represented by the general formulas: wherein R and R represent hydrogen or monovalent hydrocarbon groups such as alkyl groups containing from 1 to 6 carbon atoms, aralkyl groups containing from 6 to 10 carbon atoms, hydroxyalkyl groups of the wherein R is alkylene containing from 1 to 6 carbon atoms (the Nl LN) substituent may be substituted), groups of formula Nll mNm: Nil: wherein R is alkylene