Synthesis Of Amphiphilic Star Block Copolymer By RAFT Polymerization And Its Self-assembly Behavior

Cryopreservation protective materials with high-efficiency ice recrystallization inhibition are one of the key material technologies to be solved for the effective cryopreservation of living samples such as cells and tissues.They are of great significance to the development of cell therapy,regenerative medicine,tissue engineering,organ transplantation and other technologies.Ice control materials are adsorbed on ice crystals through hydrophilic and ice loving polar groups.It shows that they interact with ice crystals to regulate the growth process of ice crystals.The chemical and geometric structure of the material surface is the key factor to regulate the behavior of ice recrystallization.Amphiphilic block copolymers can form self-assembled particles with different morphologies in water.They can not only provide hydrophilic particle surfaces,but also give particles different surface binding structures.As a new type of ice recrystallization inhibition materials,they have attracted research attention.In this paper,the goal is to achieve efficient ice suppression by regulating the self-assembly structure and temperature sensitive behavior of morphology transformation of amphiphilic block copolymers.By regulating the multi arm star topology of segments,the following studies are carried out:1.Four Z-type raft chain transfer agents with different numbers of leaving groups（1,2,3,4）were synthesized simply and efficiently by sulfhydryl halogenated hydrocarbon click reaction.A series of amphiphilic block copolymers with different hydrophilic and hydrophobic chain length ratios were designed and synthesized by reversible addition broken chain transfer（RAFT）polymerization method,using N,N-dimethylacrylamide as hydrophilic block monomer and diacetone acrylamide as hydrophobic block monomer.In the synthesis,RAFT molecules with different departure groups were obtained.Four kinds of block copolymers with different topological structures were obtained.They were successively single arm AB type,double arm（AB）₂,three arm（AB）₃,and four arm type（AB）₄type.The copolymer was characterized by resonance nuclear magnetic hydrogen spectroscopy（¹H NMR）,gel permeation chromatography（GPC）and UV Vis spectroscopy.The controllability of star segment topology is verified.2.The amphiphilic block copolymer（AB）_n（n=1,2,3,4）of the four topologies has good dispersibility in water,and the self-assembly morphology changes with temperature,which is manifested as LCST-gel transformation.The thermal responsiveness of the subthermal mid-infrared was studied,and it was found that the temperature sensitivity of the copolymers of different topological blocks was caused by the dissociation of the hydrogen bonds inside the hydrophobic block PDAAM,which generated the driving force of the self-assembly process with temperature changes.Based on the overall molecular weight,the effect of topology on the temperature-sensitive phase transition temperature and morphology(T_gel)was studied by using the inverted vial method,dynamic light scattering method（DLS）and transmission electron microscopy（TEM）system,and the results showed that under the premise of the same overall molecular weight,increasing the number of"arms"of the star topology can reduce the phase transition temperature of the block copolymer and make it easier to obtain higher-order morphology.When a spherical and worm morphology transition occurs,a macroscopic phase transition occurs between the sol-gel.The sol-gel transition process was monitored using microfluidics,the gel structure was characterized by rheology,and it was found that when the single-arm polymerization degree and hydrophilic ratio were the same and the gel temperature was similar,the network construction rate during the sol-gel transformation process was the same and the resulting gel structure was the same,and it was not affected by the topology.3.The"splat assay"method was used to study the ice inhibition of block copolymer dispersions.Block copolymers(A₃₀B₂₀)_nwith a concentration of6wt%were assembled into spherical morphology at room temperature,and the change of arm number had no significant effect on ice recrystallization inhibition activity.Fixed hydrophilic segment chain length（DP=20）,extended hydrophobic chain length changed the assembly morphology from spherical to worm structure,and the grain size of ice recrystallization decreased significantly.Further increasing the concentration of copolymers leads to an increase in the density of worm morphological particles,and a significant modification effect of ice crystal morphology will occur,and a needle-like structure ice crystal structure will be obtained.For the four-arm structure copolymer(A₃₀B₆₀)₄,a vesicle morphology with a particle size of about 200nm can be obtained by heating to 37°C,and the grain size of the recrystallization is only 15%of DPBS buffer.Even if the crosslinked copolymer is assembled into a high-order morphology such as vesicles,its ice inhibition efficiency is still significantly lower than that of the uncrossed polymer,which is due to the transformation of the assembly morphology of the uncrossed copolymer during the ice recrystallization process,forming a dynamic interface that regulates the growth of ice crystals.