Chemistry Reference
In-Depth Information
It would be interesting to find catalysts that parallel the Zeigler-
Natta catalysts used to prepare isotactic poly(α-olefin) polymers. 46
Poly(methylphenylsiloxane) samples, for example, having sufficient iso-
tacticity (or syndiotacticity) to crystallize could be quite important. There
is promising work based on ring-opening polymerization of cyclics of iso-
tactic stereochemistry using catalysts that prevent scrambling of the
cyclic stereochemistry during polymerization. This route has yielded
poly[(methyl(3,3,3-trifluoropropyl)siloxane] samples ranging from the
usual noncrystallizable (atactic) form to materials that undergo strain-
induced crystallization to thermoplastics (which are crystalline at ambi-
ent temperatures). 47-50
Control of stereochemistry, in general, is an interesting and potentially
important subject with regard to commercial materials. 51, 52
5.2.3 Some Unusual Side Groups
Including fluoro groups in the side chains of a polysiloxane can improve
solvent resistance and modify surface properties, 53-55 as discussed in
chapter 6. Most interest has focused on replacing some hydrogen with
fluorine in n -alkyl groups—for example, the three fluorines in the trifluo-
ropropyl group -(CH 2 ) 2 CF 3 . One, two, or three fluorines have also been
substituted into the phenyl groups of poly(methylphenylsiloxane). 56, 57
Also of interest is the use of aromatic side groups different from the phenyl
groups present in the commercially important poly(methylphenylsiloxane)
or the highly intractible poly(diphenylsiloxane). 58- 65 Homopolymers and
copolymers having p -tolylsiloxane groups are of interest with regard to
the formation of liquid-crystalline phases. 66, 67
If the side groups are sufficiently hydrophilic, the polysiloxane can even
become water soluble. 68 Attaching groups such as those of ethylene oxide
to polysiloxane networks can also be used to make them hydrophilic. 69, 70
A final example is the use of optically active groups as side chains, the
simplest example being the secondary butyl group -CH(CH 3 )(C 2 H 5 ). Such
polymers could be studied using any of the numerous characterization
techniques developed to characterize optically active polymers such as the
proteins.
Other side chains include unusually long n -alkyl groups, 71, 72 cyclics (cy-
clolinear carbosiloxanes), 73 fluorocarbons,74 74 phenylethenyl substituents, 75
Cr-bound carbazole chromophores, 76 and pendant bicyclic fragments. 77
In contrast to the foregoing examples, the polysiloxane sometimes ap-
pears as side chains (e.g., on poly(phenylacetylene) main chains). 78
Search WWH ::




Custom Search