On the Mechanism of Base-Catalyzed Glycerol Polymerization and Copolymerization

Based on the herein experimental results and literature data, this article proposes a general mechanism of self-condensation of glycerol to polyglycerol in the presence of alkaline catalysts. The mechanism involves the in situ formation of glycidol and oligo-glycerols with terminal epoxy groups by intramolecular SN2 substitution reactions. We consider that terminal 1,2-glycol structures of oligo-glycerols also form terminal epoxy groups. The formation of polyglycerol consists of the addition of epoxy groups of glycidol or oligo-glycerols formed in situ, to various hydroxyl groups present in the reaction system. The formation of epoxy groups from 1,2-glycols catalyzed by bases at higher temperatures seems to be a general reaction. The proposed mechanism explains the formation of all the structures found in polyglycerols, such as linear, branched, and cyclic polyglycerol, as well as hybrid cyclic-linear and hybrid cyclic-branched polyglycerol. The self-condensation of glycerol to polyglycerol is in fact a special case of ring opening polymerization.Practical Application: While glycerol is used as an example, the results show that any compound containing 1,2-glycol structure in presence of alkaline catalysts forms a transient epoxy group, which can react with any hydroxyl group in the system. This allows generation of a large family of new ethers useful as additives or polyethers for foams, coating, and adhesives. This argument is supported by the examples of copolymers of glycerol, polyglycerols, and 1,2,6-hexane triol with triethanolamine or timethylolpropane. Mechanism of self-condensation of glycerol to PGL in the presence of alkaline catalysts involves in situ formed epoxy compounds as reactive intermediates. The formation of epoxy groups from 1,2 glycols catalyzed by bases at higher temperature seems to be a general reaction. The presented mechanism explains the copolymerization of glycerol with other polyols useful for a range of new products.