The animation you described IS possible, but in a different way.
You have to use multiple Polygon selections with multiple tags of the material used to define the "faces."
The trick is that you keyframe animate the tags ON/OFF setting. So, while you have the Material Tag with the "smiling" polygon selection keyframed ON - you keyframe the Material Tag with the "frowning" polygon selection OFF. And then alternate - turn the smiling Tag OFF and the frowning Tag ON. See attached file.
If you need to know HOW to keyframe these. Go to the animation timeline - selected the Material Tag (frown or smile) - right-click (or control-click) the RECORD button - select "Tag On/Off" - repeat for all occurrences.
Cheers