In lots of examples, it does. Let’s stick with polypropylene for now. Isotactic polypropylene is a crystalline material with a melting point around 160 °C. The crystallinity is due to the perfect arrangement of the methyl groups along the polymer backbone. This crystallinity makes the material very tough, so it’s used in all sorts of applications—from pipes to plastic chairs and carpeting. But if there are errors along the polymer chain (methyl groups pointing in the wrong direction) the melting point decreases, and the plastic loses its strength.