Either one of the above radical species can go on and initiate the crosslinking reaction. The cycle is repeated until all the peroxide is decomposed. The crosslinking of an unsaturated linear polyester by means of vinyl monomer such as styrene which undergoes a chain reaction, incorporating the unsaturated polyester as a comonomer. A trans unsaturated polyester is more stable than a cis unsaturated polyester, and the reaction therefore goes at a faster rate for the trans polyester. Glass fiber may be used with unsaturated polyester to form a composite, and the physical properties of the final product depend greatly on the type and quantity of glass fiber used.
Saturated polyesters can serve as plasticizers, or for the preparation of polyurethanes.
Often a nonohydric alcohol or a monocarboxlyic acid is included in the reactants. From this, reactive end groups eliminated and the molecular weight can be controlled. In a typical process, the mixture of reactants is heated at 200 to 250 deg C in an inert atmosphere for several hours with continuous removal of H2O. Polyesters of molecular weight up to 1000 may be prepared to serve as plasticizers. Applications of saturated polyesters - plasticizers which are less volatile, and more resistant to solvent extraction than conventional monomeric plasticizers. The preparation of saturated polyesters involves continuously heating, up to 200 deg C, the mixture of reactants in the presence of a catalyst.