Scandium-catalyzed stereoselective block and alternating copolymerization of diphenylphosphinostyrenes and isoprene

The coordination copolymerization of polar diphenylphosphinostyrenes (StPPh₂) and isoprene (IP) by a half-sandwich scandium catalyst (C₅Me₄SiMe₃)Sc(CH₂C₆H₄NMe₂-o)₂ afforded a new series of phosphine-functionalized alternating and block isoprene–styrene copolymers. In the copolymerization of p-StPPh₂ with IP, the incorporation of p-StPPh₂ into the polymer chains started after IP was almost completely consumed, yielding the p-StPPh₂–IP diblock copolymers containing a syndiotactic-rich poly(p-StPPh₂) block and a poly(IP) block containing 1,4- and 3,4-structures, probably because the coordination of IP to the catalyst is favored over that of p-StPPh₂. In the copolymerization of o-StPPh₂ with IP, the o-StPPh₂–IP alternating copolymer was exclusively formed with the coexistence of the two monomers, because the successive insertion of o-StPPh₂ was difficult due to steric hindrance caused by the phosphine/styrene chelation to the catalyst metal center. When an excess amount of IP was used in the copolymerization of o-StPPh₂ and IP, the formation of a poly(IP) block was achieved after o-StPPh₂ was consumed in the alternating copolymerization, thus affording block copolymers containing an o-StPPh₂–IP alternating block and a poly(IP) block. Moreover, the incorporation of IP in the alternating copolymerization with o-StPPh₂ took place in a regio- and stereospecific cis-1,4-fashion (95%), suggesting that the coordination (chelation) of o-StPPh₂ to the catalyst could influence the regio- and stereoselectivity of IP polymerization.