A new PDMS macroinitiator is proposed for the anionic ring-opening polymerization of lactams. This alpha,omega-dicarbamoyloxy caprolactam PDMS macroinitiator was readily obtained in quantitative yield, by an original synthesis scheme in two steps, which involved the scarcely reported reaction of isocyanates with silanol groups. It was then shown that this bifunctional macroinitiator enabled to synthesize triblock copolymers PA12-b-PDMS-b-PA12 by polymerization of lauryl lactam (LL) at high temperature (200 degrees C) in inert atmosphere under conditions compatible with reactive extrusion processes. Another related high molar weight alpha,omega-diacyllactam PDMS macroinitiator was also successfully used in the polymerization of LL under the same conditions, therefore overcoming the limitations formerly reported for this type of macroinitiators during the polymerization F-caprolactam (P-CL) at a much lower temperature (80 degrees C). Triblock copolymers with a wide range of PA12/molar weights (M-n: similar to 10,800-250,000 Da) were eventually obtained by using both types of macroinitiators. DMTA and DSC analyses showed that their thermal properties were strongly dependent upon their respective contents in soft and hard blocks. Such triblock copolymers already appear very promising for the highly effective in situ compatibilization of PA12/PDMS blends as shown by recent complementary results obtained in our laboratory. (c) 2006 Wiley Periodicals, Inc.