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 CO, 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 SO 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 degrees 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.