A Brief Discussion on Polymerization Inhibitors

  In industrial chemical synthesis, polymerization inhibitors are frequently employed. Today, we will provide a detailed introduction to what polymerization inhibitors are, as well as several common types found on the market.

  To understand Polymerization inhibitors, one must first know what a polymerization reaction is. Polymerization is the process of converting low molecular weight monomers into high molecular weight polymers, which possess important properties like plasticity, fiber-forming ability, film-forming ability, and high elasticity – characteristics that the original monomers lack. Simply put, for example, in rubber products, we require the product to have high elasticity, a property absent in low molecular weight monomers. Thus, by using polymerization reactions to synthesize them into high molecular weight polymers, we obtain a product possessing this characteristic.

   Inhibition, as the name suggests, is the prevention of the Polymerization  reaction from proceeding. When chain radicals transfer to certain substances, they form low-activity radicals. These low-activity radicals can only be initiated by radical species themselves and thus cannot initiate the polymerization of low molecular weight monomers; they can only undergo bimolecular termination with other radicals. As a result, no polymer is produced in the initial stage of the system – this is inhibition. These "certain substances" are what we call polymerization inhibitors. Inhibitors are generally used in syntheses that utilize olefinic monomer raw materials, so they can be defined as substances that can completely terminate the free radical polymerization reaction of olefinic monomers. In fact, not only in synthesis, but small amounts of inhibitors are often added during the storage and transportation of olefinic monomers, removed just before actual use, to prevent auto-polymerization during these processes.

  Polymerization inhibitors are divided into two categories: molecular inhibitors and stable radical inhibitors. Common molecular inhibitors include: 1. Hydroquinone 2. Phenothiazine 3. β-Phenylnaphthylamine 4. Cuprous chloride 5. Ferric chloride 6. p-Benzoquinone, etc. They achieve inhibition through reactions between their molecules and the monomers. Common stable radical inhibitors include: 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO). Although TEMPO is itself a radical, it is very stable and can only combine with chain radicals, eliminating them and thus achieving the inhibitory effect.

   So, how does one evaluate the quality of an inhibitor? Ultimately, it depends on the actual circumstances of its application, such as the specific type of olefinic monomer, whether it's for reaction purposes or storage/transportation, and how it will be removed afterwards, among other conditions. Generally speaking, the more readily an inhibitor reacts with chain radicals and the more stable the resulting radical is, the better its inhibitory effect tends to be. Some inhibitors work particularly well for specific olefins. For example, Inhibitor 701, namely 4-Hydroxy-       2,2,6,6-tetramethylpiperidin-1-oxyl, shows a good inhibitory effect on acrylates, methacrylates, acrylic acid, acrylonitrile, styrene, and butadiene. Its inhibition performance is superior to that of phenolic, aromatic amine, ether, quinone, and nitro compound inhibitors. It can effectively prevent the self-polymerization of olefin monomers during production, separation, purification, storage, or transportation, and controls and regulates the degree of Polymerization  of olefins and their derivatives.

How should we evaluate polymerization inhibitors? Here are a few feasible methods. First, the Visual Observation Method. This is suitable for olefinic monomers with boiling points above 70°C. The general procedure is: add pure monomer and the inhibitor to a glass reflux apparatus, heat under reflux in an electric constant temperature bath, and observe changes in the monomer solution during reflux. If phenomena such as color darkening, increased viscosity, sluggish boiling, or the appearance of polymer rings on the vessel walls occur *later* than they do without the inhibitor, it indicates higher inhibitor efficiency. Second, the Polymer Isolation Method. The general procedure for this method is: add the inhibitor to a pure monomer solution, heat for polymerization under constant temperature conditions, terminate the reaction at timed intervals, add a precipitating agent, and after precipitation is complete, filter, wash, dry, and weigh the precipitate. The efficiency of the inhibitor is judged based on the weight of the isolated polymer. Third, the Boiling Method. This method is suitable for olefinic monomers with boiling points between 70°C and 110°C. These monomers undergo bulk Polymerization  under specific conditions. The time from placing the monomer in a constant temperature bath until it automatically starts boiling and foaming is recorded. This time is compared to the time observed when an inhibitor is added. The longer the delay caused by the inhibitor, the better its efficacy.

   Since inhibitors are considered impurities relative to the production or storage/transportation process, they need to be removed after use. Here are several methods for removing inhibitors. Generally, most inhibitors are solid substances that are difficult to volatilize. They can often be removed by distilling the monomer under reduced pressure. Other methods exist; for example, an inhibitor like hydroquinone can be reacted with sodium hydroxide to form a water-soluble sodium salt, allowing its removal. Another example involves inorganic inhibitors like cuprous chloride and ferric chloride, which can be removed directly by acid washing.