What's the monomer in a polycarbonate?
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&lt;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).