Characteristics and Development of Low Unsaturation Polyether

The production technology for low-unsaturation, high-molecular-weight polyether polyols represents a significant breakthrough in polyether manufacturing. This technology is now being applied in specialized polyurethane flexible foam products and elastomer materials.

Conventional polypropylene oxide polyols (general-purpose polyether polyols) are typically produced using propylene oxide as the main raw material and potassium hydroxide (KOH) as a catalyst. This alkaline catalyst not only promotes the polymerization of propylene oxide on the initiator molecules but also induces side reactions, such as the isomerization of propylene oxide into allyl alcohol (CH₂=CHCH₂OH) or propylene glycol (CH₃CH=CHOH). The latter then acts as a monofunctional initiator, leading to propoxylation and resulting in low-molecular-weight, monohydroxy polyethers. Additionally, during polymerization, the active anionic center on the polymer chain may undergo disproportionation (chain transfer), forming polyethers with allyloxy (or propenoxy) end groups. Consequently, the actual functionality of the polyether is lower than its theoretical value. The higher the monool content, the lower the functionality (i.e., the greater the deviation from theoretical functionality). When the monools in polyether polyols react with diisocyanates or NCO-terminated prepolymers, they may terminate the growth of the polyurethane molecular chain, leading to polymers with lower molecular weights and reduced mechanical strength and other properties.

To minimize side reactions in KOH-catalyzed systems and manufacture low-unsaturation polyether polyols, researchers have developed various ring-opening polymerization catalysts, including alkaline earth metal compounds and metalloporphyrin complexes. Among these, bimetallic cyanide (DMC) catalysts have emerged as prominent alternatives, receiving significant attention both domestically and internationally. As early as the 1960s, General Tire & Rubber Company in the United States developed bimetallic cyanide catalysts. However, due to a lack of economic competitiveness, industrialization did not occur at that time. The technology was later acquired and improved by Lyondell (formerly Arco) in the United States, leading to the commercialization of a series of low-unsaturation, high-molecular-weight polyether polyols in the early 1990s, using bimetallic cyanide complexes as catalysts. Around the same time, Dow Chemical introduced the HPP series of low-unsaturation, high-molecular-weight polyether polyols synthesized using bimetallic cyanide complexes. Companies such as Asahi Glass, Olin, and BASF also launched corresponding products.

Low-unsaturation polyether polyols generally use DMC as the catalyst. Through controlled polymerization, the addition reaction of propylene oxide is significantly faster than the isomerization reaction. As a result, the polyether polyols produced by this method have extremely low unsaturation (or monool content) and a narrow molecular weight distribution, with unsaturation levels below 0.02 mmol/g.

When conventional polyether polyols are synthesized using KOH as the catalyst, the isomerization of propylene oxide becomes significant as the molecular weight of the polyether increases, resulting in unsaturated bonds at the chain ends, making it challenging to produce high-molecular-weight polyethers. Conventional polyether triols with a molecular weight of approximately 6200 have an unsaturation level as high as about 0.1 mmol/g. The molecular weight of practical, conventional polyether triols generally does not exceed 6000. The new DMC catalyst system enables the production of high-molecular-weight polyethers with relative molecular weights in the tens of thousands. For polyether polyols with practical applications in polyurethane foam, the molecular weight generally does not exceed 10,000. These high-molecular-weight polyethers exhibit low unsaturation.

Polyurethane foams and elastomers based on low-unsaturation, high-molecular-weight polyether polyols show significantly improved physical properties compared to conventional polyether polyols, which facilitates the production of soft foam products. For example, in high-resilience soft foam applications (including large block foams and molded foams), the formulation requires less TDI when using the same amount of water as with conventional polyether polyols, yielding foam with a more comfortable feel, resembling latex foam.