Simple Polymerization Reactions for Overcoming Long-Standing Challenges: From Fully Recyclable, Melt-Reprocessable Rubber Tires Containing Dynamic Covalent Bonds to Development of Broad-Temperature-Range Vibration and Acoustic Damping Materials (Invited Talk)

Innovative application of simple polymerization reactions can overcome long-standing problems as well as achieve solutions to critical needs associated with polymers. In the first example, I will demonstrate how innovative application of nitroxide-mediated polymerization, which was first developed in the 1990’s, can lead to recyclable, melt-reprocessable polymer networks or cross-linked polymers with 100% achievement (within error) of cross-link density after multiple reprocessing steps. This work relies on the reversible capping-uncapping reaction of the nitroxide radical that occurs at elevated temperature (e.g., ~120 degrees C in one incarnation) to yield dynamic covalent bonds. The approach described in this presentation can be applied to any polymer or monomer with at least one carbon-carbon double bond amenable to free radical polymerization, e.g., polymers such as polybutadiene and cis-polyisoprene used in the tire industry. I will show how a simple, one-step reaction can result in fully recyclable cross-linked polymers of the type used in the tire industry with complete property recovery after multiple recycling steps. I will also show how this simple chemistry can be used to design highly uniform polymer networks from monomer and describe how such materials may be useful in advanced engineering applications.

As a second example, I shall consider the challenge of designing polymeric materials via polymerization reactions that can yield excellent vibration and acoustic damping properties over a broad range of temperatures, from a minimum 60 degree C temperature (T) breadth to a maximum exceeding a 100 degree C T breadth. Several successful approaches will be described, all of which rely the production of polymeric materials that undergo nanophase separation with some phase mixing, thereby yielding materials with many compositions, and thus many glass transition temperatures, naturally present at equilibrium. Several cases will be detailed, including gradient copolymers yielding sinusoidal composition profiles at the nanoscale. Such gradient copolymers can be easily achieved by controlled radical polymerization methods, including nitroxide-mediated polymerization. Another case that will described will be polyhydroxyurethane (PHU) synthesized by aminolysis of cyclic carbonates. With PHU, the hydroxyl unit in the hard segment can undergo hydrogen bonding with oxygen atoms in the soft segment, thereby moderating the nanophase separation. Under optimal conditions, with enough but not too much such intersegment hydrogen bonding, excellent damping properties (as determined by tan delta > 0.3) can be tuned over exceedingly broad temperature ranges. The important role of molecular structure impacting nanoscale structure which in turn impact macroscopic properties is evident with PHUs: such properties cannot be achieved with polyurethanes lacking the hydroxyl unit present in PHUs.