The vast majority of materials engineered by humans originate from substances found in the natural environment. A material is classified as synthetic when its natural resource starting material is fundamentally altered through a chemical process to create a new substance with different properties. This contrasts with natural materials, such as wood or cotton, which are used largely in their original chemical state. Synthetic material production involves rearranging the atoms and molecules of the raw resource, transforming simple compounds into complex materials like plastics, synthetic fibers, and specialized medicines.
Primary Natural Resources as Chemical Feedstocks
The creation of synthetic materials begins with selecting a natural resource capable of providing the necessary molecular architecture. These raw substances, known as chemical feedstocks, fall into three primary categories: fossil fuels, minerals, and biomass. Fossil fuels (crude oil, natural gas, and coal) are hydrocarbons that serve as the foundational source for most synthetic polymers globally, providing simple carbon-based molecules manipulated through industrial chemistry.
Minerals like silica and bauxite are also used as feedstocks for materials such as glass and aluminum alloys, respectively. Biomass, which includes plant-derived sources like wood, cotton, corn starch, and sugarcane, offers a renewable alternative, providing compounds like cellulose and sugars. These raw materials must first be refined and broken down into simpler chemical precursors to isolate the basic building blocks required for subsequent chemical reactions.
The Core Chemical Process: From Monomers to Polymers
The foundational mechanism for creating most synthetic materials involves linking small molecules into very large ones, a process known as polymerization. The small, reactive molecules that serve as the fundamental units are called monomers. These monomers, which are derived from the natural feedstocks, are engineered to have specific chemical structures that allow them to form multiple covalent bonds. Polymerization is the chemical reaction where thousands of these monomer units join together sequentially to form a long-chain molecule called a polymer or macromolecule.
There are two primary ways this linking occurs, beginning with addition polymerization, also known as chain-growth polymerization. In this mechanism, monomers link end-to-end without losing any atoms from the original molecules, typically involving a chain reaction initiated by a reactive species like a free radical. The simplest example is the reaction of ethylene monomers, which possess a carbon-carbon double bond, linking up to form the long-chain polymer polyethylene. This process essentially adds one monomer to the growing chain at a time, extending the polymer length.
The second type is condensation polymerization, or step-growth polymerization, which involves two different types of functional monomers reacting together. As the two monomers link to form a new bond, a small molecule, most commonly water, is released as a byproduct. This mechanism is instrumental in creating materials like nylon and polyesters, where the loss of the small molecule drives the reaction forward and creates the repeating unit of the polymer chain. The final properties of the synthetic material, such as its flexibility, strength, or elasticity, are determined by the specific monomers used and the arrangement of these polymer chains.
Synthetic Materials Derived from Petrochemical Sources
The majority of modern synthetic materials are built upon petrochemicals derived from petroleum, natural gas, and coal. The process begins with the initial refining of crude oil, a complex mixture of various hydrocarbons. Fractional distillation separates the crude oil into different fractions based on their boiling points, such as gasoline, diesel, and naphtha. Since these fractions are too large for direct polymerization, they require further breakdown into simpler monomer precursors.
A process called cracking, either thermal or catalytic, breaks the larger hydrocarbon molecules in the naphtha fraction into smaller, more reactive molecules. This step yields the fundamental building blocks of the plastics industry, primarily olefins like ethylene and propylene, and aromatics like benzene and xylene. Ethylene, derived from the cracking of natural gas liquids like ethane, is the precursor for polyethylene, the world’s most common plastic, used in everything from packaging to piping. Propylene is similarly used to create polypropylene, which finds application in textiles, automotive components, and containers.
Nylon, one of the earliest synthetic fibers, is produced from petrochemical-derived aromatic compounds. The specific monomers needed for nylon, such as caprolactam or adipic acid, are created through complex chemical pathways starting from benzene or other fossil fuel intermediates. These precursors are then subjected to condensation polymerization to form the long polyamide chains that give nylon its characteristic strength and durability.
Synthetic Materials Derived from Plant-Based Resources
Renewable natural resources, primarily biomass, are increasingly used to create synthetic materials, often providing a more sustainable alternative to petrochemicals. The most common plant-based feedstock is cellulose, a large natural polymer found abundantly in wood pulp and cotton. Unlike the process for petrochemicals, which builds polymers from scratch, many cellulose-based synthetics involve chemically modifying the existing natural polymer structure.
Rayon, for instance, is a regenerated cellulose fiber where wood pulp is dissolved in a chemical bath and then extruded through tiny holes, re-forming the cellulose into continuous, smooth fibers. This chemical modification process breaks and reforms some of the bonds in the cellulose, resulting in a new material with improved textile properties. Another example is cellophane, which is a thin, transparent film created through a similar chemical treatment of wood cellulose.
Newer synthetic materials are developed from simple plant-derived compounds like sugars and starches, which can be fermented or chemically converted into bio-based monomers. This process yields precursors such as bio-ethylene and lactic acid, which can then be polymerized to form bioplastics like polylactic acid (PLA). These materials are chemically identical or functionally similar to their petroleum-derived counterparts but originate from a renewable resource, representing a significant shift in feedstock sourcing for synthetic chemistry.