Synthesis of thiophene-containing acyclic alkoxyamine for nitroxide-mediated radical polymerization of acrylates and styrene

In this study, specific acyclic nitroxide and corresponding alkoxyamine compounds containing thiophenyl group and intramolecular hydrogen bonding were designed (i.e., 2-(oxyl(2-methyl-1-(thiophen-2-yl)propyl)amino)-2-methylpropan-1-ol (HOT2NO center dot) and 2-methyl-2-((2-methyl-1-(thiophen-2-yl)propyl)(1-phenylethoxy)amino) propan-1-ol (HOT2NOPhEt)). The type of acyclic structures could render nitmxide-mediated polymerizations (NMPs) of acrylates at mild reaction temperatures that can effectively suppress side reactions of (macro)radicals. To enhance the reactivity of the initiator, one of our design to introduce a thiophenyl group is to increase hindrance/polarity feature. Another introduction of intramolecular hydrogen bonding (IHB) is to decrease the bond dissociation energy (BDE) of the "NO-C" linkage of the initiator. Accordingly, both azo- and alkoxyamine-initiated NMPs were examined. The azo-initiated NMPs (i.e., using HOT2NO center dot/AIBN) of n-butyl acrylate (BA) and methyl acrylate (MA) showed uncontrolled radical polymerization fashions, presumably due to insufficient reactivation/deactivation from HOT2NO center dot during polymerization. The alkoxyamine-initiated NMPs of BA with proper amounts of deactivator (i.e., BA/HOT2NOPhEt/HOT2NO center dot) revealed controlled/living radical polymerization fashions at 120 and 100 degrees C and afforded Ph-PBA-ON macroinitiator (M-n,M-max = ca. 38 k and PDI <1.43). Through the DFT estimations of different relevant alkoxyamines, significant suppression of the BDEs was acquired (ca. minus 10-20 kJ/mol), implying the importance of IHB. Eventually, successful chain extension of Ph-PBA-ON with styrene (St) was demonstrated and afforded well-defined PBA-b-PSt diblock copolymer (M-n = ca.12 k and PDI = 1.10).