Ⅰ. Necessity of solvent removal

   Solvent water removal is of great significance in ordinary free radical polymerization, ionic polymerization and various types of controlled polymerization. Trace amounts of moisture can affect the activity of the initiator, terminate the active chain, or cause side reactions with the catalyst, resulting in reduced polymerization rate, broadened molecular weight distribution, or even polymerization failure.

   Therefore, selecting appropriate water removal strategies for solvents of different polarities is a key step to ensure the repeatability of polymerization reactions and product quality.

Ⅱ. Commonly used water removal methods and principles

   Physical method: The adsorbent selected for physical adsorption is generally molecular sieve, and the most commonly used varieties are 3Å, 4Å, 5Å and so on. For high boiling point solvents such as toluene, atmospheric pressure distillation can be used to remove water.
   Chemical method: Commonly used desiccants include calcium hydride, sodium sulfate, and magnesium sulfate, which can be screened according to the needs of different solvents.
   If the requirements for water removal are strict and deep water removal is required, sodium blocks can also be used to remove water, but this method is more dangerous. It needs to be carried out in an inert gas atmosphere (nitrogen or argon), and it must be strictly anhydrous and oxygen-free during the operation.
Ⅲ. Water removal methods using different solvents

   The water removal method using molecular sieves, calcium hydride and sodium blocks will be explained in detail below:

Molecular sieve physical adsorption (basic method, safe and easy to operate)
• Applicable: n-hexane, toluene and other alkanes/aromatics
• Desiccant: 3Å or 4Å molecular sieve (3Å is sufficient in most cases)
• Steps:
1. Activated molecular sieve: bake in a vacuum drying oven at 300 °C for 3-5 hours. After activation, add 10-20% of the mass fraction or volume fraction of the solvent to the solvent and stir overnight.
2. End point determination: Karl Fischer moisture determination ≤50 ppm (generally ≤100 ppm is sufficient).
3. Separation and storage: Filter out the molecular sieve or directly distill it into a dry storage bottle under an inert atmosphere. For long-term use, it is recommended to store it on activated molecular sieve and place it under nitrogen/argon positive pressure.
• Safety Tips: Molecular sieves can be reactivated repeatedly; avoid inhaling dust.
Sodium metal/benzophenone reflux
• Suitable for: Toluene, tetrahydrofuran, n-hexane and other non-functional hydrocarbons (cannot be used for halogenated, nitro, and active hydrogen-containing solvents)
• Steps:
1. Preliminary preparation: Dry the three-necked flask, install a reflux condenser tube, and vent nitrogen/argon gas for protection.
2. Add sodium and reflux: add the prepared sodium block and benzophenone, add solvent and reflux for 1-3 hours. If the solution appears blue-purple and remains stable, it means it is anhydrous.
3. Distillation under reduced pressure: Distill directly from sodium metal under an inert atmosphere, discard about 5% of the first fraction, and collect the middle fraction in a dry, inert atmosphere storage bottle.
4. Post-processing: After the device is cooled, the residual sodium is separated under an inert atmosphere; quenched extremely slowly with absolute ethanol in batches, then completely quenched with water, and recycled.
• Safety Tips: Sodium is prone to spontaneous combustion when exposed to water/air. Keep away from water sources during the entire operation. It is strictly prohibited to directly use water to dispose of residual sodium.
CaH₂ drying + vacuum distillation (taking DMF solution as an example)
• Note: DMF is easy to decompose at high temperatures and is not suitable for long-term reflux at normal pressure and high temperature. It is commonly used to dry with CaH₂ or 4Å molecular sieve and then distill under reduced pressure.
• Steps:
1. Drying of CaH₂: Add DMF (1L) and CaH₂ 5–10 g/L into a drying bottle, stir at room temperature for 6–12 h; if necessary, gently heat at 50–60 °C for 2–4 h (avoid high temperature).
2. (Optional) Molecular sieve polishing: Store the distilled DMF on 4Å molecular sieves (10–20% w/v) for 24–48 h to further reduce moisture.
3. Distillation under reduced pressure: Distill under low pressure (for example, 20–30 mmHg), control the pot temperature to ≤100 °C, and collect the middle fraction into a dry storage bottle (inert atmosphere).
4. End point: ≤100 ppm measured by Karl Fischer (conventional organic synthesis is sufficient; more demanding systems can extend the standing time).
• Safety Tips: DMF has transdermal toxicity and requires protection during operation; avoid heating at high temperatures for long periods of time.
   A comparison of the above three water removal methods is as follows:

Ⅳ. Advantages and Disadvantages of Different Water Removal Methods

   The following table lists the pros and cons of different water removal methods:

Ⅴ. Storage of solvent after purification

   Even if the solvent has been strictly dehydrated and deoxygenated, if it is stored improperly, it will quickly lose its effectiveness under the action of air and moisture, and even generate peroxides (such as ether solvents).
   Storage principle: The purified solvent needs to be immediately covered with high-purity argon/nitrogen to isolate the liquid surface from the air. For non-polar solvents, activated molecular sieves can be added directly. At the same time, ensure that the solvent is sealed and stored at low temperature after the sealing is completed, and the water content of the solvent is regularly tested.
1. Inert atmosphere protection: After purification, the solvent should be immediately transferred to a drying bottle and sealed under positive pressure of inert gas such as nitrogen or argon.
2. Desiccant accompanying:
   For non-polar solvents: activated molecular sieves can be added directly to them.
   For ether solvents: can be stored on a small amount of sodium/benzophenone system (maintain blue indication), but please pay attention to safety.
   For polar solvents (such as DMF): It is recommended to store molecular sieves after polishing to avoid coexistence with strong reducing agents.
3. Light protection and temperature control:
   Light will accelerate the peroxide formation and decomposition reactions of some solvents, so they should be stored in light-proof containers (brown bottles or wrapped in aluminum foil). Some easily decomposable solvents (such as DMF) should be stored at low temperature (4–10 °C), but water vapor must be prevented from condensing into the bottle.
4. Regular testing:
   For ether solvents, the peroxide content should be tested every 1-3 months (KI-starch test paper method or kit method). If it is too high, it needs to be discarded or disposed of.
   For solvents used in key reactions, the moisture content can be confirmed by the Karl Fischer method before use.

   Solvent purification is the cornerstone of polymerization controllability. Whether it is laboratory research or industrial production, only by selecting matching purification methods for different polymerization systems and standardizing the verification process can we avoid molecular weight loss, reduced conversion rate and even safety hazards caused by impurity interference.