Synthetic polymers which possess outstanding stability to oxidative, thermal or light-induced degradation comprise as stabilizer at least a compound of the formula I ##STR1## wherein R.sub.1 and R.sub.2 are each independently of one another hydrogen or C.sub.1 -C.sub.8 alkyl, R.sub.3 and R.sub.4 are each independently of one another C.sub.1 -C.sub.12 alkyl, and R.sub.5 is C.sub.1 -C.sub.7 alkyl.