Making a plastic material involves linking hundreds of small molecules together into a long chain. We call this chain-building process polymerization. Linking molecules together releases heat, which makes the reaction want to go forward.
However, it also forces chaotic, freely moving molecules into a strict, ordered line. The universe naturally prefers chaos and disorder over strict lines. We can understand this tug-of-war using a rule called Gibbs free energy.
At low temperatures, the heat release wins out. The molecules link up to form a solid polymer. But as you turn up the heat, the desire for chaos becomes much stronger.
Eventually, you hit a specific tipping point called the ceiling temperature. Above this exact temperature, the rules flip completely. The long polymer chains actively want to unzip themselves back into small pieces.
The rate of destruction becomes faster than the rate of creation. Therefore, you cannot build a new polymer above this heat level. It does not matter what special catalysts or tricks you try to use.
The laws of thermodynamics will stop the chain from growing. This temperature limit depends heavily on how concentrated your starting materials are.