I always thought OEM bumpers were made from ABS plastic.
However I can tell you that polypropylene is a great alternative. My front bumper as well as my sideskirts are both made out of it and I couldn't be happier after dealing with so many fiberglass parts over the years. Avoid fiberglass, it cracks easily because it's so rigid. Polypropylene feels like OEM and it's sturdy, strong, and flexible.