AbstractThe synthesis of macrocyclic polystyrene‐block‐poly(2‐vinylpyridine) and macrocyclic polystyrene‐block‐poly(dimethylsiloxane) was carried out by initiation of 2‐vinylpyridine (2VP) and hexamethyl‐cyclotrisiloxane (D3) by difunctional living polystyryllithium followed by coupling with 1,4‐bis(bromethyl)benzene (1,4‐DBX) and dichloro‐dimethylsilane (Cl2SiMe2), respectively. A small portion of the living ABA precursors were protonated to serve as isobaric linear precursors. The linear and macrocyclic block copolymers were characterized by size‐exclusion chromatography (SEC). The ratios of apparent cyclic/linear SEC molecular‐weight maxima versus degree of polymerization (DP) show increases with decreasing DP varying from 0.70 ± 0.03 at high DP ≤ 200 to 0.78 ± 0.044 at low DP (≥60) whereas that of the linear ABA block copolymers decreased. Increases in glass transition temperature (Tg) were also observed for the cyclic PS‐b‐PDMS copolymers with respect to the isobaric linear precursors. The macrocycles were characterized by1H and13C NMR and in the case of macrocyclic PS‐b‐PDMS by29Si NMR as well. Broadening in the NMR absorptions of the macrocyclic block copolymers is general and is similar to that observed for the homopolymers. Differemtial scanning calorimetry (DSC) analysis of the PS‐b‐P2VP macrocycles shows increases in Tg at lower molecular weight as was