compounds represented by the formula ## str1 ## and addition polymers thereof where m is an integer from 2 to about 10 , x -- is chlorine , hydroxy , carboxy or -- ac -- r )═ ch . sub . 2 -- r is hydrogen or methyl , -- r &# 39 ; is an ethylene or propylene group and -- ac -- r )═ ch . sub . 2 is one of ## str2 ## these compounds and their addition polymers form complexes with alkali metal or alkaline earth metal salts and permit the concentration of aqueous solutions of said salts . for example , a compound of the formula ## str3 ## is polymerized with an azobis catalyst to form addition polymers of the repeating unit ## str4 ## when cross - linked , rendering it water - insoluble , such an addition polymer concentrates an aqueous solution of sodium chloride when contacted with same at about 0 ° c .