He also took classes with Daniel Mornet, Fernand Baldensperger, Paul Hazard, and Mario Roques, shifting toward studies of comparative literature and working as a lecturer on the Romanian language at the Sorbonne and the École nationale des langues orientales vivantes.
Popovici's first published book was his doctoral thesis, the 1935 Ideologia literară a lui I. Heliade-Rădulescu; this was followed later the same year by an expanded study, "Santa Cetate".
During World War II, he lived in Sibiu, having withdrawn there after the Second Vienna Award granted Northern Transylvania, including Cluj, to Hungary.
He also held courses on the history of literary ideology and of modern Romanian literature, published a volume of studies (Cercetări de literatură română) and put together critical editions of the works of Dimitrie Bolintineanu (Scrieri alese, 1942) and Ion Heliade Rădulescu (Opere, vol.
He made plans for a wide-ranging history of modern Romanian literature, of which he managed to publish just the first volume, La Littérature roumaine a l’époque des Lumières (1945).