Polysiloxane spacers

2 Main Chain Liquid Crystalline Semiflexible Polymers 

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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. 

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. 

E. Lindner, A. Jaeger, F. Auer, W. Wielandt and P. Wegner, Supported organometallic complexes. Part XIII. Catalytic studies on sol-gel processed (ether phosphine)ruthenium(II) complexes with different spacer lengths and different polysiloxane matrices, J. Mol. Catal. A Chem., 1998, 129, 91. 

Note 3 Examples of polymer backbones are polyacrylates, polymethacrylates, and polysiloxanes the spacers are usually oligomethylene, oligo(oxyethylene), or oligosiloxane. 

The introduction of organofunctional groups, i.e. the use of alkoxysilanes of the type (R O Si—X—A, where A is a functional organic group and X is a chemically inert spacer permanently linking Si and A, results in a more extensive chemical modification of the materials76. The properties of the organic functions A supplement the properties of the polysiloxane matrix formed by hydrolysis and condensation of the Si(OR )3 and Si(OR )4 units. 

Masamune et al. constructed a polysiloxane dendrimer with an oligosilane core unit [98]. A repetitive synthetic strategy, consisting in catalytic oxidation of the terminal SiH to SiOH groups and substitution of the hydroxyl groups by a spacer, provided access to polysiloxane dendrimers of up to the third generation. 

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

More recent studies on the effect of flexible spacers now include the use of either poly(ethylene oxide) or polysiloxane segments. Each such spacer brings with it certain unique characteristics to the final polymer, and it is clear that the role of the spacer... 

Polysiloxanes are known to have very low glass transition temperatures and are considered to be much more flexible than polymers of the spacer types discussed above. Quite recently, several examples of polymers using polysiloxane flexible spacers have been synthesized and were characterized by conventional methods. 

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. 

Some of the kinked units discussed in the previous section, such as —0— and — S—, are simple examples of flexible spacers that space the meso-genic units into separate structural sequences of smaller length-to-diameter ratios. Other segments often used as flexible spacers include oligomeric polymethylenes, polyoxyethylenes, polysiloxanes, and so forth. The following series of molecules, (3.5) with n = 1-8, make up one of the oldest examples of the flexible spacer concept in the molecular design of liquid crystals (Vorlander, 1927) ... 

Flexible spacer and mesogenic unit Polymethacrylate ( 3.51 ) Polyacrylate ((3.52)) Polysiloxane ((3.53))... 

The unambiguous regioselective synthesis of chiral polysiloxane-containing CyDs as chiral stationary phases, with the mono-octamethylene spacer in either the 02, 03, or 06 position was performed and the products applied to enantio-selective GC separations [99]. Subtle differences in the chemistry of the hydroxyl groups at the 2-, 3-, and 6-positions of CyDs can be exploited to direct an electrophilic reagent to the desired site. Selective monoalkylation at the primary side of a-CyDs involves the reaction of a-CyD with 4-methylamino-3-nitrobenzyl chloride in 2,6-lutidine. Monoalkylation at the 2-position of j8-CyD is accomplished by the reaction of yS-CyD with l -bromo-4-methylamino-3-nitroacetophenone [100]. 

Many reviews [1-3] of SCLCPs describe the structure of the backbone (main chain), the spacer (flexible linkage), and the side group (mesogenic unit) of the SCLCP. For example, the most widely used backbones include polyacrylates or polymethacrylates, polysiloxanes, and polyphosphazenes po-ly-ct-chloroacrylates, itaconates, and ethylene oxides have also been reported. 

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