Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is not an example of chain growth polymer? (a) neoprene (b) Buna-S (c) PMMA (d) glyptal

Step-by-Step Solution

Answer

(d) Glyptal is not a chain growth polymer, it's a step-growth polymer.

Step 1: Understanding Chain Growth Polymers

Chain growth polymers are formed by the polymerization of monomers with unsaturated bonds, usually involving vinyl groups. This polymerization process begins with the opening of the double bond of a monomer, creating a reactive site that leads to a chain reaction, where monomers add to the growing chain one at a time.

Step 2: Analyzing the Options

Let's analyze the given options one by one: - (a) Neoprene is derived from chloroprene (a vinyl compound) and is made via chain growth polymerization. - (b) Buna-S, or styrene-butadiene rubber, is produced by chain growth polymerization of styrene and butadiene. - (c) PMMA, or polymethyl methacrylate, is made by the polymerization of methyl methacrylate, a compound with a vinyl group. - (d) Glyptal is formed from polycondensation of phthalic anhydride and glycerol, which involves step-growth polymerization, not chain growth.

Step 3: Identifying the Odd One Out

Chain growth polymerization applies to options (a), (b), and (c), all of which involve vinyl monomer units leading to large polymer chains. However, option (d) glyptal is formed through a condensation reaction (a step-growth process) rather than a chain growth mechanism, making it different from the others.

Key Concepts

Vinyl Polymers

Vinyl polymers are a type of polymer that are created from monomers featuring at least one vinyl group (a carbon-carbon double bond). This bond is key to the polymerization process because it is highly reactive. When this bond is broken, it initiates a chain reaction that links multiple monomer units together, forming long polymer chains. Vinyl polymers are very common in many everyday products. Here are some examples:

• Polyvinyl chloride (PVC) – Used in pipes and cables
• Polystyrene – Found in foam cups and insulation
• Polymethyl methacrylate (PMMA) – Used as acrylic glass

Understanding vinyl polymers is important because they are a versatile group of materials. Their ability to be molded into different shapes and their durability make them useful across various industries.

Step-Growth Polymerization

Step-growth polymerization is different from chain growth polymerization. Here, polymers are formed through a process that involves reactions between functional groups of monomers, which often leads to the release of small molecules like water or methanol. This type of polymerization doesn't involve initiation at a single chain, but rather, it's more like building a structure one link at a time. Each step may involve monomers, dimers, or even larger oligomers reacting with each other, gradually building up larger molecules. Some characteristics of step-growth polymerization:

• Monomers can react at any stage with other monomers, dimers, or higher oligomers resulting in growth.
• Properties of end-products depend on achieving a high degree of polymerization.
• Typical products include polyesters and polyamides, like nylon and polyester used in textiles.

Polymerization Mechanisms

Polymerization mechanisms describe how monomer molecules join together to form polymers. Understanding these mechanisms is crucial to predicting the properties of the resulting polymer. There are two primary types of polymerization:

• Chain Growth Polymerization: This involves the growth of polymer chains through successive addition of monomer molecules to an active site of a growing chain. It's typically initiated by free radicals, cations, or anions, which start the reaction by opening double bonds of the vinyl monomers.
• Step-Growth Polymerization: Unlike chain growth, this mechanism involves all monomers reacting at any time with other reactive molecules. Each polymer chain is slower to build, starting with smaller units that link together to grow progressively larger.

The choice of polymerization mechanism impacts not only the polymer's molecular weight but also its physical properties. Thus, understanding these mechanisms helps in the synthesis of materials tailored for specific applications.