Synthetic Approaches to Semi-Sequenced Copolymers

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Nature encodes properties into large molecules, like DNA and proteins, by combining a set of building block molecules, called monomers, into specific sequences. Spider silk protein, for example, derives its exceptionally tough nature from a specific sequence of amino acid monomers. This model for controlling properties in large molecules like polymers has been difficult to emulate in non-biological materials, however, because of the challenge in preparing perfectly sequenced molecules of this type. With the support from the NSF Macromolecular, Supramolecular and Nanochemistry Program, Professor Tara Meyer at the University of Pittsburgh conducts research to enhance our understanding of how precisely the sequence needs to be controlled to attain the desired polymer properties. This work is expected to have a significant impact scientifically, technologically, and educationally. The new synthesis method and the study of monomer sequence and their resultant properties represent a considerable scientific impact. The demonstration that sequence need not be perfect to be useful could translate into the increased use of sequence in industrially-produced polymers, a potential technological benefit. Finally, the involvement of graduate and undergraduate students in the research, with a particular emphasis on ensuring broad participation of trainees from all backgrounds, should contribute to a well-prepared scientific workforce.The Meyer group has successfully prepared perfectly sequenced, biodegradable, non-toxic polymers called poly(lactic-co-glycolic acids)s (PLGAs) and demonstrated the dependence of their properties on sequence. Unfortunately, the preparation of these perfectly sequenced PLGAs, and almost all other sequenced polymers, is too complicated and expensive to scale up for most applications. The research team thinks that one way to address this barrier is to answer the following two questions: "Is it essential to have a perfect sequence?" and "Is it possible that a polymer with a sequence pattern that is 75% correct would be almost as high performing as a polymer whose sequence is 100% correct?" The goal of this project is to address these questions by developing a new, more scalable method for preparing polymers with "just enough" sequence and then testing the properties of these semi-sequenced copolymers against perfect analogues. A new parallel/successive synthetic strategy is introduced and developed with the goal of creating semi-sequenced step-growth copolymers with an intermediate level of control over monomer order. Inspired by the classic synthesis of polyurethanes, the parallel/successive strategy involves the creation, in parallel, of compositionally controlled pre-oligomers with similar backbones but differing sequences. The successive reaction of these pre-oligomers produces semi-sequenced copolymers with a homogeneous composition in terms of the types of monomers present, but a segmented structure. Careful experimental design and the implementation of the method using a flow system can be implemented to yield materials with varying degrees of fine (monomer-by-monomer) and course (segment-level) sequence control. The comparison of the properties of the semi-sequenced copolymers with those of the perfectly sequence materials improves the understanding of the various mechanisms through which sequence effects manifest. Ultimately, this research may be translated into new technologies in advanced plastics and materials manufacturing.