FIELD: organic chemistry, chemical technology. SUBSTANCE: invention describes addition agent with improved viscosity index and showing dispersing effect and stability with respect to photoelastomers. Addition agent consists of copoly(meth)acrylate synthesized by copolymerization of monomeric composition in medium of inert solvent. Monomeric composition has: (a) (meth)acrylates of the general formula (I): CH2= C(R)-COOR1 taken in the amount from 85 to 98 wt.% where R is taken among -H and CH3, and R1 is taken among direct or branched alkyl radical with amount of C-atoms 6-25; (b) (meth)acrylates of the general formula (II): CH2= C(R)-CO-XR2 taken in the amount from 1 to 6 wt.% where R has the above indicated value; -X- means O or -NH-, or NR3 where R3 is alkyl radical containing 1-5 C-atoms and R2 is taken among direct, branched or cyclic radicals where total amount of C-atoms is 4-20 and amount of tertiary N-atoms is 1-2; (c) (meth)acrylates of the general formula (III): CH2= C(R)-COOR4 taken in the amount from 1 to 9 wt.% where R has the above indicated value and R4 is taken among linear, branched or cyclic alkyl radicals where amount of C-atoms is 4-20 and hydroxyl and/or alkoxyl radicals where amount of O-atoms is 1-2 of type 1-2 being "alkoxyl" means group OR5 where R5 is direct or branched C1-C4-alkyl radical. Ratio between equivalents of oxygen in (meth)acrylates by point (c) and equivalents of nitrogen in (meth)acrylates by point (b) is in the range from 1/1 to 2/1. Invention describes also method of synthesis of copoly(meth)- -acrylate and copoly(meth)acrylate salt solution preparing. Proposed addition agent shows dispersing properties and stability with respect to fluoroelastomers, increased stability for shift, improves viscosity index and rheological behavior at decreased temperature. EFFECT: improved method of synthesis and preparing, improved properties of addition agent. 15 cl, 7 tbl, 5 ex
