Polymerization of ethylene oxide with a calixarene-based precursor: Synthesis of eight-arm poly(ethylene oxide) stars by the core-first methodology

Eight-arm poly(ethylene oxide) (PEO) stars were prepared by the core-first method with a newly designed octahydroxylated precursor. This compound was readily obtained in two steps from commercially available tert-butylcalix[8]arene. The choice of the proper solvent of polymerization proved crucial to obtain PEO star materials with a narrow distribution of molar masses. For instance, the use of dimethyl sulfoxide (DMSO) resulted in PEO samples of rather large polydispersities (PDI: 1.3-1.5). In this solvent, the calix-arene-based precursor was only sparingly soluble, and an attempt to metalate its eight hydroxyl groups produced insoluble alkoxides. In addition, the presence of a side population of low-molar-mass species attributable to linear chains was detected because of the chain transfer of propagating alkoxides to DMSO. Polymerization experiments carried out in tetrahydrofuran (THF) as solvent afforded better control over the molar masses and PDIs. This was related to the better solubility of the octafunctional calixarene-based precursor in THF and to the small tendency of the alkoxides formed to aggregate in that solvent. Under such conditions, all eight hydroxyl functions efficiently initiated the polymerization of ethylene oxide. In this way, well-defined PEO stars (PDI < 1.2) of tunable molar masses incorporating a calixarene-based core could be obtained, as it was supported by the characterization of the samples by size exclusion chromatography, NMR, and viscometry.