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Latent, thermally or UV-triggerable Catalysts for Step- and Chain Growth Polymerization (PUR Synthesis, ROMP,…)

A series of latent, thermally- or UV-triggerable pre-catalysts based on CO2­­-protected N-heterocyclic carbenes (NHCs) and NHC-Sn+II, -Zn+II, -Mg+II, -Al+III as well as NHC-Ru and NHC-Mo alkylidene complexes for step- and chain-growth polymerization have been synthesized.
The CO2-, Sn+II-, Zn+II-, Mg+II-, Al+III-protected NHCs exhibit very good thermal latency in a wide range of applications including the ring opening polymerization of lactams and lactones, polyurethane synthesis with tailored of isocyanurate content, thereby improving the thermal and mechanical properties, polymerization of methyl methacrylate (MMA) and curing of highly crosslinked anhydride-hardened epoxy resins. The active catalyst species can only be formed at elevated temperatures, an advantage that allows premixing of batch and diluted systems, important for a broad spectrum of applications, e.g. for reaction injection molding (RIM) and resin transfer molding (RTM). CO2-protected NHCs do not only show excellent activity, but also allow for metal free synthesis and therefore circumvent toxic heavy metals such as Hg+II or organotin compounds, which have sometimes toxicities comparable to cyanide.
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Figure 1:
General structure of latent CO2- and Lewis acid-protected NHCs (top) and achieved polymerization reactions catalyzed by NHCs (bottom).

Complementary, the ring-opening metatheses polymerization (ROMP) triggered by cationic NHC-Ru+II complexes in the presence of various norborn-2-ene substituted monomers can only be activated by UV irradiation. These initiators are of particular interest in technical applications of ROMP, they allow for premixing of a monomer/pre-catalyst mixture and its storage over long periods of time at elevated temperatures. Most importantly, these initiators allow for shaping and profiling of such mixtures prior to polymerization (“curing”).
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Figure 2:
Synthesized UV-labile pre-catalysts based on NHC-Ru complexes (top) and polymerized monomers (bottom).

The characteristic coalescence temperature, Tc, at which the complexes exhibit the highest activity, makes them interesting targets for applications that require latent catalyst systems. Poly(dicyclopentadiene) (poly-DCPD) at present is synthesized by a two-component catalyst system, that is mixed immediately prior to use. The employment of a one-component system, which can be premixed and stored within the substrate, i.e. DCPD would significantly facilitate the process. Molybdenum imido alkylidene NHC complexes with triazol-4-ylidenes and mesoionic carbenes turned out to be most suitable latent pre-catalysts for the polymerization of DCPD. Storage of the catalysts with the highly reactive substrate is possible and activation of the catalysts can be implemented by simple heating of the mixture.
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Figure 3: Triazol-4-ylidene complexes for latent poly-DCPD synthesis. Top: DSC curves of the polymerization of DCPD with initiators I1, I2, I3 and the reference catalyst G1.

References:
[1] S. Naumann, S. Epple, C. Bonten, M. R. Buchmeiser, ACS Macro Lett., 2, 2013, 609-612.
[2] S. Naumann, F. G. Schmidt, W. Frey, M. R. Buchmeiser, Polym. Chem., 4, 2013, 4172-4181.
[3] B. Bantu, G. M. Pawar, K. Wurst, U. Decker, A. M. Schmidt, M. R. Buchmeiser, Eur. J. Inorg. Chem., 2009, 1970-1976.
[4] B. Bantu, G. M. Pawar, U. Decker, K. Wurst, A. M. Schmidt, M. R. Buchmeiser, Chem. Eur. J., 2009, 3103-3109.
[5] S. Naumann, F. G. Schmidt, R. Schowner, W. Frey, M. R. Buchmeiser, Polym. Chem., 2013, 2731-2740.
[6] S. Naumann, M. Speiser, R. Schowner, E. Giebel, M. R. Buchmeiser, Macromolecules, 47, 2015, 4548-4556.
[7] M. R. Buchmeiser, J. A. Kammerer, S. Naumann, J. Unold, R. Ghomeshi, S. K. Selvarayan, P. Weichand, R. Gadow, Macromol. Mater. Eng., 9, 2015, 937-943.
[8] D. Wang, K. Wurst, W. Knolle, U. Decker, L. Prager, M. R. Buchmeiser, Angew. Chem., 120, 2008, 3311-3314; Angew. Chem. Int. Ed., 47, 2008, 3267-3270.
[9] D. Wang, K. Wurst, M. R. Buchmeiser, Chem. Eur. J., 16, 2010, 12928-12934.
[10] J. Beerhues, S. Sen, R. Schowner, G. M. Nagy, D. Wang, M. R. Buchmeiser, J. Polym. Sci., in press, 2017.