Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is correct about polymers? (1) polymers are substances made up of recurring structural units, each of which can be regarded as derived from a specific compound called a monomer (2) polymers made from a single monomeric species are called homopolymers, whereas, those made up of two or more different monomeric species are called copolymers. (3) very often, polymers are macromolecules, and a sample of a given polymer is characteristically a mixture of molecules with different molecular weights (a) 1 and 2 only (b) 1 and 3 only (c) 1,2 and 3 (d) 2 only

Step-by-Step Solution

Answer

The correct answer is (c) 1, 2, and 3.

Step 1: Understanding Polymer Basics

A polymer is defined as a large molecule composed of repeating structural units, typically connected by covalent chemical bonds. These repeating units are derived from molecules known as monomers. Each of the options in the list describes different aspects of polymers. We need to evaluate if each statement accurately reflects the properties of polymers.

Step 2: Evaluating Statement (1)

Statement (1) states that polymers are made from recurring structural units derived from monomers. This is true, as it is the fundamental definition of polymers. The recurring units, called monomers, link together to form a polymer chain. Thus, statement (1) is correct.

Step 3: Evaluating Statement (2)

Statement (2) details the definitions of homopolymers and copolymers. Homopolymers are made from repeating units of a single type of monomer, while copolymers are made from two or more different types of monomers. This classification is accurate according to polymer chemistry, so statement (2) is correct.

Step 4: Evaluating Statement (3)

Statement (3) states that polymers often consist of macromolecules and that a sample consists of molecules with different molecular weights. This is true, as polymers can have lengthy and complex structures contributing to varied molecular weights within a sample. Statement (3) accurately describes another characteristic feature of polymers.

Step 5: Determining the Correct Answer

Both statements (1), (2), and (3) are true based on the evaluations in Steps 2, 3, and 4. Therefore, the correct answer which includes all true statements is option (c) 1, 2, and 3.

Key Concepts

Monomers and Polymers

In polymer chemistry, understanding monomers and polymers is fundamental. Monomers are small, basic molecules that serve as the building blocks for polymers. Imagine monomers as single beads in a necklace; when they join together in long, repeating sequences, they form polymers, much like a completed string of beads forms a necklace.
This joining of monomers usually occurs through chemical reactions, most commonly by covalent bonds. The term 'polymer' itself underscores its structure — 'poly' means many and 'mer' stands for parts. So, polymers are essentially large molecules made up of many parts, or monomers.
Polymers are not limited to a single type; they can be linear, branched, or even cross-linked, depending on how the monomers are arranged and connected. Each polymer carries unique properties, which makes them extremely versatile in various fields such as packaging, textiles, and even medicine. By choosing different monomers and structural arrangements, scientists can tailor polymers for specific applications.

Homopolymers and Copolymers

Polymers can be classified based on the variety of monomers involved into homopolymers and copolymers. - **Homopolymers**: These polymers are made from a single type of monomer. Consider a polymer chain in which every link or bead is identical, just like a necklace made entirely of the same type of bead. Classic examples include polyethylene, wherein each repeating unit is derived from the ethylene monomer. - **Copolymers**: These are composed of two or more different types of monomers. Think of a necklace made from different beads arranged in a particular pattern. These make copolymers more versatile since by adjusting the types and proportions of monomers, a wide range of properties can be achieved. Variations of copolymers include random, alternating, block, and graft copolymers depending on the sequence and arrangement of the monomers. Homopolymers tend to underline simplicity in their uniformity, while copolymers offer the chance for complexity and customization, allowing scientists to achieve desired properties for specific uses.

Molecular Weight of Polymers

The molecular weight of polymers is a key concept that highlights the unique characteristics of these substances. Unlike small molecules that have a consistent molecular weight, polymers are usually mixtures of molecules with varying sizes.
This happens because polymerization — the process of stringing monomers into polymers — can yield chains of different lengths. As a result, even a single polymer sample may contain chains that range from thousands to millions of monomer units long. To describe this variety, scientists use measurements such as:- **Number Averaged Molecular Weight ( \(M_n\) )**: An average obtained by dividing the total weight of all molecules by the number of molecules present. This gives an idea of the average size of a polymer's constituent chains.- **Weight Averaged Molecular Weight ( \(M_w\) )**: This considers the size distribution and weights longer chains more heavily, providing insight into how larger chains contribute more significantly to the polymer's mass properties.Understanding the molecular weight is crucial as it influences the polymer's physical properties, such as strength, toughness, and melting temperature. A greater molecular weight often means stronger and more durable material.