Synthesis of Organic/Inorganic Hybrid Quatrefoil-Shaped Star-Cyclic Polymer Containing a Polyhedral Oligomeric Silsesquioxane Core

We report the synthesis of quatrefoil-shaped star-cyclic polystyrene, star-cyclic PS, containing a polyhedral oligomeric silsesquioxane (POSS) core via the combination of atom transfer radical polymerization (ATRP) and click chemistry techniques. The obtained star-cyclic PS represents a new chain topology in the category of nonlinear-shaped polymers. Using octa(3-chloropropyl) polyhedral oligomeric silsesquioxane, POSS-(Cl)(8), as the starting material, its azidation and subsequent click reaction with a slight excess of propargyl 2-bromobutyrate afforded octafunctional initiator, POSS-(Br)(8). 8-arm star-linear PS-N-3 was obtained by the azidation of star-linear PS-Br, which was synthesized by the ATRP of styrene using POSS-(Br)8 as the initiator. Model reaction between alpha,omega-diazido-terminated PS (N-3-PS-N-3) and difunctional propargyl ether confirmed that bimolecular click cyclization reaction can effectively occur under highly dilute conditions. Next, intramolecular click ring closure of star-linear PS-N-3, was conducted under highly dilute conditions, using propargyl ether as the difunctional linker and CuBr/PMDETA as the catalyst, affording quatrefoil-shaped star-cyclic PS. Gel permeation chromatography (GPC), H-1 NMR, and FT-IR analysis confirmed the complete consumption of azide moieties in star-linear PS-N3 and that the coupling reaction proceeded via the intramolecular manner. Differential scanning calorimctry (DSC) results revealed that star-cyclic PS possesses higher glass transition temperature (T-g) than that of star-linear PS, possibly due to the ring topology of PS arms in the former.