In Situ Polymerization via Environmentally-Borne Initiation Stimuli

The utilization of an environmentally-borne stimulus, atmospheric oxygen, has been convincingly demonstrated as a useful means for initiating in situ polymerization. As molecular oxygen is both ubiquitous, forming approximately 21% of our atmosphere, and capable of participating in reduction-oxidation reactions, it particularly well-suited for affording radicals capable of initiating radical-mediated polymerization. In order to utilize oxygen as a polymerization stimulus, three suitable initiating system were developed, including those based on alkylboranes, oxidoreductase enzymes, and thiones; these systems are capable of solidifying thiol–ene monomer formulations within seconds, minutes, or hours, respectively, after oxygen exposure. The interest in in situ oxygen-mediated polymerization is largely driven by two emerging applications. The first of these, self-healing materials suitable for utilization in space exploration, requires a reactive liquid, sandwiched between a solid support structure, that is able to flow and polymerize immediately after being punctured (by a micrometeoroid or space debris), thus sealing the puncture-induced hole. To demonstrate the suitability of oxygen mediated polymerization for self-healing materials, high-velocity ballistics testing, monitored by high-speed video and thermal imaging, was performed. The results from these experiments show that oxygen-mediated polymerization is a viable method for sealing a micrometeoroid induced puncture. The second application of interest is for the development of next-generation surgical adhesives and sealants that would be suitable for use as emergency hemostats and for other medical procedures. As a demonstration that oxygen-mediated polymerization is a useful mechanism for producing these materials, formulations based on chitosan, a polysaccharide known for its biocompatibility and bioadhesion, were produced that rapidly afford solid hydrogels after oxygen exposure.