Manufacture of low molecular weight poly-N-vinylpyrrolidone-2

#This invention relates to a process for the manufacture of low molecular weight poly-N-vinylpyrrolidone-2.

#It is known that N-vinylpyrrolidone-2 (hereinafter referred to as "vinylpyrrolidone") can be polymerized by free radical polymerization in water or alcohols, suitable catalysts being hydrogen peroxide, alkyl hydroperoxides, dialkyl peroxides and azodiisobutyronitrile. Polymerization in aqueous solution, using hydrogen peroxide, is of industrial importance, whilst polymerization in alcohols suffers from great disadvantages, above all the fact that it proceeds very slowly even if organic per-compounds are used as activators. In particular, the complete polymerization of the last residues of vinylpyrrolidone presents considerable difficulties and requires long periods. Satisfactory removal of the monomer from the polyvinylpyrrolidone, for example by extraction or by chemical means, such as saponification, is extremely involved.

#Azodiisobutyronitrile has the additional disadvantage that its decomposition product, tetramethylsuccinodinitrile, is an undesirable impurity.

#The Examples which now follow illustrate the invention, parts are by weight. The K values were determined by the method of H. Fikentscher, Cellulose-Chemie 13, 58-64 and 71-74 (1932) in 5 percent strength aqueous solution at 20.degree. C; K = k.10.sup.3.

#77 parts of toluene, 6 parts of vinylpyrrolidone and 0.24 part of di-tert.-butyl peroxide are introduced into a stirred pressure reactor. The reactor is sealed pressure-tight and is substantially freed from atmospheric oxygen by twice charging with nitrogen to 4 bars pressure, and letting down again. The contents of the reactor are now heated to 140.degree. C, whereupon the pressure assumes a value of 1.5 bars, and a mixture of 63 parts of toluene, 54 parts of vinylpyrrolidone and 2.16 parts of di-tert.-butyl peroxide is then fed in at 140.degree. - 145.degree. C for the period shown in the Table which follows. Polymerization is then continued for about 3 hours until the monomer content has fallen to less than 0.5% (based on vinylpyrrolidone employed). The batch is then cooled to 85.degree. C and diluted with 60 parts of water, and the toluene is stripped off by passing steam into the mixture. When the internal temperature reaches 98.degree. C, distillation is continued until a further 60 parts of distillate have passed over. The solution, of about 30 percent strength, is then spray-dried.

#TABLE ______________________________________ Experiment Period of addition K value ______________________________________ a 1/4 hours 23.1 b 23/4 hours 18.5 ______________________________________