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SYNTHESIS AND SELF-ASSEMBLY OF POLYPHOSPHAZENE (PP) BLOCK COPOLYMERS. DESIGN OF NEW INORGANIC NANOSTRUCTURES DERIVED FROM HIGH CRISTALLINE OR/AND CHIRAL HIGH TUNABLE PP BLOCK

Final Report Summary - AP-GAC (SYNTHESIS AND SELF-ASSEMBLY OF POLYPHOSPHAZENE (PP) BLOCK COPOLYMERS. DESIGN OF NEW INORGANIC NANOSTRUCTURES DERIVED FROM HIGH CRISTALLINE OR/AND CHIRAL HIGH TUNABLE PP BLOCK)

Summary description of the project objective

The main subject of the Project is the self-assembly of synthetic block copolymers (BCPs) into different types of nanostructures. In nature, most of the biological systems are organized by the self-assembly of both low- and high-molecular weight compounds directed by non-covalent phobic or philic interactions such as Van der Wals, hydrogen bonding, electrostatic, and chirality effects. Inspired by nature, the self-assembly of synthetic BCPs tries to mimic these interactions to generate different morphologies. The BCPs of the project are based on polyphosphazenes, [N=PR2]n (PP). This is because, beside the inherent properties conferred by the (P=N) chain (i.e. flexibility at low temperatures, thermal, radiative and oxidative stability, flame retardancy, electrical and optical features), those polymers offer a good opportunity for designing block copolymers with tuned physical and chemical properties by simply changing the R substituents. Among the routes to synthesize polyphosphazenes, only the living chain growth polycondensation of Cl3P=N-SiMe3 promoted by PCl5, allows the good control of the molecular weight and the narrow molecular weight distributions crucial in self assembly. Besides, this polymerization also provides a simple route to block copolymers via sequential monomer addition. So far, the self-assembly of these hybrid inorganic materials remains practically unexplored. Therefore, in this proposal a plan is outlined aiming to phosphazene block copolymers and to investigate their self-assembly in thin films and in selective solvents in order to understand the factors that dictate the formation of nanostructures. This is an important step towards the possibility to design materials with specific predetermined properties.
In a first approach, the use of highly crystalline poly(dialkyl)phosphazene blocks as core forming and different organic and inorganic soluble blocks for the corona were considered to generate new supramolecular materials. Further development was intended by facilitating the access to chiral polyphosphazene (PP*) for the synthesis of a new class of PP* block copolymers. It was expected that the self-assembly of these latter, both in solution and solid state, could generate new chiral-inorganic nanostructures driven by the helical structure of these systems.

Description of the work performed since the beginning of the project.

During the first period (two years) the advance of the project leads to the following results (see mid-term report):

- The successful preparation of polyphosphazene block copolymers (PP-b-PP) bearing a variety of PP blocks with different degree of crystallinity, high glass transition temperatures, chirality, and selective solubility in organic solvents (from polar solvents like alcohols to non-polar solvents like n-hexanes).
- The successful preparation of end-functionalized polystyrene with terminal phosphine groups (PS-PPh2, a telechelic polystyrene).
- The development of a new route to organic-inorganic hybrid block copolymers bearing a polyphosphazene (PP) and a polystyrene (PS) block, using the telechelic polystyrene PS-PPh2 as a macro-initiator of the PP block.
- The observation of a variety of nanoestructures such as spheres, vesicles and toroidal-shaped (donuts), generated by the self-assembly of the new block copolymers.
- A very interesting transitions from nanoporous films to nanospheres on solid state self-assembly of the chiral PP blocks.

During the second period (one year), the project also advanced under schedule with the results:

- The successful preparation of hybrid PP-b-PS block copolymers bearing chiral (N=P(R-O2C20H12), and crystalline (N=P(OCH2CF3)2) PP blocks.
- The completion of the solution and solid state self-assembly studies with the (PP)1-b-(PP)2 block copolymers where (PP)1 = [N=P(R-O2C20H12)], [(N=P(OCH2CF3)2)], and [N=P(O2C12H8)].
- The study of the self-assembly of PP-b-PS (PP = [(N=P(OCH2CF3)2)]) which led to a variety of new nanostructures including multicompartmental micelles, and cylinder networks.
- The successful synthesis of PP-b-PP having different chemical functionalities (O-, N-, and S-donor groups) randomly distributed along one of the blocks and the stabilization of Au nanoparticles selectively associated to one of the blocks.
- The selective decoration of the self-assembled nanostructured PP-b-PP and PP-b-PS with different cationic species (-H+, -SiMe3+, and Ag+).

- Description of the main results achieved.

1. We have demonstrated that the preparation of authentic two blocks polyphosphazene block copolymers (PP-b-PP), necessarily has to be carried out in the presence of an initiator blocked in one of the extremes to avoid the formation of tri-block copolymers by bidirectional chain growth. We have successfully prepared block copolyphosphazene with a variety of PP blocks including the first chiral block copolyphoshazenes bearing chemical functionalities random distributed in one block.
2. We have prepared telechelic polystyrenes bearing diphenylphosphino groups and we used it as a macroinitiators to synthesize hybrid block copolymers (poyphosphazene-b-polystyrene), combining living anionic living polymerization of the styrene and cationic living polymerization of N-silylphospharinimines.
3. We have demonstrated the formation, by self-assembly of PP-b-PP, of materials with: chiral twisted nanostructures; vesicles; nanospheres;, and mesoporous films decorated with Au nanoparticles.
4. We have demonstrated the formation, simply by changing the polymer concentration for the self-assembly of PP-b-PS, of: nanospheres; multicompartimental micelles; donut-shaped micelles; and cylinder networks,

Conclusions

Scientifically, the project has achieved the successful preparation of a variety of inorganic block copolymers based on the presence of a high tunable polyphosphazene block (PP), including chiral, crystalline, and rigid PP blocks. The self-assembly of these fascinating materials led to a variety of nanostructures such as mesoporous films, vesicles, cylinder networks or donut shaped-micelles. However, the impact of the project is also reflected in other very important results as shown in the next section.