We pioneer the synthesis of well-defined high molar mass segmented copolymers, employing a unique combination of step-growth and reversible addition fragmentation chain transfer (RAFT) polymerization. The Step-growth precursor polymer is obtained via the ambient temperature UV-light-induced Diels-Alder reaction of 6'-(propane-1,3-diylbis(oxy))bis(2-methylbenzaldehyde) (AA monomer) and di(isopropionic ethyl ester fumarate) trithiocarbonate (BB monomer). Unconventional off-stoichiometric conditions (r = [AA](0):[1313](0) = 1.5-1.75) are employed to ensure a sufficiently high incorporation of BB in the step-growth product (1200 <= M-n/g mol(-1) <= 3950). The optimum r value is based on a detailed product distribution analysis, comparing experimental and bivariate kinetic Monte Carlo generated data, using a scheme of over 200 reactions. The analysis highlights the unexpected occurrence of AA homopolymerization and the ligation of the resulting AA segments at higher reaction times. The precursor step-growth polymer is successfully transformed into a segmented copolymer via insertion of styrene by RAFT polymerization at 60 degrees C (11200 <= mol(-1) <= 53400), as confirmed both experimentally and through simulations.