Problem 55 - Chemistry and Chemical Reactivity

Question

Saran is a copolymer of 1,1 -dichloroethene and chloroethene (vinyl chloride). Draw a possible structure for this polymer.

Step-by-Step Solution

Answer

The polymer consists of repeating units of alternating 1,1-dichloroethene and chloroethene: \[-[CH_2-C(Cl)_2-CH_2-CHCl]-_n\].

Step 1: Understanding the Monomers

The copolymer Saran is made of two monomers: 1,1-dichloroethene (also known as vinylidene chloride) and chloroethene (vinyl chloride). We need to begin by understanding the structure of these monomers. 1,1-dichloroethene has the structure \[CH_2=C(Cl)_2\]and chloroethene has the structure \[CH_2=CHCl\].

Step 2: Identifying the Copolymerization process

Copolymers are formed by the chemical reaction of repeating different monomer units. In this case, 1,1-dichloroethene and chloroethene will form a polymer chain through addition polymerization, typically involving free-radical or ionic mechanisms.

Step 3: Drawing the Copolymer Structure

The polymer structure can be visualized as a repeat unit where monomers alternate or are distributed randomly. A possible simple structure for the copolymer can show alternating monomers for clarity:\[-[CH_2-C(Cl)_2-CH_2-CHCl]-_n\]This represents a polymer structure where the vinyl chloride and vinylidene chloride repeat along the polymer chain. Subscript \(n\) denotes that this unit repeats multiple times to create the polymer chain.

Step 4: Highlighting the Structural Features

In this structure, notice the presence of the repeating single bonds (")") within the polymer chain and the occasional double chloride substitution linked to the carbon backbone in \(CH_2-C(Cl)_2\) units. These features explain the gas-barrier properties of Saran as a material.

Key Concepts

1,1-dichloroethene

1,1-dichloroethene, also known as vinylidene chloride, is a small organic molecule. Its structure is characterized by a double bond between two carbon atoms, where one carbon atom is bonded to two chlorine atoms. This unique setup can be represented as \(CH_2=C(Cl)_2\).
The presence of two chlorine atoms gives it important chemical and physical properties. These chlorines can influence the polymerization process and the final material characteristics. Keep in mind:

• The molecule's symmetry due to the chlorines can affect how it reacts in polymer chains.
• Vinylidene chloride often acts as a strong barrier against gases, which is valuable for materials like Saran wrap.
• It can readily undergo polymerization due to the presence of a double bond.

Chloroethene

Chloroethene, commonly referred to as vinyl chloride, is another important organic molecule in the creation of copolymers. Its structure features a double bond between two carbon atoms, with one of the carbons also bonded to a single chlorine atom: \(CH_2=CHCl\).
Vinyl chloride is simpler than vinylidene chloride, with only one chlorine atom, but it still plays a crucial role in polymer chemistry:

• This single chlorine atom impacts the behavior of the molecule in the polymer creation process.
• The double bond in chloroethene allows for easy participation in polymerization reactions.
• It contributes flexibility and provides unique properties to the polymer, especially when used in combination with vinylidene chloride.

Addition Polymerization

Addition polymerization is a fundamental process used to create polymers from monomers like 1,1-dichloroethene and chloroethene. This method involves the combination of monomers without the formation of by-products, forming long chains or networks.
Here's how it typically works:

• Monomers containing double bonds, such as 1,1-dichloroethene and chloroethene, are ideal candidates since their double bonds are reactive sites.
• The process can be initiated by free radicals or ionic mechanisms, which break these double bonds to form new linkages.
• Through addition polymerization, the monomers create a repeating unit structure, represented as \( -[CH_2-C(Cl)_2-CH_2-CHCl]-_n \), where \(n\) indicates a large number of repeating units.

If you consider the physical properties of the resulting polymer, you'll see that they derive directly from this process. The repetition of the monomer units leads to unique thermal, mechanical, and barrier properties in products like Saran wrap.