Length ratio, spacers

Some remarkable examples of positive cooperativity in helicate formation have also been reported by Ken Raymond from the University of California, Berkley, USA. Raymond s systems use the deprotonated 1,2-dihydroxy benzene dianion moiety as a basic ligating motif. Based on the identity and length of spacer units between two such binding domains, a large array of triple helices and coordination clusters (cf. Section 10.5.3) self-assemble selectively. In one example, a mixture of three such ligands in the presence of an appropriate ratio of Ga3+ results in the selective formation of three individual, homoleptic triple helices (Scheme 10.26), 23... 

The kinetics of iron(III) dissociation from a series of dihydrox-amate siderophores and siderophore mimics, including rhodo-torulic acid (3) and alcalagin, have been investigated (52,127,128, 177,178). ESI-MS studies show that these systems form multiple species as a function of pH and siderophore/iron ratio (128). The lability of these systems and the resultant multiple species leads to several parallel paths to iron(III) dissociation (177). Both the distribution of structures and kinetics of dissociation were shown to be dependent on the length of the spacer chain between the dihydroxamate donor groups (52,127). 

The membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are... 

The spacer arm length between the hapten and the carrier is in the range of 6 to 8 A, which should eliminate any steric interference with carrier side chains. According to the protocol described by Schultz the haptens were coupled to BSA and KLH via A-(3-dimethylaminopropyl)-A -ethylcarbodiimide or the N-hydroxysuccinimide ester at pH 5.5 in water. Other coupling strategies include substitution with diazonium salts and reductive amination. The ratio of hapten-carrier range between 8 and 15 haptens per carrier. 

This technique overcomes the drawbacks of unidirectional pressing and produces a charge of uniform density and homogeneity. In this method, the explosive formulation in the mold is compressed into position in two modes. In the first mode, pressure acts from top by the downward movement of the ram. In the second stage of pressing, the spacer is removed and the base plate moves upwards on application of pressure. Thus the formulation experiences equal pressure throughout the length. The L/D ratio is two in this case. 

Thermotropic behaviour is observed in many polymers that contain rigid-rod units alternating regularly with relatively flexible spacer units. A first requirement for nematic melt formation is that the rigid-rod unit should exceed a certain length and length diameter ratio. The shortest rigid-rod unit so far reported 52) to permit nematic melt formation is... 

The nematic phase stability is higher the greater the length and length diameter ratio of the rigid-rod unit, and the shorter the flexible spacer unit, and rises ultimately to values above the temperature of decomposition of the polymer. 

A large length/width ratio was also important for the achievement of liquid crystallinity in the poly(ethers). All five of the R=H and all five of the R=Me materials were LC, exhibiting robust birefringence and stir opalescence. For R=MeO only three of five were LC, and for R=EtO, only the two mixed-spacer length polymers were LC. 

A BINOL-dimethylaminopyridine hybrid was seen to be efficient in mediating the MBH reaction (Table 5.14) [96], with optimal reaction conditions being found as —15 °C with a mixed solvent system consisting of toluene and cyclopentyl methyl ether (CPME) in a 1 9 ratio. The reaction was sensitive to the structure of the catalyst 112, the position of the Lewis base attached to BINOL, the substitution pattern of the amino group, and the length of the spacer. It should be noted that the bulky i-Pr substituent on the amino group showed the best selectivity and kinetic profile (Table 5.14, entry 5) [98]. (For experimental details see Chapter 14.10.4). 

For a spacer length of 11 methylene units, all polymers showed smectic mesophases except for poly-(II-ll) and poly-(VII). Even the flexible poly-(cy-clooctene) main chain prevented a smectic mesophase. Compared to all of the other polymer architectures, poly-(II-ll) and poly-VII present the lowest ratios of mesogens to atoms in the main chain. It can therefore be assumed that smectic phases will only be formed when there is sufficient mesogen density. For the norbornene chain, it is notable that a high Z/E-ratio and a high tactic-ity increased the stability of the smectic A phase. 

Several different structural factors influence the properties of the mesophase in these polymers, including dipolar effects, the planarity and rigidity of the mesogenic unit, and its length-to-width ratio among others. These factors are difficult to quantify, either absolutely or relatively, but some idea of their influences can be obtained by comparing the properties of polymers with different mesogenic units when combined with the same flexible spacer. This comparison has already been made for the dyad and triad esters in Table 2, and in this section it will be extended to other types of liquid crystalline polymers which contain a common decamethylene spacer. 

Roviello and Sirigu prepared copolymers with a single mesogenic group by varying the ratio of two spacers of different length, as follows ... 

In practice we take the ratio H/D less than 0.8, where H is the length of the pressed composition and D the diameter. To avoid such an inclined distribution of the density another mould like in Fig.65 may be used. The mould has two plungers,A and C, and a spacer D. On pressing D is removed and the composition is pressed from both sides, A and C, but this type of mould is not so popular, because it is somewhat inconvenient for... 

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