Raw Material Introduction of Polyether Polyols: Ring-Opening Polymerization Catalysts

Ring-opening polymerization catalysts for epoxy compounds include alkali metal hydroxides, Lewis acids, metal alkoxides, metal alkoxide + metal halide combinations, metal alkyls, activated alumina, aluminum silicate + metal alkyls, alkali metal carbonates, and fluoroborate esters. The commonly used catalysts for the ring-opening polymerization of oxiranes, such as propylene oxide and ethylene oxide, can be classified into three main types: anionic catalysts, cationic catalysts, and metal complex catalysts.

In the production of polyether oligomers used for polyurethane, the primary catalysts are alkali metal hydroxides, which are anionic catalysts, and Lewis acids, which are cationic catalysts. The former is mainly used to prepare regular polyether polyols with lower molecular weights, while the latter is used for the production of high molecular weight polyether polyols and special polyether polyols from the copolymerization of tetrahydrofuran. Metal complex catalysts are mainly used for preparing ultra-high molecular weight isotactic polyether polyols, though they are currently only used in small quantities in the synthesis of polyether polyols for polyurethane applications.

1. Anionic Polymerization Catalysts  

   Anionic catalysts include potassium hydroxide, sodium hydroxide, potassium alcoholate, and sodium alcoholate. Hydroxides are the most commonly used basic catalysts, producing polyether polyols with relatively low molecular weights. The most commonly used alkali metal hydroxides are potassium hydroxide and sodium hydroxide, with potassium hydroxide being the most popular due to its lower required amount. The end groups of the resulting chains are mainly hydroxyl groups, with a few alkoxide metal salts. When proton exchange occurs between these end groups, the ionic charge preferentially transfers to the shorter chain, resulting in a relatively narrow molecular weight distribution for the resulting polyether polyols. Of course, the width of the molecular weight distribution of polyether polyols is also influenced by the type of initiator, monomer, and synthesis process conditions. Industrially, nearly all catalysts for polymerization reactions are strong bases, used in amounts of 0.1% to 1.0% of the total feed, with reaction temperatures ranging from 100 to 120°C. Additionally, a small amount of special amines, such as dimethylamine, can also be used as catalysts in the synthesis of polyethers.

2. Cationic Polymerization Catalysts  

   Common cationic catalysts include Lewis acids such as FeCl3, AlCl3, SnCl4, BF3-ether complex, chlorosulfonic acid, and fluorosulfonic acid. The drawback of acidic catalysts is that they can easily cause side reactions during the synthesis of polyether polyols, leading to the formation of dioxane and dioxolane, and there is a possibility of the hydroxyl groups being replaced by acidic anions. These polymerization reactions are carried out at low temperatures ranging from 0 to 20°C. Industrially, these acidic catalysts are used in the polymerization of tetrahydrofuran to produce polytetramethylene ether glycol.

3. Metal Complex Catalysts

   Many metal ion complexes can be used as catalysts for the polymerization of propylene oxide, with the most active being double metal cyanide catalysts (DMC). The most successful catalyst system is the zinc-cobalt cyanide complex series, which is used in amounts of only a few parts per million of the polyether. Double metal cyanide complexes, such as zinc hexacyanocobaltate, are generally suitable for preparing high molecular weight isotactic polyether polyols. In recent years, there has been a growing interest in low unsaturation, high molecular weight polyethers due to their low unsaturation, high average functionality, and narrow molecular weight distribution. The resulting soft polyurethane foams and elastomers exhibit better performance.