Synthesis of Linear Poly(oxazolidin-2-one)s by Cooperative Catalysis Based on N-Heterocyclic Carbenes and Simple Lewis Acids

Abstract

The synthesis of isocyanurate-free, linear poly­(oxazolidin-2-one)­s starting from diepoxides and aromatic as well as aliphatic diisocyanates is reported. N-Heterocyclic carbenes (NHCs), liberated in situ from thermally labile CO2 adducts, in combination with Lewis acids of the simplest kind (metal halides such as LiCl and MgCl2) were employed in a cooperative manner to prepare linear polymers with molecular weights (M n) ranging from 6 to 50 kg/mol. Crucially, it is demonstrated that action of either NHC (Lewis base) or metal halide (Lewis acid) alone entails the formation of significant amounts of trimerized isocyanates (isocyanurate) and concomitant gelling of the thus cross-linked material, highlighting the advantages of a cooperative, dual catalytic approach. Reactions were conducted at 200 °C with low NHC loadings (0.5 mol %) to deliver isolated yields of 60–90% within 3–8 h polymerization time. Investigations regarding regioselectivity revealed that exclusively 5-substituted oxazolidin-2-one was formed. Notably, these transformations can be catalyzed by a combination of 1,3-dimethyl­imidazolium-2-carboxylate, a readily accessible and robust NHC-precursor tolerant toward atmospheric conditions, and well-available LiCl. A mechanism is proposed whereby the high molecular weights and the selectivity for oxazolidinone formation over side reactions are attributed to the high nucleophilicity of the NHC, cooperative monomer activation by the metal halide, and specifically chosen reaction conditions.

Alternating Copolymerization of Epoxides with Isothiocyanates

“…As common characteristics, comonomers such as CO 2 , carbonyl sulfide, and anhydrides that were copolymerized with epoxides are known to be unable to homopolymerize. More recently, alternating copolymerization of epoxides was successfully carried out with isocyanates which are other heteroallenes that are very challenging to polymerize . Adriaenssens and co-workers indeed first reported the alternating copolymerization of epoxides and isocyanates using a dimagnesium catalyst.…”

Dual catalysis with an N‐heterocyclic carbene and a Lewis acid: Thermally latent precatalyst for the polymerization of ε‐caprolactam

“…We already reported on the synthesis of polyurethanes, on the AROP of ε‐CLA and on poly(oxazolidinone) 31,32,42,43 using a “dual catalysis” approach. Additionally, dual catalysis in the ROP of propylene oxide, lactones, and carbamates, especially related to PA6 formation, deserves to be mentioned 21,44–46 .…”

A Spirocyclic Parabanic Acid Masked N‐Heterocyclic Carbene as Thermally Latent Pre‐Catalyst for Polyamide 6 Synthesis and Epoxide Curing

“…[ 34,35 ] In accordance with the literature, the trimerization to isocyanurates and the formation of oxazolidinones seem most conceiving. [ 18,33,42 ] On a final remark it is worth mentioning that all mixtures, stored at ambient temperature and under nitrogen atmosphere, were usable as latent resins for seven days, indicating sufficient pot life.…”