In polyurethane chemistry, the type of hydroxyl group (primary vs. secondary) at the terminal ends of polyether polyols significantly influences their reactivity with isocyanate (-NCO). This difference is a critical factor in formulating designs, selecting raw materials, and adjusting process parameters (e.g., reaction rate, gel time, and curing time).

Below is a comparison of the reactivity of primary hydroxyl groups (from EO) and secondary hydroxyl groups (from PO) in polyethers with isocyanate:

1. Reactivity Comparison

Primary Hydroxyl (-CH₂OH, mainly from EO capping): Significantly higher reactivity.

Reason: Lower steric hindrance. Primary hydroxyls are attached to primary carbon atoms (-CH₂-), where the surrounding space is relatively open, making it easier for the isocyanate group to approach.

Electronic effect: The oxygen atom in primary hydroxyls has a relatively higher electron density (due to inductive and conjugative effects), making it more susceptible to nucleophilic attack by the electrophilic carbon of isocyanate.

Secondary Hydroxyl (-CH(CH₃)OH or >CHOH, mainly from PO capping): Significantly lower reactivity.

Reason: Higher steric hindrance. Secondary hydroxyls are attached to secondary carbon atoms (-CH(CH₃)- or >CH-), where adjacent methyl groups (-CH₃) or other substituents create greater spatial obstruction, hindering the approach of isocyanate groups.

Electronic effect: The electron-donating inductive effect of alkyl substituents (e.g., methyl) does not increase the electron density on the oxygen atom as significantly as in primary hydroxyls (or may even slightly reduce it due to steric factors), and the charge is more dispersed.

2. Quantitative Difference

Under similar conditions (temperature, catalyst, etc.), the reaction rate of primary hydroxyls with isocyanate is typically 3 to 4 times faster than that of secondary hydroxyls. This means polyether polyols with a higher proportion of primary hydroxyls react much more rapidly with isocyanates.

3. Impact on Polyurethane Processing

Reaction/Curing Speed: Formulations using polyethers with high primary hydroxyl content (e.g., EO-capped polyethers) generally exhibit shorter cream time, gel time, and demolding time, leading to faster curing. This is crucial for production lines requiring rapid demolding (e.g., molded foams, elastomers).

Catalyst Requirement: To achieve the same reaction speed, formulations based on secondary hydroxyl polyethers (e.g., pure PO polyethers) often require higher concentrations or more potent catalysts (e.g., organotin or amine catalysts). EO-capped polyethers are also more sensitive to catalysts.

Final Properties: While the hydroxyl type primarily affects reaction kinetics, the backbone structure of the polyether (PO homopolymer, PO/EO copolymer, EO capping ratio) has a greater influence on the final polymer's soft-segment properties (flexibility, low-temperature resistance, hydrophilicity). Although primary hydroxyls do not directly determine the final properties, they indirectly influence performance (e.g., the speed at which mechanical properties develop) by affecting the reaction extent and crosslinking network uniformity.

4. Common Polyether Types and Hydroxyl Groups

PO Homopolyethers: Terminated mainly with secondary hydroxyls. Lower reactivity, typically requiring strong catalysts. Examples include PPG (polypropylene glycol/triol).

PO/EO Copolyethers (random or block): Terminated mainly with secondary hydroxyls (unless explicitly EO-capped). Slightly higher reactivity than PO homopolyethers (due to increased chain flexibility from EO incorporation), but still significantly lower than primary hydroxyls.

EO-Capped Polyethers: Synthesized by replacing terminal PO units with EO units. Terminated mainly with primary hydroxyls. This is the most common and effective method to enhance polyether reactivity. For example, many high-resilience (HR) foams and CASE (coatings, adhesives, sealants, elastomers) applications use EO-capped PO-based polyethers. The primary hydroxyl content (typically 70–85%) is a key specification for such products.

PEG (Polyethylene Glycol): Fully polymerized from EO, terminated with primary hydroxyls, offering the highest reactivity and hydrophilicity.

Summary Comparison Table

Conclusion

In polyether polyols, primary hydroxyl groups (mainly from EO units, especially EO capping) exhibit much higher reactivity with isocyanates than secondary hydroxyl groups (mainly from PO units). This reactivity difference is a fundamental aspect of polyurethane chemistry, directly influencing formulation design, catalyst selection, and production parameters. By controlling polyether synthesis (particularly through EO capping), the reactivity of polyols can be precisely tailored to meet the requirements of diverse applications.