formed.

Eg: Polyethylene

Discuss the structure of natural rubber.

i)   The empirical formula of natural rubber is C5H6

ii) On heating natural rubber in air gives only CO2 and H2 So it is a hydrocarbon.

iii)  On heating natural rubber in the absence of air or O2 gives isoprene i.e., 2 – methyl – 1, 3 -butadiene)

Hence natural rubber is considered as a polymer of isoprene.

iv) Ozonolysis of natural rubber indicates that isoprene units are joined by head – to – tail by 1, 4 – links

The remaining double bonds are arranged between C2 and C3 of each isoprene unit.

v) X – ray studies indicate that rubber is composed of long chains of isoprehe units arranged in cis form

  1. vi) The weight average Molecular weight of rubber varies between 1,30,000 to 3,40,000.vii) Between the rubber molecules only van der Waal’s attractive forces exists because in the rubber molecules polar groups are absent. The van der Waal’s attractive forces between rubber molecules are weak because of the cis configuration about the double bonds. The cis configuration will not allow the molecules to come closer. Hence the rubber molecules donot exist as a straight chain but exist as a coiled structure. Therefore it can be stretched like a spring and imparts elastic nature to the polymer.

Vulcanisation of Rubber : When natural rubber is heated after mixing with sulphur at 373 – 415K it becomes hard. This process is called Vulcanisation of rubber. Vulcanisation of rubber is catalised by zinc oxide, zinc stearate and some organic compounds. During vulcanisation of natural rubber the rubber molecules are cross linked with sulphur atoms at double bonds. The hardness of vulcanised rubber depends upon the percentage of sulphur. The hardness of rubber increases with increase in percentage of sulphur. The rubber containing about 40 – 45% sulphur sets into non elastic hard material called ebonite.

How are polymers classified? Give one example to   each type.

Ans:-Polymers can be classified in many ways depending on the characteristic selected.

1)  Depending on the source: Polymers are classified depending on the source as follows.

a) Natural polymers: These occur in nature. Ex: Cotton, silk, wool, rubber etc.

b) Semisynthetic polymers: Chemically modified naturally occurring polymers.

Ex: Cellulose rayon, cellulose nitrate.

c) Synthetic polymers Ex : Polythene, PVC.

2) Depending on structure:

  1. a) Linear polymers : These contain linear polymer chains.

Ex : Polythene, PVC.

b) Branched polymers : These contain branched polymeric chains.

Ex: Low density polyethylene.

c) Cross linked or Network polymers: These contain cross linkages between polymer chains.

3. Depending on mechanism of polymerisation:

a) Condensation polymers: These are formed by the condensation of similar or different type of monomers.

Ex -Nylon 6,6 –( NH – (CH2)6 NH CO (CH2)6 – CO)n

b) Addition polymers: These are formed by the addition of similar or different monomers.

Ex: Polythene   – (CH2 – CH2)n

4)  Depending on heat treatment:

a) Thermoplastics: These soften on heating and harden on cooling reversibly. !

Ex: PVC, polythene.

b) Thermosetting plastics: These soften once and then harden irreversibly on heating.

Ex : Bakelite.

5)  Depending on polarity:

  1. a) Cationic polymerization polymers Ex:
  2. b) Anionic polymerization polymers Ex: Buna type synthetic rubbers.

6)  Depending on molecular forces :

  1. a) Elastomers: They have spring like structure between the rings.of the coil there exists weak hydrogen bonds. Ex : Buna – N, Buna – S.
  2. b) Fibres: They contain long polymeric chains held together with strong hydrogen bonds. Ex: Polyamides.
  3. c) Thermoplastics : Polythene, polystyrene.
  4. d) Thermosetting plastics: Bakelite.

7)   Depending on chemical nature:

  1. a) Inorganic polymers like glass, silicones.
  2. b) Organic polymers like leather, cellulose.

 

How do double bonds in rubber molecules influence their structure and reactivity?

Ans:-Rubber is a polymer of isoprene and its derivatives. Isopresene polymerizes in Cis 1,4 – addition polymerisation method.

The inermolecular attractions are largely limited to van der Waals interactions. These interactions are weakened due to the cis configurations about the double bond which do not allow the polymer chains to come close together. Hence the cis polyisopresene molecule cannot be present as a straight chain and it is existing as a coiled structure. It can therefore be stretched like a spring. This impacts elastic nature to the polymer rubber.

The double bonds in natural rubber polymer provide the reactive sites and also determine the configuration of the polymer. The – CH2 group present next to the double bond, known as allylic – CH2 group, It is very reactive. Vulcanization takes place at these active centers and sulphur forms cross-links at these centres. Thus the rubber gets stiff. The intermolecular movement of rubber coils is prevented and the physical properties change. The extent of stiffness of vulcanized rubber material depends on the amount of sulphur added.

The tentative structures of vulcanized rubber molecules are given below.

Write on the modes of a) Anionic polymerization,   b) Cationic polymerization.

Ans:-a) Anionic polymerisation: In the anionic polymerisation, the polymerisation is initiated by an anion, forming an anionic intermediate for propagating  the addition chain process.   In the anionic polymerisation the monomers containing more electronegative groups like phenyl, nitrile etc partici­pate.  The typical initiators for anionic polymerisation is potassium amide KNH2; alkyl lithium RLi.

In the anionic polymerisation the chain terminating step is absent.

b) Cationic polymerisation: In the cationic polymerisation, the polymerisation is initiated by a cation, forming a cationic intermediate for propagating the addition chain process. Generally cationic polymerisation is initiated by protonic acids like HCl, H2S04 or Lewis acid like BF3, AlCl3, SnCl etc.

Eg : Vinyl polymers are formed by this process. Cationic polymerisation takes place in three steps as shown below.

i) Chain initiating step :

ii) Chain propagating step

iii)  Terminating step:

What is the vulcanization? Write the consequences of vulcanization of rubber.

Ans:-Vulcanization : When natural rubber is heated after mixing with sulphur at 373-415K it becomes hard. This process is called vulcanization of rubber.

Vulcanization of rubber is catalysed by zinc oxide, zinc stearate and some organic compounds. When the proportions of sulphur are increased the rubber can be hardened. If sulphur amount is about 40 – 45% the rubber sets into a non- elastic hard material known as ebonite. The rubber becomes cross-linked with sulphur during the process of vulcanization. This restricts -the intermolecular move­ment and thus its physical properties changes.

Vulcanized rubber has excellent physical properties like elasticity; minimum water abosorbs tentency, high resistence to chemical oxidation as well as organic solvents.

What is a “biodegradable polymer”? Discuss about any two of the artificial biode­gradable polymers.

Ans:-The polymers that degrade quickly by enzymatic chemical reactions like oxidation of hydrolysis are called biodegradable polymers. Ex: Carbohydrates, proteins etc.

1) PHBV: PHBV is poly –β -hydroxybutyrate – Co – β – hydroxy valerate. It is a copolymer of 3 – hydroxybutanoic acid.

PHBV is a biodegradable polymer.   So it is used in making capsules.  The medicine inside the capsule is released when the polymer is degraded. It is also used in special package materials and in orthopaedic devices.

2) Polyglycolic and polylactic acid : These are polymers of glycolic acid and lactic acid.

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