(c) Urea-formaldehyde is used to manufacture unbreakable cups and laminated sheets. It is an example of condensation polymer in which water molecule is removed from urea and formaldehyde forming an amide linkage. (a) cis-poly isoprene is natural rubber. (b) Melamine - formaldehyde polymer used in the manufacture of unbreakable crockery. (d) Phenol-formaldehyde resin used as good wood adhesives for plywood and particle board.