Propanoic acid,2-[(2,6-diethyl-4-hydroxy-2,3,6-trimethyl-1-piperidinyl)oxy]-,2,2-bis[[2-[(2,6-diethyl-4-hydroxy-2,3,6-trimethyl-1-piperidinyl)oxy]-1-oxopropoxy]methyl]-1,3-propanediyl ester