There are a lot of plastics that are disposable. The first plastics over 100 years ago were made from milk (I suspect the Romans knew the process - it wasn't industrially useful though). Many decomposable plastics have been made over the year. The PLA commonly used in 3d printers is decomposable (in the right environment).

Oil based plastics are generally a lot cheaper than the above though, and they are typically not decomposable. Depending on your use this can be good or bad (I don't want my plastic plumbing pipes to decompose, but other plastics are used up and I want them to decompose)