Plastic is a synthetic material used in countless products, from packaging to construction components. However, it is not a single substance that can be described with one chemical formula. The term “plastic” describes a massive family of materials, all classified as polymers. Because the chemical composition and structure vary widely, understanding the fundamental chemistry of plastics requires breaking down the concept of polymers.
Understanding Polymers and Repeating Units
The lack of a single formula stems from plastic’s structure as a polymer, a term derived from the Greek words poly (many) and meros (parts). Unlike simple compounds, such as water (\(\text{H}_2\text{O}\)) or salt (\(\text{NaCl}\)), polymers are giant molecules, or macromolecules. They are built from thousands of small, identical units chemically linked together in long chains.
These chains consist of a constitutional repeating unit, the smallest segment that, when repeated, forms the polymer chain. The process of linking these units is called polymerization. Because the number of repeating units in any plastic molecule is a variable, massive quantity, the chemical formula is always represented with a subscript ‘n’.
The subscript ‘n’ indicates that the repeating unit is present an indeterminate and very large number of times, typically ranging from thousands to hundreds of thousands of units. This variable quantity means a simple, fixed chemical formula for the entire molecule cannot be written. The value of ‘n’ determines the final physical properties of the plastic, such as its strength, flexibility, and melting point.
Core Monomers The Building Blocks of Plastic
The constitutional repeating unit is derived from an individual small molecule called a monomer. The monomer acts as the initial building block for a specific plastic. Different monomers have distinct chemical structures, and the choice of monomer fundamentally determines the properties of the resulting polymer.
Four foundational monomers produce many common commercial plastics. For example, the gas ethylene is the monomer that creates polyethylene, known for its toughness and flexibility. Propylene is the monomer used to form polypropylene, often chosen for its heat resistance in bottle caps and microwavable containers.
Other monomers include vinyl chloride, the precursor for polyvinyl chloride, and styrene, which is polymerized to make polystyrene. These monomers are simple organic molecules, often derived from petrochemicals through cracking. Subtle differences in the chemical structure of each monomer are amplified when linked into long chains, yielding plastics with vastly different characteristics.
Formulas for Common Commercial Plastics
To illustrate this concept, we examine the general formulas for two widely produced plastics: Polyethylene (PE) and Polyvinyl Chloride (PVC). Polyethylene is formed by polymerizing the ethylene monomer (\(\text{C}_2\text{H}_4\)). The resulting repeating unit is \(\text{CH}_2-\text{CH}_2\), and the general chemical formula is \(\text{(C}_2\text{H}_4\text{)}_n\).
Polyethylene’s structure is a simple chain of carbon atoms, each attached to two hydrogen atoms. This uncomplicated structure makes PE a versatile plastic, used widely in plastic bags and bottles. The ‘n’ subscript represents the degree of polymerization, resulting in long chains that provide the material its strength.
Polyvinyl Chloride (PVC) offers a clear example of how a small change in the monomer alters the final plastic. PVC is made from the vinyl chloride monomer (\(\text{C}_2\text{H}_3\text{Cl}\)), which is nearly identical to ethylene but has one hydrogen atom replaced by a chlorine atom (\(\text{Cl}\)). The repeating unit for PVC is \(\text{CH}_2-\text{CHCl}\), and its general formula is \(\text{(C}_2\text{H}_3\text{Cl}\text{)}_n\).
The addition of that single chlorine atom gives PVC significantly different properties than PE, making it dense, rigid, and fire-resistant. This difference is why PVC is used for durable construction materials like pipes and window frames, while polyethylene is used for flexible packaging. Ultimately, the chemical formula for any plastic must include the formula of its specific repeating unit, enclosed in parentheses, followed by the subscript ‘n’.