| In this paper, we studied the synthesis of cyclic PS-b-PMPCS copolymers by thecombination of self-assembly and “click” reaction. The high dilute concentration, which isrequired for the intramolecular cyclization synthesis of cyclic block copolymers by “click”reactions, is achieved by controlling the critical micelle concentration (CMC) of blockcopolymers in selective solvent. With the dynamic equilibrium between micelles andunimers, the pseudo high dilution concentration is achieved. The influence of the criticalmicelle concentration (CMC) and the micelle (as “storage”) concentration on theintramolecular cyclization were investigated. The properties of the cyclic polymers wereinvestigated in detail, comparing to their linear precursors. The detailed research works areoutlined as following:(1) The alkyne and bromide end-functionalized polystyrene macroinitiator wassynthesized by atom transfer radical polymerization (ATRP) using3-butynyl2-bromo-propionate as initiator, CuBr and N,N,N’N’,N’-pentmethy-ldiethylenetriamine(PMDATA) as the ligand, and styrene as monomer. PS-b-PMPCS-Br was converted intoPS-b-PMPCS-N3by reacted with NaN3. At last, the cyclic-PS-b-PMPCS was successfullysynthesized via Cu-catalyzed “click” reaction at traditional high dilute concentration. Theeffect of block copolymer concentration on cyclization was studied to find the bestcondition for the intrachain reaction, which should belower than0.28mg/mL. The cycliccopolymers were characterized and confirmed by GPC, FT-IR and NMR.(2) The critical micelle concentration (CMC) of PS-b-PMPCS-N3block copolymer insolvent mixture of toluene and cyclohexane was determined by the laser light scattering atdifferent temperature (25°C,40°C,50°C and60°C). The measured CMCs are among10-3~10-1mg/mL increased with the increasing of the temperature or toluene content. (3) The influence of solvent mixture, concentration and temperature on the cyclizationof linear-PS-b-PMPCS-N3by click reaction in micelle solution of block copolymer in themixture solvent of oluene and cyclohexane is studied. GPC, FT-IR, NMR had beenemployed to verify the formation of the cyclic polymers. GPC and CMC results suggestedthat the control of the CMC was very important to the unimer cyclization. Only a rightconcentration range is perfect for the intrachain reaction. With the results of the GPC andweight-average molecular weights (Mw) determined by SLS, it proved that influence of themicellar concentration could not be neglected when the concentration was high enough, inwhich the molar concentration of the micelle was higher than that of the unimer. However,purified cyclic polymers could still be facilely prepared at relatively high concentrationafter processed with azido resin.(4) The thermostability and glass transition temperature of linear and cyclicPS-b-PMPCS were determined by thermal gravity analysis (TGA) and differentialscanning calorimetry (DSC). The results shows the cyclic block copolymers have lessthermal stability compared to linear precursor, but higher glass transition temperature.(5) The self-assembled behaviours of linear and cyclic PS-b-PMPCS in toluene (goodsolvent), and mixed solvent of toluene and cyclohexane were investigated by transmissionelectron microscopy (TEM), dynamic (DLS) and static (SLS) light scattering methods. TheDLS and SLS results in toluene revealed that cyclic PS-b-PMPCS had smallerhydrodynamic volume (Rh), similar Mwcompared with the linear precursor. Besides, cyclicPS-b-PMPCS had larger second virial coefficient value (A2), indicating that the cyclicPS-b-PMPCS had better affinity to solvent. At same condition, cyclic PS-b-PMCS hadlarger CMC, Rh, Rg, A2(negative) and smaller Mwcompared to their linear precusor. Bothfor linear and cyclic PS-b-PMCS, Rh, Rg, A2decreased with the increasing of thecyclohexane content, while the Mwincreased. By the combination of DLS and SLS results,the value of ρ parameter is calculated, and it indicates the formation of core-shell micellenanostructure, agrees well with the TEM results. |
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