Unsaturated polyesters are produced from Propylene glycol and Maleic anhydride. Some specialised resins that need high chemical resistance use Fumeric acid.
Other acids in conjunction with the unsaturated Maleic type to prevent resins from being too reactive. Ortho-phthalic anhydride is most common for general purpose formulations. Iso-phthalic acid is being used where better chemical resistance is required. Adipic acid is used where flexibility is required. Halogenated acid (tetra bromo phthalic anhydride) can be used to produce reduced flammability in mouldings.
Other glycols like Dipropylene glycol, Diethylene glycol give some degree of flexibility. Neopentyl glycol offers better chemical resistance.
Methylmethacrylate can be used as part replacement for Styrene in the monomer portion of the resin.
Unsaturated polyester resin is used by fibre-glass industry to build boats and car bodies, for encapsulating electrical components. There is huge production of synthetic marble and buttons manufacturing with UPR.
In all of these applications the UPR is poured into a mould and a free radical initiator such as MEKP (methyl ethyl ketone peroxide) or benzoyl peroxide added to initiate cross-linking. In general a release agent is necessary to apply on mould before pouring resin.