Aliphatic hydroxyacids

Table 33 Actinide(IV) Compounds with Aliphatic Hydroxyacids ---------------------------------------------------1-----------...

Table 74 Some Dioxoactinide(VI) Compounds with Aliphatic Hydroxyacids...

Liquid crystal polyesters are made by a different route. Because they are phenolic esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenolic hydroxyl groups are acylated with a lower aliphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxylic acid, sometimes in the presence of a catalyst. The phenolic polyester forms readily as the volatile lower acid distills from the reaction mixture. Many liquid crystal polymers are derived formally from hydroxyacids (97,98) and their acetates readily undeigo self-condensation in the melt, stoichiometric balance being automatically obtained. 

Cork from the cork-oak (Quercus suber L.) differs chemically from wood, mostly by the presence of suberin as a major structural component (ca. 60% of extractive free cork) in addition to lignin and polysaccharides (6).The structure of suberin is not fully elucidated yet. It is a cross-linked polymer with a polyester linked aliphatic domain containing fatty acids, alcohols, hydroxyacids and diacids and a phenolic, probably lignin-like domain. 

This contribution places the emphasis on aliphatic polyesters and, as the main protagonists, on poly (2- and 3-hydroxyacids). Poly(2-hydroxypropionic acid) and poly(3-hydroxybutyric acid) which can be regarded as their prototypes, will be specially comparatively discussed concerning their chances and risks. Herein they will be called polylactic acid or PLA and PHB. 

The pioneering work carried out in Moore s laboratory in the 1970s and 1980s represents the first systematic approach to furan polyesters (after numerous isolated and rather qualitative publications on the topic), which were prepared fi om 2,5-furandicarboxylic derivatives in conjunction with aliphatic, furanic and tetrahydrofuranic diols as well as with bisphe-nol A [4d, 4e]. In the same period, the furan hydroxyacid 10 was polymerized to give both linear and cyclic oligomers at relatively low temperatures, but its condensation with ethylene glycol produced black intractable resins [4d, 38]. 

Lanolin steryl ester determinations Lanolin is the wool grease secreted by the sheep sebaceous glands, and represents a complex mixture of high molecular mass lipids, including fatty acids and alcohols, sterols, hydroxyacids, diols, and aliphatic and steryl esters. Lanolin is widely used in cosmetic and pharmaceutical formulations. The study of minor lipids... 

These aliphatic polyesters can be obtained by catalyzed dehydration of hydroxyacids and, more efficiently, by ring opening polymerization of the cyclic esters of hydroxyacids (equations (1) and (2)). Catalysts are generally used to facilitate the polymerization. Among the effective catalysts are Lewis acids in the form of metal salts of Sn, Zn, Ti, Al, and rare earth metals [17-23] alkali metal alkoxides and super-molecular complexes [20,24,25] and acids [26]. 

As far as biodegradable aliphatic polyesters are concerned, a main classification divides them into two groups. In the first group, the repetitive unit of the polymer is constituted by a hydroxyacid unit, while in the second one it is constituted by a diol... 

Biodegradable Aliphatic Polyesters with a Hydroxyacid Repetitive Unit... 

---