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

A series of cholesterol end-capped poly(ethylene glycol) (Ch(2)PEG) were sy nthesized by coupling cholesterol at each end of PEG (molecular weight = 40 00, 10 000, 20 000 and 35 000 g/mol) with hexamethylene diisocyanate. Unlik e hydrophobically modified PEGs, which are end-capped with flexible hydroca rbons or fluorocarbons, Ch(2)PEGs are not soluble in water, although they d o swell significantly, and the swelling ratio increases with molecular weig ht. Analysis of the swelling ratios via the Flory-Rehner equation indicates that, as PEG molecular weight increases, the Flory-Huggins interaction par ameter decreases slightly from 0.53(4) to 0.49(5) and becomes constant with in experimental error when the PEG molecular weight reaches 10 000 g/mol. A ddition of small amounts of a co-solvent such as n-propanol converts this i ntractable opaque material to a completely homogeneous, optically transpare nt, highly elastic fluid whose viscoelastic properties are those of a trans ient 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.