Preparation of Low-Unsaturation Polyether - Polypropylene Oxide Polyol

The preparation of polyether polyols can be divided into several stages, including heating and dehydration, catalyst activation, and polymerization.

1. Heating and Dehydration Process

   DMC (double metal cyanide) and the initiator (either a small molecule polyol or a low-molecular-weight polyether polyol with a molecular weight around 500) are added to the reactor. After purging with nitrogen several times, the mixture is heated to 105–120°C, and trace water is removed under reduced pressure.

2. Catalyst Activation (Induction) Process

   Add 1/10 to 2/10 of the initiator mass in PO (propylene oxide) to the dehydrated mixture of DMC and initiator. The pressure in the reactor quickly rises. After a period of time at 105–150°C, DMC begins to activate, causing a rapid drop in the vapor pressure of PO. When the temperature increases, it indicates that the induction is complete, and PO starts to polymerize.

3. Polymerization Reaction Process

   The remaining PO is added to the reactor to continue the polymerization reaction, with the internal pressure maintained at 0–0.6 MPa. Due to the high catalytic activity of DMC, PO can even be introduced at near-zero pressure. The reaction temperature is maintained between 105–150°C. When the PO pressure in the reactor stabilizes, it signifies that PO has fully reacted, and the reactor is cooled to discharge the product. Early DMC formulations had low activity and required only a trace amount, resulting in longer induction periods and higher metal ions in the crude polyether, which required further purification through conventional polyether production methods.

Increasing the initial reaction pressure during polyether polyol synthesis can shorten the induction period but may negatively impact the physical properties of the resulting polyether polyol. A method of gradually introducing PO at low pressure during induction benefits the quality of the final product. Additionally, raising the induction temperature appropriately and adding PO slowly at low pressure yields a polyether polyol with more desirable properties.

The induction time significantly shortens as the catalyst amount increases, with minimal impact on polyether properties. Experiments show that with a catalyst concentration of 70*10⁻⁶, a small amount of gel forms in the polyether polyol, as the reaction rate increases too quickly under high initial pressure, causing rapid molecular chain growth without sufficient chain transfer. Reducing the PO addition rate improves the outcome.

Experiments show that conducting the induction reaction at a lower temperature results in a longer induction period, slightly higher polyether viscosity, and a broader molecular weight distribution. At the same pressure, a lower temperature (below 105°C) leads to more liquid-phase PO, which can cause local overheating once the reaction initiates. At a higher initial pressure (e.g., 0.25 MPa), the reaction proceeds very rapidly, making it difficult to control in practice. Therefore, in production, the initial temperature is controlled between 130–145°C, and the initial pressure is kept as low as possible.

Increasing the initial pressure of PO shortens the induction period but slightly reduces polyether quality. When the initial pressure exceeds 0.4 MPa, impurities with ultra-high molecular weight often appear in the polyether polyol. This is because the active catalytic sites depend on PO activation during polyether polyol synthesis. The higher the PO pressure or concentration in the reactor, the shorter the induction period; however, during induction, the active catalytic sites have not yet reached maximum levels, making higher PO concentrations the source of ultra-high molecular impurities.