Flexible spacer

Mesogenic diols, such as 4,4 -bis( CO-hydtoxyaLkoxy)biphenyls, ate used with 2,4-TDI or 1,4-diisocyanatobenzene (PPDI) to constmct Hquid crystalline polyurethanes (7). Partial replacement of the mesogenic diols by PTMG shows that the use of lower molecular weight flexible spacers form polymers that have a more stable mesophase and exhibit higher crystallinity (8). Another approach to Hquid crystal polyurethanes involves the attachment of cholesterol to the polyurethane chain utilizing the dual reactivity in 2,4-TDI (9). 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

F3 see Siloxanes Flame resistance 38, 73 Flexible spacers 47, 68 Fluctuations in composition 164 Free-radical polymerization 54-57, 60 Frontier orbitals 196 Functional polymers 148, 155, 164 Functionalizations 155-157... 

Bied-Charreton (63) showed by XH NMR spectroscopy that a zinc porphyrin bearing a primary amino group attached via a flexible spacer to the meta position of a /neso-phenyl ring spontaneously dimerizes in solution via NH Zn coordination. The corresponding ortho derivative shows an equilibrium between the dimeric form and the intramolecu-larly coordinated monomer. ESR spectroscopy was used to demonstrate the same behavior in the corresponding cobalt porphyrin (64). 

In this paper two new polymerized vesicle systems have been presented. The first lipid can be polymerized in vesicle through UV irradiation. Because the second lipid contains a flexible spacer group it can be prepolymerized in benzene and then converted to vesicles by ultrasonication in water. The polymerization improves the stabilities of the synthetic liposomes. Since there is a acetal linkage between the... 

Figure 6. General structure for phosphazenes with mesogenic side groups. Example is a mixed substituent polymer (VII) where R represents the trifluoroethoxy group and the mesogen with flexible spacer is represented by the curlicue and rectangular box.

The side chains in these structures can rearrange into positions either parallel or perpendicular to the deformed chain backbone. [178]. The outcome depends particularly on the nature and length of the flexible spacer connecting the mesogenic groups to the chain backbone. As expected the physical properties can become strongly anisotropic [179]. 

Disklike rc-conjugated mesogen Mesogenic side group Flexible spacer Solubilizing side chain... 

Bifluorophores consisting of two different fluorescent dyes linked by a flexible spacer containing heteroatoms (oxygen, nitrogen or sulfur atoms) can bind cations. This results in a decrease of the distance between the two fluorophores and, consequently, to an increase in efficiency of photoinduced energy transfer between the two moieties (Figure 2.13) provided that the emission spectrum of the donor (D) overlaps the absorption spectrum of the acceptor (A).(36) The transfer efficiency depends on the distance according to Forster s theory ... 

Fig. 3. Dendrimers. The branching units can be directly attached to each other in generations or shells (left), hut can also be connected via flexible spacers of identical length (right)...

Both thermotropie and lyotropic liquid crystal polymers exhibit eharacteristic micro-strueture features [9,10]. Anisometrieal monomers such as rods or disks are conneeted to ehains in an appropriate manner. These anisometrieal monomers are considered the mesogens and may be part of main ehain LCP, side chain LCP, or of both types together (Fig. 6). Flexible spacers of nonmesogenic character are located between the mesogens. A sufficient flexibility is a prerequisite for the liquid crystal formation with an increase in temperature or solvent concentration. 

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