The main factors affecting the reactivity of polyether hydroxyl groups (-OH) are multifaceted and can be summarized into the following categories:

1.Type of Hydroxyl Group (Primary vs. Secondary Hydroxyl)

Primary Hydroxyl Groups: This is the most significant factor affecting reactivity. Primary hydroxyl groups (-CH₂OH) generally exhibit higher reactivity because the carbon atom they are attached to has less steric hindrance, and their α-hydrogen atoms are more easily activated by basic catalysts.

Secondary Hydroxyl Groups: Secondary hydroxyl groups (-CH(OH)-) are notably less reactive than primary hydroxyl groups due to greater steric hindrance around the attached carbon atom and fewer α-hydrogen atoms (only one), making them harder to activate.

Source of Influence: In polyether synthesis, the ring-opening of ethylene oxide (EO) primarily yields primary hydroxyl groups, whereas the ring-opening of propylene oxide (PO) mainly produces secondary hydroxyl groups. Thus, the proportion of EO in the polyether molecular chain is the most critical factor determining its average hydroxyl reactivity. Polyethers with higher EO content contain a greater proportion of primary hydroxyl groups, resulting in higher reactivity.

2.Steric Hindrance Effects

Bulky substituents or molecular chain structures near the hydroxyl group increase steric hindrance, impeding the approach of nucleophilic reagents to the hydroxyl oxygen or blocking the catalyst's access to the hydroxyl's α-hydrogen.

This effect is particularly pronounced near rigid structures or branching points at the chain ends. For example, polyethers synthesized from aromatic initiators (e.g., bisphenol A) or high-functionality initiators (e.g., sucrose) typically exhibit greater steric hindrance around their hydroxyl groups compared to those derived from small diols (e.g., propylene glycol) or glycerol.

3.Molecular Weight

End-Group Concentration: Higher molecular weight reduces the number of hydroxyl end groups per unit mass or volume (i.e., lower hydroxyl value), decreasing the number of reactive sites. While this does not affect the intrinsic reactivity of individual hydroxyl groups, it reduces the overall reaction rate (fewer hydroxyl groups participating in the reaction per unit time).

Diffusion Limitations: Higher-molecular-weight polyethers generally have higher viscosity, leading to slower chain movement and reduced diffusion rates of reactants to hydroxyl sites, thereby slowing the macroscopic reaction rate. High viscosity also makes it harder to dissipate heat generated during the reaction.

4.Molecular Chain Flexibility

Greater chain flexibility and segmental mobility help hydroxyl groups and reactants overcome steric hindrance, facilitating collisions and reactions.

Polyether backbones are typically flexible (especially polyoxypropylene segments), but the presence of rigid structures or high branching reduces flexibility, thereby lowering reactivity.

5.Catalysts

Catalysts have a profound impact on polyether hydroxyl reactions.

Alkali Metal Catalysts: Commonly used organotin (e.g., dibutyltin dilaurate) and amine catalysts (e.g., triethylene diamine) significantly accelerate reactions. Different catalysts may exhibit varying efficiencies for primary and secondary hydroxyl groups. For example, certain amine catalysts are more effective at activating secondary hydroxyl groups.

Catalytic Mechanism: Catalysts typically accelerate reactions by activating the hydroxyl group or the reactive group interacting with it. The choice and dosage of catalysts are among the most effective and commonly used methods to regulate reaction rates.

6.Temperature

Increasing temperature is a universal method to enhance the rate of chemical reactions, including those involving polyether hydroxyl groups.

Effects of Higher Temperature:

Increases molecular thermal motion and collision frequency/energy.

Reduces system viscosity, improving diffusion.

Enhances catalyst efficiency.

Caution: Excessive temperatures may intensify side reactions or pose safety risks.

7.Impurities

Moisture (Water, H₂O): Water contains active hydrogen and can preferentially react with highly reactive groups, consuming reactants and potentially generating gas, interfering with the main reaction. This significantly reduces the effective reaction rate and affects product quality. Strict control of moisture content is essential.

Acids: Residual acidic substances can neutralize commonly used basic catalysts, deactivating them and severely inhibiting reaction rates.

Other Active Hydrogen Impurities: Alcohols, amines, etc., can also consume reactants.

8.Concentration

Higher reactant concentrations increase the likelihood of effective collisions per unit volume, accelerating the macroscopic reaction rate.

Summary:

Intrinsic Factors: Hydroxyl type, steric hindrance (molecular structure), and molecular weight (affecting end-group concentration and viscosity) are critical.

Catalysts: The most powerful and commonly used external means to regulate reaction rates.

Temperature: A universally applicable accelerator.

Strict Removal of Moisture and Acidic Impurities: A prerequisite for ensuring expected reactivity.

In practical applications, the reactivity of polyether hydroxyl groups is precisely controlled by carefully selecting polyether types, catalyst systems, and process conditions (temperature, mixing efficiency, vacuum dehydration), as well as strictly monitoring raw material quality (low moisture, low acid value). This ensures compliance with the performance and processing requirements of different products.