Synthesis of reponsive homo- and block copolymers - application to the generation of inorganic-organic nanohybrids
Pierre-Eric Millard (08/2004-10/2010)
Support: Axel H. E. Müller
Responsive homopolymers and multi-responsive block copolymers were prepared via reversible addition-fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Self-assembly in solution depending on environmental stimuli was investigated and exploited to create responsive micelles. New cross-linking strategies were thoroughly performed in aqueous solution to allow a controlled preservation and a high shape-persistence of the colloid particles, even when exposed to non-selective environmental conditions.
The synthesis of poly(N-isopropylacrylamide) (PNIPAAm) was investigated by ATRP for subsequent polymer-protein nanohybrid generation. This temperature-responsive polymer was polymerized directly in pure water at a low temperature (4 ºC) by using a functional ATRP initiator which allows post-polymerization conjugation. Without the addition of Cu(II), the kinetics were extremely fast, typically less than one minute for a full conversion. By adjusting the ratio of Cu(I)/(Cu(II) and selecting a very active ligand, all polymerizations proceeded in a controlled fashion to near quantitative conversion without evidence of termination.
N-isopropylacrylamide and acrylic acid (AA) were also homopolymerized by RAFT in aqueous media using a novel strategy. Instead of using a diazo-initiator, which generally decomposed at high temperatures, gamma-irradiation was used to initiate polymerization at ambient temperature. This type of radiation has many advantages. A very tiny and constant amount of radicals can be generated, which is perfect for the RAFT process. Moreover, the rate of initiation only has a low level of dependence on temperature and can be used in a wide range of temperatures. Finally, compared to UV-initiation, gamma-irradiation can penetrate the reaction solution deeper and without evidence of irreversible decomposition of the dithioester end group. Therefore, RAFT polymerizations of NIPAAm and AA were achieved with a very good level of control, even at high monomer conversions.
This new process was then extended to many other water-soluble monomers for generating homopolymers and block copolymers. Among these, acrylamide, N,N-dimethylacrylamide, 2-hydroxyethyl acrylate and poly(ethylene glycol) methacrylate gave the best results. This technique proved to be very efficient at generating very long and narrowly distributed polymers (up to a degree of polymerization of 10,000) and at designing block copolymers.
High molecular weight PNIPAAm-b-PAA copolymers, synthesized by RAFT polymerization under gamma-radiation, were used to generate multi-responsive cross-linked micelles. These block copolymers were self-assembled in water at pH 7 by increasing the temperature over the lower critical solution temperature. The PNIPAAm became hydrophobic and formed the micellar core and the hydrophilic PAA block generated the corona which prevented full aggregation of the system. Then, by amidification at elevated temperatures of the carboxylic moieties via a trifunctional primary amine, the structure was found to remain even after cooling down the system. The shell-cross-linked micelles formed were utilized to generate inorganic-organic nanohybrids by the in situ reduction of gold or silver salts to generate nanoparticles inside the nanocarrier.
Another strategy of cross-linking was also investigated by using amino-functional silsesquioxane nanoparticles. In water around neutral pH values and room temperature, these particles interacted with the carboxylic groups of a high molecular weight PNIPAAm-b-PAA by hydrogen bonding and ionic interactions to generate an insoluble complex. Due to the presence of the hydrophilic PNIPAAm block, defined spherical micelles were obtained. The inorganic-organic particles were successfully cross-linked by subsequent amidification to preserve the structure, even at a high pH. Different temperature properties of the hybrids were observed depending on the pH value, due to the residual charge in the micellar core. At a neutral pH, shrinking of the corona was observed, while at a high pH (pH 13) a fully reversible aggregation of the system occurred.