Method for preparing PVPK90 based on solution polymerization method

The invention relates to a method for preparing PVPK90 based on a solution polymerization method, which comprises the following steps: 1) stirring and heating to a reaction temperature under the protection of inert gas, then dissolving an initiator azodiisobutyronitrile in NVP, and adding a solvent, an NVP monomer and the azodiisobutyronitrile into a reactor according to the related proportion of the solvent, the NVP monomer and the azodiisobutyronitrile; 2) after the reaction is carried out for 30-60 minutes, supplementing the initiator azodiisobutyronitrile, then supplementing the azodiisobutyronitrile once every 60-120 minutes, and supplementing the azodiisobutyronitrile for 3-5 times in total; and 3) after the total reaction lasts for 300-420min, adding a proper amount of ammonia water to terminate the polymerization reaction, and carrying out vacuum drying and grinding to obtain the product. According to the method, the addition time and the addition amount of an initiator in the polymerization process a