NHS ATRP initiator NHS protected Bromoisobutyrate NHS-BiB N-Hydroxysuccinimidyl 2-bromo-2-methylpropionate 2-Bromoisobutanoic acid N-hydroxysuccinimide ester 98% (GC) 1-(2-Bromo-2-methyl-1-oxopropoxy)-2,5-pyrrolidinedione 2,5-Dioxopyrrolidin-1-yl 2-bromo-2-methylpropanoate 2-Bromo-2-methyl-propanoic acid 2,5-dioxo-1-pyrrolidinyl ester