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A homopolymer is a type of polymer that has only one repeating unit. Homopolymers can be broken apart into smaller pieces of one chemical species, which may or may not be the physical monomer used to make the polymer. The four most used plastics (polyethylene, polyvinyl chloride, polypropylene, and polyethylene terephthalate) are all homopolymers.
A homopolymer is defined based on its structure, not how that structure is made. Polyethylene terephthalate is made from two "real monomers", terephthalic acid and ethylene glycol, but these compounds always react in an alternating pattern, making an "implicit monomer" that repeats in the structure. These polymers are copolymers because they are made of multiple real chemical species, but homopolymers because the final structure can be broken down into copies of juts one piece.
Polymers made by chemical changes of another polymer after polymerisation have "hypothetical monomers". One example is polyvinyl alcohol: the hypothetical monomer is vinyl alcohol, but this enol is too unstable to polymerise. The polymer must be made by hydrolysis of polyvinyl acetate.
