Scope of the copper halide/bipyridyl system associated with calixarene-based multihalides for the synthesis of well-defined polystyrene and poly(meth)acrylate stars

Novel multihalide compounds which were readily obtained by derivatization of 4-tert-butylcalix[4,6,8]arenes were used to initiate the atom transfer radical polymerization (ATRP) of styrene, tert-butyl acrylate (t-BuA), and methyl methacrylate (MMA), in the presence of CuX (X = Br or Cl) and 2,2'-bipyridyl. Well-defined polystyrene (PS) stars constituted of precisely four, six, and eight arms could be synthesized with a narrow polydispersity in this way. For instance, octafunctional polystyrene stars exhibiting molar masses as high as 600 000 g mol(-1) could be prepared. However, the polymerization had to be restricted to low conversion, typically below 15-20% to prevent stars from mutually coupling and avoid their contamination with species of higher functionality. Besides the concentration of polymeric radicals [P-.], the factor that was found to play a significant role in the occurrence of star-star coupling is their overlapping concentration (C*): intermolecular coupling indeed appeared to be enhanced whenever stars crossed their C*, but ways to alleviate this phenomenon of star-star coupling were also reported. In the case of hexa- and tetrafunctional PS stars, well-defined samples could be obtained within a large range of conversion because of the lower probability of such hexa- and tetraarmed species to get coupled. With octaarmed poly(methyl methacrylate) stars as well, the polymerization had to be discontinued at relatively low conversion in order to isolate samples with the expected functionality of 8, although this class of polymeric radicals tend to disproportionate rather than to recombine upon reaction. In contrast, no such side reactions could be detected while synthesizing poly(tert-butyl acrylate) stars, because the ATRP of t-BuA is associated with a much lower equilibrium constant (K-eq) between dormant and active species-and therefore to a lower concentration of growing radicals for a same initial concentration of initiator-than in the case of styrene or MMA. The functionality of the stars obtained was checked by comparing their molar mass with that of their individual arms isolated after hydrolysis of the ester functions of the central core. The viscometric characterization also attested to the well-defined character of the star polymers generated from the calixarene derivatized initiators, as confirmed by the comparison of their intrinsic viscosity with that of linear homologues of same molar mass.