Abstract: Isocyanate compounds are highly effective activators for anionic polymerization of caprolactam （CL）, but often lead to crosslinking reactions due to their dual active sites. To prevent this, monofunctional p-toluene isocyanate （p-Ti） as an activator with sodium hydride （NaH） catalyst was used to study the anionic ring-opening polymerization behavior of CL. The p-Ti reacted completely with CL to form acyl lactam active centers, resulting in a reaction system devoid of free isocyanate groups, which effectively suppressed crosslinking reactions, and the molecular weight of the polymerization product cloud be controlled by the amount of p-Ti. In addition, at 150 ℃, the NaH concentration had no effect on the relative viscosity of the product. However, at 180 ℃, as the NaH mole fraction rose from 1% to 4%, the relative viscosity of the polymerization product increased from 2.5 to 3.0. This is due to the deprotonation of the active center by NaH at high temperatures, resulting in the formation of acyl lactam anions. This causes a decrease in their nucleophilicity, a reduction in active centers, an increase in the molecular chain length of the polymer product, and an increase in relative viscosity. Changing the feeding sequence to prioritize the consumption of NaH, or using N-acetylcaprolactam （N-AcL） activator that does not react with NaH can reduce side reaction, resulting in that the relative viscosity of the product does not change with increasing NaH concentration.