It is well known that comb copolymers with densely grafted side chains in a good solvent can adopt a worm-like cylindrical brush conformation, in which the side chains are stretched in the direction normal to the backbone owing to the excluded volume interaction. Polymerizations of macromonomers provide regular multi-branched polymers with dense branching. Since both degree of polymerization and the length of branches are varied, poly(macromonomer)s, often so-called “polymer brushes,” are interesting models for the study of the branched polymers [1–9]. We have reported the synthesis and dilutesolution properties of poly(diblock macromonomer)s: polystyrene-block-polyisoprene (PS-block-PI) [10] and PS-block-poly(2-vinylpyridine) (P2VP) diblock poly (macromonomer)s [11]. These types of poly(diblock macromonomer)s have the advantage of controlling the stiffness of the main chain parts by crosslinking inner PI or P2VP blocks. Free-radical copolymerization of binary macromonomers will form copolymers with not only combshaped but also hetero-armed structures. In this preparation process, the relevant copolymerization parameters are very important in determining the branch distribution in the resulting comb-shaped copolymers. Free-radical copolymerizations of binary vinylbenzylterminated PS and PI macromonomers were carried out in benzene, varying the molecular weight of the macromonomers [12]. The reactivity ratios of these systems were close to an azeotropic copolymerization, in the case of the same degree of polymerization (DP) for binary macromonomers. Subsequently, free-radical copolymerizations of vinylbenzyl-terminated PS (PSVM; M1) with maleate-terminated poly(ethylene oxide) (PEO-MA; M2) were carried out in benzene, in order to achieve alternative reactivity [13]. The reactivity ratios (r1 = 0.765 and r2 = 0.064) obtained for such binary macromonomer systems were different from the corresponding model system of styrene and maleic acid ethyl ester (r1 = 0.13 and r2 = 0.035) due to phase separation. More recently, free-radical copolymerizations of PS-VM (M1) with methacryloyl-terminated PEO (PEO-MC; M2) were carried out under various copolymerization conditions [14]. As a result, the reactivity ratios (r1 = 0.25 and r2 = 0.02) obtained for the binary copolymerization of PS-VM with PEO-MC/tin tetrachloride (SnCl4) complex showed strong alternation (Scheme 1). It was suggested the alternation results from homopolymerization of 1 : 1 complexes formed Scheme 1