what is the monomer of sucrose
It's not a monomere but a dimere, because it is made up of two (di) mono's. The sucrose or saccharose molecule is a junction of a glucose and a fructose monomere: I would hardly call this a poly-mere (poly=many). Usually these are called di-saccharides, there are also tri-saccharides (=three), oligo-saccharides (=a few).