For polymer functionalization reactions, or other types of post-polymerization reactions involving high molecular weight polymers, the extent of reaction of the mers in a particular chain depends on the length of time that chain is in the reactor. Thus, except for plug flow residence time distributions (RTD), polymer functionalization in flow reactors gives rise to a composition distribution (CD) of functional groups with the breadth of the distribution proportional to the breadth of the RTD. This is also true in micromixed reactors since high molecular weight polymers behave as segregated elements even when the chains are mixed on the molecular level. In this paper equations;a;re derived for characterizing functional group CD breadth for 1 to 3 CFSTRs in series for reactions which are first order in mer concentration. When there are equal rate constants in each reactor the equations can be obtained readily, but for the more realistic case of unequal rate constants, a method involving integrals of delta functions was employed to obtain the distributions. The methods developed for characterizing CD are also applicable to obtaining distributions in other instances of reactions in segregated CFSTRs. The results showed that distribution breadth is a very strong function of mer conversion. Relatively narrow distributions are only obtained in the limits of very high or very low conversion, and the broadest distribution results at a conversion of about 60%. As expected, narrowing the RTD by using tanks in series narrows the CD. For tanks in series, the breadth of the CD is narrowest when the mer conversion is the same in each reactor in the train.