Synthesis Mechanism of Polyether Polyols

Different catalysts can lead to different reaction pathways in the ring-opening polymerization of alkene oxides to produce polyethers.

There are various theories regarding the polymerization mechanism of organic α-oxides such as ethylene oxide and propylene oxide. Initially, some believed that the polymerization of α-oxides occurred through a hydrogen atom transfer mechanism. However, this explanation does not account for the following two phenomena:

1.The activation energy for the ring-opening polymerization of alkene oxides decreases in the presence of H⁻ and OH⁻ ions.

2.Higher molecular weight polyethers can be obtained by increasing the acidity or basicity of the catalyst.

It is now clear that the ring-opening polymerization of alkene oxides, like ethylene oxide and propylene oxide, follows an ionic reaction mechanism. According to the ionic mechanism, the polymerization process of polyether polyols includes three stages: initiation, chain growth, and chain termination, regardless of whether the catalysis route is anionic or cationic.

1.Anionic Catalysis Mechanism

For the ring-opening polymerization reaction using epoxides, such as ethylene oxide, the process mainly follows an anionic polymerization mechanism. This mechanism can be divided into three steps:

·Initiation: The alcohol (or amine) initiator reacts with a strong base catalyst to form an alkoxide ion, making the three-membered ring of the epoxide easily cleavable and rapidly attaching to the initiator. When propylene oxide undergoes base-catalyzed ring-opening polymerization, the nucleophile tends to attack the less substituted carbon atom due to steric effects, leading to the formation of polyethers predominantly with secondary hydroxyl end groups.

·Chain Growth: This occurs through the transfer of anionic charge, releasing a significant amount of heat.

·Chain Termination: When the desired chain length is reached, the reaction can be terminated by neutralizing the chain-end charges through proton exchange or by adding water or acid.

Base catalysts not only catalyze the addition of propylene oxide to the initiator molecule but also catalyze side reactions, such as the isomerization of propylene oxide into propylene glycol (CH₃CH=CHOH) or allyl alcohol (CH₂=CHCH₂OH). Propylene glycol, as a mono-functional initiator, can undergo propoxylation under KOH catalysis, producing low molecular weight polyether monoalcohol impurities. Additionally, the active center of the molecular chain anion can undergo rearrangement, resulting in polyethers with allyl (or vinyl ether) end groups.

Therefore, polyether polyols produced using strong base catalysts have a broad molecular weight distribution, high unsaturation, low functionality, and difficulty in achieving high molecular weights (with molecular weights per hydroxyl group above 2000). While such polyethers are widely used in the production of polyurethane foams where high polyether quality is not required, they are inadequate for polyurethane soft foam products that require high molecular weight polyethers. Later, metal complex catalysts were developed, which can produce high molecular weight polyethers with low unsaturation.

2.Cationic Catalysis Mechanism

The ring-opening polymerization process can be explained using 1,4-butanediol as the initiator and boron trifluoride etherate as the catalyst. Initially, the Lewis acid catalyst and the initiator form a complex with a hydroxy complexed anion and a carbocation. The alkene oxide monomer (such as tetrahydrofuran) reacts with the carbocation to initiate chain growth. The reaction can be terminated by adding substances like ammonia water once the desired polymerization degree is reached.

Preferred Lewis acids include boron trifluoride, aluminum chloride, or iron chloride combined with thionyl chloride. The polyether obtained from the cationic polymerization of propylene oxide may contain both primary and secondary hydroxyl end groups.

The drawback of acidic catalysts is the occurrence of numerous side reactions, which can result in the formation of dioxane and dioxolane, and the possible replacement of hydroxyl groups by acidic anions. This polymerization reaction is conducted at low temperatures, ranging from 0°C to 20°C. In industrial applications, acidic catalysts are commonly used for the polymerization of tetrahydrofuran into polytetramethylene ether glycol, as well as the polymerization of epichlorohydrin and trichlorobutylene oxide.

3.Metal Complex Catalysis Mechanism

Studies on the polymerization mechanism suggest that zinc-cobalt bimetallic cyanide complexes catalyze the polymerization of propylene oxide through a living polymerization system. The true active center of the polymerization is a zinc ion coordinated with five oxygen atoms. The polymer's molecular weight depends only on the molar ratio of propylene oxide to the initiator, not on the amount of catalyst used.

During the polymerization of epoxy compounds, the bimetallic cyanide catalyst (DMC) is first activated by the propylene oxide, forming an active structure with the initiator. This structure promotes the growth of molecular chains by reacting with alkene oxides. The active structure formed between the initiator, propylene oxide, and the catalyst undergoes a substitution reaction, where the substitution rate exceeds the active chain growth rate. This ensures that all initiators simultaneously undergo chain growth, resulting in polyether polyols with a very narrow molecular weight distribution.