SCALE DEPOSIT INHIBITOR, PROCESS FOR ITS PRODUCTION, POLYMERIZER WHOSE INSIDE WALL IS COVERED WITH THE INHIBITOR, AND PROCESS FOR PRODUCTION OF VINYLIC POLYMERS BY THE USE OF THE POLYMERIZER

A scale adhesion preventing agent for use in a polymerization reactor, consisting essentially of a condensate of a phenol represented by the following formula (1), a 1-naphthol and an aldehyde, which is soluble in an alkaline aqueous solution: wherein R and R are the same or different and each a hydroxyl group, alkyl group having 1 to 8 carbon atoms, halogen atom, hydrogen atom, alkoxycarbonyl group whose alkyl moiety has 1 to 8 carbon atoms or phenyl group, with the proviso that a hydrogen atom is existent at at least two of ortho-positions and para-positions with respect to the hydroxyl group. When a vinyl-based monomer is to be polymerized using a polymerization reactor having a film of this scale adhesion preventing agent on the inner wall, the vinyl-based polymer can be produced by preventing the adhesion of a scale.