(3ee) Studying and Engineering the Biological-Material Interface with Molecular Resolution

Synthetic polymers play a foundational role in many of the materials used today. Their programmed destruction provides opportunities ranging from the clearance of biomaterials in the body to the breakdown of commodity plastics in the environment. Key to developing better strategies to break down these materials are new approaches to engineer polymer structure at the molecular level.

Here, I discuss how we engineer backbone-degradable versions of existing polymers through new classes of cleavable additives. Our work so far has focused on polynorbornenes, prepared by the ring-opening metathesis polymerization (ROMP) of norbornene-based monomers. These materials can be found in many areas, ranging from probes for modulating living systems to thermosets produced on the industrial scale. Due to the mechanism of olefin metathesis, these polymers have backbones connected solely by carbon-carbon bonds. This prevents these materials from breaking down under mild conditions, resulting in unwanted persistence within living systems or the environment.

To address this problem, we have invented a new class of co-monomers that introduce cleavable bonds throughout the polymer backbone. Through tailored co-monomer design, we can tune the degradation rate of these materials under biologically relevant conditions by many orders of magnitude. This degradation was shown to be enabling in the context of new polymeric drug delivery systems, where we could promote the rate of clearance of polymeric drug carriers in vivo through engineered degradability.

We next applied our co-monomers to the industrially relevant thermoset poly-dicyclopentadiene (pDCPD) prepared by the ROMP of dicyclopentadiene. Thermosets play a key role in the modern plastics industry, comprising ~18% of polymeric materials and a current worldwide production of 65 billion tons per year. Their high density of chemical crosslinks result in excellent mechanical properties for high-performance applications, but also prevent them from being readily reprocessed once formed. As a result, no general solution for thermoset recycling exists. We made the surprising discovery that only a small quantity of our co-monomer additive was sufficient to allow pDCPD to be broken down into soluble fragments. At these low co-monomer loadings, we maintain the thermal and mechanical properties of the parent thermoset. In the context of sustainability, soluble fragments derived from thermoset breakdown can then be recycled back into the parent thermoset. Moreover, we can easily recover embedded carbon fiber from composite materials through mild and selective thermoset degradation.

Our results are uniquely enabled by where we introduce our cleavable bonds, highlighting the importance of cleavable bond location as a design principle in degradable crosslinked polymer networks. We developed a theoretical model to show that all classes of polymer networks can be degraded at minimal co-monomer loading as long as suitable co-monomers structures can be found. Beyond high-performance thermosets, crosslinked polymer networks are used in 3D-printed structures prepared through stereolithographic printing and crosslinked hydrogel coatings used in medical devices. New strategies to better break these materials down, either gradually over time or through externally applied stimuli, will open the door to new opportunities in these adjacent fields.

Moreover, there have been tremendous advances in tools to characterize the interactions of small-molecules within living systems. We will leverage analogous engineering principles to develop high-throughput and high-resolution methods to characterize the interaction of synthetic materials within living systems. These opportunities will open the door to next-generation materials for both sustainability and medicine.

Research Interests

Polymeric materials, biomaterials, sustainability.

Teaching interests

Polymer chemistry, biomaterials, molecular engineering.