Micellar copolymerization of styrene with poly(ethylene oxide) macromonomer in water: Approach to unimolecular nanoparticles via pseudo-living radical polymerization

Micellar copolymerizations of amphiphilic omega-methoxy-alpha-p-styrylalkyl poly(ethylene oxide) (PEO) macromonomers 1 (C-1-PEO-C-m-S-n, m = 1, 4, 7 a nd n = 18) with a limited amount of styrene solubilized therein have been i nvestigated. The reactions were carried out with 4,4'-azobis(4-cyanovaleric acid) (AVA) as a water-soluble initiator and 2,2'-azobis(isobutyronitrile) (AIBN) as an oil-soluble initiator at 60 degrees C. Fractional time-evolut ion experiments revealed that the copolymerization rate of styrene is great ly enhanced by the presence of macromonomer micelles and that the rate incr eases with hydrophobicity (increase of m) of alpha-end group of 1. The char acterization of the resulting graft copolymers revealed that the organized copolymerization proceeds in the manner of a pseudo-living radical copolyme rization with highly limited terminations between compartmentalized (isolat ed) propagating radicals. That is, the molecular weight of the graft copoly mers increased with the conversion of monomers. At an equimolar styrene wit h 1 (n = 18, m = 7) the copolymerization proceeded apparently transparently and azeotropically to afford a highly branched graft copolymer of poly(sty rene-graft-PEO), which in water formed a "unimolecular particle" with 15 nm radius and 7.4(4) x 10(6) apparent molecular weight. A plausible mechanism for the present organized copolymerization behavior is presented and discu ssed.