Self-association of cholesterol-end-capped poly(sodium 2-(acrylamido)-2-methylpropanesulfonate) in aqueous solution

Poly(2-(acrylamido)-2-methylpropanesulfonic acid) sodium salt end-capped wi th a cholesterol moiety (Chol-PAMPS) was prepared by free radical polymeriz ation of 2-( acrylamido)-2-methylpropanesulfonic acid (AMPS) initiated by a cholesterol substituted azo compound, 4,4'-azobis(4-cyano-1-cholesteryl)pe ntanoate, and the associative behavior of Chol-PAMPS in aqueous solution wa s studied by H-1 NMR, size exclusion chromatography (SEC), static light sca ttering (SLS), quasielastic light scattering (QELS), and fluorescence probe techniques. For a reference polymer (AIBN-PAMPS), polyAMPS of a similar mo lecular weight with that of Chol-PAMPS was prepared in the presence of 2,2' -azobis(2-methylpropionitrile) as an initiator. The degree of polymerizatio n (DP) of Chol-PAMPS was estimated to be about 70 from a H-1 NMR spectrum ( in DMSO-d(6)) by assuming disproportionation for the termination. This DP v alue agreed fairly well with that estimated by SEC in water/acetonitrile us ing sodium poly(styrenesulfonate)s as standards. QELS indicated that Chol-P AMPS formed multipolymer aggregates at polymer concentrations (C-P) higher than 0.5 g/L in 0.1 M NaCl. Fluorescence emission and excitation spectra fo r pyrene probes solubilized in the aggregates of Chol-PAMPS suggested the p resence of a critical micelle concentration (cmc) around C-P approximate to 0.6 g/L in water. Above this C-P, the micellelike aggregates coexist with unimers over a wide range of C-P. in the C-P regime of 1.0-5.0 gn, hydrodyn amic radii for the aggregates were practically constant at about 50 nm. How ever, when C-P was increased beyond this C-P regime, the size increased con siderably with C-P. Given DP approximate to 70 for Chol-PAMPS, these sizes for the aggregates are obviously too large for a single spherical micelle w ith cholesterol groups in the core and extended polyAMPS chains in the coro na. The structures of these multipolymer aggregates remain to be an open qu estion, but these results are explained by considering that some polymer ch ains possess cholesterol groups at both chain ends and spherical micelles a re bridged by these polymer chains.