Synthesis And Characterization Of Multicomponent Miktoarm Star Polymers And Dendrimer-like Polymers

Miktoarm star polymers and dendrimers-like polymers have attracted great attentiondue to their new shapes, unique properties and variable chemical components of polymersegments. Much emphasis has been paid on developing synthetic methodologies to thesepolymers, and investigating their self-assembly in aqueous solution. This study wasprimarily aimed at synthesis and properties of ABCDE-type miktoarm star polymers anddendrimers-like polymers comprising poly（ethylene glycol）（PEG）, poly（ε-caprolactone）（PCL） and poly（acrylic acid）（PAA）. To this end,“living”/controlled polymerizationtechniques such as reversible addition-fragmentation chain transfer （RAFT） polymerization,atom transfer radical polymerization （ATRP） and ring-opening polymerization （ROP） andcopper（I） catalyzed azide-alkyne cycloaddition （CuAAC） reaction were combined tosynthesize the target star and dendrimers-like polymers, their chemical structures werecharacterized by^<sup>1H NMR and GPC, and their self-assembly properties in aqueous solutionwere also investigated. The major contents in this study were listed below.The first part was focused on synthesis and characterization of5-arm ABCDE starpolymers. A trifunctional initiator HPCP comprising alkynyl, hydroxyl and C（=S）S moietyand MPEG-（N₃）₂（A segment） with two terminal azide functionalities in one side weresynthesized and characterized by¹H and¹³C NMR, FT-IR and GPC. Themid-alkyne-functionalized BC and DE-type diblock copolymers were synthesized bysuccessive RAFT polymerization and ring-opening polymerization based on HPCP initiator.On this basis, excess monomethoxy polyethylene glycol containing two azide end groups（A） was clicked with BC diblock copolymer to obtain core-azide-functionalized3-armABC star, which was further reacted with excess DE diblock copolymer to prepare5-armABCDE star polymers. The results revealed that the target ABCDE star polymers withwell-defined structures and low polydispersity could be successfully achieved. In somecases, however, fractional precipitation and a further click reaction with azide-functionalized resin were necessary to obtain the target stars with high purity. Allmiktoarm star polymers were characterized by¹H NMR, GPC, FT-IR and DSC.Furthermore, the characteristic assembled structures in aqueous solution were observed bytransmission electron microscopy （TEM） and dynamic light scattering （DLS）. Preliminaryresults indicated5-arm PEG-PCL-PSt-PDMAEMA-PLLA star was liable to formonion-like micelles with phase separation in the core or polymer vesicles with phaseseparation in the membrane in aqueous solution. These aggregates had average diametersin the range of100-200nm, and their size distributions were usually less than0.25.The second part was aimed at synthesis and characterization of dendrimers-likepolymers with PEG, PCL and PAA segments. The iterative methodology usually containedthree step reactions:（a） hydroxyl was converted to alkyl bromide;（b） thiol-bromo clickreaction was used to converted one alkyl bromide to two hydroxyl functionalities;（c）polymer segments such as PCL or PtBA was introduced by ring-opening polymerization orATRP. Starting from poly（ethylene oxide）（PEO） with molecular weight of1000g/mol,generation three dendrimer-like polymer with16alkyl bromide moieties in out layer wassynthesize by iterative method, and PtBA segments were introduced by a following ATRPof tert-butyl acrylate （tBA）. After further hydrolysis, dendrimers-like copolymers withPEO as inner core, PCL as repeating units, and PAA as surface segments were successfullyachieved. The dendrimer-like polymers with different generations were characterized by¹H NMR, GPC, and FT-IR. The self-assembly behaviors of the amphiphilic dendrimer-likepolymer PEG-（PCL）₄-（PCL）₈-（PAA）₁₆ was investigated by TEM and DLS. The resultsshowed the aggregates had a uniform distribution and average particle size of about100nm. Because of their internal cavity, the dendrimer-like polymers have a potential in drugloading and release.