Polyurethane (PU) is used in a wide range of applications due to its diverse chemical and physical properties. To meet the increasing demands on thermal and mechanical properties of PU materials, polyisocyanurates (PIRs) have been introduced in PU materials as crosslinkers and due to their high decomposition temperature. We prepared a liquid PIR prepolymer with high PIR content by co-trimerization of 4,4′-methylene diphenyl diisocyanate (4,4′-MDI) and mono-isocyanates. The mono-isocyanate was synthesized via reaction between a 4,4′-MDI and a 2-ethyl-1-hexanol. The PIR prepolymer obtained was further reacted with long chain polyols and chain extenders in both solvent and solvent-free conditions, leading to PIR elastomers that exhibited good thermal stability with high char formation, and improved mechanical properties with much higher Young's modulus. This work demonstrates that the liquid PIR prepolymer can potentially be used in various large-scale industrial applications.