Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the order of decreasing reactivity of the following monomers towards anionic polymerization? (1) \(\mathrm{CH}_{2}=\mathrm{CHCN}\) (2) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\) (a) \(1>2>3\) (b) \(3>2>1\) (c) \(2>3>1\) (d) \(3>1>2\)

Step-by-Step Solution

Answer

The correct order is 1 > 3 > 2, aligning most closely with option (d).

Step 1: Identify Electron Withdrawing Groups

In anionic polymerization, monomers with strong electron-withdrawing groups tend to be more reactive because they stabilize the negative charge on the growing polymer chain. Examine the monomers: 1. CH₂=CHCN has a nitrile group (CN), which is a strong electron-withdrawing group. 2. CH₃CH=CH₂ lacks significant electron-withdrawing groups. 3. C₆H₅CH=CH₂ has a phenyl group, which has some electron withdrawal through resonance but is less effective than nitrile.

Step 2: Determine Reactivity Based on Electron Withdrawing Capability

Since monomers with stronger electron-withdrawing groups will react more rapidly, predict the reactivity order based on the groups: - CH₂=CHCN (with a CN group) is likely the most reactive. - C₆H₅CH=CH₂ (with a phenyl group) is expected to be less reactive than CH₂=CHCN but more than CH₃CH=CH₂. - CH₃CH=CH₂, lacking strong electron-withdrawing groups, is predicted to be the least reactive.

Step 3: Compare with Given Options

Now, compare the order from Step 2 to the provided answer choices: - (a) 1 > 2 > 3 (b) 3 > 2 > 1 (c) 2 > 3 > 1 (d) 3 > 1 > 2 The predicted order based on electron-withdrawing ability is 1 > 3 > 2. However, none of the options match this precisely, so reconsider the influence of phenyl vs. alkene: Revised assumption: 1 > 3 > 2 (d) might be more applicable.

Key Concepts

Monomer Reactivity

In polymer chemistry, understanding the reactivity of monomers is essential for predicting and controlling the polymerization process. Monomer reactivity in anionic polymerization can be influenced by several factors, including the presence of electron-withdrawing groups. Monomer reactivity is the ability of a monomer to participate in polymerization reactions. Reactive monomers tend to form polymers quickly under suitable conditions. In anionic polymerization, monomers can react differently depending on the substituents attached to them. This is because particular functional groups can either stabilize or destabilize the negative charge on the growing polymer chain. Knowing the order of reactivity helps in choosing monomers for desired polymer properties. It's essential to assess each monomer's substituents and how they could influence electron flow and charge distribution. By analyzing functional groups, scientists can predict how fast or slow a particular monomer will polymerize in various conditions.

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) play a crucial role in anionic polymerization by stabilizing the negative charge on the carbon atom of the growing polymer chain. Let's unpack why these groups increase monomer reactivity. When a monomer contains an electron-withdrawing group, such as a nitrile group (CN) or a phenyl group (C₆H₅), these groups can pull electron density away from the negatively charged carbon atom. This leads to a more stabilized intermediate during polymerization.

Types of Electron Withdrawing Groups

• Nitrile groups (CN): Very strong electron-withdrawing by induction. This makes monomers like acrylonitrile highly reactive in anionic polymerizations.
• Phenyl groups (C₆H₅): Moderate electron withdrawal through resonance. Although not as strong as nitrile groups, phenyl groups still increase reactivity compared to non-electron-withdrawing substituents.

Recognizing these groups in monomers helps predict and control reactivity and thus optimize the polymerization process.

Polymer Chemistry

Polymer chemistry involves the study of large molecules—polymers—and their synthesis, structure, properties, and applications. Anionic polymerization is a type of chain-growth polymerization where a negatively charged initiator adds to a monomer, propagating a reaction and forming a polymer.

Key Polymer Chemistry Concepts

• Chain-Growth Mechanism: Monomers add one by one to the active site of a growing polymer chain, usually at a rapid rate once initiated.
• Control of Molecular Weight: The reaction conditions, including the type of monomer and initiator, can significantly influence the molecular weight and distribution of the resulting polymer.
• Applications: Understanding and optimizing anionic polymerization has practical applications in making materials ranging from soft elastomers to hard plastics, depending on the selected monomer and reaction conditions.

Given the strategic role of monomers and electron-withdrawing groups, scientists tailor polymers for specific needs by manipulating the chemistry during synthesis. This highlights the importance of reactivity insights and understanding polymer chemistry fundamentals for advanced material development.