Synthesis And Performance Of Responsive Catalytic Microgels

Homogeneous metallic catalysts are attracting increasing attention for application in catalysis and energy conversion,due to their high activities and reaction selections.However,the difficulty in purity and recycling restricts their more extensive applications.Enhanced environmental consciousness has promoted efficiency of the chemical reactions under benign conditions with recycling and reuse of the catalysts,a trait that has become an integral part of chemical research today.Polymer microgels,colloids of three-dimensional crosslinked polymer networks that combine the properties of solids and fluids,are suitable for molecular transition,and thus have various applications for catalysis and biomedicions.Especially,stimuli-responsive microgels,which can respond to external stimuli and switch their properties,have attracted much attention in recent years.The introduction of the stimuli-responsive microgels into the metallic catalyst systems is a fascinating approach to intelligent catalytic system,which can display the merits of both homogeneous(high efficient catalytic activity)and heterogeneous(recyclability)catalysis in organic synthesis.Moreover,the catalytic performance of catalyst can be tuned via stimuli-responsive volume phase transition of the microgels.Despite the exciting progress,the challenge of facilitating the catalytic microgels with high catalytic acticity,long-term stability,easy to separate and good regulated performance still remains.Herein,we developed a series of glucose-responsive microgels and temperature-responsive microgels under reasonable design,studied the stimuli-responsive conformational/volume phase transition behavior,and thus understood the internal mechanism of the recognition and stimuli-responsive properties of microgels.From that,a series of responsive catalytic microgels were fabricated through the combination of stimuli-responsive microgels and catalytic metal nanoparticles/ions.The catalytic performance of catalyst tuned via stimuli-responsive volume phase transition of the microgels was studied and thus the synergistic effect of recognition,response and catalysis was explored.The works can be summarized as follows:(1)To address the issue of poor glucose selectivity of poly(phenylboronic acid)(pPBA)gels,we developed a class of pPBA microgels with high glucose selectivity at a physiological pH,as inspired by the binding of saccharides to proteins in nature for carbohydrate recognition.The strategy behind is to introduce suitable aromatic moieties(perylene bisimides)into pPBA microgels to create specialized chain structures that have perylene bisimides adjacent to the PBA groups.(2)We developed a class of glucose responsive catalytic microgels composed of Au nanoparticles embedded in PBA-containing polymer microgels.In view of the fact that the mesh size and the hydrophobicity of the polymer gel networks and thus mass transfer process can be tuned in a well-defined manner by the concentration of glucose,glucose was used as an additive to mediate the catalytic chemical reactions with the microgels as catalysts.Upon adding glucose into the reaction media,the model Au-catalytic reduction of hydrophilic 4-nitrophenol was accelerated,while the reduction of relatively more hydrophobic nitrobenzene slowed down.This work may provide a new insight into mimic enzyme catalysis.(3)By simply stirring the microgel dispersion with copper sulfate,copper was immobilized on a temperature-responsive polymer microgel made of O-carboxymethylated chitosan,crosslinked with genipin.The ensuing catalyst was highly active for a model azide-alkyne[3+2]-cycloaddition reaction;the catalytic activity can be tuned in a non-monotonous way via swelling-deswelling transitions of the polymer gels upon change of temperature.(4)A class of ligand-based temperature-responsive microgels was prepared by copolymerizaton of oligo(ethylene glycol)methyl ether methacrylate with vinyl modified phosphine ligand.Then palladium was immobilized on the microgel by simply stirring the microgel dispersion with palladium acetate.The ensuing catalyst showed enhanced activity and excellent recyclability in a model Suzuki coupling reaction;the catalytic activity can be tuned in a non-monotonous way tuned via temperature stimuli-responsive volume phase transitions of the polymer gels.