Stereochemically Variable Polysiloxanes

In unsymmetrically disubstituted chains, the substituents of one type can be on the same side of the all-trans chain, on opposite sides, or on either side in a random arrangement, yielding isotactic, syndiotactic, and atactic forms. Poly(methylphenylsiloxane) was one of the chains chosen to illustrate this stereochemical variability. The relatively large Si-0 bond length and Si-O-Si bond angle place apposed side groups at distances of separation (ca. 3.8 A) at which there is a favorable energy of interaction. 

Cyclization measurements have also been conducted on other stereo-chemically variable polysiloxanes [-Si(CHg) R-0-], where R is H, CH CHg, CHjCHjCHg, and CHjCH CFg. The conclusion from these investigations was that such polymers are generally atactic and that an increase in the length or size of the side chains increases the characteristic ratio.  

Control of stereochemistry, in general, is an interesting and potentially important subject with regard to commercial materials.  

If the side groups are sufficiently hydrophilic, the polysiloxane can even become water soluble. Attaching groups such as those of ethylene oxide to polysiloxane networks can also be used to make them hydrophilic. -  

A final example is the use of optically active groups as side chains, the simplest example being the secondary butyl group -CH(CHg)(C2Hg). Such polymers could be studied using any of the numerous characterization techniques developed to characterize optically active polymers such as the proteins. 

---

An example of such modeling of a stereochemically variable polysiloxane focused on poly[methyl(3,3,3-trifluoropropyl)siloxane],137 a polymer much studied with regard to the already-mentioned new synthetic techniques for controlling stereoregularity and thus crystallizability.102-104 138... 

---