A novel cleavable multiblock copolymer was synthesized by ring-opening metathesis polymerization (ROMP) of cyclooctene (COE) and a flexible 27-membered macrocyclic olefin (MCO), which is acted as the spacer to collect the polymer structure block by block. MCO 2 was prepared via ring-closing metathesis of the long chain alkyldiene, and then 2 was well-conducted ROMP with COE to provide the multiblock copolymer [Poly(COE)-2](m) consisting of homo-Poly(COE) blocks and ring-opened 2 segments with different molecular weights (M-n = 30.0 - 249.6 x 10(3)) and polydispersity index (PDI) within 1.45-1.67 as variation of the feed ratio of COE to 2. The multiblock copolymer chain containing weak ester linkage can be cleaved under alkali condition to afford the carboxyl-telechelic Poly(COE) blocks with much lower molecular weights (M-n,M-h = 3.6-35.7 x 10(3)) and slight higher PDIs (1.65-1.88). The average block number on multiblock copolymer chain was obtained from the ratio of M-n to M-n,M-h and was reached up to the value of 7-16. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 380-388, 2010