Polymers

    POLYMERS

The word polymer is derived from two Greek words 'poly=many' & 'mer=unit or parts '.The term polymer is defined as very large molecules having high molecular mass.These are also refered to as macromolecules,which are formed by joining of repeating structural units on a large scale.The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds. This process of formation of polymers from their respective monomer is called as polymerisation.

Example:Nylon 6 , polythene,Buna-S, Buna-N etc.

          

 Classification of polymers

1. Based on the source: classification based on the source is divided into three categories given as follows -

A. Natural polymers:The polymers which are found in plants and animals are called as natural polymers Ex: proteins,cellulose,starch,resins,rubber ,etc

B. Semi-synthetic Polymers:Cellulose derivatives as cellulose acetate(rayon) and cellulose nitrate ,etc are called as semi-synthetic polymers.

C. Synthetic polymers: A variety of synthetic polymers as plastic(polythene),Nylon 6,6 ,Buna-s, etc.

2.Based on the structure of polymer: Classification based on the structure of the polymer is divided into three categories given as follows:-

A. Linear Polymers : These polymers consists of long and straight chains ,some examples are high density polymers,polyvinyl chlorides etc. These are represented as: 

  

B. Branched chain polymers; These polymers contain linear chains having some branches  eg: low density polythene.These are represented as ;

C. Cross linked or Network polymers; These are usually formed from bi-functional and tri-functional monomers and contain strong covalent bond between various linear polymer chains eg; Bakelite,melamine etc. These polymers are represented as :

3. Based on mode of polymerization: Classification based on the mode of polymerization is divided into two categories given as follows:

A.Addition polymers :The addition polymers are formed by the repeated addition of monomer molecules possessing double or triple bonds eg: The formation of polythene  from ethene and polypropene  from propene. However, the addition polymers formed by the polymerization of a single monomeric species are known as homopolymers eg: polythene,polypropene 

• polypropene:-

•  polythene:-

 

The polymers made by addition polymerization from two different monomer are termed as copolymers, eg: Buna-S Buna-N etc .

• Buna-S :-
  

  
• Buna-N:-
  

B. Condensation polymers :- Condensation polymers are formed by the repeated condensation reaction between two different bi-functional or tri-functional monomeric units.In these polymerization  reactions, the elimination of small molecules such as water, alcohol,hydrogen chloride{HCL},etc take place.example Dacron,Nylon 6,6 ,Nylon 6.

• Nylon 6 6:-
  

4. Based on Molecular forces:- Classification based on the molecular forces is divided into four categories given as follows:-

A. Elastomers:- These are rubber-like solids with elastic properties,In these elastomeric polymers the polymer chains are held together by the weakest inter-molecular forces.These weak binding forces permit the polymer to be stretched,a few cross-links are introduced in between the chains, which help the polymer retract to its original position after the force is released as in vulcanization rubber. ex Buna -S, Buna -N,Neoprene etc.

   B. Fiber:  Fibers are the thread forming solids which possess high tensile strength high modulus. These  characteristic can be attributed to the strong inter molecular forces like hydrogen bonding. these strong also lead to close packing of chains and thus impart crystalline nature.ex: polyamides (Nylon 6 6), Teryline etc.

• Nylon 6 6:-

C. Thermoplastic polymers :  These are linear or slightly branched long chained molecules capable of repeatedly softening on heating and hardening on cooling. These polymers posses intermolecular forces of attraction intermediate between elastomers and fiber ex polystyrene, polyvinyl.

• P.V.C :-

 D. Thermosetting polymers :-  These polymers are cross linked or heavily  branched molecules,which on heating undergo extensive cross linked in molds and again becomes infusable. These cannot be reused. ex: Bakelite, urea-formaldehyde resin.

   5. Based  on the growth polymerization :- The Addition and Condensation Polymers are now days also referred as chain growth and step growth  Polymers depending on the type of polymerization mechanism they undergo during their formation.
1. Free radical mechanism:-  the process start with the addition of the phenyl free-radical formed by the peroxide to the ethene double bond thus generating a new and larger free radical . This step is called as chain inhibiting step.
As this radical react with another molecules of ethene,another bigger sized radical is formed. The repetition of this sequence with new and bigger radical carries the reaction forward and the step is termed as chain propagation step .

Ultimately at some stage the product radical thus formed react with another radical to form the polymerized product. this step is called the chain terminating step. 

Preparation Of Some Important  Addition  Polymers 

1. polythene: There are two types of polythenes :

A: Low density polythene 

B. High density polythene 

Low density polythene 

It is obtained by the polymerization of ethene under high pressure of 1000 to 2000 atm a temperature of 350 k to 570 k in the presence of traces  of di oxygen or peroxide inhibitor . these are chemically inert and tough but  flexibility and poor conductor of electricity used in  squeeze bottles, toys , flexible  pipes.

High Density polythene 

It is chemically inert  and more tougher and harder. it is used for manufacturing buckets, dustbins ,bottles ,pipes etc.

2. Teflon (polytetra fluroethene):  Teflon is manufactured by heating tetrafluroethene with a free radical at high pressure. It is chemically inert and instant to attack by corrosive reagents. It is used in making Oil Seals, Gaskets, Non-Stick Surface coated Utensils.

3.Polyacronytrile(PAN): The addition polymerization of acronytrile in presence of a peroxide catalyst leads to the formation of polyacronytrile (PAN) n. It is used as a substitute for wool in making commercial fibres.

Condensation Polymerization

In these reactions the product of each step is again a bi-functional species and the sequence of condensation goes on  since, each step produces a distinct fiunctionalized species and is independent of each other , this process is also called as step growth polymerization, The formation of terylene by the interaction of ethylene glycol and terphthalic acid is an example of step growth polymerization. 

1. Polyamides: There are two polyamides given as follows:
A. Nylon 6 6 :- It is prepared by the condensation polymerization of hexamethylenediamene with adipic acid under high pressure and at high temperature .
B. Nylon 6 :-It is obtained by heating copralactum with water at a high temperature. it is used for the manufacture of tyre cords , fabrics and ropes.

2. Polyester: Dacron and terylene is the best known example of polyester. It is manufactured by heating a mixture of ethylene glycol and terepthalic acid at 420 k to 469 k in the presence of zinc acetate antimony trioxide catalyst as per the reaction given earlier . Dacron fiber (Terylene ) is crease resistant and is used in blending with cotton and wool fibers & also as glass reinforcing materials in safety helmets etc .

3. Bakelite and related polymers (Formaldehyde polymers ): These are obtained by the condensation reaction of phenol with formaldehyde i n  the presence ofether an acid or a base catalyst . The reaction starts with the initial formation of O-and /or P-hydroxymethyl phenol derivatives , which further react with phenol to form compounds having rings joined to each other through -CH2 group ,the initial products could be a linear products Novalac used in paints .

Novalac on heating with formaldehyde undergoes cross linkage to form a infusible solid mass called Bakelite. It is used in making combs,Phonograph records,Electrical switches and Handles of various Utensils.

4. Melamine: Melamine formaldehyde polymers is formed by the condensation polymerization of melamine & Formaldehyde . 

Copolymerization

Copolymerization  is Polymerization reaction in which a mixture of more than one monomeric species is allowed to polymerize and form a copolymer .The copolymer can be made not only by chain growth polymerization but by step growth polymerization also ex Buna -S etc. \

1.rubber: rubber is of two types given as follows :

A. Natural rubber:  Natural rubber may be considered as linear polymers of isoprene and is also called as cis-poloupropene molecules consists of various chains held together by weak vander waals interaction and has a coiled structure thus it can be stretched like a spring and exhibits elastic properties. 

Vulcanization of rubber 

Natural rubber becomes soft at high temperature (>335) and brittle at low temperature (<283) & shows high water absorption capacity .it is soluble in non-polar solvents and is non-resistant to be attacked  by oxidizing agent.This process consists of heating a mixture of raw rubber with sulpher and an appropriate additive at temperature range 373k to 415 k on vulcanization  , sulphur forms cross links at the reactive sites of double bond & thus the rubber gets stiffed. 

In manufacture of tyre rubber , 5% of sulphur is used as cross linking agent, the probable structure of vulcanization rubber molecules.is 

2.Synthetic rubber: Synthetic rubber is any vulcanizable rubber like polymer,which is capable of getting stretched to  twice its length. However, it returns to its original shape and size as soon as the external stretching force is released. thus synthetic rubbers are either homo-polymers of 1,3-butadiene  or copolymer of 1,3 - butadiene.some examples are neoprene,Buna-N .

Biodegradable Polymers 

The polymers which contain functional  group similar to the functional  groups present in bio-polymers .ex Aliphatic polyester are important class of biodegradable polymers.

1.Poly B- Hydroxy butyrate-co- b - hydroxy valerate (PHBV): it is obtained by the copolymerisation of 3-hydroxy butanoic acid and 3-hydroxy pentanoic acid.PHBV  is used in speciality packaging,orthopaediv devices and in controll release of drugs. PHBV undergoes bacterial degradation in the environment . 

2. Nylon 2-Nylon -6: It is alternating  polyamide copolymer of glycine (H2N-CH2-COOH) & amino caproic acid [H2N(CH2)5COOH] and is biodegradable.