Flexibility, siloxane homopolymers

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

Analogous to the described homopolymers, the copolymers are mainly based on stereoirregular methacryloyl or, less frequently, acryloyl derivatives containing cholesterol as the intrinsically chiral component. A series of linear and crosslinked (elastomeric)" copolymers based on the highly flexible siloxane backbone has been investigated in great detail and in all cases the copolymers have been found to assume a cholesteric structure. 

A series of elegant studies by Percec et al has demonstrated some subtle, but significant, effects in copolymers based upon constitutional isomer pairs in methacrylate, acrylate and siloxane materials. The acrylic copolymers tended to be non-crystalline but the more flexible siloxanes were all crystalline, whatever the spacer length (Fig. 7.16, x O, y O, n>3). Controlled synthesis provided three series of materials (n = 3, 6 and 11) each including both homopolymers (x = 0, y= 100 and x= 100, y = 0 ) and copolymers of known composition. Each series provided its own information as follows. 

Glucose Sensors. Siloxane polymers are known to be extremely flexible. This flexibility will, of course, be sensitive to the amount of side-chain substitution present along the polymer backbone. For instance, in the homopolymer used in these studies (polymer A), the presence of a ferrocenylethyl moiety bound to each silicon subunit should provide an additional degree of steric hindrance, and thus a barrier to rotation about the siloxane backbone, in comparison with the copolymers, which have ferrocene relays attached to only a fraction of the Si atoms. Because these siloxane polymers are insoluble in water, their flexibility is an important factor in their ability to facilitate electron transfer from the reduced enzyme. Relays contained within more rigid redox polymers, such as poly(vinylferrocene), cannot achieve close contact with the enzyme s redox centers and are thus less effective as electron transfer mediators (25,34). The importance of this feature can be seen quite clearly by comparing the mediating ability of the homopolymer A with that of copolymers B-D, as shown in Figures 4 and 5. 

Homopolymers. Flexibility. The most important siloxane polymer is poly(dimethylsiloxane) (PDMS), [-Si(CH3)20-] (3). PDMS is also one of... 

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 ratio of ferrocene-modified siloxane subunits to unsubstituted siloxane subunits rrv.n ratio) was varied as was the length (a ) of the alkyl side chain onto which the ferrocene moiety was attached as shown in Fig. 3.3. The electrode containing co-polymer with m n ratio of 1 1 or 1 2 was the more efficient electron relay systems. The ferrocene-modified homopolymer on the other hand loses flexibility due to steric hindrance caused by the side chain substitution by ferrocene, preventing efficient electron transfer from the enzyme to the electrode. The length of the alkyl side chain onto which the ferrocene moiety is attached was also found to influence the electron transfer efficiency of the electron relay system. Maximal current density was measured... 

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