Poly vinyl pyrrolidone is divided into four levels according to its average molecular weight. It is usually expressed by K value, and different K values represent the corresponding PVP average molecular weight range. The K value is actually a characteristic value related to the relative viscosity of the PVP aqueous solution, and the viscosity is a physical quantity related to the molecular weight of the polymer. Therefore, the K value can be used to characterize the average molecular weight of PVP. Generally, the greater the K value, the greater the viscosity and the stronger the adhesion.
Poly vinyl pyrrolidone is obtained from the monomer vinylpyrrolidone (NVP) through bulk polymerization, solution polymerization and other methods. During the preparation process of bulk polymerization, due to the high viscosity of the reaction system, the polymer is not easy to diffuse, the heat of the polymerization reaction is not easy to remove, resulting in local overheating and other problems, and the resulting product has a low molecular weight, high residual monomer content, and the majority of it is yellow and has little practical value. The industry generally uses solution polymerization to synthesize poly vinyl pyrrolidone.
The first is the solution polymerization of NVP in an organic solvent, followed by steam stripping. The second route is aqueous solution polymerization of NVP monomer and water-soluble cationic, anionic or nonionic monomer. Heat the NVP monomer directly above 140℃, or add an initiator to the NVP solution for heating, or add an initiator to the NVP solution to polymerize by free radical solution. The solvent can be water, ethanol, benzene and so on. PVP homopolymer can be obtained by direct light irradiation of NVP monomer or its solution. Different polymerization methods result in different polymer structures and properties. Among them, the polymer composition and structure obtained by free radical solution polymerization are relatively uniform. The performance is also relatively stable. It is the most commonly used method for NVP homopolymerization. By adjusting the reaction conditions such as monomer concentration, polymerization temperature, initiator dosage and so on, poly vinyl pyrrolidone homopolymers with different molecular weights and different water solubility can be obtained.
Process 1
Configure NVP into a solution with a mass fraction of 50%, use a small amount of hydrogen peroxide as a catalyst, and initiate polymerization at 50°C under the action of azobisisobutyronitrile, so that almost all NVP is converted into PVP. Then add ammonia water to the polymer to decompose the remaining azobisisobutyronitrile. The monomer polymerization conversion rate is nearly 100%, and the solid content is 50%.
Process 2
Add 0.4g dispersant P (NVP-co-VAc) and 80g dispersing medium ethyl acetate into a 250 ml four-neck flask, stir and dissolve in a 70℃ constant temperature water bath, then add 20g monomer NVP and 0.15g initiator AIBN, react for 6h under nitrogen atmosphere, cool and filter, put the insoluble matter in a vacuum drying box, and vacuum dry for 24h to obtain white poly vinyl pyrrolidone solid powder.
Most of the polymerization of poly vinyl pyrrolidone uses AIBN as the initiator. There is no literature on the use of water-soluble azo initiators to initiate the synthesis of PVP, but some people are doing this. Since both NVP monomer and PVP are soluble in water, water-soluble azo initiators can be used to initiate polymerization to form linear PVP polymers. Moreover, AIBN contains cyano groups that are harmful to humans, while most water-soluble azo initiators do not contain cyano groups, and PVP is mostly used in products that are in direct contact with the human body, so water-soluble azo initiators have more advantages than AIBN.
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