Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

The monomer of the polymer (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}^{2}\)

Step-by-Step Solution

Answer

The monomer base is (b) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH}_{2}\).

Step 1: Identify the Monomer Structure

Monomers are small molecules that can join together to form a polymer. In these questions, the monomers provided have carbon-carbon double bonds (alkenes), which are a common feature in monomers used in polymerization.

Step 2: Examine Each Option for Suitable Features

Analyze each compound to determine if it has a structure that can lead to polymerization.(a) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCH}_{3}\) is a diene, meaning it has two vinyl groups on each end, allowing it to behave like monomers connecting to each other, forming a polymer.(b) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH}_{2}\) is an alkene, with one vinyl group that can participate in polymerization. This is often considered the simplest form of a monomer, like ethylene turning to polyethylene.(c) \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{C} = \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) has no vinyl groups. Despite having double-bonded carbons, its structure is too bulky to form standard linear chains via polymerization.(d) \(\mathrm{H}_{2} \mathrm{C} = \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) contains two methyl groups on one end, allowing it to potentially form systematic polymer chains.

Step 3: Determine the Correct Option from the Structures

Among the given structures, only (d) has a basic structural foundation typically seen in monomers that can create polymer chains. The radical can easily reach a less hindered end position on \(\mathrm{C} = \mathrm{C}\), providing good propagation conditions for forming polymers. Option (b) is also a common monomer base, particularly straightforward and similar for forming polymers alone or in copolymers.

Step 4: Choose the Simplest Monomer Base for Polymerization

Considering simplicity and availability of sources, the most recognized straightforward option often used in forming polymers could be found in (b) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH}_{2}\), primarily recognized for bearing simplicity for radical chains compared to others.

Key Concepts

Monomer structure

Monomers are the building blocks of polymers, acting like individual links in a long chain. When considering polymer formation, the monomer’s structure is vital. A monomer is typically a small molecule that can undergo polymerization. Polymerization is the process where these small monomer units join together, creating a larger and complex molecule known as a polymer. Understanding the specific arrangement of atoms in a monomer, especially the presence of reactive sites, is key. These sites, often the carbon-carbon double bonds, allow monomers to link, enabling the growth of a polymer chain. Small variations in monomer structure can lead to significant differences in the properties of the resulting polymer.

Carbon-carbon double bonds

The presence of carbon-carbon double bonds is a crucial feature in many monomers used for polymerization. These double bonds, which are characteristic of alkenes, are sites of high reactivity. They allow the bonds to open during the polymerization process, facilitating the linking of monomers. This bond opening occurs because the double bond contains one stronger sigma bond and a weaker pi bond. During polymerization, it is typically the pi bond that is broken, enabling the chain reaction that leads to polymer formation. The reactivity and accessibility of these double bonds are what make certain molecules, like alkenes, particularly suited for forming polymers.

Alkenes

Alkenes are hydrocarbons characterized by carbon-carbon double bonds. This feature is not only critical for their reactivity in chemical reactions but also makes them excellent candidates for polymerization.In a typical alkene polymerization, the double bonds in the monomers open up, allowing them to connect and form long chains. A common example of an alkene used in polymerization is ethylene (\(\mathrm{CH}_2=\mathrm{CH}_2\)), where the polymer polyethylene is derived. Alkenes provide simplicity and efficiency in forming homopolymers or, in some cases, copolymers when combined with other types of monomers.

Diene

Dienes are hydrocarbons with two carbon-carbon double bonds. This structure allows for unique polymerization properties, as each double bond can participate in linking monomers together to form polymers. These double bonds can react with each other or with other types of monomers to create different polymer structures. The presence of two double bonds increases the versatility and reactivity of the diene monomers, which can lead to the formation of crosslinked polymers. One application of dienes in polymerization is in the production of synthetic rubber, where their structure helps in adding elasticity and resilience to the material.

Vinyl groups

Vinyl groups are important structural features found in certain monomers used for polymerization. A vinyl group consists of a carbon atom double-bonded to another carbon atom, with a single bond to a hydrogen atom (\(\mathrm{-CH=CH_2}\)).These groups are commonly part of the monomers that undergo addition polymerization, a process where all the atoms in the vinyl group contribute to the polymer backbone. Vinyl groups are the active sites in many polymerization reactions because their double bonds readily break and form new linkages. Common examples of polymers formed from vinyl monomers include polyvinyl chloride (PVC) and polyvinyl acetate, each displaying unique properties influenced by the presence of the vinyl group.