Polymethylene spacers

The effect of the polymethylene flexible spacer on the liquid-crystalline behavior of 

Evidently, the even members allow for a more favorable packing of the polymer chains in the mesophase this results in higher transition temperatures. This effect is best understood by assuming the conformation of the methylene spacer to be all trans. This conformation has the lowest energy, and there is considerable evidence that it is in fact a very likely conformation of these spacers in both liquid-crystalline and crystalline phases. The conformational distribution of a linear polymethylene chain, when 

2 Main Chain Liquid Crystalline Semiflexible Polymers 

Various mesogenic groups were connected through oligomers of polyfethylene oxide), even with a polydisperse distribution of chain lengths. However, to understand the individual effect exerted by the polyfethy-lene oxide) spacers it is necessary to employ monodisperse molecular species. It is im- 

Within each series, the isotropization entropies for homologs with the same parity lie on smooth curves which increase with increasing spacer length, even members giving the upper curve. The isotropization entropies of the members of the series with n = 2 and 4 containing the same number m of methylene units are very similar to each other, but significantly lower than those of the corresponding members of the series with = 3. In addition, the slope of the iso- 

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The distribution of chain sequence extension, calculated by using RIS models, is compared with isotropic-nematic transition characteristics for a number of thermotropic polymers comprising rigid groups connected by polymethylene spacers. The distribution depends strongly not only on the odd-even character of the number of methylene units of the spacers, but also on the specific groups (or atoms connected at the ends of polymethylene spacers. 

Table 20 presents the results pertaining to the synthesis and properties of the gels. The gels are seen to sorb heparin from plasma solution. Their capacity (the maximal amount of heparin sorbed) increases as the mobility and accessibility of the cholesterol fragment for the macromolecules of heparin is increased by varying the length of the polymethylene spacer. The data in Table 20 illustrate that the capacity varies from 0.7 to 0.9 and 1.3 mg/mg of immobilized UChD for cholesterol esters of N-meth-acryloyl-fS-alanine (n — 2), N-acryloyl-co-aminoenantic acid (n = 6), and N-meth-acryloyl-co-aminolauric acid (n = 11), respectively. 

Thus, the remoteness of mesogenic groups from the backbone provided by a polymethylene spacer secures them sufficient autonomy from the main chain. On the other hand, the fact that mesogenic groups are chemically linked with the main chain of the macromolecule assists their cooperative interaction. This is why comblike polymers have come to be accepted as convenient matrices for constructing LC polymers. Already a few hundred liquid-crystalline polymers with various mesogenic side groups have been synthesized. 

The comparison of optical anisotropy and of Kerr constants for polymers 1, 3, 4, and 5 (Table 16) shows that the spacing out of mesogenic groups from the main chain by insertion of a polymethylene spacer leads to a drastic decrease in correlation of side group and backbone orientation, and, consequently, to a worse intramolecular ordering. 

In summary, the dimeric lipids (29a-29h) with low m-value (3-4) and high m-value (20-22) showed exceptional thermal, lipid-packing and cholesterol-association properties. Obviously the introduction of a polymethylene spacer chain at the level of headgroup brought about dramatic effect on the aggregation behavior, membrane organization and lipid packing of 29. 

Roelfes and coworkers first assessed the catalytic performance of their DNAzyme in asymmetric Diels-Alder reactions of cyclopentadienes with a dienophile that binds to the Cu(II) center through a pyridyl group. The length of the polymethylene spacer and the R substituent proved to be crucial, both for the enantioselectivity as such and also for the sense of stereoinduction (Figure 29). 

Table 12. Selected photophysical properties for the dicopper(I) knots with the interchromophoric polymethylene spacer ...

The virtually universal even-odd behavior of both melting and clearing temperatures for liquid crystalline polymers having polymethylene spacers has not yet been explained theoretically, but similar trends have also been observed for low molecular weight LC compounds with alkyl terminal groups. Of course, odd-even effects are... 

We prepared polymers containing the mesogenic structure 25 of Table 1, both in the homopolymer form and as copolymers containing polysiloxane and decamethylene flexible spacers, and these polymers were compared in their properties to the earlier polymers prepared with a polymethylene spacer only The polymer with mesogenic structure 32 and the siloxane spacer showed a low molar enthalpy for the melt transition, while the copolymer and the polymer with the polymethylene spacer showed quite high melt enthalpies. 

The range of mesophase stability and melting temperature was greater for the polymer with a polymethylene spacer than that with a siloxane spacer however, the size of the entropy of clearing was reversed. In all such measurements, the properties of the copolymer fell between these two extremes, except for the case of the range of mesophase stability, which was greater for the copolymer than the two homopolymers. In all cases, the mesophases were identified as nematic from characterization by polari-zed-light microscopy. 

To conclude this discussion, it is quite obvious that the flexible spacer plays a very important role in determining not only the transition temperature but also the type of mesophase. In almost all cases, an increase in flexible spacer length leads to a decrease in transition temperatures. For the polymethylene spacers, this decrease is associated with an even-odd effect, with lower transition temperatures for the odd-numbered members of the series. For some mesogenic types, this effect can also be accompanied by a change from nematic to smectic mesophases. 

Polymers with poly(ethylene oxide) spacers were shown to have transition temperatures similar to those for polymers with polymethylene spacers of the same length, but the mesophases formed could be quite different, which again emphasizes the importance of the spacer in determining mesophase morphology. Furthermore, polymers with polysiloxane spacers and mesogenic units, identical to those present in polymers with polymethylene spacers having a comparable number of bonds, showed different mesophase structures in addition to much lower transition temperatures. 

The polymers under study have a chemical repeat comprised of three phenyl rings linked by azomethine groups forming the rigid unit and a flexible polymethylene spacer (1 to 16 methylene units) linked to the rigid units by ether oxygens. 

On the basis of the observed results, macroporous polystyrene/DVB matrix stannylated with a polymethylene spacer (CH2) or with a CH2—O—(CH2) spacer (with n > 3) seem to be the most promising. However, it is also clear that gentle mechanical shaking (rather than stirring with magnetic stirrer) and... 

Absolute Configuration Involving Stereocenters Separated by a Polymethylene Spacer... 

First, recent examples of the determination of structure including the absolute configuration of bioactive natural products have been discussed, emphasizing the techniques to solve stereochemical problems among compounds with remote stereogenic moieties separated by a polymethylene spacer. Case studies with pheromones and acetogenins have been given to illustrate the problems and solutions. 

The ease of forming the smectic mesophase by this class of side-group type liquid crystalline polymers has rendered a great possibility in synthesizing polymeric chiral smectic materials useful in non-linear optics, transducers, pyroelectric detectors and display devices (Chapter 6). The first polymer forming a chiral smectic-C phase was synthesized by Shibaev et al. (1984). It has a polymethacrylate main chain, a long polymethylene spacer, and a mesogenic unit attached at the end with a chiral moiety (polymer (3.60)). Since then, a lot of polymers with chiral mesophases have been synthesized and studied (Le Barny and Dubois, 1989). 

C.Ober, J.I. Jin, and R.W.Lenz, Thermotropic polyesters with either dyad or triad aromatic ester mesogenic units and flexible polymethylene spacers in the main chain, Pol3mier J. 14 9 (1982). 

Yoon, D. Y, and Bruckner, S., Configurational characteristics of thermotropic polymers comprising rigid groups connected by polymethylene spacers. Macromolecules, 18, 651-657... 

In this article, in connection with our previous endeavor, we would like to report synthesis and properties of new copolyesters having ordered comonomer sequences consisting of mesogenic units and polymethylene spacers, i.e., thermotropic aromatic-aliphatic copolyesters having sequentially ordered structures ... 

In polymers 1 and 6, the mesogenic group is separated from the main chain by a flexible polymethylene spacer. For these polymers, the optical anisotropy and the Kerr constant are lower by one order of magnitude than for polymers with a similar structure of the mesogenic side chain but without a flexible unit. 

An interesting exploitation of the OH side groups borne by these copolymers is provided by a recent study in which poly(a-terpineol-co-MMA) was modified by grafting phenyl benzoate mesogenic groups, bearing vinyl terminated polymethylene spacers, to give liquid crystalline materials [88]. 

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