JNTU B.Tech IV POLYMERIC MATERIALS Previous Question Paper (October/November - 2020 - R16)

Polymers are classified based on several criteria to understand their structure, properties, and applications:

1. Based on Source:

• Natural Polymers: Found in plants and animals, such as cellulose, starch, proteins, and natural rubber.
• Synthetic Polymers: Man-made polymers synthesized in laboratories or industries, such as polyethylene, nylon, and PVC.
• Semi-synthetic Polymers: Chemically modified natural polymers, such as cellulose nitrate and vulcanized rubber.

2. Based on Structure:

• Linear Polymers: Long, straight chains with high density and packing, such as HDPE and PVC.
• Branched Chain Polymers: Linear chains with side branches, resulting in lower density, such as LDPE.
• Cross-linked or Network Polymers: Chains linked by covalent bonds, forming a rigid 3D structure, such as Bakelite and melamine-formaldehyde.

3. Based on Polymerization Mechanism:

• Addition Polymers: Formed by the repeated addition of monomers without the loss of small molecules, such as polyethene and polystyrene.
• Condensation Polymers: Formed by the repeated condensation reaction between bifunctional monomers with the loss of small molecules like water or alcohol, such as nylon-6,6 and polyester.

4. Based on Molecular Forces:

• Elastomers: Weak intermolecular forces allowing elasticity, such as rubber.
• Fibers: Strong intermolecular forces (hydrogen bonding) leading to high tensile strength, such as polyamides.
• Thermoplastics: Soften on heating and harden on cooling, allowing reshaping, such as polystyrene.
• Thermosetting Polymers: Undergo chemical changes upon heating to form an infusible, insoluble network, such as epoxy resins.

5. Based on Tacticity (Stereochemistry):

• Isotactic: Functional groups are on the same side of the chain.
• Syndiotactic: Functional groups alternate regularly on opposite sides.
• Atactic: Functional groups are arranged randomly along the chain.

Polymerization is the chemical process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. It involves the formation of covalent bonds between repeating structural units.

The mechanism of polymerization is primarily classified into two types: Chain-Growth Polymerization and Step-Growth Polymerization.

1. Chain-Growth Polymerization (Addition Polymerization): This process occurs through the rapid addition of monomers to an active center (free radical, cation, or anion). It typically involves three distinct stages:

• Initiation: Formation of an active species (e.g., a free radical) using an initiator, which then attacks the double bond of the monomer.
• Propagation: The reactive center adds successive monomer units rapidly, extending the chain length while maintaining the active site at the end of the chain.
• Termination: The active center is destroyed by combination or disproportionation, stopping the growth of the chain.

2. Step-Growth Polymerization (Condensation Polymerization): This process involves the reaction between functional groups of monomers, often accompanied by the elimination of small molecules like water or methanol. Key characteristics include:

• The reaction occurs in steps where monomers react to form dimers, trimers, and eventually long chains.
• Any two species in the mixture can react with each other at any time.
• The molecular weight of the polymer increases slowly throughout the reaction, and high molecular weight polymers are only achieved at very high conversion rates.

The choice between these mechanisms depends on the chemical structure of the monomers and the desired properties of the final polymeric material.