Spacer polymethylene

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

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. 

The polymethylene linkers between the two diphenylmethane moieties are very flexible leading to a poorly preorganised host. Spacers that are more rigid should result in greater selectivity. 

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). 

A series of compounds, illustrated by (137), was evaluated for in vitro affinity for a, and a,-adrenoceptors by radioligand-binding assays (83). All compounds showed good affinities for the a, adrenoceptor, with K- values in the low nanomolar range. The polymethylene chain spacer between furoylpiperazinylpyradiz-inone and aryl piperazine moieties was shown to influence the affinity and selectivity of these compounds. A gradual increase in affinity for the a, adrenoceptor was observed, by length-... 

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

Most of the subject polymers that have been studied to date have contained flexible spacers composed of polymethylene chains. The monomers or precursors for these spacers are readily available as either diols or diacids or as dihalides, and this availability has led to the preparation of several homologous series of thermotropic polymers with different mesogenic units. Indeed, the first reported main chain, thermotropic LC polymer contained a polymethylene flexible spacer... 

Another series of polymers was prepared by reaction of the diacetoxy ester of meso-genic structure 33 in Table 1 with a series of a,(B-polymethylene diacids The spacer length was increased from 3 to 12 methylene groups, and examples with spacers of 14 and 20 repeat units were also prepared. These polymers had a mesogenic unit that was shorter than that for the series just discussed (18A vs 13A), and this probably led to the reported formation of only the nematic mesophase in this polymer series. 

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. 

The lower transition tempera ture of the siloxane-containing LC polymers compared to the polymers with polymethylene and poly(alkylene oxide) spacers can be attri-... 

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. 

Diamino-polycarboxylates are analogues of EDTA and can have a polymethylene chain of variable length as a spacer between the amine nitrogen atoms, in which branching, ethereal functionalities, or a fifth -COO group can also be present (Figure 14). 

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. 

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