Polymer:
Polymer is a complex, giant macromolecules made from repeating units which are derived from small molecules called as monomer.
Classification of polymers:
Classification of polymers is based on :
- Source of origin.
- Chemical structure.
- Mode of polymerization.
- Molecular forces.
- Types of monomers.
- Biodegradability.
1.Classification on the basis of source of origin: Polymers are divided into three categories: a. Natural b. Synthetic c. Semisynthetic
a. Natural polymers: The polymers obtained from natural source are said to be natural polymers. They are further subdivided into two types.
• Plant polymers: These are obtained from plants. For example Natural rubber is manufactured from the latex obtained from bark of rubber trees.
• Animal polymers: These are derived from animal sources. For example, wool is obtained from hair of sheep.
b. Synthetic polymers: These are man made polymers. These polymers are artificiallyprepared by polymeruzation of one monomer or copolymerization of two or more monomers. Nylon, terylene,neoprene are synthetic polymers. These are further divided into three subtypes, namely fibres, stnthetic rubber and plastic.
c. Semisynthetic polymers : These are derived from natural polymers. These are also called regenerated fibres. Cellulose derivatives such as cellulose acetate rayon, cellullose nitrate, viscose rayon are few eaxamples of this categary.
Semisynthetic polymers are used in preparation of non- flammable photographic films, cinema films, varnishes, etc.
2. Classification of polymers on the basis of structure : Depends upon how monomer are linked together nature of linkage depends upon the nature and number of functional groups in them
a. Linear or straight chain polymers:
When the monomer molecules are joined together in a linear arrangement the resulting polymer is straight chain polymer. It is obtauned from bifunctional monomer or alkene.
For example: PVC, high density polyethylene.
b. Branched chain polymers: The second most common arrangement is that of branched chain. Monomer having 3 functional groups or already having side chain give rise to branched chain polymers. For example: low density polyethene.
c. Cross linked polymers: Third type of arrangement is said to be cross linked or network polymers where cross links are produced between linear chains. Cross linking results from polyfunctional monomers. For example, bakelite.
3. Classification of polymers on the basis of polymerization: Polymerization is the fundamental proce2ss by which low molecular mass compounds are converted into high molecular mass compound by linking together of repeating unit with covalent bonds.
There are three modes of polymerization:
a. Addition polymerization:
Addition polymerization is a process of formation of polymers by addition of monomers with out loss of any small molecule. Compounds containing double bond undergo addition polymerization. It is also reffered as vinyl polymerization, since majority of monomers are from vinyl category. Addition polymerization produces high molecular mass polymeric materials without formation of any intermediate low molecular mass polymeric materials. Free radicals mechanism is most common in addition polymerization. It is also called chain reaction which involves three distinct steps chain initiation, chain propogation and chain termination.
Step 1: Chain initiation: The chain reaction is initiated by free radic. An initiater such as benzoly peroxide, acetyl peroxide, tert-butyl peroxide., ect. can be used to produce free radical. For example acetyl peroxide generates methyl radical as shown below: Step 2. Chain propogation: The new radical formed intitiation step reacts with another molecule of vinyl monomers, forming another still bigger sized radical, which in turn reacts with another monomer molecule. The repetition of this sequence takes place very rapidly. It is called chain propogation. This step is very rapid and leads to high molecular mass radical.
Step 3. Chain termination: Ultimately, at some stage , termination of the growing chain takes place. It may occur by several processes. One mode of termination is by combination of two growing chain radicals.
b. Condensation polymerization:
Consider the formation of terylene(dacron) , a poly ester polymer, from ethylene glycol and terephthalic acid.
In this reaction an alcoholic OH group in ethelene glycol confenses with a carboxyl group in terephthalic acid by eliminating a water molecule to form an ester linkage.
The process of formation of polymers from polyfunctional monomers with the elimination of some small molecules such as water, hydrochloric acid, methanol, ammonia is called condensation polymerization.
In this type of polymerization the chain growth occurs by a series of condensation steps. Therefore it is also referred to as step growth polymerization. This process is continued until a high molecular mass polymer is obtained.
c. Ring opening polymerization: The third type of pymerization is ring opening polymerization. Cyclic compounds like lactums, cyclic ehters, lactones,etc. polmerize by ring opening polymerization. Strong acid or base catalyze this reaction.
Elemental composition of the repeating unit in the polymer resulting from ring opening polymerization is same as that of the monomers. Ring opening polymerization is a step growth polymerization .
4. Classification of polymers on the basis of intermolecular forces :
Mechanical properties of polymers such as tensile strength, toughness, elasticity differ widely depending upon the intermolecular forces. Polymers are classified into various categories on the basis of intermolecular forces as follows:
a. Elastomers:
Elasticity is a property by which a substance gets stretched by external force and restores its original shape on release of the force. Elastimers, the wlastic polymers have weak van der Waal type of intermolecular forces which permit the polymer to stretched. A few crosslinks between the ahains help the stretched polymer to retract to its original position on removal of applied force. For example: valcunized rubber, bunna-S,buna-N, etc.
b. Fibres:
Polymeric solids which form threads are called fibres. The fibres posses high tensile strenght which is a property to have resistance to break under tension. This character is due to the strong intermolecular forces like hydrogen bonding and strong dipole-dipole forces. Due to these intermolecular forces the fibres are crystalline in nature. For example : polyamides, polyesters,etc.
c.Thermoplastic polymers:
Plastucity is a property of being easily shaped or moulded. Thermoplastic polymers are capable of repeated siftening on heating and hardening on cooling. These polymers posses moderately strong intermolecular forces that are intermediate between elastomers and fibres. For example: polythene, polystyrene, etc.
d. Thermosetting polymers: Thermosetting polymers are rigid polymers. During their formation they have property of beinh shaped in heating; but they get hardened while hot. Ince hardened these become influsible; cannot be softened by heating and therefore cannot be remoulded. This characteristic is the result of extenssive cross linking by covalent bonds formed in the moulds during hardening process while hot. For example: bakelite, urea,etc.
5. Classification of polymers on the basis of type of different monomers: Polymers are divided into two classes:
a. Homopolymers:
The polymers which have only one type of repeating unit are called homopolymers. Usually they are formed from single monomer. In some cases the repeating unit is formed condensation of two distinct monomers. For example: polythene, polypropene,etc.
b. Copolymers:
The polymers which have two or more types of repeating units are called copolymer. They are formed by polymerization of two or more different types of monomers in presence of each other. The different monomer unit are randomly sequenced in the copolymer. For example: Buma-S, Buna-N.
6. Classification of polymers on the basis of biodegradability: Most of the synthetic polymers are not affected by microbes. These are called non- biodegradable polymers. These in the form of waste material which stays in the environment for very long time and pose pollution hazards. Most natural fibers in contrast are biodegradable. In attempt to tackle the environmental problems, scientists have developed biodegradable synthetic polymers.
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