(255ad) Synthesis and Single Molecule Studies of DNA-PNIPAM Copolymers

Polymer chain architecture encompasses a wide range of shapes and chain types, including linear polymers, branched polymers, ring polymers, and polymer networks. In addition to chain topology, the chemical composition of polymers also plays a key role in determining emergent properties. To this end, copolymers and block polymers with simple linear chain architectures can give rise to intricate structural ordering and assembly across multiple length scales. Polymer architectures play a critical role in supramolecular assembly of advanced materials with applications in encapsulation for drug delivery and catalysis, surfactants for emulsions, and materials for chemical separation. For example, PEG-PLA is an important drug delivery carrier that can encapsulate hydrophobic drugs such as camptothecin when mixed with drug solutions. Recently, the development of hybrid polymers that contain both synthetic and natural components have enabled new materials with higher compatibility, enhanced biodegradation, wide functionalities, and unprecented levels of sequence-control. A recent area of interest has focused on the self-assembly of hybrid polymers, especially DNA-based hybrid polymers, into various architectures such as micelles, vesicles, and tubes. Despite recent work, however, we still lack a complete understanding of this process.

In this work, we study the real-time transition of trigger-responsive hybrid polymers using single molecule techniques. In particular, we synthesize DNA-PNIPAM copolymers and study the supramolecular assembly of these materials using fluorescence microscopy. First, we use polymerase chain reaction (PCR) to produce 10 kb DNA with 1% substitution of chemically modified nucleotides containing DBCO, which is a strained alkyne for Cu-free click chemistry. In a separate reaction, we use reversible addition-fragmentation chain-transfer (RAFT) polymerization to synthesize fluorescent PNIPAM copolymerized with rhodamine monomers. In addition, the PNIPAM polymers contain a terminal azide group, which facilitates click reaction with the DBCO-modified DNA. In this way, we generate DNA-PNIPAM comb and bottle-brush polymers, and after purification, these materials show a sizable shift via agarose gel electrophoresis denoting the formation of the desired product. In this way, we effectively synthesize semi-sequence controlled DNA-PNIPAM polymers with different architectures, and we can directly study these materials using single molecule fluorescence microscopy, including the dynamic transition process for temperature responsive materials. In particular, upon increasing the temperature above the lower critical solution temperature (LCST) of PNIPAM around 32C, the material transitions from hydrophilic to hydrophobic character. In this way, we directly observe the spontaneous formation of higher order structures in solution by dynamically increasing the temperature above the LCST. As a control, we also used scanning electron microscopy (SEM) to characterize the self-assembled structures as a function of temperature, and we found micron-sized vesicle structures compared with the control group. Finally, we study the single chain dynamics of DNA-PNIPAM polymers using fluorescence microscopy, both in equilibrium conditions and under flow. Using this approach, we characterize single chain dynamics as a function of the temperature-sensitive dynamic transition of DNA-PNIPAM above and below the LCST of PNIPAM.