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Air-Free Technique and Moisture-Free Manipulation

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Ⅰ. The influence of the presence of oxygen on the reaction during the polymerization process

In polymerization reactions, oxygen can have a variety of negative effects. First, the dissolved oxygen in the solvent will inhibit polymerization and significantly reduce the reaction rate and conversion rate. Secondly, the presence of oxygen will also lead to a decrease in product quality. For example, in the synthesis process of polyvinyl chloride resin, the participation of oxygen will reduce the thermal stability of the product and make the product prone to discoloration.

In addition, oxygen may also participate in side reactions. Vinyl chloride monomer will absorb oxygen to generate vinyl chloride peroxide, which is easily decomposed into hydrogen chloride, formaldehyde and carbon monoxide under polymerization conditions. These decomposition products will reduce the pH value of the reaction medium, cause "poisoning" of the dispersion system and produce coarse materials, and will also increase the pressure of the polymerization reaction, posing safety risks.

This article will comprehensively introduce the common deoxidation methods in the polymerization process and analyze its deoxidation steps in polymerization in detail.

Ⅱ. Common oxygen removal methods in laboratories

First, prepare some experimental equipment:

Nitrogen bubbling method

This is a physical replacement method. The principle is to continuously introduce high-purity inert gas (such as nitrogen, argon) into the liquid, and use the air flow to bubble and stir the liquid to "carry" out the oxygen (and other volatile impurities) dissolved in it. At the same time, an inert gas protective layer is formed above the entire liquid surface to prevent air (oxygen) from redissolving, and is suitable for removing dissolved oxygen in the reaction liquid.

Generally, it is necessary to insert a long needle connected to nitrogen gas below the reaction liquid level and continue to ventilate it for about 30 minutes (the specific time depends on the volume, viscosity and sensitivity of the liquid to oxygen). The purpose is to completely replace the air in the headspace of the reaction bottle with nitrogen. At the same time, connect a bubbler to the outlet of the reaction device and put a small amount of silicone oil or mineral oil into it to determine the flow rate of nitrogen. The laboratory operation process is shown in Figure 1.1 below. The flow direction of nitrogen can be seen by the arrows in the figure.

Figure 1.1

Freeze-extraction-thaw cycle

This is a very efficient and thorough method of oxygen removal, especially suitable for systems that are extremely sensitive to oxygen or in laboratory research-grade synthesis. The principle of the freeze-thaw cycle is to cleverly combine the three steps of freezing (phase change), vacuuming (reducing partial pressure) and thawing (phase change again). Through multiple cycles, the concentration of oxygen in the system is exponentially reduced. The steps are as follows:

• Freezing: Place the reaction solution to be deoxygenated into a Schlenk bottle and place it in a liquid nitrogen bath to allow the liquid to completely solidify.

• Vacuuming: After the liquid is completely frozen, turn on the high vacuum pump, evacuate the system to a lower pressure, and maintain the vacuum for 5-10 minutes. At this time, the volatile impurities in the freeze, including oxygen, will be extracted. At this point the volatile impurities in the freeze (including oxygen) are extracted.

• Melting: Close the valve between the vacuum pump and the system, remove the cold bath, and allow the frozen sample to melt naturally or use a warm water bath to accelerate the melting. When melted, gases dissolved in the liquid are released. Note: If a large number of bubbles are produced during melting, it means there is still more gas and this cycle will work very well.

• Cycle: Repeat the above operation, usually 3-5 times. Each cycle can effectively reduce the dissolved oxygen content in the system. After the last cycle of vacuuming, the system is backfilled with inert gas to normal pressure under vacuum.

Vacuum and inert gas displacement

The principle is mainly through physical dilution and replacement. This method is often used in kinetic test sampling. Taking cleaning a Schlenk flask as an example to describe the oxygen removal operation:

As shown in Figure 1.2. When zone a has been ensured to be an oxygen-free atmosphere, it is crucial to ensure that no new air atmosphere is introduced during the sampling process.

Figure 1.2

Before the long sampling needle enters the inside of the flask, insert a syringe at the outlet b on the side wall of the flask and always connect it to nitrogen to maintain the entire nitrogen atmosphere. Then insert the long sampling needle to replace air and nitrogen (Figure 1.3). Repeat this several times and then open the flask piston (Figure 1.4 a), take the sample. After the sampling is completed, quickly close the piston and seal the position b with tape (Figure 1.5). The nitrogen must be kept flowing during the entire process. Repeat the above operation for multiple subsequent samplings.

Figure 1.3

Figure 1.4

Figure 1.5

The advantages and disadvantages of the above three methods are summarized in the following table:

Table 1.1 Analysis and comparison of three oxygen removal methods

Ⅲ. Oxygen removal effect evaluation and method selection

There are various methods to evaluate the oxygen removal effect:

• The dissolved oxygen meter can monitor the dissolved oxygen content in liquid online;

• Gas chromatography can accurately measure the oxygen content in the headspace of the system;

• The oxygen removal effect can be indirectly evaluated by analyzing the product molecular weight distribution and residual monomer content.

There are several factors to consider when choosing an oxygen removal method:

• Polymerization reaction types (radical, anionic, cationic, etc.) have different sensitivity to oxygen;

• The production scale (laboratory small test, pilot test, industrial production) is suitable for different oxygen removal methods;

• Cost factors (equipment investment, operating costs, maintenance costs)

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