Functionalized polyesters cross reactivity

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

The hydroxyfunctional hyperbranched polyesters have been characterized with respect to their mechanical and theological properties, both as thermoplastics and in cross-linked networks. The high number of terminal groups in hyperbranched polymers has a large impact on the properties, and also makes it easy to functionalize the polymers for various applications. One option is to attach reactive groups at chain ends, forming a cross-linkable polymer. Variations in functionality and the type of functional groups will affect both the polymer properties and the final cross-linked material properties. 

A large number of halogen-containing reactive diols, polyols, anhydrides, and other functional groups containing intermediates have been used to produce flame-resistant unsaturated polyester resins. Flame-retardant polyester resins have been made by using broraostyrene as partial replacement of styrene for cross-linking (21). 

Two major developments occurred that overcame the problem. Coating research laboratories developed high-solids low-viscosity coatings based on polyester intermediates or oligomers. Those lower molecular weight polymers have higher functionality. In addition, new melamines were developed to cross-link the more reactive systems (37). 

As stated in Section 2.4, the condensation reaction may be the basis for the mechanism of stepwise polymerization. As long as the functionality equals 2 (di-acid and di-alcohol in polyester or di-amine in polyamide), only linear chains are obtained. However, once polyfunctionality prevails (appearance of tri-alcohol like glycerol, or tri-acid), reactive branches are formed that may interact and lead to a three-dimensional structure, called cross-linked (gelation). This is the basis for thermosetting polymers on one hand, or for stabilizing the elastomeric chain on the other hand (replacing vulcanization). 

In covalent immobilization of biomolecules onto the fibers, chemical modifications are made in electrospun polyester in order to produce reactive functional groups in its chain. Primary amine and carboxylate are chemical groups frequently used as intermediates of reaction. Through this strategy, the amino or carboxyl groups present on biomolecules are cross-linked to free carboxyl or amino groups on activated electrospun polyesters. The l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and M-hydroxysuccinimide (NHS) are the most used intermediary reagents in activation reactions of polyesters. 

The hydroxy-functional polyurethane dispersions can be cross-linked with MF resins or blocked isocyanates. Blocked isocyanates can also be used with water-reducible anionic acrylic or polyester resins instead of an MF resin oxime blocked isocyanates are commonly used, owing to their high reactivity. Oxime blocked aliphatic isocyanates are more stable to hydrolysis than oxime blocked aromatic isocyanates (32). As mentioned earlier alcohol blocked isocyanates are used in cationic electrodeposition coatings. 

Linear unsaturated polyesters are prepared commercially by the reaction of a saturated diol with a mixture of an unsaturated dibasic acid and a modifying dibasic acid (or corresponding anhydrides). In principle, unsaturation desired in a polyester can be derived from either an unsaturated diol or an unsaturated acid for economic reasons the latter is invariably preferred. As mentioned previously, the unsaturated acid provides sites for subsequent cross-linking the function of the modifying acid is to reduce the number of reactive unsaturated sites along the polymer and hence to reduce the cross-link intensity and brittleness of the final product. Some acids and anhydrides which are used to modify polyesters are, in fact, unsaturated but the double bonds are not sufficiently reactive to represent sites for subsequent cross-linking. 

---