Synthesis Of Polyoxometalate Based Miktoarm Supramolecular Star Copolymers And Their Self-Assembly In Solution

Amphiphilic star-shaped copolymers can self-assemble in selective solvents to form complicated micellar constructs as a synergistic result of both the topological constraints and relative volume fractions of the arms.Although the self-assembly of amphiphilic star copolymers in nonselective solvents has been observed,both the structural detail and formation mechanism of these associates are not clear yet.To clarify these issues,we have synthesized a series of polyoxometalate based polystyrene-poly?ethylene glycol??PS-PEG?miktoarm star supramolecular copolymers?SEW-1-5?by coupling a Keggin-type polyoxometalate of K₄[?-SiW₁₂O₄₀]with 1,2,3-triazolium bridged block copolymers of PS_n-b⁺-PEG_mI^-?n=17,26,39,57,81;m=45?through ionic exchange reactions,respectively.In the first part,we mainly investigate the self-assembly behaviors of the above-mentioned five miktoarm supramolecular star copolymers in nonselective?THF and chloroform?and slightly selective?toluene?solvents.The resulting TEM images,combined with both water contact angles and ¹H NMR spectral data reveal that SEW-2-5 self-assemble in the nonselective solvents of both THF and chloroform to create micellelike aggregates ranging from cylindrical to spherical with a PS corona and a PEG core,while in the case of SEW-1,reverse bilayers are captured with a PEG corona and a PS core.Among these aggregates,the Keggin clusters of[?-SiW₁₂O₄₀]^4-localize at the core-corona interfaces between PS and PEG.Similar morphological transformations are also observed when SEW-1-5 are dispersed in toluene that is a slightly poorer solvent for PEG than both THF and chloroform.These unexpected aggregates should originate from the topological constraints of the chemically different arms of PS and PEG in the miktoarm stars,where the weak incompatibility between the PS and PEG arms is intensified appropriately.The presence of the reverse bilayered structures of SEW-1 is due to the magnified steric hindrance of the PEG₄₅arm accompanied by the decrease in the degree of polymerization of the PS₁₇ arm.However,to the best of our knowledge,these are first examples clearly indicating that miktoarm star copolymers self-assemble in common good solvents or slightly selective solvents to generate micellelike aggregates.This scenario is in sharp contrast to the intuitively considered behavior of unimolecular miktoarm stars in nonselective solvents.In the second part,we mainly investigate the self-assembly behaviors of SEW-2-5 in three selective solvents?THF/methanol,toluene/methanol and chloroform/methanol?,while methanol is a good solvent for PEG block and a poor solvent for PS block.TEM imaging results show that SEW-2 self-assembles into fiberlike aggregates in the THF/methanol mixture solvents.SEW-3-5 tend to form spherical aggregates with longer PS chains than SEW-2 when dispersed in this mixture with high methanol content to minimize interfacial energy.These stars self-assemble to form form free-standing sheets as a result of inter-cylinder van der Waal attractions between the PS_n coronas in the toluene/methanol mixture solvent with a methanol composition of 50 vol%.The morphology of the assembly eventually evolves into vesicles with the increase of methanol content and the length of PS chains.In the chloroform/methanol mixture solvent with high methanol composition,the greater the degree of polymerization of PS chains,the easier it is for these stars to form vesicles with PS_n cores and PEG₄₅ coronas.This picture is in sharp contrast to the conventional self-assembly behaviors of amphiphiles in selective solvents.The specific mechanism of the aggregate formation concerns a delicate competition of tunable intra-and inter-star steric repulsions and van der Waals attractions in the toluene/methanol mixture solvents with worsening quality.