Synthesis Of Multi-arm Star Thermo-responsive Polymers And Topology Effect On Phase Transition

Thermo-responsive polymers have been widely reported due to their application value in fields such as biomedicine and intelligent device manufacturing.Their phase transition behaviors are affected by both internal factors such as degree of polymerization,end group,topological structure,and external factors such as polymer concentration,additives and solvent,of which topological structure is undoubtedly an important concern in the design and preparation of thermo-responsive polymers.In this work,linear and 2,3 and 4-arm thermo-responsive homopolymers and amphiphilic block copolymers nanoparticles with different topology structures were designed and synthesized,and the influence of topology on polymerization kinetics and thermal phase transition of polymers was investigated,and the details are as follows:1.Synthesis of multi-arm star thermo-responsive homopolymers and topology effect on phase transition.Firstly,linear and 2,3,4-arm chain transfer agents with different numbers of functional groups were synthesized,and then they were employed in solution RAFT polymerization to prepare linear and star thermo-responsive polymers of poly?N-acryloylsarcosine methyle ster?[?PNASME?_n] and poly?N-isopropylacrylamide?[?PNIPAM?_n]with the arm number n=1,2,3 and 4.Then we studied topology effects on polymerization kinetics and thermal phase transition behavior by turbidity analysis,variable-temperature ¹H NMR analysis.It was found that under the same conditions of polymerization,the reaction rate of preparing linear polymers through RAFT polymerization was faster than that of preparing star polymers.PNASME chains didn?t completely dehydrate during the phase transition,and the maximum dehydration rate p_max gradually increased with the increase of arm number.The topology structure had a significant effect on the phase transition of the homogeneous polymer?PNASME?_n,and PTT decrease with the increase of arm number,we also verified this conclusion with the most widely studied?PNIPAM?_n.2.Synthesis of star thermo-responsive amphiphilic block copolymer nano-assemblies and topology effect on thermoresponse.In this section,we synthesized the macro-RAFT reagents?PNIPAM-TTC?_n?n=1,2,3,4?with similar chain length but different topology structures of PNIPAM first,and then nano-assemblies of well-defined multi-arm star block copolymers of[poly?N-isopropylacrylamide?-block-polystyrene]_n[?PNIPAM-b-PS?_n]with a similar DP and chain density of the PNIPAM blocks but different topology?arm number?structures were synthesized via polymerization-induced self-assembly employing the mono-and multi-functional macro-RAFT agents under dispersion condition.Turbidity analysis,DLS analysis and variable-temperature ¹H NMR analysis all showed that PTT and T_cp decreased with the increase of the arm number.Different from the free PNIPAM chain in the homopolymer?PNIPAM-TTC?_n,the topology structure of the PNIPAM chains tethered on the hydrophobic PS surface in the nanoparticle system of?PNIPAM-b-PS?_n has a greater influence on the thermo-responsive phase transition.We hypothesized that it?s caused by inter-molecular and intra-molecular entanglement in the star structure.The variable-temperature ¹H NMR analysis showed that the maximum dehydration rate of the linear and 2-arm polymer nanoparticles is greater than that of the 3-arm and4-arm nanoparticles,and the rheological analysis showed that the viscosity range of the linear polymer during the phase transition was greater than that of the 3-arm one,which all verified the reliability of our hypothesis.