Well, this is a broad question, so I'm not quite sure what answer you're looking for, but the simplest answer, which used to get me confused, is: the monomer is a repeat unit of the polymer. Let's start by analyzing the words monomer and polymer. Mono = one, and poly = many. Now, lets go back to the science.
History of the United States with 150+ answers. According to one source: Twelve American presidents owned slaves, eight while serving in office, and at least 25 presidents count slave owners among their ancestors.. http://www.slate.com/articles/li... However, to put this into perspective this would be 37 that were either slave owners themselves or descended from slave owner ancestors - or 84%.
Of these, 3,776 free Negroes owned 12,907 slaves, out of a total of 2,009,043 slaves owned in the entire United States, so the numbers of slaves owned by black people over all was quite small by comparison with the number owned by white people.