We described the obtention of A(2)B star block copolymers through the use of a new heterotrifunctional initiator. That way, well-defined (PCL)(2)-arm-PtBuMA and (PCL)(2)-arm-PS star block copolymers have been synthesized from a heterotrifunctional initiator bearing two hydroxyl groups able to initiate ROP of CL (with AlEt3 or Sn(Oct)(2) as coinitiator) and a bromide function able to initiate ATRP of tBuMA or styrene. Firstly, we have proceeded using a sequential process (two-steps), leading to an intermediate macroinitiator. Secondly, attempt to polymerize these two monomers in a simultaneous process (one-step), that is directly from the mixture of monomers, initiator, coinitiators, and solvent, has been realized and has shown that some interferences between the two polymerizations occurred, leading to an inhibition of ATRP when Sn(Oct)(2) was used and an unexpected increase in control when AlEt3 was used as catalyst for the ROP (obtention of well-defined (PCL)(2)-arm-PtBuMA with pdi of 1.18). (c) 2006 Wiley Periodicals, Inc.