Strictly speaking, polycarbonate is not just one type of polymer but is a whole family of them. The name simply indicates that the monomers that reacted to form the polymer did so by forming carbonate groups:

But in most cases, "polycarbonate" refers to a specific polymer that is typically made by reacting two monomers - bisphenol A (BPA) and phosgene. A given molecule of BPA will react with two molecules of phosgene on opposite ends of the BPA. Each of the reacted phosgene molecules will then add on another BPA molecule, which then react with more phosgene...alternating back and forth between BPA and phosgene. (The reaction is a little more complicated, in that there by products formed, and even the BPA needs to be modified initially in order to react with the phosgene. But since no one is going to be trying this on their own, I'll gloss over that.)
The resulting structure has none of the chemical properties of either BPA or phosgene,

The most common tradename for polycarbonate is Lexan. In my mind, it's becoming a generic term much like Plexiglas is for polymethyl methacrylate. This might have contributed to "Lexan" being the most searched plastics on the IDES database in 2009. Or maybe it was because of the BPA scare.
But let's be perfectly clear on one thing here: BPA is not a plastic, it is not even added to plastic. It is used to make a plastic.
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