What do we call a short chain of monomers that is not long enough to be considered a polymer?

The correct answer is "Oligomer." An oligomer is defined as a short chain of monomers that does not reach the length necessary to be classified as a polymer. Typically, polymers are composed of many repeating units and have molecular weights significantly higher than those of oligomers. Oligomers can vary in their composition and can have unique properties that differ from those of full polymers.

Understanding the distinction between oligomers and polymers is important in fields like chemistry and materials science, as it influences the performance and application of various substances. In contrast, the other terms listed do not pertain specifically to the definition of a short chain of monomers. A polymer refers to a larger chain of molecules, a photoinitiator is a substance that initiates the polymerization process when exposed to light, and an adhesive is a material used for bonding substances together. Each of these has its own specific roles that are distinct from the concept of oligomers.