The presence of foam in the petroleum industry causes many problems in gas-liquid separators and its complete elimination, by employing a silicon-free defoamer agent, poses a significant challenge. In this research, a series of high molar masses polyethers was synthesized using different epoxide monomers (1,2-propylene oxide PO, 1,2-butylene oxide BO, 1,2-hexylene oxide HO and 1,2-dodecylene oxide DDO), varying their molar masses from 10,000 to 120,000 g/mol. The synthesis was performed by anionic ring-opening polymerization (AROP) using as initiation system an ammonium salt and triisobutylaluminum as complexing agent and monomer activator. All polymers were characterized by H-1 NMR, SEC and TGA, experimentally evaluated as defoamer agents of gasified heavy crude oil (13.08 degrees API) and compared to a silicon based defoamer. Results revealed that PBO and PHO with molar masses above 80,000 g/mol show high activity as foam suppressors, similar to that of commercial silicon based defoamer agents.