Another important class of condensation polymers are polyesters.  They arise from the reaction of a carboxylic acid and an alcohol. An example is polyethyleneterephthalate, the common plastic PETE (recycling #1 in the USA):
:n HO-X-OH + n HOC-Y-COH  →  [O-X-OC-Y-C(O)]  +  (2n-1) HO