Stabilization of synthetic polymers

A synthetic polymer composition stabilized against photo- and thermal deterioration wherein there is incorporated, in a sufficient amount to prevent said deterioration, a compound having the formula ##STR3## wherein R and R', which may be the same or different, and each represents hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, a hydroxyalkyl group, an alkoxyalkyl group, an aliphatic or aromatic acyloxyalkyl group, a cyanoalkyl group, a halogenoalkyl group, an epoxyalkyl group, an alkoxycarbonylalkyl group, an aliphatic acyl group, an alkoxycarbonyl group or an aralkoxycarbonyl group, or an acid addition salt thereof.