Polyesters with Lateral Substituents

Different types of lateral substituents can be incorporated by monomers with substituents of different size, flexibility, and polarity. In addition, the number of substituents and the symmetry of the substitution can result in a different positional isomerism along the polymer backbone (Fig. 11). For example, in polyesters derived from mono-substituted monomers, the substituents can point towards each other (syn) and away from each other (anti), resulting in a random 

1 Aromatic Main Chain Liquid Crystalline Polymers 

As flexible substituents, linear and branched alkyl, alkoxy, or thioalkyl side chains of different lengths have been utilized to modify para-linked aromatic polyesters. The main difference to the systems discussed above is the limited thermal stability caused by the substitution with alkyl chains. Also, the mechanical poperties are substantially lowered with increasing 

363 °C. The clearing temperatures from the nematic melt are between 377 and 391 °C. In some cases glass transitions were found. 

As already mentioned, positional isomerism is important for the solubility and fusibility of aromatic LC polyesters. Consequently, polyesters made from symmetrical 2,5-disubstituted or 2,3,4,5-tetrasubstituted monomers should result in polymers that are less soluble and less fusible. This is in general the case with short lateral substituents. Ballauff and others reported that the series of poly( 1,4-phenylene-2,5-dialkoxy tereph-thalate)s with long flexible alkoxy side chains at the terephthalic moiety result in tractable LC polyesters [20] (Fig. 12). These polyesters exhibit a high degree of crystallinity with melting temperatures below 300 °C. Polyesters with short side chains (2 m 6) form nematic melts with clearing temperatures T. 350°C for m = 2 

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Later workers, chiefly in Japan, used naphthalimides with alkoxy substituents at the 4- or 4,5-positions and obtained brighteners with good lightfastness for polyester substrates and good chlorite fastness for polyacrylonitriles. The first commercial product was 4-methoxy-iV-methylnaphthalimide (64) [3271-05-4] [113], Table 7.10 lists the most important compounds. 

Ringsdorf s research group have prepared novel types of rigid-rod polyesters and polyamides 56 with a disc-like mesogen in tte mainchain [75]. Most polymers with six lateral substituents appear to be thermotropic liquid crystals. Polyamides with Z = H and having four substituents on the diamine component are not liquid crystalline. The two substituents on the diacid component seem to contribute to decrease further the intermolecular hydrogen bonding. 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

A. eutrophus, P. oleovorans, and several other organisms are able to form and store polyesters from monomers other than 3-hydroxyalka-nes or alkanoic acids under appropriate conditions. P. oleovorans can use a much wider variety of substrates than microorganisms in the Alcaligenes family. In addition to homopolymers and copolymers, ter-polymers have been created in this manner. For example P. acidovo-rans fed with 1-4-butanediol and pentanol creates a terpolymer of 3 hydroxybutyrate (3HB), 4HB, and 3HV. It is also possible to incorporate potentially reactive substituents in the R side chain, which might be used for later cross-linking reactions or some other type of deriva-tization to modify polymer properties. 

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