I know what the repeating unit is, but what's the monomer in a polycarbonate?
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$\begingroup$ A polymer name is defined by the repeating unit. Polycarbonates (which is a large group of polymers) further are a nice example where the same repeating unit can be created in a number of ways, from different monomers. Your question is a bit ambigous with regard to this. Please elaborate. What polycarbonate are we talking about? $\endgroup$ – Karl Jul 15 '18 at 21:10
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$\begingroup$ Not all polymers are made of single monomers. Polycarbonates usually consist of at least 2 so asking what "the" monomer is isn't quite correct. $\endgroup$ – matt_black Jul 16 '18 at 13:06
Like Dr J said there is no single polycarbonate polymer, the most common one is formed from phosgene (or another carbon dioxide reagent) and bisphenol A. The repeating unit is $\ce{C6H4C(CH3)2C6H4-O-C(=O)-O -}$
In Nagahata, R.; Sugiyama, J.; Goyal, M.; Goto, M.; Asai, M.; Ueda, M.; Takeuchi, K. X-ray analysis of the bisphenol-A-type macrocyclic carbonate trimer. Polym. Adv. Technol. 2000, 11 (6), 294–300. DOI: 10.1002/1099-1581(200006)11:6<294::AID-PAT979>3.0.CO;2-C. (OCAMUA) a tetramer of the repeat unit of this polymer has been reported. This is a small cyclic polymer (oligomer).
There is no one single answer to your question as posed. Polycarbonates (PC) are a group of polymers, the most common of which would probably be the polycarbonate of bisphenol A. The monomer of this PC can be seen here https://en.wikipedia.org/wiki/Polycarbonate. There are hundreds of other polycarbonate resins that exist, and they will each have their own unique structure and unique monomer.