Interaction characteristics controlling catalytic performances of Ni (II) and Cu (II) phthalocyanines immobilized on bentonite clay surface in redox-initiated polymerization of methyl methacrylate in aqueous medium

Abstract

Different catalytic performances have been shown for nickel (II) phthalocyanine (1.8 wt%)/bentonite/HSO3− and copper (II) phthalocyanine (1.8 wt%)/bentonite/HSO3− initiating systems in the early stage of methyl methacrylate polymerization in aqueous medium, in the temperature range, 40–60 °C. Redox–free radical mechanisms, suggesting dual-site initiation in presence of nickel phthalocyanine system and single-site initiation by using copper phthalocyanine system, were discussed in view of different interaction and dispersion profiles of the immobilized complex systems. Both complexes exhibited strong interaction with bentonite, assisted by n-butyl amine solvent, during the immobilization process. This amine could modify bentonite surface through eliminating its narrower pores, expanding its sheet structure and using up its strongest Bronsted acid sites. Nickel phthalocyanine molecules, intercalated into bentonite clay galleries by n-butyl amine, could be located parallel to silicate layer planes in an even distribution and higher dispersion. Cation exchange or substitution mechanism involving stronger interaction between Ni2+ ions and Al3+ sites in the octahedral sheet encouraged dual-site initiation; two active Ni sites, accessible from one side each, coordinated with two monomer molecules. Copper phthalocyanine molecules, drawn in an inclined edge-on orientation, displayed a strong interaction between N’s of the macro-ring system and the clay external edge OH sites (asymmetric Bronsted interactions). This allowed facile access of Cu sites to the monomer and HSO3− cocatalyst from both sides, resulting in smoother initiation effect.