Nano-Carrier Synthesis via Z-RAFT Star Polymerisation

Abstract

In this project the establishment of nano-scaled and monomolecular objects working as carrier system was investigated. Such nano-carriers find application in drug delivery, catalysis or as templates for hybrid nanomaterials. As globular platform for these nano-carriers, amphiphilic diblock star-shaped copolymers were used. Star polymers containing a central core surrounded by radiating arms provide various requirements requested to yield the proposed nano-carriers: they are unimolecular, equal sized and monodispersed, chemically stable in different modification steps and easily synthesised by means of radical Reversible Addition-Fragmentation Transfer (RAFT) polymerisation. In contrary to self-assembling systems, e.g. micelles, stability in various solvents and dilution, at different pH-values and temperatures as well as in the presence of charges is given. The arms of the polymer stars can be tuned for the different fields of application as RAFT is versatile regarding monomers and solvents and enables the defined synthesis of polymers in size and composition. By cross-linking the arms, the carrier's shell should be formed. Removal of the RAFT-template enables the storage of the desired compounds within the shell. Within this project RAFT-agents with six RAFT-groups represented the template for the star polymer. The number of RAFT-units corresponded to the number of arms the star consisted of. The Z-group approach was applied which enabled the arms to grow from the centre and thus prevent star-star coupling. To form the carrier's shell, [2+2]-cycloaddition by UV-irradiation was chosen. Therefore the outer block of the diblock copolymer arms had to consist of units of a basic monomer and a monomer carrying a unit which undergoes a [2+2]-cycloaddition. At first a suitable polymerisation system had to be found: monomers based on N-alkyl-3,4-dimethylmaleimid (DMI), cinnamic acid (CA) and courmarins were tested in firstly in RAFT copolymerisations with n-butylacrylate (BA), N-isopropylacrylamide (NIPAam), methylmethacrylate (MMA) and methylacrylate (MA). It was found that BA and NIPAam along with the DMI-based monomer 1.1 yield copolymers with well-defined molecular weight and narrow polydispersities. The existence of six-armed polymer stars could be proven by AFM-measurements. The composition of the polymers and the copolymerisation parameters for both systems were determined by NMR-spectroscopy indicating that a gradient polymer was formed. Two kinds of amphiphillic star polymers were investigated: stars with a hydrophobic corona made of p(BA-co-1.1) and a hydrophilic pNIPAam-core and the reversed case with a hydrophilic shell. Different UV-lamp systems were tested for cross-linking reactions induced by UV-irradiation. The success of the reaction was monitored by SEC-measurements. It was found that the RAFT centres of the star polymers were destroyed by energy entries above 150 W resulting in linear polymers, i.e. loose arms. Lower irradiation energies yielded star polymers with a closed shell. As a last step the removal of the RAFT-template by aminolysis was investigated and verified by SEC-measurements.