Compounds of the formula ##STR00001## wherein, provided that at least one of R.sub.1, R.sub.2, and R.sub.3 is not hydrogen, R.sub.1, R.sub.2, and R.sub.3 each, independently of the other, is hydrogen, alkyl, aryl, arylalkyl, or alkylaryl, and wherein R.sub.1 and R.sub.2 can also be alkoxy, aryloxy, arylalkyloxy, alkylaryloxy, polyalkyleneoxy, polyaryleneoxy, polyarylalkyleneoxy, polyalkylaryleneoxy, silyl, siloxane, polysilylene, polysiloxane, or ##STR00002## wherein X is a direct bond, oxygen, sulfur, --NR.sub.40-- wherein R.sub.40 is hydrogen, alkyl, aryl, arylalkyl, or alkylaryl, or --CR.sub.50R.sub.60-- wherein R.sub.50 and R.sub.60 each, independently of the other, is hydrogen, alkyl, aryl, arylalkyl, or alkylaryl.