Design of advanced polymer architectures by tuneable thermoresponsive LCST-UCST copolymers

For advanced technical applications, the bio-inspired development of modern materials is of great importance and is inter alia encouraged by the discovery of liquid-liquid phase transitions, as main driving force for molecular structure formation of membrane-less organelles in living organisms. Motivated by this background, the present thesis is dedicated to the molecular design of advanced, modular polymer architectures based on versatile, thermoresponsive copolymer systems, bearing tuneable miscibility gaps in aqueous media in consequence of their lower or upper critical solution temperatures (LCST/UCST). These responses serve as external trigger to dynamically change “on-demand” the solvation properties of the macromolecular building blocks, which were exploited for several innovative applications in this thesis: (a) complex solution behaviours of dual-thermoresponsive LCST-UCST block copolymers, (b) characterization of thermally switchable membranes, (c) control of dynamic coordination structures in aqueous solution by thermoresponsive macromolecular coordination ligands and (d) non-viral copolymer vectors for gene delivery.
For this purpose, the LCST polymer poly(N-isopropyl acrylamide) (poly(NIPAAm)) and UCST analogue poly(acrylamide-co-acrylonitrile) (poly(AAm-co-AN)) are introduced as basic thermoresponsive systems, whose contrary solvation behaviours in aqueous polymer solutions are attributed by sharp and robust transitions, which can be monitored by turbidity measurements of their cloud points (Tc). The herein presented macromolecular structures are functionally designed by the choice of specific comonomers and were realized by the reversible addition-fragmentation chain transfer (RAFT) polymerization, which allows the fabrication of defined building blocks with variable end groups, controlled architectures and tuneable thermoresponsiveness.
The UCST system of poly(AAm-co-AN) was thoroughly studied to gain accurate knowledge about the prediction of the copolymer’s thermoresponsive properties in free and controlled radical polymerization giving access to the subsequent research fields. In contrast to the literature, in which polar DMSO with high boiling point is used as solvent, the copolymer was synthesized in a mixture of water and 1,4-dioxane in order to simplify the common time-consuming purification (dialysis). Therefore, the polymerization parameters in terms of conversion and monomer incorporation were determined for the new reaction mixture in dependence of the reaction time to guarantee a suitable copolymerization behaviour of the monomers acrylamide (AAm) and acrylonitrile (AN). For the copolymers, the Tc could be synthetically adjusted by the choice of the proper acrylonitrile fraction in the monomer feed. An additional terpolymerization with the monomer N-(4-benzoylphenyl) acrylamide (BPAAm) provided access to the fabrication of photo-crosslinkable copolymers, which could be either covalently attached to surfaces or transformed to hydrogels by intramolecular network formation via subsequent irradiation with UV light. These entire study results in an accurate prediction for the adjustment of desired cloud point temperatures within the copolymerization of AAm, AN and BPAAm, tremendously improving the applicability of this copolymer system.
The already extensively-studied LCST system of poly(NIPAAm) was employed as robust thermoresponsive counterpart, which was copolymerized with BPAAm and hydrophilic comonomers to yield linear photo-crosslinkable copolymers with appropriate transition temperatures, enabling the subsequent fabrication of responsive hydrogels.
These two main polymer systems were modified for the use in the following four application fields.
In the first research field, the contrary thermoresponsive systems above were combined in a dual-thermoresponsive LCST-UCST block copolymer by pursuing the sequential RAFT copolymerization pathway. This novel system of poly[(AAm-co-AN)-block-NIPAAm]-EMP showed a complex solvation behaviour in aqueous solution dependent to the adjustable hydrophobicity of its UCST block. While the thermoresponsiveness in previously reported systems is explained on basis of the simple behaviour of a homopolymer chain, here the analysis of the complex interdependent phase transitions also includes the intramolecular influence between both thermoresponsive blocks and their particular solvation state. The marked shift of the transition temperatures, when comparing the homoblocks with the corresponding block copolymers, is caused by the abrupt changes in the hydrophilicity during the coil-to-globule transitions for each block and therefore depends on the thermal history (heating or cooling cycle). Thus, the double-thermoresponsive copolymers were characterized with respect to their thermal behaviours, aggregate dimensions and calorimetric transitions via UV-Vis turbidity measurements, dynamic light scattering (DLS), and modulated differential scanning calorimetry (MDSC).
The second application demonstrated a successful combination of anodized aluminium oxide substrates (AAO) with thermoresponsive copolymers for the fabrication of thermally switchable nanopore membranes, to investigate reversibly alterable diffusion processes through nanoscopic channels by the external stimulus of temperature. While the inner pore surface of reported systems is usually functionalized with polymers via ATRP, here linear RAFT copolymers of characterized composition and thermoresponsiveness are deposited in adjustable quantities from solution, before they are photo-immobilized as hydrogels in the pores and forming highly defined membranes. The copolymer mass of the filling was quantified in thermogravimetric measurements with dynamic heating rate, while the thermoresponsive switching states in aqueous solution (in form of LCST transitions) could be evidenced inside the nanopores by MDSC for the first time.
In a third application, the reversible switchability of responsive polymer systems is married with the convenience of assembling highly defined molecular scaffolds by complexation of metal ions with specifically tailored ligands. Novel macromolecular coordination ligands (MCLs) with LCST (published as: https://doi.org/10.1039/D1PY00847A) or UCST character in aqueous media have been designed by conjugation of thermoresponsive polymer systems poly(NIPAAm) or poly(AAm-co-AN) with 1,2,4-triazole coordination sites. These triazole units were integrated via the RAFT method into two fundamentally different MCL architectures following two synthetic strategies: (I) The customized chain transfer agent 1-{[3-(4H-1,2,4-triazol-4-yl)propyl]amino}-2-methyl-1-oxopropan-2-yl dodecyl carbonotrithioate (DMP-APTRZ) was employed for hemi-telechelic MCLs with a single triazole end group. (II) A tailored comonomer N-[3-(4H-1,2,4-triazol-4-yl)propyl] methacrylamide (APTRZMAAm) provides access to multidentate MCLs with a controllable number of triazole side groups along the polymer backbone. The thermally controlled variation of the MCL volume demand was exploited for reversible aggregate formation upon Fe2+ complexation with respect to their chain state in aqueous solution driven by the LCST or UCST behaviour. Thermal response was studied via UV-Vis turbidity measurements, aggregate dimensions were determined via DLS, while the aggregate morphology was analysed via customized transmission electron microscopy (TEM).
Finally, the copolymer system of AAm/AN was utilized for the first time in the scope of gene delivery research as basis of polycationic, non-viral vectors with UCST character. The RAFT methodology is suitable for the preparation of variable vector designs with a controlled integration of pDNA binding sites via the copolymerization with the known comonomer N-(3-aminopropyl) acrylamide (APAAm) to introduce free amine side groups to the polymer backbone. In contrast to the literature, which focuses on LCST polymers as thermoresponsive base, here the contrary thermoresponsivness of UCST vectors allows the reverse switching of the solvation states from hydrophobic to hydrophilic chain behaviour in aqueous media upon external temperature stimuli, leading to dynamic interactions between vector and pDNA. Those synthesized vectors demonstrated marked abilities to complex pDNA and further transfect several cell strains (C2C12, A549 and HeLa) in strong dependence of their molecular composition and skeletal architecture: In the latter, the APAAm units were either distributed (I) randomly, (II) as concise end block, or (III) as hybrid-structure along the macromolecular chain.