In polymer chemistry, a comonomer refers to a polymerizable precursor to a copolymer aside from the principal monomer.  In some cases, only small amounts of a comonomer are employed, in other cases substantial amounts of comonomers are used.  Furthermore, in some cases, the comonomers are statistically incorporated within the polymer chain, whereas in other cases, they aggregate.  The distribution of comonomers is referred to as the "blockiness" of a copolymer.