Harald,

It is very hard to be specific because of the lack of information, design and
application.  However, this may help in directing your search.  You are looking
for a thermoplastic like  PVC, PE and PET.  These materials are used for such
things as "living hinges", "stents" and "springs".  Reversible processes around
the glass transisition temperature are generally used in "smart materials".

50% is a little on the high side for a reversible process, but the 150C is
workable but also on the high side.

At this point you should define your criteria (loads, temperatures, dimensions,
environment, application process,...), and start talking with vendors in your
area.  If your potential product volume is high they will bend over backwards to
help.  If this is  research and small quantities then you will need to start
your own literature search in some of the plastic journals and handbooks.

Good luck, Rob