Problem 88 - Chemistry The Central Science

Question

Write the chemical equation that represents the formation of $$ \begin{array}{l}{\text { (a) polychloroprene from chloroprene (polychloroprene is }} \\ {\text { used in highway-pavement seals, expansion joints, con- }} \\\ {\text { veyor belts, and wire and cable jackets) }}\end{array} $$ $$ \begin{array}{c}{\mathrm{CH}_{2}=\mathrm{CH}-\underset{\mathrm{C}}{\mathrm{C}=\mathrm{CH}_{2}}} \\\ {\text { Chloroprene }}\end{array} $$ $$ \begin{array}{l}{\text { (b) polyacrylonitrile from acrylonitrile (polyacrylonitrile }} \\ {\text { is used in home furnishings, craft yarns, clothing, and }} \\ {\text { many other items). }}\end{array} $$ $$ \begin{array}{c}{\mathrm{CH}_{2}=\mathrm{CH}} \\ {\mathrm{I}} \\ {\text { Acrylonitrile }}\end{array} $$

Step-by-Step Solution

Answer

The chemical equations for the formation of the given polymers are: (a) Polychloroprene from chloroprene: \[ \mathrm{n} \left( \mathrm{CH}_{2}=\mathrm{CH}-\underset{\mathrm{C}}{\mathrm{C}=\mathrm{CH}_{2}} \right) \longrightarrow \mathrm{-CH_{2}-CH-}{\underbrace{{\text -} \mathrm{C}-(-\mathrm{CH_{2})_{n}}}_{\text {polychloroprene}}} \] (b) Polyacrylonitrile from acrylonitrile: \[ \mathrm{n} \left( \mathrm{CH}_{2}=\mathrm{CH}{\text -}\mathrm{CN} \right) \longrightarrow \mathrm{-CH_{2}-CH{-}}_{n}\mathrm{CN} \]

Step 1: (a) Identifying the functional group of chloroprene

Chloroprene is shown as: \[ \mathrm{CH}_{2}=\mathrm{CH}-\underset{\mathrm{C}}{\mathrm{C}=\mathrm{CH}_{2}} \] We can see that there is a vdinylic moiety in the.chain

Step 2: (b) Identifying the functional group of acrylonitrile

Acrylonitrile is shown as: \[ \mathrm{CH}_{2}=\mathrm{CH}{\text -}\mathrm{CN} \] It contains a vinyl group and a cyano (nitrile) group. ##Step 2: Determine the polymerization reaction##

Step 3: (a) Polymerization reaction for chloroprene

Chloroprene undergoes addition polymerization via a radical chain-growth mechanism.

Step 4: (b) Polymerization reaction for acrylonitrile

Acrylonitrile also undergoes addition polymerization via a radical chain-growth mechanism. ##Step 3: Writing the chemical equation for polymer formation##

Step 5: (a) Formation of polychloroprene from chloroprene

The chemical equation for the formation of polychloroprene from chloroprene is: \[ \mathrm{n} \left( \mathrm{CH}_{2}=\mathrm{CH}-\underset{\mathrm{C}}{\mathrm{C}=\mathrm{CH}_{2}} \right) \longrightarrow \mathrm{-CH_{2}-CH-}{\underbrace{{\text -} \mathrm{C}-(-\mathrm{CH_{2})_{n}}}_{\text {polychloroprene}}} \]

Step 6: (b) Formation of polyacrylonitrile from acrylonitrile

The chemical equation for the formation of polyacrylonitrile from acrylonitrile is: \[ \mathrm{n} \left( \mathrm{CH}_{2}=\mathrm{CH}{\text -}\mathrm{CN} \right) \longrightarrow \mathrm{-CH_{2}-CH{-}}_{n}\mathrm{CN} \]

Key Concepts

Polychloroprene

Polychloroprene is a type of synthetic rubber, more famously known as Neoprene. It is created via addition polymerization from its monomer, chloroprene. This polymerization process transforms chloroprene, a molecule with carbon-carbon double bonds, into long chain polychloroprene. The basic structure of chloroprene is

• CH₂=CH-
• C=CH₂

Chloroprene’s structure makes it particularly suitable for addition polymerization. During polymerization, the double bonds open up, allowing each monomer to connect to another, forming an extended polymer chain. This process is what creates the elastic and flexible properties of polychloroprene, useful in practical applications like seals, joints, and protective coatings. The resulting polymer aligns and stretches, providing durability and resistance against environmental stressors.

Polyacrylonitrile

Polyacrylonitrile, abbreviated as PAN, is another synthetic polymer produced from the monomer acrylonitrile. Acrylonitrile possesses a vinyl group, indicated by the

• CH₂=CH-

In addition to this, it has a cyano group, represented as CN, which contributes to its unique chemical properties. Polyacrylonitrile is formed through a similar addition polymerization process as polychloroprene, with the monomers joining together forming long fibrous chains. This process eliminates the unsaturation of the double bonds, allowing polymerization to occur effectively.
Polyacrylonitrile is valued for its strong, durable nature and resistance to various chemicals. Consequently, it finds widespread application in fibers for clothing, home textiles, and even outdoor gear due to its resilience.

Radical Chain-Growth Mechanism

Both polychloroprene and polyacrylonitrile are formed through a specific type of addition polymerization known as the radical chain-growth mechanism. This process begins with the generation of a radical, an atom or a molecule that has an unpaired electron which makes it highly reactive. The initiation of this radical is the first step in the polymerization.
During the propagation phase, these radicals attack the carbon-carbon double bonds in the monomer, causing the bond to open and allowing the monomer to attach to the growing polymer chain, forming new radicals in the process. This continuous linkage extends the chain and propagates the reaction across many monomers, eventually forming long polymer chains.
The termination of this mechanism can occur in different ways, such as by radical coupling, where two radical ends join to form a stable, completed polymer chain. This method efficiently produces polymers with high molecular weights, which are essential for the desired physical properties of both polychloroprene and polyacrylonitrile.