Transition-metal compatibilization of poly(4-bromostyrene) and polybutadiene via palladium(0) catalyzed reactions

Abstract

Immiscible blends of 1,2-polybutadiene and poly(4-bromostyrene) can be compatibilized by rather low concentrations of Pd (0) [P(C 6 H 5 ) 3 ] 4 at ambient temperature and 60 °C under argon. Two distinct glass-transition temperatures merge into a single glass-transition temperature at high enough concentrations of Pd (0) (i.e., 2 or 3 mol %). Compatibilization does not occur if Pd (0) is absent, triphenylphosphine is added without Pd (0) or polystyrene is not functionalized. The methodology described herein is also useful for inducing melting-point depression of 2,7-dibromofluorene in ternary complexes with 1,2-polybutadiene and Pd (0) A 72/28 complex of poly(4-bromostyrene) and 1,2-polybutadiene with 5.5 mol % Pd (0) exhibits a reinforced rubbery response with a modulus of 1.2 × 10 7 N/m 2 , a fracture strain of 235%, and a single glass-transition temperature. Mechanical properties of these compatibilized ternary systems compare well with those of styrene-butadiene block copolymers, particularly above 100% strain. A five-step mechanism that includes oxidative addition, olefin coordination, migratory insertion, β-hydride elimination, and reductive elimination in the coordination sphere of the transition metal is proposed to illustrate how either poly(4-bromostyrene) or 2,7-dibromofluorene is linked covalently to alkene side groups in the diene polymer via the Heck reaction.