Some webbing might be made of polypropylene but I would suspect that most of it is nylon, especially if you buy it in bulk off a roll. It would be difficult to tell the difference until the material is wet. The PP would still be tight while the nylon would stretch. Therein may be what people are seeing when they report that their straps don't stretch when wet. They may be using 'nylon' webbing but, in actuality, they are using polypropylene.