Why Are There No Perfluorosiloxanes?

There are fluorocarbons, but they only have a little fluorine in them, attached to carbon-based groups attached to the siloxane backbone. These materials aren't much different
than regular silicone, except they cost more and emit a toxic gas if you get them too hot. Why are there no polymers which consist of a siloxane backbone with a fluorine at every available site (i.e. replacing the methyl groups in PDMS).
It seems to me that this material, if possible, would have amazing properties. It would be an elastomer which should be stable indefinitely at 500 degrees C or even more!
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