In the spite of organic-chemistry, inorganic-chemistry, ... , environmental-chemistry, ... , I would like to propose a tag to be called "material-sciences" or "polymer-chemistry". (In line with physics.se as this is an overlapping field.)
The scope of plastics is not defined yet, there is no tag wiki and not even an excerpt. At the same time we have the materials, where I have created a very short description some weeks ago. I am no expert in the field, so I cannot be sure that i defined that well. When is comes to polymers it just states
A large molecule (macromolecule) composed of repeating structural units.
This is also very broad. I understand that people in that field will be able to distinguish between those tags naturally, but will an average asker be able to?
Another question is, would those tags attract different users to read them? We are not an experts only site - this might come as chemistry.se develops into something bigger. I am thinking along the line of math.se and math overflow.
As with theoretical-chemistry, quantum-chemistry and quantum-mechanics I would suggest merging those tags into a more general tag. (We have not reached a conclusion there).