Chain Growth Polymerization of Isoprene and Stereoselective Isoprene-Styrene Copolymerization Promoted by an ansa-Bis(indenyl)allyl-Yttrium Complex

The ability of rac-[{Me2C(Ind)(2)}Y(1,3-(SiMe3)(2)C3H3)] (Ind = 9-indenyl) (1) to catalyze isoprene polymerization, either as a single-site component or in combination with an alkyl metal as a chain transfer agent (CTA) via coordinative chain transfer polymerization (CCTP), has been investigated. Complex 1 alone catalyzes the highly 1,4-trans-stereoselective polymerization of isoprene with moderate activity. The addition of ZnEt2 provides an efficient catalytic system for the reversible CCTP of isoprene with retention of the 1,4-trans stereoselectivity. By contrast, in the presence of Mg(nBu)(2), with increasing amounts of CTA, the 1,4-trans specificity gradually decreases at the advantage of the 3,4-polymer. The copolymerization between isoprene and styrene has been also studied, resulting in the formation of materials with unprecedented architecture. Simultaneous copolymerization of isoprene and styrene with 1 provides copolymers with a wide range of compositions (chi(s) = 0.30-0.98) and a stereospecific blocky distribution of the monomers in the copolymer chain. The reactivity ratios (average values: r(1,Ip) = 2 and r(2,St) = 18) determined by NMR spectroscopy confirmed the propensity to form multiblock materials. Microstructural C-13 NMR analysis showed the presence of stereoregular 1,4-trans-polyisoprene and isotactic polystyrene segments. The copolymerization via sequential monomer addition allowed the synthesis of well-defined 1,4-trans-polyisoprene-block-isotactic polystyrene, which contains two crystalline blocks. The authenticity of these new di- or multiblock copolymer materials was assessed by combined NMR, GPC, and DSC characterizations.