Synthesis and solution properties of cholesterol end-capped poly(ethylene glycol)

A series of cholesterol end-capped poly(ethylene glycol) (Ch(2)PEG) were synthesized by coupling cholesterol at each end of PEG (molecular weight = 4000, 10 000, 20 000 and 35 000 g/mol) with hexamethylene diisocyanate. Unlike hydrophobically modified PEGs, which are end-capped with flexible hydrocarbons or fluorocarbons, Ch(2)PEGs are not soluble in water, although they do swell significantly, and the swelling ratio increases with molecular weight. Analysis of the swelling ratios via the Flory-Rehner equation indicates that, as PEG molecular weight increases, the Flory-Huggins interaction parameter decreases slightly from 0.53(4) to 0.49(5) and becomes constant within experimental error when the PEG molecular weight reaches 10 000 g/mol. Addition of small amounts of a co-solvent such as n-propanol converts this intractable opaque material to a completely homogeneous, optically transparent, highly elastic fluid whose viscoelastic properties are those of a transient network with relaxation times in the range from 0.1 to 10 s, depending on co-solvent content and temperature. (C) 7000 Elsevier Science Ltd. All rights reserved.