Problem 11 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which one of the following is a chain-growth polymer? (a) Starch (b) Nucleic acid (c) Polystyrene (d) Protein

Step-by-Step Solution

Answer

(c) Polystyrene

Step 1: Understanding Chain-Growth Polymerization

Chain-growth polymerization involves the reaction of an active species, such as a radical, cation, or anion, with monomer molecules to form polymer chains. The process involves three key steps: initiation, propagation, and termination.

Step 2: Analyzing the Options

Let's evaluate the polymers given in the options: (a) Starch is a polysaccharide formed by linking glucose monomers through glycosidic bonds, which is a step-growth polymerization. (b) Nucleic acids are formed by linking nucleotides through phosphodiester bonds, which is step-growth polymerization. (c) Polystyrene is synthesized from styrene monomers, typically involving radical polymerization, a type of chain-growth polymerization. (d) Proteins are formed by linking amino acids through peptide bonds, another form of step-growth polymerization.

Step 3: Identifying the Chain-Growth Polymer

From the analysis, only polystyrene (option c) involves chain-growth polymerization, as it is created by the polymerization of styrene monomers where the forming chain builds by rapidly adding monomer units.

Key Concepts

Polystyrene

Polystyrene is one of the most common types of plastics used today and is a chain-growth polymer. It is made from the monomer styrene, a liquid hydrocarbon that is industrially produced from petroleum and natural gas by-products.
Styrene monomers link together in a chain in a process called polymerization.

• Polystyrene can be produced as both solid and foam material.
• It is highly valued for its ease of production and cost-effectiveness.

The solid form is used in products such as plastic utensils and the casing for electronic appliances, while the foam form, often called Styrofoam, is used in insulation and protective packaging materials.
Additionally, polystyrene is favored for being recyclable, although the recycling process can be complex compared to other plastics.

Radical Polymerization

Radical polymerization is a vital method for the creation of many everyday plastics, including polystyrene. In this process, polymerization begins when free radicals are introduced to the polymerization system.
These reactive molecules initiate a chain reaction, allowing styrene monomers to link together and form long chains.

• Initiation: Free radicals form, typically using a radical initiator such as benzoyl peroxide.
• Propagation: The radical reacts with a styrene monomer, adding it to the growing polymer chain in a stepwise fashion.
• Termination: The process ends when two radical sites combine, stopping the chain growth reaction.

This method is chosen due to its ability to be conducted under a variety of conditions and to produce polymers with a wide range of properties. Radical polymerization is versatile and highly efficient for creating polymers like polystyrene where the polymer chains can grow quickly and achieve large molecular weights.

Polymerization Mechanism

The polymerization mechanism, particularly in the context of radical polymerization, is an exciting and fundamental concept in polymer chemistry. This process encompasses several phases, each crucial to forming the final polymer.

• Initiation Phase: Starts with the generation of radical sites that become the reactive centers.
• Propagation Phase: The active radical reacts with monomers, consecutively joining them to the growing chain.
• Termination Phase: The process concludes, typically when two radicals combine, effectively capping the chain and removing the radical character.

This mechanism is distinct from step-growth polymerization, where monomers will only join after being partially activated into reactive intermediates. This systematic and continual addition of monomer units in chain-growth polymerization, like in the production of polystyrene, helps in achieving strong and durable synthetic polymers used in countless products ranging from plastic cups to automobile parts.