Preparation Of Polyurethane Microspheres By Precipitation Polymerization And Their Palladium Immobilization And Applications As Catalyst

The precipitation polymerization has received great attentions because of its production of microspheres with clean surfaces,but surfactants or stabilizers are not required in this process,the polymerization can only produce uniform microspheres with a lower monomer concentration.This is an important hindrance to industrial production and the subsequent application.In order to increase the yield,different versions of precipitation polymerization have been proposed,such as reversible addition-fragmentation chain transfer?RAFT?precipitation polymerization,distillation-precipitation polymerization,reflux precipitation polymerization,and step-growth precipitation polymerization.Among them,the step-growth precipitation polymerization,based on the step-growth polymerization mechanism,is of particular interests.In this process,the monomer conversion is usually higher than in the others.Polyurethane?PUN?is one of the most widely used synthetic materials,has been widely used in many fields.In contrast,studies on the preparation of PUN microspheres by precipitation polymerization are hardly available.Herein,highly uniform PUN microspheres were prepared by precipitation polymerization of toluene diisocyanate?TDI?and trimethylolpropane?TMP?in acetonitrile with high yield in comparison with previously reported precipitation polymerizations.PUN-supported Pd?Pd@PUN?were prepared by immobilizing Pd-nanoparticles?NPs?onto the surface of PUN microspheres and used as catalyst in 4-nitrophenol reduction and dye degradation with high catalytic activity.Under optimized condition,the maximum monomers concentration to prepare monodisperse PUN microspheres was increased to 25 wt%with a microsphere yield of96.92%;and the monomers were completely polymerized within 2 hours.Compared with the previously reported free-radical precipitation polymerization,great improvements have been achieved in terms of microsphere yield and polymerization time.The effects of reaction temperature,the amount of catalyst and monomer on the formation of microspheres were discussed.It was found that all of them affected the polymerization rate and thus the microsphere morphology and size.Using PUN as the support,immobilization of Pd?OAc?₂ on the surface of PUN microspheres was carried out in dichloromethane.Pd?OAc?₂/PUN composite microspheres were prepared.Effects of Pd?OAc?₂/PUN ratio,Pd?OAc?₂ and PUN concentration,and immobilizing time were investigated with reagrd to the immobilization.Under optimized condition,immobilization of Pd?OAc?₂ reached the maximum within 24 h of contact of Pd?OAc?₂ to PUN microspheres.The composition and morphology of the catalyst were characterized.The results showed that there was no significant change in the morphology by the immobilization.The presence of Pd on the surface of PUN microspheres was determined by SEM,EDX and TEM.Coordination of Pd atoms with N and O atoms on the surface of PUN microspheres was determined.Pd dispersion in Pd@PUN was determined to be 10.6%through chemisorption,and the average diameter of Pd nanoparticles was 10.57 nm.Both PUN microspheres and Pd@PUN catalysts have good heat and solvent resistance.The composites microspheres Pd@PUN were used as catalyst in 4-nitrophenol?4-NP?reduction with high catalytic activity.The reaction followed the pseudo-first-order kinetics,with a kinetics constant of 7.45 min^-1,a value higher than many of those reported for similar studies.The effects of the catalyst and sodium borohydride?NaBH₄?on the reduction were discussed.The reusability test of the catalyst revealed that its high catalytic activity was slightly decreased in the first 4 recycled uses,and an obvious deterioration was observed in the subsequent reuses.This was attributed to the formation of a coordinated complexe between Pd atoms on Pd@PUN surface and the product of the reduction,and the conclusion was affirmed through different experiments.