What do you call a short chain of monomers that has not yet become a polymer?

The term that describes a short chain of monomers that has not yet become a polymer is "oligomer." An oligomer consists of a few repeating units—typically between two and several dozen monomers—linked together. Unlike polymers, which are often long chains formed by many repeating monomers, oligomers do not reach the extensive lengths and comprehensive properties characteristic of full polymers.

Oligomers can serve as intermediates in the formation of polymers or can have specific properties that make them useful in various applications, such as adhesives or coatings. The other terms refer to different concepts: a polymer is made up of long chains of monomers, a monomer is the basic building block that combines to form larger molecules, and a copolymer is a polymer made from two or more different types of monomers. Each of these plays a distinct role in the chemistry of materials, but in the case of a short chain that has not yet become a polymer, oligomer is the correct classification.