Here is a calibrated material model for a generic polypropylene (PP) tested at room temperature. To use this material model you will need to first install the MCalibration software, and then export the model to you favorite FE software. The calibrated material model is the PolyUMod Three Network (TN) model.
Note, you can get a free trial license to MCalibration and PolyUMod using the link on the main page.