It is common for people to use the terms polymer and plastic interchangeably, leading to confusion about their precise scientific relationship. While all plastics are fundamentally composed of polymers, the reverse is not scientifically accurate. The distinction lies in whether the term refers to a chemical structure found throughout nature or a specific type of manufactured material with particular properties.
What Defines a Polymer?
A polymer is formally defined as a substance composed of very large molecules, or macromolecules, built from many repeating subunits called monomers. These monomers link together chemically through a process known as polymerization to form long, chain-like structures. The resulting macromolecule possesses a high relative molecular mass, meaning it is substantially larger than the simple molecules that formed it.
Polymers exist widely in both the natural world and in manufactured materials. In nature, biopolymers perform biological functions, such as cellulose, which forms the structural integrity of plant cell walls and wood. Proteins, built from amino acid monomers, and nucleic acids like DNA and RNA, which store genetic information, are also examples of natural polymers.
Synthetic polymers are human-made, typically derived from petrochemicals and designed for specific uses. Controlling the length and arrangement of the monomer units allows chemists to engineer materials with a wide range of physical properties, including toughness, elasticity, and chemical resistance. Polyethylene, for instance, is formed by linking thousands of ethylene monomers into a long chain.
What Defines Plastic?
Plastic, in contrast to a polymer, is a term used to describe a wide range of synthetic or semi-synthetic materials based primarily on organic polymers. The defining characteristic of plastic is its plasticity, a property that allows the material to be permanently molded, extruded, or pressed into solid objects without breaking. This malleability is achieved by applying heat and pressure during the manufacturing process.
These materials are valued for their adaptability, durability, low density, and resistance to corrosion, making them suitable for widespread industrial use. Common types include polyethylene (PE), used in shopping bags and bottles, and polyvinyl chloride (PVC), used in drain pipes and window frames. Most manufactured plastics are sourced from crude oil or natural gas, which provide the hydrocarbon monomers needed for polymerization.
Plastic materials are often classified by their reaction to heat. Examples include thermoplastics, which can be melted and remolded repeatedly, and thermosets, which harden permanently after initial formation. The specialized chemical structure of the underlying polymer chain dictates these functional properties. The term plastic is applied to a material engineered for a specific physical function, rather than simply describing its chemical composition.
The Essential Difference: Polymers That Aren’t Plastic
The essential difference is that “polymer” is a chemical term describing a molecular structure, while “plastic” is a materials science term describing a specific type of synthetic material. All plastics are polymers because they are structurally built from long molecular chains of repeating units. Conversely, not all polymers qualify as plastic, as they do not meet the functional criteria of being a moldable, manufactured material intended for consumer or industrial use.
Natural polymers provide the clearest examples of this distinction, meeting the chemical definition without the functional classification. For instance, silk fiber is a polypeptide polymer, but it is not considered plastic because it lacks the required synthetic origin and processing. Similarly, cellulose, which makes up cotton and wood, is a polysaccharide polymer, but it is a natural structural component, not a synthetic plastic.
DNA and proteins, which are fundamental to all life, are also polymers that are clearly not plastics. The term plastic is reserved for the subset of polymers that are predominantly synthetic and possess the unique property of being easily formed into a permanent shape. Consequently, the term “polymer” is a broad chemical category, and “plastic” refers to a specific application of a polymer.