Investigating the active phase of Ca-based glycerol polymerization catalysts: On the importance of calcium glycerolate

•The active phase of calcium-based catalysts for glycerol polymerization was investigated.•Ca-glycerolate was identified as being the in situ formed solid active phase.•A mechanism is proposed to explain the in situ formation of calcium glycerolate.•A mechanism is proposed for the polymerization of glycerol catalyzed by Ca-glycerolate.