Kinetic and Sequence Regulation in Copolymerization Through Emergent Reactant Ordering

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Hamblin, Ryan, Chemistry - Graduate School of Arts and Sciences, University of Virginia
DuBay, Kateri, AS-Chemistry (CHEM), University of Virginia

Sequence control in synthetic copolymers remains a tantalizing objective in polymer science, as the phase behavior, self-organization, and bulk material properties of copolymers are intrinsically dependent on their primary comonomer sequences. Achieving precise control over monomer sequence in synthetic copolymerizations is challenging, as sequence determination is influenced not only by the reaction conditions and the properties of the reactants, but also by the statistical nature of the copolymerization process itself. Further, characterizing the primary sequence of a synthetic copolymer is a significant challenge, making the experimental study of sequence development intractable with current methods. Despite these difficulties, greater understanding of sequence development throughout the polymerization process will aid the design of simple, generalizable methods to control sequence and tune supramolecular assembly. To this end, this dissertation utilizes a reactive, Langevin dynamics model of copolymerization to directly observe sequence development in silico throughout the reaction. This allows for direct comparison to standard statistical theories of copolymerization processes, as well as direct control over system parameters which may influence the reaction. We particularly target conditions in which reaction driven phase change behaviors occur, which lead to reactant organization and emergent heterogeneity not accounted for in traditional theories. We find that differences in non-bonded attraction strengths between comonomers on the order of thermal fluctuations drive a reactant assembly process, leading to a shift in reaction kinetics, molecular weight distribution, and primary sequence that is not captured by Mayo-Lewis and Flory-Schulz theories. We further explore how differences in solvent selectivity may give rise to such self-organization, leading to sequence biasing and the formation of polymer structures with a wide range of morphologies and composition distributions. Additionally, we examine the influence of chain stiffness in concert with these self-assembly behaviors, exploring a transition to nematic ordering that occurs for oligomers of sufficient persistence length. Such liquid crystalline ordering introduces a characteristic length scale into the system, which both significantly enriches the formation of specific chain and sequence repeat lengths and shifts in response to the relative diffusive and reactive timescales within the system. This work provides new fundamental insights into the impacts of collective and emergent reactant behaviors on the kinetics and sequence development of copolymers which are not captured by current theories. It develops provides greater understanding of reaction conditions which produce particular sequence behaviors, allowing for the informed design of a sequence-controlled copolymerization in a simple one-pot synthetic method.

PHD (Doctor of Philosophy)
copolymerization, sequence controlled polymers, molecular dynamics
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