Elucidation of the intra-chain radical mechanism in poly(norbornene imide) single-chain nanoparticle formation Justin P. Cole, Jacob J. Lessard, Christopher K. Lyon, Bryan T. Tuten, and Erik B. Berda. Department of Chemistry, University of New Hampshire. An Unexpected Mechanism for SCNP Formation Summary and Conclusions Introduction Characterization of NP1 Attempting to design an efficient and scalable method for producing functionalized single-chain nanoparticles, we investigated intra-chain radical polymerization of pendant methacryloyl decorated poly(norbornene imides). Polymer P1 was synthesized via ROMP. To initiate intra-chain polymerization of the pendant methacryloyl groups, we subjected P1 to a range of radical initiator (AIBN) concentrations under ultra-dilute conditions as is typical in the SCNP literature. References Acknowledgements The author would like to graciously thank the Army Research Office for support through award W911NF , and NIST for support through award 70NANB15H060 as well as Dr. Erik Berda, Dr. John Tsavalas, and Dr. Gary Weisman for sharing their time and expertise. As a control experiment we synthesized polymer P2, which contains no pendant methacryloyl units, and subjected it to the same cross-linking conditions as P1. We were quite surprised to observe once again shifts in retention time consistent with the formation of single-chain nanoparticles. SEC-MALS traces for the parent chain P2 and corresponding single-chain particles NP2 are shown in Fig. 2. Here again there is no evidence of large multi-chain aggregates. Our initial thought was that polymerization through the backbone olefins could be driving SCNP formation here, however examining the 1 H NMR spectra again revealed no discernable loss of olefin resonances or other changes between P2 and NP2. Figure 2: SEC overlay of P2 and NP2. Figure 3: Effect of backbone hydrogenation on pNBI radical cross-linking. We found that poly(norbornene imides) decorated with polymerizable methacryloyl side chains can be converted to single-chain nanoparticles by polymerizing through the methacryloyl side chains, however this process is not as straight-forward or effective as we originally hoped. We also discovered that small concentrations of adventitious oxygen in this system, when exposed to a radical source, can react with sites of unsaturation in the NBI backbone to promote intra-chain cross-linking, presumably by oxygen bridging. Our work to unveil this mechanism more fully and exploit this chemistry to create functional nanostructures is ongoing. Scheme 1: Synthetic route to nanoparticle NP1 1.Cole, J. P.; Lessard, J. J.; Lyon, C. K.; Tuten, B. T.; Berda, E. B., Intra-chain radical chemistry as a route to poly(norbornene imide) single-chain nanoparticles: structural considerations and the role of adventitious oxygen. Polymer Chemistry Figure 4: Intra-chain cross-linking experiments on P3. Subjecting a hydrogenated polymer (P2h) to the previously described radical cross-linking conditions resulted in no change in the SEC trace (Fig. 3). This result strongly supports the involvement of backbone olefins in the intra-chain radical cross-linking chemistry we are seeing in this system. We again applied this hydrogenation strategy to synthesize a similar material, P3, with a fully saturated backbone. Doing so permits the isolation of any cross- linking behavior that may have occurred due to the methacryloyl groups in our original example. Fig. 4 shows these results: a broadening of the SEC-MALS trace, indicating a competing intra-chain collapse and interchain coupling. 1 H NMR shows a decrease in the methacryloyl resonances in the photochemical reaction but not in the thermally initiated case. Oxygen’s Role in Collapse To understand the role of adventitious oxygen in this system we examined the polymers described above after rigorous degassing via multiple freeze–pump–thaw (FPT) cycles. Fig. 5A compares the methacryloyl-functionalized polymer P1-FPT before and after exposure to radical initiation with rigorous oxygen exclusion. Here, a mix of intra-chain and inter-chain cross-linking are observed. Fig. 5B highlights similar experiments on the hexyl- functionalized homopolymer P2-FPT. This result confirms that polymerization or coupling through the methacrylate pendants is occurring and that without oxygen present the backbone olefins do not participate in this process. Figure 5: Exposure of P1 (A) and P2 (B) to radical cross-linking conditions after oxygen exclusion via freeze–pump–thaw cycles. Figure 1:(A) SEC overlay of P1 and corresponding NP1, (B) NMR overlay of N1 and NP1 highlighting the olefinic region.