a reactive polyester resin composition comprising a polyester resin having ethylenically unsaturated terminal groups , and a reactive , aliphatic plasticizer compatible with the resin and having ethylenically unsaturated terminal groups . by virtue of the terminal ethylenic unsaturation , the composition can be cross - linked with , e . g ., styrene to form a tough , flexibilized , corrosion resistant polymer . the high level of corrosion resistance is maintained notwithstanding the impressive degree of flexibility imparted to the composition .