Introduction to Raw Materials of Polyether Polyols: Initiators

In the production of polyethers, initiators are compounds containing active hydrogen atoms. Unlike initiators in free radical polymerization, the initiators in polyether production act as the starting point for polyether chain segments, where epoxides like propylene oxide undergo chain growth.

The initiators used for synthesizing polyether polyols are diverse and numerous. They can be broadly classified into two main categories based on the nature of their active groups: hydroxyl-containing compounds and amine-containing compounds. These initiators are typically small molecules with multiple reactive sites, including polyols, polyamines, or polyol-amines. Commonly used initiators include propylene glycol, trimethylolpropane, glycerol, mannitol, sorbitol, pentaerythritol, sucrose, xylitol, ethylenediamine, triethanolamine, and toluenediamine. Sometimes, mixed initiators are used to achieve polyether polyols with specific functionalities and viscosities.

Initiators play a crucial role in determining the functionality and molecular weight of polyether polyols:

Control of Molecular Weight: Polyether polyols are composed of initiators and epoxides. If the total amount of epoxides remains constant, increasing the amount of initiator leads to lower molecular weight polyether polyols, while reducing the initiator amount results in higher molecular weight polyether polyols.

Control of Functionality: The functionality of polyether polyols can be adjusted by varying the number of active hydrogen atoms in the initiator. For example, typical di-functional alcohol initiators include ethylene glycol, diethylene glycol, and propylene glycol. Glycerol and trimethylolpropane are the main initiators used industrially to produce tri-functional polyethers with branched structures. To achieve higher degrees of branching, initiators like pentaerythritol, sorbitol, mannitol, glycosides, and dextrins can be used. Decomposition products of starch and cellulose can also serve as initiators.

Xylitol, often derived from agricultural by-products like corncobs through hydrolysis, hydrogenation, and crystallization, is abundant and inexpensive.

Water, as an initiator, provides two functional groups. It reacts with epoxides to form diols, which act as the initiator. Therefore, it is crucial to remove water from the initiator to prevent it from affecting the final functionality of the polyether polyol.

Amine initiators can produce highly reactive basic tertiary amine polyethers. Common amine initiators include alkylamines (e.g., ethylenediamine), alkanolamines (e.g., triethanolamine), and aromatic amines (e.g., toluenediamine). Each hydrogen atom directly bonded to nitrogen in polyamine initiators can act as an initiating site, determining the functionality of the polyether. For instance, polyether polyols synthesized from ethylenediamine have a functionality of four, while those from diethylene triamine have a functionality of five. These tertiary amine-containing polyether polyols, known as "amine ethers," have catalytic properties and are often used in formulations for polyurethane rigid foam spray applications.

In addition to using conventional alkyl alcohol or amine compounds as initiators, initiators containing other elements or structures can impart special properties to the resulting polyether polyols. For example, reacting phosphorus oxychloride with glycerol produces a phosphorus-containing diglycerol ester with a functionality of three. This initiator results in polyether polyols containing phosphorus, which can impart flame-retardant properties to polyurethane materials.

Phenols and polycarboxylic acids can also serve as initiators. Using aromatic or heterocyclic polyols or polyamines as initiators in polyether synthesis introduces aromatic rings or heterocyclic structures into the polyether polyol. This feature can enhance the dimensional stability, heat resistance, and flame retardancy of the resulting polyurethane materials. Common initiators in this category include bisphenol A, bisphenol S, toluenediamine, tris(2-hydroxyethyl) isocyanurate, and phenol-formaldehyde or aniline-formaldehyde condensates.