A process for preparing a thermoplastic polymer involves reacting at least components (a) to (b), in the presence of a catalyst composition (c). Component (a) is an isocyanate composition containing at least one uretdione diisocyanate (a-i), and component (b) is an epoxide composition containing at least one diepoxide (b-i). The catalyst composition (c) contains at least one ionic liquid (c-i), preferably selected from 1-ethyl-3-methyl imidazolium bromide, 1-benzyl-3-methyl imidazolium chloride, 1-butyl-1-methylpiperidinium chloride, 1-ethyl-2,3-dimethylimidazolium bromide, 1-(2-hydroxyethyl)-3-methyl imidazolium chloride, butyl-1-methylpiperidinium acetate, or mixtures of two or more thereof. A thermoplastic polymer obtained or obtainable from the process is useful for the preparation of a fibre or a molded article or as a modifier for another thermoplastic material.