Synthesis And Properties Of Multiarm And Miktoarm Star Polymers With A Cleavable Branched Core

As compared with their linear analogues, star polymers are liable to exhibit unique structure and functions, and thus they have attracted much attention recently. Among them, one important tendency lies in introducing stimuli-responsive segments and cleavable linkages into star polymers, which can not only extend the functions and applications but endow polymers with stimuli-triggered topological transition. From the viewpoint of material applications, the development of facile synthetic methods is very promising since it can underlie preparation and practical application in large scale. The important progress in hyperbranched polymers enables the synthesis of target polymers with controllable molecular weight and degree of branching, and tunable reactive sites in branching points, branches and terminal groups. Therefore, hyperbranched polymers can act as an important building block such as internal core to construct functional star polymers. In this study, “living”/controlled polymerization techniques such as reversible addition-fragmentation chain transfer(RAFT) polymerization and ring-opening polymerization(ROP) and linking reactions were combined to construct Am-type multiarm stars, and AmBn and AxByCz-type miktoarm stars with a stimuli-cleavable branched core. On this basis, the structure-property correlations including the influence of external stimuli on structural transition and aggregation behaviors were investigated, and their potential applications in smart materials were explored. The main contents and results are listed below.The research in Part 1 aimed at synthesis and properties of amphiphilic(PEG)m(PCL)n(m ≈ n ≈ 23) miktoarm stars with a disulfide-linked branched core. The miktoarm stars were synthesized by three step reactions, namely, RAFT copolymerization of 2-((2-(acryloyloxy)ethyl)disulfanyl)ethyl 4-cyano-4-(phenylcarbonothioylthio)pentanoate(ACP) and glycidyl methacrylate(GMA) afforded hyperbranched PGMA, a subsequent epoxy-carboxyl coupling reaction between PGMA and carboxyl-terminated poly(ethylene glycol)(PEG-COOH) gave multiarm star PEG with a branched core, and the newly generated hydroxyl moieties were further used to initiate ring-opening polymerization(ROP) of ε-caprolactone(CL) to generate the target stars. On this basis, postpolymerization modification was conducted to attach some reactive moieties such as coumarin, alkyne and alkyl bromide onto the branched core. The results given by 1H NMR and GPC-MALLS revealed that the multiarm stars and their precursors had controlled molecular weight and relatively low polydispersity(PDI = 1.13-1.44). To reveal the influence of topology, location of coumarin moieties and solvent polarity, the aggregation behaviors and fluorescence properties of coumarin-bearing star before and after reduction-cleavage were investigated. In water and THF/water mixtures, coumarin-modified star copolymer could aggregate into some intriguing morphologies including hyperbranched micelles and large vesicles due to the influence of solvent polarity on aggregation behaviors. Owing to the differences in isolation of fluorophores from solvent and restricted molecular motion, coumarin-functionalized star exhibited adjustable fluorescent properties in water and THF/water mixtures, and its aqueous solution had a maximum quantum yield(ΦF = 44.2%). The solutions of star copolymer and its reduction-cleaved copolymer were of different hydrodynamic diameters, and the ΦF(star)/ΦF(cleaved copolymer) values were 1.85(in THF), 3.00(in water) and ranged between 1.29 and 2.58 in THF/water mixtures, revealing the aggregation behaviors and fluorescent properties were strongly dependent on polymeric architecture, location of fluorophores and solvent polarity. Our study affords a versatile method to construct functional miktoarm stars with a multi-reactive branched core, and coumarin-functionalized star copolymer may have a great potential as solvent polarity and reduction dual-sensitive imaging materials in “green” ink, coating and nanocarriers for biomedical applications.The study in Part 2 aimed at synthesis and properties of multi-stimuli responsive(PNIPAM)x(PAA)y(PCL)z star terpolymer with a disulfide-linked branched core. RAFT copolymerization of ACP and GMA afforded hyperbranched PGMA, and followed by RAFT chain extension polymerization of NIPAM to give star PNIPAM with a branched core. On this basis, ring-opening reaction between 2-bromopropionic acid and epoxy-functionalized star PNIPAM was performed to synthesize bromide and hydroxyl functionalized star PNIPAM. Finally, successive atom transfer radical polymerization(ATRP) of tert-butyl acrylate(t BA), ROP of CL and selective hydrolysis of Pt BA segments were used to construct(PNIPAM)x(Pt BA)y(PCL)z(S4) and(PNIPAM)x(PAA)y(PCL)z(S5) star terpolymers. The target star copolymers had controlled molecular weight and polydispersity with the range of 1.12-1.28. As S4 was subjected to reduction using Bu3 P, the star copolymer could degrade into mixtures of low-molecular-weight star, graft and block copolymers. Upon thermo, p H and reduction stimuli, S5 micelles could perform reassembly and form microphase-separation micelles and branched micelles with different sizes and particle size distributions. S5 aggregates could efficiently encapsulate DOX and perform stimuli-triggered delivery, and the cumulative releases at 24 h were 23.4%(25 oC), 39.1%(p H 5.3, 37 oC), 55.7%(10 m M DTT, 37 oC) and 63.4%(p H 5.3 + 10 m M DTT, 37 oC), respectively. Consequently, S5 star could exhibit multiple stimuli responsivenesses and have a great potential in nanocarrier for smart drug delivery system.The research in Part 3 aimed at synthesis and properties of multiarm star PNIPAM with acetal and cyclic azobenzene functionalized branched core. RAFT copolymerization of 4-(1-(2-(acryloyloxy)ethoxy)ethoxy)butyl 4-(benzodithioyl)-4-cyanopentanoate(ABCP) and cyclic azobenzene-functionalized methacrylate(CAMA) afforded branched poly(ABCP-co-CAMA)(abbreviated as PAC), and followed RAFT chain extension polymerization to give star PNIPAM with a branched core. Meanwhile, linear PCAMA was also synthesized by RAFT polymerization of CAMA mediated by 2-cyanopropan-2-yl 1-dithionaphthoate(CPDN). These polymers had controlled molecular weight and relatively low polydispersity(PDI = 1.33-1.46). The aggregation behaviors of star PNIPAM were affected by some factors such as molecular weight, topology, types and number of end groups and Z-E isomerization of cyclic azobenzene, and micelles and compound micelles could be formed under different conditions. Moreover, the influence of topology on photoisomerization was investigated. For various solutions of polymer in THF, the rate constants of the E-to-Z photoisomerization reaction(ke) upon UV irradiation(365 nm, with light intensity of 0.69 m W cm–2) were 0.0285 s–1(linear PCAMA), 0.0621 s–1(branched PAC) and 0.0650 s–1(star PNIPAM), and the rate constants of the Z-to-E photoisomerization reaction(kH) upon visible light irradiation(435 nm, with light intensity of 1.2 m W cm–2) were 0.0315 s–1(linear PCAMA), 0.0449 s–1(branched PAC) and 0.0686 s–1(star PNIPAM). In addition of some factors such as photoisomerization mechanism and tunnel and energy difference, the differences in stacking density of cyclic azobenzene and chain entanglement resulting from topology may account for this phenomenon. Our study further extends the utilization of cyclic monomer in construction of branched and star polymers. Preliminary results revealed Z-E photoisomerization of cyclic azobenzene could be affected by topology, and the resultant branched and star polymers may have a potential in smart materials.In summary, some functional multiarm and miktoarm star polymers have been successfully synthesized by combination of controlled polymerization and linking reaction. Owing to the presence of stimuli-cleavable linkages in the internal branched core, star polymers can be degrade into other topological polymers with addition of external stimuli, and they can exhibit stimuli-triggered changes in aggregation behaviors, photo properties and drug release kinetics and thus have a great potential applications in a wide range of fields. Our study can not only afford facile methods to prepare functional star polymers and enrich the types of smart stars but favor to extend the potential of star polymers in smart biomedical and fluorescent imaging materials.