AbstractThe kinetics of propagation in the polymerization of propylene oxide proceeding by the activated monomer (AM) mechanism is studied. Propagation in this system involves addition of a protonated monomer molecule to the terminal hydroxyl group of the growing oligomer. A kinetic analysis of the reaction allowed the determination of the equilibrium constantK, governing the distribution of protons between monomer and HO‐ groups, which in turn enables to calculate the propagation rate constantkp. This composite value has been further separated into the rate constants of four reactions, involving two different modes of ring opening in reactions with two different types of hydroxyl groups, viz. primary and secondary ones. The corresponding values are (all rate constants in mol−1· L · s−1and for CH2Cl2at 25°C):K= 5,3;k1,1= 0,170,k1,2= 0,190,k2,1= 0,105,k2