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Long-Time Stability of Menthyl and Thymyl Vinyl Ethers in the Koh–Dmso–Acetylene System: Product Loss, Byproducts, and Polymer/Resin Formation

This study demonstrates that menthyl vinyl ether exhibits superior long-term stability and lower resin formation compared to thymyl vinyl ether in KOH–DMSO–acetylene systems, reaching its optimal yield at 300 minutes versus 180 minutes for the thymyl derivative, which suffers significantly higher post-peak degradation and polymerization.

Original authors: Makhammadjon Soliev, Suvonkul Nurmonov, Shukhrat Nuraliev, Vokhidkhuja Azizov, Gulmira Parpieva

Published 2026-10-14

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CC BY 4.0

Original authors: Makhammadjon Soliev, Suvonkul Nurmonov, Shukhrat Nuraliev, Vokhidkhuja Azizov, Gulmira Parpieva

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of chemical manufacturing, creating a useful substance is only half the battle; keeping it stable long enough to be useful is often the harder part. Imagine a chemical reaction as a factory assembly line where raw materials are transformed into a valuable product. If the line runs too briefly, the product is incomplete. If it runs too long, the product might begin to break down, turn into waste, or stick together into a useless solid. This delicate balance is especially critical for a class of chemicals called vinyl ethers. These molecules are prized for their ability to be turned into high-performance plastics, coatings, and adhesives, but they are also chemically fragile. Because of their structure, they can easily react with themselves or other impurities, turning from a liquid product into a sticky tar or a hard resin if left in a hot environment for too long. Understanding exactly when to stop the reaction is the key to efficiency, ensuring that the maximum amount of valuable product is harvested before it begins to degrade.

Researchers at institutions in Uzbekistan set out to solve this timing puzzle for two specific types of vinyl ethers made from natural plant compounds: one derived from menthol and the other from thymol. These compounds are created by reacting the plant alcohols with acetylene gas in a specialized mixture of potassium hydroxide and dimethyl sulfoxide, heated to 130 degrees Celsius under pressure. The team did not just watch the reaction start; they monitored it continuously over a period of eight hours, taking samples every hour to see how the amount of desired product changed, how much waste was created, and how much solid residue formed. They treated each time point as a separate, independent experiment to ensure that the solid residue they measured was a snapshot of that specific moment, not a pile-up of waste from earlier in the process.

The results revealed a clear difference in how long each reaction could safely run. The reaction using menthol climbed steadily, reaching its highest concentration of the desired product after five hours. At this peak, the yield was just over 93 percent. Even after running for another three hours, the menthol-based product remained remarkably stable, losing only a tiny fraction of its value. In contrast, the reaction using thymol reached its peak much earlier, at just three hours, with a yield of 73 percent. After this point, the situation changed dramatically. If the thymol reaction was allowed to continue, the amount of useful product dropped significantly, falling to less than 50 percent by the six-hour mark. The researchers found that for the thymol reaction, every minute spent past the three-hour peak increased the risk of the product turning into a solid, sticky resin or breaking down into unwanted byproducts.

To understand why this happened, the team looked closely at the relationship between the disappearing product and the appearing waste. For the thymol reaction, there was a perfect match: whenever the amount of useful product dropped, the amount of solid resin increased by a corresponding amount. This suggests that the product was actively transforming into this sticky residue. For the menthol reaction, this direct link was not as clear, which aligns with its greater stability. The solid residue in both cases was measured as a dry mass, representing a mix of polymerized chains and other secondary reactions, though the exact chemical structure of this residue was not identified in this study. The data showed that while resin formation increased over time for both substances, the thymol system was far more sensitive to prolonged heating.

The practical conclusion from this work is a specific recommendation for stopping the process. For the menthol-based reaction, the ideal time to stop is around five hours, as extending the process offers no benefit and only a small risk of loss. For the thymol-based reaction, the window is much tighter; the process should be halted at three hours to capture the maximum yield. Continuing beyond these points does not create more product; instead, it encourages the valuable liquid to turn into solid waste or break down into other chemicals. This study provides a clear guide for industrial chemists, showing that while both reactions start similarly, their long-term behaviors are distinct, and knowing the precise moment to stop is essential for maximizing efficiency and minimizing waste.

Technical Summary: Long-Time Stability of Menthyl and Thymyl Vinyl Ethers in the KOH–DMSO–Acetylene System

Problem Statement
Vinyl ethers, synthesized via the direct addition of alcohols or phenols to acetylene, are valuable intermediates for polymers, coatings, and functional materials. However, their electron-rich C=C-O unit renders them chemically sensitive, prone to cationic polymerization, rearrangement, and degradation under the harsh basic and thermal conditions required for their synthesis. While previous studies have optimized reaction parameters such as temperature, pressure, and catalyst loading to maximize yield, a comparative analysis of long-term residence-time stability—specifically focusing on post-peak product loss, byproduct formation, and polymer/resin accumulation—has been lacking. This study addresses the need to define optimal "stopping times" to prevent the consumption of target ethers into secondary products and solid residues in a KOH–DMSO superbase system.

Methodology
The research compared the long-time stability of menthyl vinyl ether and thymyl vinyl ether over a 480-minute period. The reaction was conducted in a GPXF-4×250 mL system using 145 mL charges containing approximately 1.00 mol L⁻¹ substrate (menthol or thymol), 50 mol% KOH, and dried DMSO, under acetylene pressure at 130°C and 14 atm with 240 rpm agitation.

Key methodological features included:

Independent End-Point Charges: Unlike cumulative sampling, each time point (0, 60, 120, 180, 240, 300, 360, 420, and 480 min) was derived from three independent reactor charges (n=3). This ensured that polymer/resin mass represented the dry residue of a specific reaction duration rather than a cumulative accumulation.
Analytical Techniques: Concentrations of substrate ( $C_{sub}$ ), product ( $C_{product}$ ), and byproducts ( $C_{byproduct}$ ) were quantified using GC–FID with biphenyl as an internal standard. Polymer/resin was measured as the gravimetric mass of the dry residue.
Statistical Modeling: A piecewise approach separated product formation from post-peak loss. An apparent loss constant ( $k_{loss}$ ) was derived descriptively from the slope of $\ln(C_{max}/C_t)$ versus time. Non-parametric correlations (Spearman $\rho$ and Kendall $\tau$ ) were used to assess relationships between product loss, time, and resin formation, acknowledging the small sample size and treating $k_{loss}$ as a descriptive metric rather than a definitive kinetic constant.

Key Results

Product Maxima and Stopping Times: Menthyl vinyl ether reached its maximum concentration ( $0.93087 \pm 0.00145$ mol L⁻¹; 93.09% yield) at 300 minutes. Thymyl vinyl ether peaked earlier at 180 minutes with a lower maximum ( $0.73000 \pm 0.00209$ mol L⁻¹; 73.00% yield).
Post-Peak Stability: Extending the reaction beyond the peak revealed significant differences in stability. At 480 minutes, menthyl vinyl ether retained 96.48% of its maximum yield (3.52% loss), whereas thymyl vinyl ether retained only 79.29% (20.71% loss).
Degradation Kinetics: The apparent post-peak loss constant ( $k_{loss}$ ) for thymol (0.0620 h⁻¹) was 6.3 times higher than that for menthol (0.0099 h⁻¹). However, statistical fits were modest ( $R^2$ values of 0.672 and 0.531 respectively, with $p > 0.05$ ), leading the authors to treat these constants as descriptive trends rather than intrinsic kinetic parameters.
Polymer/Resin Formation: Both substrates showed a significant positive correlation between reaction time and polymer/resin mass (Kendall $\tau$ = 0.778 for menthol and 0.833 for thymol). For thymol, there was a perfect rank association (Spearman $\rho$ = 1.00) between product loss and resin mass at post-peak points, indicating that product degradation directly correlates with resinification. This strong association was not observed for menthol ( $\rho$ = -0.60).

Significance and Claims
The study establishes that prolonged residence time in the KOH–DMSO–acetylene system leads to the secondary consumption of formed vinyl ethers, particularly for the thymyl derivative. The authors claim that process optimization must prioritize residence-time control alongside substrate conversion.

The primary practical contribution is the definition of approximate stopping times: 300 minutes for menthol and 180 minutes for thymol. Continuing the reaction beyond these points does not improve target yield; instead, it promotes the diversion of the product into byproducts and polymeric/resinous residues. The paper concludes that thymyl vinyl ether is significantly more sensitive to long-time secondary transformations and resinification than menthyl vinyl ether under these specific conditions. The authors explicitly note that without structural characterization (e.g., SEC/GPC, NMR), the "polymer/resin" remains an operational designation for the dry residue, and the observed degradation likely involves a complex sequence of cationic chain growth and other secondary reactions.

Technical Summary: Long-Time Stability of Menthyl and Thymyl Vinyl Ethers in the KOH–DMSO–Acetylene System

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