Trifunctional acids, cross-linked

Collagen cross-links. Besides amide bonds between amino acids in the same a chain, bonds between amino acid side chains of different a chains can form "cross-links". These bonds originate from enzymatically-oxidized side chains of lysine and hydroxylysine residues. The oxidized residues react with other lysine and hydroxylysine residues, forming difunctional products. Reactions of such products with oxidized lysine or hydroxylysine yield trifunctional cross-links (Reiser et al., 1992). 

Alkyds were synthesized by Kienle in the 1920s from trifunctional alcohols and dicar-boxylic acids. Unsaturated oils, called drying oils, were transesterified with the phthalic anhydride in the reaction so that an unsaturated polymer was obtained which could later be reacted producing a cross-linked product. 

RIS theory, in the form appropriate for branched molecules, is used to calculate the mean-square unperturbed radius of gyration, < s2>0, for cross-linked polyglycine, poly(L-alanine),poly(L-proline),poly(i-alanyl-D-alanine),poly(i.-prolyl-L-pro lylglycine),poly(L-prolyl-i.-alanylglycine ,poly(glycyl-L-alanyl-L-pro line), andpoly(L-aianyl-L-alanylgIycine).Thecentral amino acid residue in each polypeptide chain is replaced by the L-cysteinyl residue involved in cross-link formation. Each cross-linked molecule is considered to contain two trifunctional branch points, the a-carbon atoms of the two... 

Phthalic acid and ethylene glycol are both bifunctional. Consequently, only linear polymers are produced from the reaction between these monomers. On the other hand, the reaction between phthalic acid and glycerol leads initially to molecules that are either linear, branched, or both. But since glycerol is trifunctional, cross-linking ultimately takes place between these molecules leading to an insoluble and infusible material. 

If a bifunctional compound (e.g., adipic acid) is condensed with a trifunctional one (e.g., glycerol) in a functional group molar ratio of 1 1, then after a certain time the viscous mass changes into an elastic gel. This transition is sufficiently sharp for it to be possible to speak of a gel point. Immediately after passing the gel point, this gel contains a fraction that is insoluble in all solvents plus a soluble fraction. It is thus only partially cross-linked. As the conversion increases, the soluble fraction becomes progressively smaller. 

Carboxyhc acid-functional PUDs can also be cross-linked with polyaziridines in 2K coatings. For example, a group of patents cover use of trifunctional aziridines, such as trimethylolpropane tris(2-aziridinylpropionate), in 2K coatings for plastics (43). Carboxyhc acid-fimctional PUDs can also be cross-linked with polycarbodiimides (44). 

It is possible to control the degree of cross-linking by the use of trifunctional acids and the molecular weight of the resin. The alkyl resin, called alkyd paint, dries faster than the corresponding oil which is one of its major advantages. 

Another type of cross-linked polyesters can be prepared by using a trifunctional alcohol or trifunctional acid (or both) to give a polymer network. For example, glycerol has been used to form cross-linked polyesters with citric acid and aspartic acid [198,199]. Polymer degradation was found to coincide with total release of a low MW drug (methyldopa). 

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