What is the definition of a polymer?

A polymer is defined as a large molecule composed of repeating structural units, typically connected by covalent chemical bonds. These structural units, known as monomers, undergo a process called polymerization, where they chemically bond together to form long chains or networks that exhibit unique properties.

Polymers can be natural, like rubber and DNA, or synthetic, like nylon and polyethylene. The defining characteristic of a polymer is the presence of these repeated units, which distinguishes them from other materials.

Regarding the other options, a mixture of metals would refer to an alloy, which does not involve the polymerization process. A type of covalent bond describes a molecular interaction rather than a type of material. A composite of multiple materials refers to a material made from two or more constituent materials with different physical or chemical properties, which again is not a polymer but rather a composite structure. Thus, the purity and structure created through polymerization are what make the definition of a polymer accurate and comprehensive.