A mixture of: 7,9,9-trimethyl-3,14-dioxa-4,13-dioxo-5,12-diazahexadecane-1,16-diyl-prop-2-enoate7,7,9-trimethyl-3,14-dioxa-4,13-dioxo-5,12-diazahexadecan-1,16-diyl-prop-2-enoate