Trifunctional alcohols, cross-linked

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

One way of obtaining the more useful cross-linked polyurethanes is by using a trifunctional reagent. Thus either the TDI can react with a triol or the propylene oxide can be polymerized in the presence of a triol. Then the isocyanate-alcohol reaction would of course give a cross-linked urethane. 

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. 

Glycerol is a trifunctional alcohol. It uses two of its OH groups in a growing chain. The third OH group cross-links with another chain. The more cross-linked a polymer, the more rigid it is. 

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). 

Monomers with Tri- (or Higher) Functionality The above examples are for polymers formed from difunctional reactants. If either reactant has only one functional group, no polymer can result. However, monomers with functionalities higher than two are also used to make polymers. For example, glycerin is a trifunctional molecule (three alcohol groups) that can be used in polyesterification. Molecules with three or more functional groups tend to make network (or cross-linked) polymers, as the reaction can proceed to form a three-dimensional network instead of just a linear polymer as difunctional monomers do. 

Tris(o-hydroxyphenyl)triazine possesses structural features similar to 8-hydroxyquinoline. Linear polymers can be prepared by reaction of this trifunctional ligand with divalent metal salts in solution (32, 34). Reaction in bulk leads to the formation of cross-linked network polymers. Exactly how the difference in polymer structures [31] and [32] was determined was not made clear because both products were insoluble and infusible. It was postulated that the linear structure was obtainable because it precipitated from the reaction solvent before reaction with a third metal ion. Dimethylformamide was used in conjunction with an alcohol, such as methanol or ethanol, for the solution reaction. Temperatures up to 330°C were used for the bulk reaction. 

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