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Introduction
Polymers are macro-sized molecules of relatively high molecular mass, which find extensive use in our daily life. Polymers are large but single chain-like molecules in which the repeating unit derived from small molecules called monomers are covalently linked.
Classification Based on Source of Availability
Polymers are classified based on their source of availability as follows:
1. Naturally occurring polymers
2. Semi synthetic polymers
3. Synthetic polymers
Classification Based on Mode of Polymerization
Based on the mode of polymerization, polymers are classified as:
1. Homopolymers and copolymers
2. Addition and condensation polymers
Classification Based on Molecular Forces
Elastomers are polymers in which the polymer chains are held by weakest intermolecular forces. These forces permit the polymers to be stretched. A few cross links are introduced between the chains to help the polymer retract to its original position after the force is released.
General Methods of Polymerization
Two major methods generally used for preparing polymers are:
a) Addition polymerization
b) Condensation polymerization.
Free Radical Addition Polymerization
For chain growth polymerization to occur by a radical mechanism, a radical initiator must be added to the monomer to convert it into a radical. The initiator breaks homolytically into radicals and each radical adds to an alkene monomer, converting it into a radical. The radical site reacts with another monomer which further reacts and thus the process keeps on going, propagating the chain. The process is repeated over and over.
Vinyl Polymerizations
Most of the commercial addition polymers are vinyl polymers obtained from alkenes and their derivatives.
Polyethylene or Polythene Formation
It is obtained by polymerization of ethylene under a high pressure of 1000 to 2000 atmospheres at a temperature of 350 to 570 K in the presence of traces of oxygen or a peroxide which initiates the polymerization.
Conjugated Diene Polymerization
1, 3 butadiene, a conjugated diene can be polymerized like a simple alkene but there are two modes by which this process can take place.
Ionic Addition Polymerization
Vinylic monomers can undergo addition polymerization through the formation of ionic intermediates instead of free radicals. Here the initiator is an ion source and not a free radical source.
Cationic Addition Polymerization
When the initiator is cationic in nature, on addition to the double bond, it would generate a cationic intermediate for propagating the addition chain process and is called cationic addition polymerization.
Anionic Addition Polymerization
An anionic initiator will generate a carbanionic intermediate and thus the polymerization is of anionic addition type. Here the active centre of the propagating species is negatively charged. Hence it occurs easily with monomers containing electron withdrawing groups such as phenyl, nitrile etc., which are able to stabilize the propagating species.
Copolymerization
If a mixture of more than one monomeric species is allowed to polymerise, a copolymer is formed and it contains multiple units of each monomer used in the same polymeric chain.
Polyolefins
Polydienes
Polyhalo - Olefins
Commercially Important Condensation Polymers
Terylene or Dacron is manufactured from ethylene glycol and terephthalic acid. The reaction is carried out at 420 - 460 K in the presence of catalyst zinc acetate and antimony trioxide.
Commercially Important Condensation Polymers (Continued)
Bakelite is made from phenol and formaldehyde in the presence of a base catalyst. It involves formation of methylene bridges at the ortho and para positions. The reaction starts with the initial formation of ortho and/or para-hydroxymethyl phenol derivatives which further react with phenol to form compounds where the rings are formed to each other with -CH2 groups.
Natural Rubber
Natural rubber is an excellent example of a natural polymer and an elastomer in particular. Elastomers are substances that can be readily stretched. They retract rapidly to their original form when released. It undergoes long range reversible extension under relatively small applied force. This elasticity makes it valuable for variety of uses. Natural rubber is also called plantation rubber.
Properties of Natural Rubber
The most important property of natural rubber is its elasticity. When stretched, it expands and attains its original state, when released. This is due to its coil-like structure. The molecules straighten out when stretched and when released, they coil up again. Therefore applying a stress can easily deform rubber. Note that when this stress is removed, it retains its original shape.
Vulcanization of Rubber
Charles Goodyear discovered the process of vulcanization in 1893 to modify the properties of natural rubber. Vulcanization is the addition of right amount of sulphur to natural rubber to impart high elasticity, tensile strength and resistance to abrasion.
Synthetic Rubbers
Synthetic rubbers are made by the polymerization of dienes, in the presence of Zeigler-Natta catalyst. These rubbers are tougher, more flexible and more durable than natural rubbers.
Neoprene or Polychloroprene
The first commercially successful rubber substitute, manufactured in the US is neoprene (1931). It is prepared by the polymerization of chloroprene (2-chlorobutadiene).
Molecular Mass of Polymers
Generally a polymer sample contains chains of varying lengths and therefore its molecular mass is always expressed as an average. In contrast, natural polymers such as proteins, contains chains of identical length and hence have definite molecular mass.
Biopolymers
Nature has many polymeric species which are essential for life and are called biopolymers. Polysaccharides, proteins and nucleic acids are examples.
Summary
Addition polymerization are usually performed on alkenes and their derivatives and proceed through chain growth mechanism involving radicals, cations and anions as intermediates.
