Clearing lateral substituents

The combined results of kinetic studies on condensation polymerization reactions and on the degradation of various polymers by reactions which bring about chain scission demonstrate quite clearly that the chemical reactivity of a functional group does not ordinarily depend on the size of the molecule to which it is attached. Exceptions occur only when the chain is so short as to allow the specific effect of one end group on the reactivity of the other to be appreciable. Evidence from a third type of polymer reaction, namely, that in which the lateral substituents of the polymer chain undergo reaction without alteration in the degree of polymerization, also support this conclusion. The velocity of saponification of polyvinyl acetate, for example, is very nearly the same as that for ethyl acetate under the same conditions. ... 

Our interpretation of the results was that the para-substituent was involved in a specific interaction (we define this more clearly later) with the receptor. We then found (this is the work of Dr R. H. Davies) that if we divided our (3-blockers into classes according to the type of para-substituent. we were then able to do a Hansch type regression (l2) (usingit and d ) on each class and get quite meaningful results. 

The decrease of the clearing point induced by lateral fluorination can be understood in terms of reduction of the length-to-breadth ratio of the liquid crystal molecule caused by any lateral substituent. As rule of thumb, for most compounds a decrease of the clearing temperature of 30-40 K is observed for each lateral fluorine atom. 

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 (2350°C for m = 2... 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

A particularly striking example of shielded lateral substituents occms in the naphthoic acids (compounds 84-87). Even large lateral substituents cause an increase in clearing point because of the efficient filling of space which enhances intermolecular attractions. The smectic tendency of the naphthoic acids (85-87) with the lateral substituents is very high. 

As to the lateral L groups in (l.xix), even small ones usually perturb the structure of a mesophase and cause a significant depression in the clearing point. Moreover, they have a different effect on the thermal stability of the nematic and smectic mesophases. Smectic phases are much more infiuenced because lateral substituents strongly prevent side-by-side packing of rigid molecular cores favorable for the smectic ordering. 

The position of lateral substituents is quite important [154, 159-161], as can be seen from the examples given in Table 34. Because they do not reduce the conjugation of the rings, substituents in the positions 3 and 3 depress N-I far less than do those in positions 2 and 2. On the other hand, the influence of the size of the substituent can be nicely observed. The influence of lateral substituents on the stability of the smectic phase by far exceeds that on the nematic phase. This is clearly demonstrated by compounds 4 in Table 1. Suppression of the... 

Table 33. Decrease in the melting and clearing temperatures by the addition of lateral substituents.

There are compounds in which the lateral substituents are shielded, so that they are less effective in broadening the molecule. Table 35 shows some impressive examples of this kind. There are also compounds with axial CN groups in cyclohexane [162, 163] or dioxane [164] rings, in which partly the shielding effect and partly the increase in density due to the polar group may be responsible for the unexpectedly high clearing temperature. 

In more recent papers, compounds have been described in which the lateral substituents are attached via spacers [191-192]. As the spacer length is increased, the clearing temperatures are seen to alternate distinctly (Figure 11). 

A further example of lateral substitution, but this time with respect to extension of a lateral chain and change in substituent polarity, is shown in Fig. 22 for the 3-substi-tuted-1,4-bis-(4-n-octyloxybenzoyloxy)ben-zenes. It can be seen that for the parent system (X = H) smectic C and nematic phases are formed, but when a lateral substituent is introduced at the 3-position smectic phase behavior is suppressed. The clearing points are considerably lowered by increasing the length of a lateral aliphatic substituent, whereas for smaller polar substituents the reduction in clearing point is not as great [32]. 

The use of latoal substituents in liquid crystals has proved to be very important, initially in nematic material and lata- in smectic C matoials. Clearly, anything that sticks off the side of a rod-like molecule will tend to reduce the liquid crystal phase stability, and genmlly the larg the lateral substituent the greater the reduction in liquid crystal phase stability. Usually, the smectic phase stability is much mmre affected than that of the nematic phas especially by larger substitumts because of the obvious reduction in lateral attractions, but increased lateral attractions associated with polar substituents cause a smaller reduction in smectic phase stability (see compounds 52-57) [46]. 

The effect of lateral methyl groups in the spacer on the phase behavior has been studied in several polybibenzoates [18,19] derived from poly(tetramethy]ene p,p bibenzoate), P4MB. The branched polymers display transition temperatures significantly lower than P4MB. Moreover, the substituents have a clear effect on the kind of mesophase formed. Thus, P4MB displays a smectic A mesophase, while the lateral methyl groups... 

Another interesting observation was made by Bagal et al. a year later (1992). In the reaction of 4-nitrobenzenediazonium ions with various 4-phenylazophenols, with or without substituents in the 2- and 3-positions of the phenolic ring and in the 4 -position of the phenylazo ring, in addition to azo coupling in the 6-position they obtained a product that had the same atomic composition as 2,4-bis(4 -nitrophenyl-azo)-phenol (Ci8Hi4N605), but whose 13C NMR spectrum clearly showed a tetrahedral and a carbonyl carbon in the 4- and 1-positions. This product must therefore be the compound 12.153. 

---