We currently live in the Polymer Age, where polymers are universally implemented into every imaginable application from bullet-proof vests to cooking pans and everywhere in between. The ubiquity of polymers arises from the incredible breadth of material properties, ease of processing and low-cost. This wide range of attributes originates from the underlying chemical structure. One the most fundamental structural property of a polymer is its molecular weight distribution (MWD). A polymer’s molecular weight distribution impacts simultaneously the processability, mechanical strength, and morphological phase behavior of the polymer. The presence of low molecular polymers provides ease of processing, while high molecular weight imparts high mechanical strength. This structure function relationship is of critical importance for the production of commodity objects as well as for new emerging areas of applications for polymers (like 3D printing). So through tuning of the MWD, an ideal balance of material properties and processability can be achieved. Unfortunately, to date the synthesis of polymers with specific MWD distributions has remained mostly impossible.
In our recent report, we present a methodology that enables the independent control over molecular weight, MWD breadth, and MWD shape using an automated flow reactor. Our approach is built on the first attempts made at MWD control following a simple blending approach (mixing polymer batches of different molecular weights). We use a flow reactor to produce a quasi-infinite number of polymer batches with very specific molecular weights and narrow MWD, which accumulate in a collection to produce the desired MWD. This approach goes far beyond the concept of dispersity (width of a polymer distribution), which is normally used to describe polymers MWD. It enables the synthesis of any MWD that we could draw. To deliver on this design-to-synthesis concept, we implemented an automated tubular flow reactor with controlled polymerizations. Living polymerization, reactor engineering principles and automation were key to the success of our approach.
Ultimately, this approach provides complete design freedom and reaches the theoretical limit of design resolution for constructing any arbitrary MWD with any polymer accessible via living polymerization. We hope that this methodology will aid in fundamental material property studies, as well as, aid in the tuning of materials for specific applications. Additionally, this work has enabled the synthesis of non-cylindrical bottlebrushes [1, 2].
Full article is available here.
(Walsh, D. J., Schinski, D. A., Schneider, R. A. & Guironnet, D. General Route to Design Polymer Molecular Weight Distributions Through Flow Chemistry. Nat. Commun. (2020). doi:10.1038/s41467-020-16874-6)
 Walsh, D. J. & Guironnet, D. Macromolecules with programmable shape, size, and chemistry. Proc. Natl. Acad. Sci. 116, 1538–1542 (2019).
 Walsh, D. J., Dutta, S., Sing, C. E. & Guironnet, D. Engineering of Molecular Geometry in Bottlebrush Polymers. Macromolecules 52, 4847–4857 (2019).