For those wondering - like me - MPI is Modified Polyimide.
From what I can gather (I have designed antennas on LCP materials), I know there was a delivery problem with LCP for one major manufacturer as a sub-supplier stopped production of certain base materials.
LCP had good performance up to 77GHz IIRC.
I presume MPI may allow easier integration with flex-rigid designs - perhaps also better heat handling in solder phases and probably lower cost (for Apple profitz)
I thought the antennas were the metal bands firming the perimeter of the phone. Where would there be an antenna made from imide / Kapton?