Multifunctional coupling agents for living cationic polymerization .7. Synthesis of amphiphilic tetraarmed star block polymers with alpha-methylstyrene and 2-hydroxyethyl vinyl ether segments by coupling reactions with tetrafunctional silyl enol ether

By the polymer coupling reaction with a tetrafunctional silyl enol ether [1; C{CH2OC6H4C(OSiMe(3))=CH2}(4)], amphiphilic tetraarmed star polymers (6) with alpha-methylstyrene-2-hydroxyethyl vinyl ether (alpha MeSt-HOVE) block arm chains have been synthesized: C{CH2OC6H4COCH2-[CH(OCH2CH2OH)CH2](n)-[C(CH3)(C6H5)CH2](m)-}(4). The synthesis consisted of the following steps: (i) the sequential living cationic polymerization of alpha MeSt and 2-[(tert-butyldimethylsilyl)oxy] ethyl vinyl ether (SiVE) into an alpha MeSt-SiVE living block copolymer (4); (ii) the coupling of four chains of 4 with the quencher 1 into a tetraarmed polymer (5); and (iii) the transformation of the poly(SiVE) segment into the poly(HOVE) chain by the fluoride-catalyzed deprotection. In step i the living polymerization was carried out at -78 degrees C in methylene chloride with a binary initiating system comprising tin tetrabromide and the hydrogen chloride adduct of 2-chloroethyl vinyl ether. After step iii, the tetraarmed amphiphile 6 (alpha MeSt/HOVE = 27/27 in <(DP)over bar(n)> in each arm) was soluble in ethanol and chloroform, in which its H-1 NMR spectra changed dramatically by responding to solvent polarity. For example, in the former polar solvent, the signals of the hydrophobic alpha MeSt units were hardly detectable, whereas they were clearly observed in chloroform. This responsiveness was attributed to the rigidity of the poly(alpha MeSt) segment, which contrasts with the more flexible poly(vinyl ether) arm chains in the previously reported tetraarmed amphiphiles.