Clusto-Supramolecular Block Copolymer:Synthesis And Self-Assembly Behavior

Block copolymers, as an important class of macromolecules constructed by linking two or more linear homopolymer segments, have attracted intense attention because of their ability to self-assemble into a variety of nanostructured morphologies in bulk and in solution through micro-phase separation, thus leading to diverse applications in biomedicine, energy storage and nanophotonics. Living polymerization is a popular method to synthesize well-defined block copolymers, in which the polymer segments are joined by covalent bonds. To further enrich the compositions and properties of block copolymers, various noncovalent motifs based on hydrogen bonding arrays, metal-ligand coordination complexes, host-guest inclusion complexes, and ion-association complexes etc., have also been explored as supramolecular junctions, which results in the formation of supramolecular block copolymers. This new polymer system provides a promising platform to combine the phase separation behaviour of conventional block copolymers and the dynamic assembly property of supramolecular polymers. However, as a new polymer system, challenges still remain to expand its scope, which should resort to the development of new supramolecular junctions.Polyoxometalates(POMs) are a class of single molecule clusters which are comprised of early-transitionmetals, bridged by oxygenatoms. Because of its rich varietychemical composition and topologies, POMs have a variety of physical and chemical properties which make it have a broad application prospect in the light, electricity, magnetic and catalytic materials and other related fields.And POMs are considered as a very promising block units for supramolecular polymers. What’s more, POMs are macroanionic clusterswith the negatively charged on their surface, whichmake them could process supramolecular assembly with catonic polymeric segments through electrostatic interaction to form inorganic clusters hybrid supramolecular polymers. On the other hand, because of poor workability,it limits the application and development of POMs. The combination of the polymer and POMs also provide a broad space for development and application field for improving processability and explore the function of the POMs.In this paper, we develop an effective suitable way to fabricate a kind of Clusto-supramolecular block copolymers(abbreviated as CBCs)in which POMs serve as ionic cluster junctions in the block copolymer through combination of electrostatic modify and “grafting from” controlled polymerization, and conduct a comprehensive study for this supramolecular polymers self-assembly behaviors in solution and analyse the dependence of assemble morphology with molecular weight, concentration, temperature and solvent polar. Working content is as follows:Firstly, for building CBCs, we selected reversible addition–fragmentation chain transfer(RAFT) polymerization to graft polymer segments on the surface of the POMs.Detailed process is that we used a divalent hexatungstate cluster [W6O19]2-(abbreviated as W6) as a bridge to modify two cationic RAFT agents with trithioester group through electrostatic interaction to get supramolecular chain transfer agent with inorganic cluster,and then through thermal initiate the polymerization of styrene to get a series of CBCs with different degree of polymerization 26 and 67, named as CBC-1 and CBC-2. Through NMR,MALDI-TOF, GPC, IR, EA and so on, we confirm the molecular structure of CBCs and discover that their PDI is less than 1.1.Secondly, we study the self-assembly behaviors of CBCs in selective solvents system and then analyse and discuss its related assembly mechanism and molecularpacking model.Combination with the characterization methods of DLS, TEM and AFM, we verify that the self-assembly morphology of CBC-1 and CBC-2 respectivelyis vesicle and micelle. And then, through the analysis of their packing parameters, we present that the packing model of CBC-1 is a symmetric shape, but CBC-2 is a wedge-like shape. Note that the conformational difference of different molecular weight of grafted chains led to the difference of their packing modes. In addition, DLS results indicate that the CBCs display similar self-assembled behaviours in a series of solvent systems such as chloroform,chloroform/methanol and toluene/butanol, which demonstrate the universality of thisassembly phenomenon.Thirdly, we further study concentration, temperature and solvent polar effect on self-assembly behaviors of CBCs in solution. When solution concentration is between 0.2and 5.0 mg/mL, the vesicle of CBC-1 don.t have any change, but the self-assembly of CBC-2 exhibit the feature of concentration dependence. When its concentration is decreased to 0.5 mg mL-1, the dilution induces partial CBC-2 micelles to transform to vesicles. We think this transformation is because of PS chains on W6 favour a more flexible conformation at the condition of low concentration, and thereby one of the PS chains turns over to the other side of W6,resulting in a symmetrically packed CBC-2 that further forms unilamellar vesicles. Interestingly. The vesicle of CBC-2 exhibit the feature of temperature dependence.With the temperature from 25 to 45 o C reciprocating, the Rhof CBC-2 vesiclesgradually decreasing from 150 to 70 nm, and this process is well reversible.We think that increasing temperature can give rise to a larger a PS because the PS chains become more relaxed in these situations. As a consequence, a looser packing state of CBC-2 is caused, in which the lateral electrostatic interaction between W6 and adjacent PS chains should be weakened, thus resulting in the decreased size of CBC-2 vesicles.In a word, we utilized the controlld\living free radical polymerization RAFT via“grafting from” strategy to get the well-defined CBCs with narrow PDI. We have comprehensivly analysed and discussed for self-assembly behaviors of these CBCs in the solution and related molecular packing mode and assembly mechanism. This work for the design of organic/inorganic hybrid supramolecular block copolymer provides a new way of thinking.