CAREER: Supramolecular Polycondensation of Polymer Brushes via DNA Hybridization

Northeastern University, Boston MA

Investigators

Abstract

NON-TECHNICAL SUMMARY Living organisms are very proficient at utilizing nanoscopic objects, such as proteins, as building blocks to create materials that they need. Indeed, protein-protein interactions are the foundation of all cells because they are highly directional, specific, and reversible; these are characteristics that synthetic chemists have yet to fully emulate. The goal of this project is to emulate some of the precision-assembly ability that natural proteins exhibit by extracting the requirements for such assembly, and recreating them in new forms accessible to modern polymer and organic chemistry. Towards this goal, the PI's team will design bottlebrush-like synthetic polymers (where long polymer molecules have short appendages across them like the bristles in a bottlebrush) and connect them with short strands of DNA. Because DNA recognition of a complementary strand is well defined, one can in principle design the interconnectivity of the brush polymers with one another accurately. With this approach, it is possible to create a new class of "superpolymers", i.e. polymers whose building blocks are nanoscale-sized polymeric objects. The PI will explore the kinds of superpolymers that can be formed, and identify rules governing their formation. Because of the specificity of DNA recognition, polymer building blocks with various properties, such as size, electrical conductivity, or biological interactions, can be linked together in a sequence-specific and spatially defined manner, to create functional materials previously difficult or impossible to access. This synthetic technique also has the potential to be mediated by innate nucleic acids in cells, and therefore has important implications in medicine. These research activities will intertwine with educational programs to provide laboratory training to graduate/undergraduate students (including undergraduates from institutions lacking research capabilities) and to promote learning and awareness in science-related careers among high school students and STEM teachers. TECHNICAL SUMMARY Nanoscopic objects such as macromolecules and colloidal nanoparticles can form ordered structures when there exist attractive forces. Because sphere-like particles uniformly interact across their surfaces, directional assembly to form 1D or 2D structures represents a significant challenge. In this project, co-funded by the Polymers and Biomaterials Programs in the Division of Materials Research, the PI will utilize DNA recognition as a functional group-equivalent to program the bottom-up assembly of made-to-order polymeric building blocks that can interact with each other only in defined directions. These limited-valency building blocks will be used to study topologically regulated supramolecular polymerizations. The proposed self-assembly strategy involves the synthesis of triblock brush polymers as "macromonomers", which will be conjugated with nucleic acids. The hybridization between the nucleic acid strands allows the monomers to self-assemble head-to-tail, connecting them either linearly or with branching, to form higher order assemblies. By systematically studying a library of brush polymers with different side-chain length, DNA duplex, and reactive block length, mechanistic insights about linear assembly of the brush building blocks will be obtained. In addition, superpolymer sequence control will be studied using multiple monomer systems (e.g. AA+BB), paving the ground for creating multi-component functional materials. Concepts from polycondensation reactions that define small-molecule polymerization will be tested against superpolymer synthesis, utilizing mono- and multi-valency macromonomers, which are expected to provide control over architecture, average degree of polymerization, and chain-end functionality. The capabilities gained from this project will open a wide spectrum of designer materials whose sequential and topological characteristics are well-defined on the nanoscopic scale, and will bring about new knowledge in polymer science and create opportunities to study yet unknown properties.