.2).sub.m or (CH.sub.2).sub.n wherein R.sub.2 and R.sub.3 are each hydrogen or alkyl of 1 to 15 carbon atoms, R.sub.4is an alkylene group of 1 to 15 carbon atoms, m is an integer of 1 to 20, and n is 0 or an integer of 1 to 20; and U is a hydrogen atom, ammonium radical or an alkali metal atom.2.