Abstract: The invention relates to the N-substituted acyl-lactam compound of the formula: ##STR1## where R' is a bivalent cyclic or non-cyclic alkyl, aralkyl, alkaryl or aryl radicalR" a bivalent radical having the following general formula ##STR2## wherein R'" is H, alkyl, cycloalkyl, aryl, alkaryl or aralkyl,R.sup.iv is H, alkyl, cycloalkyl, aryl, alkaryl or aralkyl whereR'" and R.sup.iv may jointly form a substituted or non-substituted cycloalkyl residuen is 0 or 1(-L) represents a non-opened lactam ring andx a number>1.It has been found that this group of compounds is very suitable as activator in the preparation of nylon polymers, more specifically in the so-called RIM (Reaction Injection Moulding) or RRIM (reinforced RIM) systems, in which it is highly essential for the polymerization to take effect within a very short time.