Thermoresponsive Helical Polymeric Nanoparticles

Author:Zhao Yuan Bo

Supervisor:zhang a fang


Degree Year:2018





Polymeric nanoparticles possess outstanding properties,including small size and volume,large surface to volume ratio,high diffusibility and mobility,and stable dispersions,which have gathered considerable research attention.Optically active helical polymers are asymmetric,stereo and accurately ordered,which play an important role in life systems.Chiral polymeric nanoparticles combine the characteristics of nanosize and chirality in one matter,which can provide a confined environment to switch chirality and promote transformations.They have made great progress as one kind of bio-materials and have promising applications in bio-related fields such as biological medicine,medical testing,drug control and release,and biochemistry.Herein,we reported on fabrication of optically active polymeric nanoparticles(PNPs)from linear dendronized copolymers carrying dendritic oligoethylene glycol(OEG),L-alanine groups and aldehyde groups,and investigation of their tunable chirality upon external stimuli,such as temperature,pH,amino acids and metal ion complexation.The chemical structures,secondary conformations,thermoresponsive properties and morphologies of the synthesized compounds,copolymers and nanoparticles were characterized by NMR,UV/Vis,CD,DLS and AFM.The details are described as following:(1)Linear dendronized copolymers carrying L-alanine groups,aldehyde groups and dendritic oligoethylene glycol(OEG)were designed and successfully synthesized.These copolymers have the properties of thermosensitivity and helical conformation.There helicity decrease or even inverted upon thermally induced phase transitions.(2)By virtue of the thermally induced aggregation of these OEG-based polymers,the nanoparticles were successfully prepared by adding adipic dihydrazide(ADH)as the cross linker.The resulted nanoparticles show typical thermoresponsiveness and their helical conformations were stable with increase of temperature.Meanwhile,the DLS data displayed that the size of the nanoparticles increased along with the rise of the molecular weights of the polymers.(3)Polymeric nanoparticles were also fabricated via one-pot suspension copolymerization of phenylacetylene derivatives.This method is more convenient than the former and the yield is higher as well.Obtained PNPs also show typical thermoresponsiveness and the stable helical structure.We found that the size of nanoparticles is affected by the terminal groups of the OEGs and the stirring speed.(4)The tunable chirality of these PNPs upon external stimuli,such as temperature,pH,amino acids and metal ion complexation was investigated.It was found that the intensity of nanoparticle’s helicity decreased with increase of temperature at basic conditions.At neutral and acidic conditions,the helicity of nanoparticle remains quite stable.Coordination with mental ions can be shielded by OEG dendrons below their LCST,which can be reversibly switched off just by increasing solution temperature above their Tcps.Chirality of polymeric nanoparticles showed significant changes upon addition of excess D-Phe,demonstrating specific recognition to D-Phe.(5)Achiral nanoparticles terminated with methoxyl were prepared via one-pot approach through suspension copolymerization.This chemical structure was changed by amide bond instead of L-alanine.Through the dynamic covalent chemistry between different chiral amino acids and aldehydes,it was successfully mediate helical conformation reversibly and chirality transferring from small compounds to conjugated polymer main chains.The results show that L-Amino acids can induce right handed helix,while D-Amino acids can induce left handed helix.The helical structures were destroyed by increasing temperature,which can be recovered completely when temperature was decreased and kept up to 8 hours.(6)1,4-Diethynylbenzene was used as the cross linker instead of ADH for nanoparticle preparation.The influence of external stimuli such as metal ions,pH and amino acids on secondary structure of PNPs was explored.The results show that it has the same performance as the former methods in metal ions and acid or base solution,however,no special amino acid recognition exists.In this work,polymeric nanoparticles carrying dendritic oligoethylene glycol(OEG),L-alanine groups and aldehyde groups were synthesis successfully.Combination of the dynamic covalent chemistry and thermally induced aggragation behavior,thermoresponsive helical polymeric nanoparticles were successfully prepared.Also,these PNPs were prepared by suspension polymerization at room temperature,and the effects of external stimuli on their secondary conformations were investigated in detail.The present work provides important research value and application prospects in the field of chiral recognition,chiral separation,biomimetic materials,molecular devices,asymmetric catalysis,biological probes and the like.Figure A:The helical switching of polymeric nanoparticles by external stimulus