Polymer based disc brakes

I'm not an expert, but the disk in a disk brake system needs to absorb enourmous amounts of heat to dissipate the energy of motion. It then needs to transfer that heat to the air. Polymers generally are poor heat conductors and have relatively low heat capacity. Add to this lower strength and you have a poor choice for disk material. As Jim Clark mentions in his comment thermosetting polymers don't melt and can have higher degradation temperatures, but they will burn. All in all, it is hard to justify any polymer over something cheap and effective like cast iron.