Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a chain growth polymer? (a) proteins (b) starch (c) nylon-6 (d) polythene

Step-by-Step Solution

Answer

(d) polythene

Step 1: Understand Chain Growth Polymer

Chain growth polymerization involves the growth of polymer chains from monomers that react successively. This process typically starts with an initiator that creates a reactive site, allowing monomers to add one by one to form a long chain. The polymer grows continuously until the monomers are consumed or the reactive sites are deactivated.

Step 2: Analyze Option (a) Proteins

Proteins are formed through the polymerization of amino acids via condensation reactions. This process is known as step-growth polymerization, where the polymer molecules grow step by step without the continuous chain reaction characteristic of chain growth polymers.

Step 3: Analyze Option (b) Starch

Starch is a polysaccharide composed of glucose monomers, connected by glycosidic linkages. It is formed through step-growth polymerization, similar to proteins, as it involves condensation of monomers rather than continuous addition in a chain reaction process.

Step 4: Analyze Option (c) Nylon-6

Nylon-6 is produced through the ring-opening polymerization of caprolactam, which proceeds in a chain-growth manner. The polymer chain grows by successively adding monomers to the reactive end, characteristic of chain growth polymers.

Step 5: Analyze Option (d) Polythene

Polythene (polyethylene) is produced by the polymerization of ethylene (an alkene) via a free-radical or coordination polymerization process. This is a classic example of chain growth polymerization where the polymer grows by continuous addition of ethylene monomers.

Key Concepts

Polymer Chains

Polymer chains are long, repeating sequences of molecules connected together to create a larger and often more complex structure. Imagine a train made up of many individual carriages; similarly, a polymer chain is made up of many small units called monomers. These monomers link together in various ways to form a chain, which can then be extremely long, sometimes consisting of thousands or even millions of units.

Polymers can be found all around us, from the plastics that package food items to the natural polymers like DNA in living organisms. The properties of the resulting polymer chain depend greatly on the type of monomers used and how they are linked together. These properties can include strength, flexibility, and heat resistance.

Understanding polymer chains is fundamental to grasping complex processes like polymerization, where these chains are formed or modified. Chain growth polymerization is one such process where these chains grow gradually, monomer by monomer, creating a long chain quickly.

Free Radical Polymerization

Free Radical Polymerization is a popular method of polymerizing certain monomers, especially in the creation of plastics like polythene (polyethylene). It begins with the generation of free radicals, which are highly reactive molecules with unpaired electrons. These free radicals play the role of initiators, starting the polymerization process by opening the double bond of a monomer.

Once the process starts, many monomers join one after the other, creating a long and stable polymer chain. The reaction continues until the monomers are exhausted or the free radicals become paired, halting the reaction. The simplicity of the setup and the ability to carry out at different conditions make it a widespread choice for synthetic polymer production.

Free radicals in the reaction can be sensitive to oxygen and other impurities, which could potentially stop the process or produce unexpected results. Hence, controlling the environment in which free radical polymerization occurs is crucial for obtaining the desired polymer properties.

Ring-opening Polymerization

Ring-opening polymerization describes a process in which cyclic monomers react and open their rings to form polymer chains. For example, this method is employed in the polymerization of materials like nylon-6, where caprolactam, a ring molecule, opens up to form a polymer chain.

This technique is essential for producing polymers from monomers that come in cyclic shapes - the ring structure of the molecule becomes a part of the backbone of the polymer as it "opens" up to join with other monomers. As the polymer chain grows, each added monomer helps to stabilize the polymer structure.

Ring-opening polymerization offers more precise control over the structure of the final polymer. This results in products with highly defined molecular weights and reduced chances of any unexpected formations, often resulting in a desirable set of mechanical and chemical properties.

Condensation Reactions

Condensation reactions are distinct from chain growth polymerization and involve the reaction between two monomers with the simultaneous loss of a small molecule, commonly water. This type of polymerization often produces step-growth polymers, where the polymer chains grow more slowly in a step-by-step fashion.

Proteins and starch are classic examples of polymers formed through condensation reactions. In such reactions, an amine group can react with a carboxylic acid group, releasing a molecule of water and forming a peptide bond in the case of proteins. The resulting polymers often exhibit different physical properties compared to those formed by chain growth polymerization.

Condensation reactions are sometimes slower and require precise conditions to occur effectively, but they allow the formation of a wide variety of stable and functional polymers in both natural and synthetic forms.