Overview of Various Polyethers—Polypropylene Oxide Tetrol

Due to differences in the types of initiators used, polypropylene oxide tetrol (tetrahydroxy polyether) is typically categorized into two types: ethylenediamine-based polyether polyol and pentaerythritol-based polyether polyol.

When ethylenediamine is used as the initiator and propylene oxide is polymerized through ring-opening at 100–110°C without a catalyst, the resulting tetrahydroxy polyether is a type of “amine ether.” This nitrogen-containing polyether polyol exhibits certain tertiary amine basicity and polyhydroxy properties, which accelerate its reaction with isocyanates, making it widely used in rigid foam formulations as a catalytic polyol ingredient for on-site spray applications.

In the synthesis of polypropylene oxide tetrol, ethylenediamine not only acts as the initiator but also serves as a basic catalyst. As the polymerization proceeds and molecular weight increases to a certain level, catalytic activity decreases. For producing high molecular weight polyether tetrol, additional KOH (potassium hydroxide) as a basic catalyst is required. When the mass ratio of propylene oxide to ethylenediamine is 100/(3.7–4.0) with a KOH amount of 0.5%–1.0%, an ethylenediamine polyether with a hydroxyl value of 280–320 mgKOH/g and a relative molecular weight of 700–900 can be obtained. When the propylene oxide to ethylenediamine mass ratio is 100/(2.0–2.1) with a KOH amount of 0.7%, an ethylenediamine-based polypropylene oxide tetrol with a hydroxyl value of 85–93 mgKOH/g and a relative molecular weight of 2400–2600 is produced. Due to the presence of polar tertiary amine groups and numerous hydroxyl groups, the viscosity of ethylenediamine-based polypropylene oxide tetrol is very high.

The preparation conditions of pentaerythritol-based polyether polyol are largely similar to those of glycerol-based polyether polyol. Since pentaerythritol is a crystalline solid with limited solubility in propylene oxide, the initial induction period of the reaction is longer compared to when glycerol is used as the initiator. For pentaerythritol polyether with relative molecular weights of 400, 500, and 600 (hydroxyl values of 560 mgKOH/g, 448 mgKOH/g, and 374 mgKOH/g, respectively), the viscosity is approximately 2800 mPa·s, 1500 mPa·s, and 1140 mPa·s. Pentaerythritol-based polyether polyol is primarily used in general rigid foam formulations. Compared to triol polyethers, pentaerythritol polyethers have higher functionality, resulting in rigid foams with improved heat resistance and dimensional stability.