One-pot synthesis of isotactic-capped syndiotactic polystyrene with a bimetallic homogeneous catalytic system

INTRODUCTION Polystyrene can be obtained in atactic (aPS), isotactic (iPS) or syndiotactic (sPS) form by a variety of catalytic systems1,2 and initiators. For instance, it is known that (pentamethylcyclopentadienyl)titanium derivatives activated by methylaluminoxane (MAO) afford a highly sPS, with %m (defined as meso diads content, calculated by 13C-nuclear magnetic resonance (NMR)) below the instrumental detection levels, and Tm1⁄4271 1C, contaminated by small amounts of aPS by-product.3 On the other hand, nickel derivatives/MAO systems afford an isotactic-rich polystyrene (%m1⁄470–80), lacking any melting temperature;4 also in this case aPS is formed as a by-product. On the other hand, block copolymers of styrene with diolefins or acrylates are also well known.5,6 To our knowledge and according to the recent literature on the subject,1 the preparation and characterization of polystyrene with blocks of different tacticities have not been reported before; the only published paper on a polystyrene copolymer containing both syndiotactic and atactic sequences is that describing the two-step synthesis of an sPS–graft–aPS, prepared by syndiospecific polymerization (catalyst: CpTiCl3/MAO) of styrene with atactic vinyl-terminated styrene macromonomers obtained by anionic polymerization.7 As dual catalytic systems have been found to be able to produce other stereoblock polymers (for example, polypropylene, polymethyl methacrylate, etc.),8–12 we decided to test a homogeneous bimetallic nickel-titanium catalytic system in the polymerization of styrene. Here we report the synthesis of polystyrene using the [Z-(CH3)5 C5]TiCl3/Ni(Z-CH3COCHCOCH3)2/MAO (Cp*TiCl3/Ni(acac)2/MAO) system (Figure 1), either using or not using triisobutylaluminum as an additional component. EXPERIMENTAL PROCEDURE Materials Styrene was purified by overnight treatment with calcium hydride followed by percolation trough activated basic alumina. Toluene was refluxed and distilled in the presence of sodium/benzophenone and stored over activated molecular sieves. Nickel bis(acetylacetonate) and pentamethylcyclopentadienyl(titanium) trichloride were used as received. MAO (10% solution in toluene) was treated at 60 1C under vacuum for 2 h to drive off most of the free trimethylaluminum, and re-dissolved in anhydrous toluene before use. Triisobutyl aluminum (1 M solution in toluene) was used as received.