A chitin derivative with an alternating N-plithaloyl group was prepared via chitinase-catalyzed polymerization. A chitobiose oxazoline derivative with a N-phthaloyl group at the C2' position was designed as a transition-state-analog substrate monomer for chitinase belonging to the glycoside hydrolase family 18. The monomer was successfully polymerized by the enzyme catalysis, providing the corresponding chitin derivative. This implies that the recognition at the C2 position of the substrate by the family 18 chitinase in the -2 and +1 subsites is quite loose.