. . "The terms "alkyl", "alkoxy", "alkoxyalkyl", "aryl", "alkaryl" and "aralkyl" in the definitions of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 herein are used to denote alkyl radicals of 1-8, preferably 1-4 carbon atoms, e.g. methyl, ethyl, propyl," . .