Polyester homopolymer, structure

Polyester homopolymer C5C10, structurally analogous to the main-ch block, formed nematic and smectic C mesophases (8,9). Within each series, the block copolymers are designated with a letter from a to d referring to the different amount of the methacrylate monomer present in the feed mixture. 

Aromatic polyesters that do not contain any flexible structural units are often nonmeltable or extremely high melting polymers that cannot be processed. Copolymerization is a way to obtain processable wholly aromatic polyesters The Tm versus copolyester composition curve is a U-shaped curve exhibiting a minimum that is generally well below the Tm of corresponding homopolymers. Liquid crystalline aromatic polyesters, for instance, are usually copolymers.72 An example is Ticona s Vectra, a random copolyester containing 4-oxybenzoyl and 6-oxy-2-naphthoyl units in ca. 70 30 mol ratio. This copolymer melts at ca. 

PAs have also been copolymerized with other polymer systems and, in particular", with polyesters and poly ethers. In the copoly esteramides the crystallinity is decreased by copolymerization, as the crystalline structure of the amide unit is very different from the ester unit. However, alternating polyesteramides behave as homopolymers with a glass ttansition temperature and a melting temperature intermediate to the polyester and the PA polymer (Figs. 3.10 and 3.11).23,24 Polyesters, such as PBT and PET, modified with a small amount of diamide are also copolymers that have a high order.24,73... 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

Several wholly aromatic polyesters are available. As expected, they are more difficult to process, stiffer, and less soluble, but are employed because of their good high-thermal performance. Ekonol is the homopolymer formed from p-hydroxybenzoic acid ester (structure 4.51). Ekonol has a Tg in excess of 500°C. It is highly crystalline and offers good strength ... 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

Strzelecki and coworkersfound that the use of (+) 3-methyladipic acid in the synthesis of liquid crystal polyesters led to a cholesteric polymer. They prepared a homopolymer and a series of copolyesters of the following structures ... 

Aromatic polyesters undoubtedly represent the most important class of thermotropic nematics. Fully aromatic rod-hke homopolymers such as poly(p-oxybenzoate) or poly (p-phenylene terephthalate) melt at temperatures which are too high to form a stable nematic mesophase. However if the regular chemical structure of the homopolymer is disrupted, the melting temperature is reduced and it is possible to obtain thermotropic nematics. ... 

The polycondensation of difunctional oligomers leads to the preparation of well-defined polymer structures. Monomers in this type of reactions must be soluble in the reaction mixture and stable when the reaction is carried out in the melt, which is the case for some aromatic polymers prepared by polycondensation [22]. As previously described, polycondensation can occur with monomers bearing the same or a different functional group at both ends of the molecule. When one of the reactive functional groups is a hydroxyl moiety, several types of materials can be prepared, such as polyethers, polyesters, and polyurethanes, independently if they are used to form homopolymers, copolymers, or hyperbranched polymers. 

A large number of commercially important condensation polymers are employed as homopolymers. These include those polymers that depend on crystallinity for their major applications, such as rylons and fiber-forming polyesters, and the bulk of such important thermosetting materials like phenolics and urea-formaldehyde resins. In many applications, condensation polymers are used as copolymers. For example, fast-setting phenolic adhesives are resorcinol-modified, while melamine has sometimes been incorporated into the urea-formaldehyde resin structure to enhance its stability. Copolyesters find application in a fairly broad spectrum of end uses. 

Interesting investigation on study of structure characteristics of polyesters and copolyesters of -oxybenzoic acid have been performed in Refs. [35, 36]. The introduction of links of -oxybenzoic acid results in high-molecular compounds of ordered packing of chains, similar to the structure of high-temperature hexagonal modification of homopolymer of n-... 

The number of new polyester-a-diketones in the form of homopolymers and copol5mers has been produced [402]. Polyester-a-diketones have the amorphous structure, include a-diketone, a-hydroxyketone and ester groups, they are soluble in organic solvents. The films with lengthening of 5-87%, solidity and module under tension, respectively 54-83 MPa and 1.6-3 GPa are produced by casting fi-om chloroform solution. 

A large number of biodegradable polyesters are based on petroleum resources, obtained chemically from synthetic monomers (Vert et al. 1995 Bigg 1996 Sinclair 1996 Lunt 1998 Albertsson and Varma 2002 Okada 2002 Steinbuchel and Doi 2002). According to the chemical structures (see Fig 17.2), we can distinguish (see Table 17.2) homopolymers (polycaprolactone, etc.), aliphatic or aromatic copolyesters. All these polyesters are soft at room temperature. 

PHB homopolymer is a completely stereoregular polyester and it is not surprising, therefore, that it is capable of achieving high levels of crystallinity. The crystal structure has been extensively studied by X-ray diffraction " " and found to consist of polymer chains... 

The physical and thermal properties of bacterial PHA copolymers can be regulated by varying their molecular structure and copolymer compositions. The P(3HB) homopolymer is a relatively stiff and brittle material. The introduction of hy-droxyalkanoate comonomers into a P(3HB) chain greatly improves its mechanical properties. The PHA family of polyesters offers a wide variety of polymeric materials, from hard ciystalline plastic to elastic rubber. The PHA materials behave as thermoplastics with melting temperatures of 50-180°C and can be processed by conventional extrusion and molding equipment (Holm 1988). 

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