Depends on what kind of plastic you buy. This is also true of the rod and tube in these grades. Acrylic, such as Plexiglas, is sold this way. The table below should help you be more informed about what you are getting. Since the beginning of the creation of plastics, many myths have been perpetuated about the longevity of plastics, especially outside in the elements.
All plastics come from petroleum and natural gas.
Sunlight, especially ultraviolet radiation, has a disastrous effect on most plastics. PE can easily be recycled. Many children's toys are made from PE and get brittle and crack when left outside. American-made acrylic does NOT yellow in the sunlight. There are three other clear plastics that do yellow in the sun and get confused with acrylic - Styrene, PETG, and Polycarbonate.