valve stem seals
 

why on my new built motor are 2 exhaust valve teflon seals breaking into pieces

I'm going to guess here...

Either the springs are rubbing on them, or you have a machine shop who can't setup valve heights correctly, and the retainers are smashing them into the guide.

x2 Many times the maching shop will cut the guides for a .500 or .531 seal but not reduce the total height of the guide. The valve height will also be reduced when installing new valves. The first time the engine rotates, the retainer smashes the seal.
Dave