When the bulk oligomerization of 1,3-dioxolan-2-one (ethylene carbonate, EC) and 4-methyl-1,3-dioxolan-2-one (propylene carbonate, PC) with the 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A, BPA)/base system (bases such as KHCO3, K2CO3, KOH, Li2CO3, and t-BuOK) was investigated at elevated temperature, significant differences were observed. Oligomerization of EC initiated by BPA/base readily takes place, but the oligomerization of PC is inhibited. The very first propylene carbonate/propylene oxide unit readily forms a phenolic ether bond with the functional groups of BPA phenolate, but the addition of the second monomer unit is rather slow. The oligomerization of EC yields symmetrical oligo(ethylene oxide) side chains. According to IR studies the oligomeric chains formed from PC with BPA contain not only ether but also carbonate bonds. The in situ step oligomerization of the BPA dipropoxylate was also identified by SEC, and a possible reaction mechanism is proposed.