Consecutive chain extensions via the RAFT process have been carried out, and polymers were investigated via ESI-MS to elucidate for the first time the chain end distributions in high detail. In this manner, the number of chains carrying living chain ends is assessed as well as the number of polymers carrying both RAFT typical R and Z groups. Polymerizations were carried out to practically full conversion, and two subsequent chain extensions have been carried out for each obtained material under systematic variation of the AIBN concentration between 1 and 10 mol % with regards to RAFT agent. Generally, very high percentages of living chains were observed the lower the AIBN concentration in the polymerization. With 1 mol %, up to 96% of chains (triblock structure) could be confirmed to be of living nature. In contrast, with 10 mol % of AIBN, only 85% of chains are living after addition of the third polymer block. When looking at the alpha-omega pure materials (carrying exclusively the RAFT-derived R group instead of an AIBN-derived end group), much lower values are deduced, yielding abundances of 90% for the low and 70% of the higher AIBN concentration. Loss of R groups indicates loss of the polymer block structure and must thus be seen as equally important as the information on livingness of the polymer. For multiblock copolymer synthesis it is thus strongly recommended to assess both the Z but also the R group integrity of the polymer.