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Common polymerization inhibitors Polymerization inhibitor is a substance that can completely terminate the free radical polymerization reaction of vinyl monomers. This effect is called polymerization inhibition. The polymerization inhibitor molecules react with chain free radicals to form non-radical substances or low-activity free radicals that cannot be initiated, thereby terminating the polymerization. In order to avoid polymerization of vinyl monomers during storage, transportation, etc., a small amount of polymerization inhibitor is often added to the monomer and removed before use. Generally, polymerization inhibitors are solid substances with low volatility and can be removed when the monomer is distilled. The commonly used polymerization inhibitor hydroquinone can react with sodium hydroxide to form a water-soluble sodium salt, so it can be removed by washing with 5% to 10% sodium hydroxide solution. Inorganic polymerization inhibitors such as cuprous chloride and ferric chloride can also be removed by pickling. The categories and functions of polymerization inhibitors are generally divided into molecular polymerization inhibitors and stable free radical polymerization inhibitors. The former mainly include: Hydroquinone (see structural formula a), p-benzoquinone (b), phenothiazine (c), β-phenylnaphthylamine (d), p-tert-butylcatechol (e), methylene blue (f): Inorganic substances such as cuprous chloride, ferric chloride and sulfur can also be used as polymerization inhibitors. Stable free radical polymerization inhibitors mainly include 1,1-diphenyl-2-picrylhydrazyl DPPH (g), 2,2,6,6-tetramethyl * * Nitrogen radical TMP(h): Although they are also free radicals themselves, because they are very stable, they cannot initiate monomer polymerization and can only effectively combine with chain free radicals to make the chain free radicals disappear. Taking DPPH as an example, the reaction is as follows: In this reaction, one DPPH molecule binds to a chain radical P• almost quantitatively. The reaction material changes from purple to colorless. The amount of DPPH participating in the reaction can be measured using spectrophotometry to calculate the concentration of free radicals. Therefore, this method is often used to determine the initiation rate. Trityl radical (structural formula is as follows) can also be regarded as a free radical polymerization inhibitor, but its stability is poor and it can initiate polymerization at higher temperatures, so it is rarely used. The most commonly used polymerization inhibitor in industry and laboratories is hydroquinone, generally added in an amount of 0.001% to 0.1% of the monomer amount. Its polymerization inhibition mechanism is to first convert it into benzoquinone under the action of oxygen, and then react with free radicals.: For this reason, when using hydroquinone as a polymerization inhibitor, the container should not be filled with monomers. A little space should be reserved to store air and provide the oxygen needed for polymerization inhibition. Generally speaking, highly efficient polymerization inhibitors are first of all easy to react with chain free radicals, and the free radicals generated are very stable and have no ability to initiate again. Let M represent the vinyl monomer and Z represent the polymerization inhibitor, then the chain growth reaction to generate polymers can be expressed as: The polymerization inhibition reaction can be expressed as: The conditions for an effective polymerization inhibitor should be: ,=0. where is the initiation rate constant of PZ•. The ratio of the polymerization inhibition rate constant to the growth reaction rate constant is called the polymerization inhibition constant, represented by, =/. If a certain polymerization inhibitor has a large inhibitory effect on a certain monomer, it means that the inhibitor has a high efficiency in inhibiting the polymerization of this monomer. The values of several polymerization inhibitors (or retarders) for several commonly used monomers are shown in the table (temperature is about 50C)" class=image>. Self-blocking allyl monomers (such as allyl acetate) polymerize slowly and produce low molecular weight polymers, as do many other allyl monomers. This is due to the conjugation effect of the allyl radical generated by the reaction of the free radical with the allyl monomer, which becomes stable.: In the formula, X is halogen. Reaction (1) is an addition reaction. The generated free radicals are not conjugated and are highly active. The addition reaction can continue. ； Reaction (2) is a transfer reaction. The generated allyl free radical is conjugated and stable, and can no longer perform addition polymerization. It often undergoes double-radical termination with the primary free radical (or itself).: The result is a decrease in polymerization rate and the production of low molecular weight polymers. This automatic inhibition of allyl monomers is called degenerate chain transfer. Oxygen polymerization inhibition and initiation Oxygen molecules are paramagnetic, and their structure is that each oxygen atom has an unpaired electron, so it is a diradical. Oxygen molecules react very easily with chain free radicals to generate peroxyl radicals POO, which are relatively stable and generally cannot add to double bonds. They can only be combined with another P or transfer a hydrogen atom from other substances to generate peroxides or hydrogen peroxides.: Reaction (3) eliminates free radicals, and reaction (4) terminates the active chain. As a result, the polymerization rate decreases and the degree of polymerization decreases. When the temperature is not too high, POOP or POOH are stable, but at higher temperatures, they will decompose into active free radicals PO or OH that can initiate monomer polymerization. Therefore, for free radical polymerization, oxygen often plays a dual role, acting as a polymerization inhibitor at lower temperatures and as an initiator at higher temperatures (above 100C). Free radical polymerization is generally carried out at a lower temperature (40-80C), and oxygen acts as a polymerization inhibitor, so before polymerization, the oxygen in the system must be eliminated with nitrogen. The polymerization inhibition mechanism of copper salts and iron salts is generally considered to be a single electron transfer reaction, taking CuCl or FeCl as an example.: The reaction result of CuCl+R→RCl+Cu FeCl+R→RCl+FeCl is the disappearance of free radicals. Since acrylic acid and methacrylic acid have good solubility in metal salts, copper salts and iron salts are commonly used as polymerization inhibitors for these monomers.