Plastic is the most versatile and widely used synthetic material on the market today. They are polymers, that is, large molecules (molecular weight ~10,000+) which consist of long repeating chains of smaller molecules (molecular weight ~100) known as monomers. Common monomers used in the production of plastics, such as vinyl acetate, styrene, butadiene and vinyl chloride, are extracted from crude oil or natural gas. Monomers fall into the class of chemicals known as petrochemicals.
Plastics are produced using a process called polymerization, where many thousands of monomers are joined together to form a polymer chain. Monomers are made of atoms like chlorine, nitrogen, oxygen, hydrogen and sulphur. Monomers are easily extracted from abundant organic sources, though as the price of crude oil increases, so does the price of plastics.
There are two main types of plastic; thermosetting plastics and thermoplastics. Both are produced by pouring liquid monomers into a mold where they undergo polymerization. Thermosetting plastics are permanent once molded — they do not deform under heat. Thermoplastics will melt under heat and can be reformed repeatedly. The polymerization process increases the molecular weight of the constituent molecules, turning liquid monomers into solid polymers. (Increase of molecular weight causes the substance to undergo a phase change.)
During the molding process, the monomer soup is superheated and condensed under many atmospheres-worth of pressure. This causes polymerization to occur and ensures that the plastic is solid and uniform, lacking any internal air bubbles (a big problem which had to be circumvented in the early days of plastics). The result is a finished plastic product. Well-known plastics include Bakelite®, nylon, PVC, synthetic rubber, acrylic, polyethylene (Tupperware®), Formica®, and Teflon®. The downside of their plastics is their impact on the environment. They take many thousands (or even millions) of years to biodegrade. In recent decades, the recycling of plastics has been encouraged, though only about 5% of plastics in the US today are actually recycled.