Polymers are common in nature which is generally seen in the form of wood, rubber cotton, leather, silk, proteins, anzymes, starches and cellulose. The key factor of polymer usage is its low production cost along with wide range of useful properties.
The enormous growth of synthetic polymers is due to the following reasons.
– Polymers are lightweight materials.
– Polymers act as insulators for electricity and heat
– Polymers cover a wide range of properties from soft packaging materials to stronger fibres than steel
– Polymers allow for relatively easy processing.
Most polymers are organic in nature and formed from hydrocarbon molecules. These molecules can have single, double or triple carbon bonds. A saturated hydrocarbon is one where all bonds between adjacent carbon atoms are single.
Isomers are molecules that contain the same molecules but in a different arrangement.
Polymer molecules are huge macromolecules that have internal covalent bonds. For most polymers, these molecules from very long chains. The backbone is a string of cargon atoms, often single bonded.
Polymers are composed of basic structures called mer units. A molecule with just one mer called a monomer.
Chemistry of Polymer Molecule
Chains are represented straight but in practice they have a three-dimensional zig-zag structure.
When all the mers are the same, the molecule is called a homo polymer. When there is more than one type of mer present the molecule is called a copolymer.
The mass of a polymer is not fixed, but is distributed around a mean value, since polymer molecules have different lengths. The average molecular weight can be obtained by averaging the masses with fraction of times they appear.
Polymers are usually no-linear; bending and rotation can occur around single C-C bonds. Random kings and coils lead to entanglement, like in the Spaghetti structure.
– Linear (end-to-end, flexible, like PVC, nylon)
The regularity and symmetry of the side-groups can affect strongly the properties of polymers. Side groups are atoms or molecules with free bonds called free-radicals, like H, O, methyl, etc.
– If the radicals are linked in the same order, the configuration is called isostatic.
– In a stereoisomer in a syndiotactic configuration, the radical groups alternative sides in the chain.
– In the atactic configuration, the radical groups are positioned at random.
In copolymers, polymers with at least two different types of mers can differ in the way mers are arranged.
Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density.
Thermoplastic and Thermosetting Polymers
Thermoplastic Polymers soften reversibly when heated (harden when cooled back). Thermoplastic are solids, which are formed using heat and pressure. Thermosetting resins usually begin as liquids, which are converted to solids by chemical reaction only.
Thermosetting polymers harden permanently when heated, as cross-linking hinder bending and rotations. Thermosetting polymers harden permanently when heated, as cross-linking hinder bending and rotaions. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts.
Plymerization is the synthesis of high polymers from raw materials like oil or coal. It may occur by
– addition polymerization, where monomer units are attached one at a time
– condensation polymerization, by step-wise intermolecular chemical reactions that produce the mer units.
The Mechanism of polymerization is divided into following two types :-
Chain Growth Reactions – In the growth reaction, vinyl polymerization initiates by radicals, cations, anions, or organometallic catalysts.
Step Growth Reactions – In this mechanism, reaction proceeds to normal functional group reactions of multi-functional monomers.