Synthesis of a Well-Defined Glycopolymer by Nitroxide-Controlled Free Radical Polymerization

This is the first report of the synthesis of a well-defined glycopolymer by free radical polymerization. N-(p-vinylbenzyl)-[O-β-d-galactopyranosyl-(1→4)]-d-gluconamide (VLA), a styrene derivative with an oligosaccharide moiety, was polymerized in N,N-dimethylformamide solution at 90 °C by the nitroxide-mediated free radical polymerization technique. Acetylated VLA gave polymers with a molecular weight from about 2000 to 40 000, an M w/M n ratio of about 1.1 in all cases, and a conversion of up to about 90%, where M w and M n are the weight- and number-average molecular weights. Indispensable for this success were (1) the use of di-tert-butyl nitroxide (DBN) rather than other nitroxides such as TEMPO, (2) the acetylation of VLA, and (3) the use of a radical initiator DCP (dicumyl peroxide) as an accelerator. DBN provided a well-controlled polymerization of VLA at 90 °C (VLA becomes unstable at higher temperatures, e.g., >120 °C). The acetylation effectively prevented the chain transfer that leads to dead polymers and broad polydispersities. DCP remarkably accelerated the rate of polymerization (the rate of chain extension), which otherwise was impractically slow, without causing any appreciable broadening of polydispersity.