Glycerol, cross-linked polyester

Phthalic anhydride + Glycerol Cross-linked polyester... 

Polyesters [2] find use in fibers [poly(ethylene terephthalate), poly(ethylene oxybenzoate), poly(ester ethers), poly(ester amides), etc.] [1], coatings (especially unsaturated polyesters) [4], plasticizers, adhesives, polyurethane base resins, films, etc. Cross-linked polyesters prepared from glycerol and phthalic anhydride (alkyd resins) have been reviewed [20], High-melting poly aryl esters have been investigated for high-temperature applications. 

Reaction of phthalic anhydride with glycerol gives a cross-linked polyester called aglyptal resin. Draw a partial structure, showing the cross-linking. 

The reaction of phthalic anhydride with a diol (ethylene glycol) is described in the procedure. This linear polyester is compared with the cross-linked polyester (Glyptal) prepared from phthalic anhydride and a triol (glycerol). 

In this A is a hydrocarbon radical and A is an organic or inorganic acid radical (for example, polyethylene terephthalate, nucleic acids). They are prepared by polycondensation of glycols with dibasic acids or their anhydrides, or hydroxy acids. Branched polyesters (for example, alkyd resins) or cross-linked polyesters are produced by using polyhydric alcohols (more than two OH groups for example, glycerol, and pentaerythritol and various polyols). 

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). 

Smith condensed phthalic anhydride with glycerol in 1901 to prepare a glassy brittle material. Kienle in 1924 started investigating these resins and called them alkyds, He modified the polyesters with drying oils. By this modification they become soluble, on exposure to oxygen rapid cross-linking occurs and... 

The use of triols or tricarboxylic acids leads to cross-linked or network polyesters. For example, an alkyd resin is formed by the reaction of glycerol with phthalic anhydride [19]. 

Glyptal is also a polyester condensation product, but glycerol (HOCH2CHOHCH2OH) produces a cross-linked thermosetting resin. In the first stage a linear polymer is formed with the more reactive primary OH groups. 

Glyptal polyesters were first prodnced in 1901 by heating glycerol and phthalic anhydride. (See Figure 3.) Because the secondary hydroxyl is less active than the terminal, primary hydroxyl in glycerol, the initial product formed is a linear polyester. A cross-linked product is produced by further heating through reaction the third alcohol. 

All cross-linked polymers tend to be difficult to disrupt, and they can be regarded as one giant molecule. This makes them of use when durability is needed. Many of the polymers mentioned above can be modified so as to form extensive cross-linking by increasing the number of reactive groups on the monomers. For example, the polyester formed by phthalic acid and glycerol (Scheme 6.5) can cross-link to form a polymer that is used in bake-on car paints. 

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 condensation polymerizations involving formation of polyesters or polyurethanes, cross-linking can be achieved by use of some of a trihydroxylic alcohol, such as glycerol, in addition to the dihydroxylic alcohol. 

The other hpid polymer, suberin, is a heteropolymer, consisting of an aliphatic polyester associated with cross-linked polyaromatics and embedded waxes. Upon transesterification of suberin, the monomers released include C16-C28 m-hydroxy fatty acids and C16-C26 ct, -dicarboxyhc acids, the latter of which are diagnostic monomers, unsubstituted very-long-chain fatty acids (VLCFAs C>i8) and alcohols, glycerol and ferulate. Usually the major components of suberin are -hydroxy derivatives of palmitic and/or oleic acids, but in some cases oo-hydroxy C220 also is a dominant component [37]. Dicarboxylic FAs derived from further oxidation of the -hydroxy-FAs are also found in suberin. 

The feasibihty of the Diels-Alder coupling was shown on the example of biodegradable polyester by Ninh and Bettinger [47]. In this case, hyperbranched poly(glycerol-co-sebacate) with pendant furan groups was coupled with a bifunctional maleimide cross-linker to produce an elastomeric material. POSS moieties may serve as cross-linking sites, as well. Ishida et al. [48] have shown how a... 

Suppose, however, that glycerol is used as the cross-linker in the synthesis of a polyester. Then the functionality of the cross-link site will be 3. Use of trimethylol propane trimethacrylate or pentaerythritol tetramethacrylate results in functionalities of 6 and 8, respectively (66). 

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