Basic question. Yet if the historian wants to expand it with the context and popularization, it will be great.
Thanks again to everyone involved in AskHistorians. Best subreddit ever.
Edit1: Based on the clarifying answer r/erath_droid gave, I would like to reformulate the original question: When was synthetic plastic discovered, and how did the popularization process happened (considering that the vast majority of items in a XXI Century household are synthetic plastic-made)?
Not a historian, but a biochemist- this is a very broad question, and I don't have the historical answers but I do possess some of the technical knowledge to help explain why this is a broad question and break out plastics into its various definitions and hopefully a historian can come along and flesh out the answers.
"Plastic" is a very broad definition. Plastics are basically long (mostly) carbon chain polymers, but there are a LOT of different things that qualify as plastics.
If you are referring to "natural" plastics (like rubber and shellac) that are purely naturally occurring compounds, those have been in use for centuries. Technically tar and pitch could be considered plastics as well, and none of these were invented so much as discovered.
There are also "semi-synthetic" plastics, which are derived from non-petroleum sources (such as cellulose) but have been chemically altered from their naturally occurring state, which have been in use since the early-to-mid 1800's. Examples of this would be polystyrene (originally derived from tree sap) and vulcanized rubber- a compound that had been in use for centuries but was chemically altered to be different from naturally occurring rubber.
The final class of plastics would be purely synthetic plastics- i.e. those derived entirely from fossil fuels, which include things like polystyrene- identical in chemical structure to the semi-synthetic version but instead of being made from (essentially) tree sap was synthesized entirely from petroleum.
So the question of "When was plastic invented?" would have a drastically different answer depending on what exactly you mean by "plastic" and "invented." You may want to clarify what you mean by "plastics" so that the historians here can provide a more specific answer to your question.
I hope this helps.
I apologize if this isn't worthy of a top-level answer, but the question is one that is very non-specific and I'd hoped to provide a little perspective to help narrow down (or expand) the question so that it could be adequately answered.