Problem 10 - Introductory Chemistry Atoms First

Question

In the polymer polyethylene, there are no double bonds. Why, then, is it called polyethylene?

Step-by-Step Solution

Answer

Polyethylene is called so despite no presence of double bonds because it is made up of repeating ethylene units. During the polymerization process, the carbon-carbon double bond of ethylene (C₂H₄) monomers breaks, and forms single bonds with other ethylene molecules, creating a long chain-like structure. The name "polyethylene" is derived from the monomer "ethylene" and the prefix "poly-", which denotes the presence of multiple ethylene units in the polymer structure. The definition of polyethylene refers to its origin and composition of ethylene monomers rather than the presence of double bonds in the final polymer structure.

Step 1: Ethylene Monomer Structure

Ethylene (C₂H₄) is an organic compound that consists of two carbon atoms connected by a double bond and each carbon atom is connected to 2 hydrogen atoms. Its chemical structure can be represented as follows: \[ \mathrm{H_2C=CH_2} \]

Step 2: Polymerization of Ethylene

Polyethylene is formed by polymerization of ethylene monomers. During the polymerization process, the carbon-carbon double bond of ethylene molecules breaks and forms single bonds with other ethylene molecules, creating a long chain-like structure composed of repeating ethylene units.

Step 3: Structure of Polyethylene

After the polymerization process, the resulting polyethylene structure consists of a backbone made up of carbon atoms, which are each connected to two hydrogen atoms. The structure can be represented as follows: \[ \mathrm{-(CH_2-CH_2)_n-} \] Where 'n' denotes the number of repeating ethylene units in the polymer chain.

Step 4: Naming of Polyethylene

Despite the fact that there are no double bonds present in the polyethylene polymer, it is still called polyethylene because it is made up of repeating ethylene units. The name "polyethylene" is derived from the monomer "ethylene" (C₂H₄) and the prefix "poly-", which denotes the presence of multiple ethylene units in the polymer structure. The definition of polyethylene, therefore, refers to its origin and composition of ethylene monomers rather than the presence of double bonds in the final polymer structure.

Key Concepts

Ethylene Monomer

The journey of understanding polyethylene begins with its building block, the ethylene monomer. Ethylene, or ethene, is a simple organic compound with the formula \(\mathrm{C_2H_4}\). It’s the simplest alkene—a hydrocarbon with a carbon-carbon double bond. Ethylene's structure is composed of two carbon atoms joined by a double bond, with each carbon also bonded to two hydrogen atoms. This configuration is represented by the chemical formula \(\mathrm{H_2C=CH_2}\).
Through a fascinating transformation in the polymerization process, these double bonds are key to creating the polymer's chain-like structure. Despite its ultimate conversion to a different bonding structure in the final polymer, the reference to ethylene in the name 'polyethylene' recognizes this molecule’s pivotal role.

Polymerization Process

The polymerization process is a chemical reaction where small molecules, known as monomers, bind together to form a long repeating chain, becoming a polymer. In the case of polyethylene, this process is particularly interesting. The double bonds in the ethylene monomers are broken, and the carbons form new single bonds with other carbons from neighboring ethylene units.
This connection of many ethylene units forms the durable and versatile polymer known as polyethylene. The process requires specific conditions, like high pressure and the presence of a catalyst, to proceed. The result is a transformation from a small, doubly-bonded molecule to a large, stable chain without double bonds that characterizes many everyday materials, from plastic bottles to grocery bags.

Chemical Structure of Polymers

Diving into the chemical structure of polymers like polyethylene unveils a world of strong, flexible materials shaped by their molecular architecture. Polyethylene consists of a carbon atom backbone with two hydrogen atoms attached to each carbon. This can be represented as \(\mathrm{-(CH_2-CH_2)_n-}\), where 'n' is the degree of polymerization or the number of repeating ethylene units.
This seemingly simple structure is what imparts polyethylene its appealing characteristics: it's lightweight, resistant to chemicals, and an excellent insulator. The lack of double bonds in the finished product contributes to its stability since double bonds are typically more reactive than single bonds. Understanding the structure is critical for grasping how properties like tensile strength or durability arise in polyethylene and similar polymers, underpinning countless applications in multiple industries.