A new synthetic strategy, the combination of living polymerization of ylides and ring-opening polymerization (ROP), was successfully used to obtain well-defined polymethylene-b-poly(epsilon-caprolactone) (PM-b-PCL) diblock copolymers. Two hydroxyl-terminated polymethylenes (PM-OH, M-n = 1800 g mol(-1) (PDI = 1.18) and M-n = 6400 g mol(-1) (PDI = 1.14)) were prepared using living polymerization of dimethylsulfoxonium methylides. Then, such polymers were successfully transformed to PM-b-PCL diblock copolymers by using stannous octoate as a catalyst for ROP of c-caprolactone. The GPC traces and H-1 NMR of PM-b-PCL diblock copolymers indicated the successful extension of PCL segment (M-n of PM-b-PCL = 5200-10,300 g mol(-1); PDI = 1.06-1.13). The thermal properties of the double crystalline diblock copolymers were investigated by differential scanning calorimetry (DSC). The results indicated that the incorporation of crystalline segments of PCL chain effectively influence the crystalline process of PM segments. The low-density polyethylene (LDPE)/PCL and LDPE/polycarbonate (PC) blends were prepared using PM-b-PCL as compatibilizer, respectively. The scanning electron microscopy (SEM) observation on the cryofractured surface of such blend polymers indicates that the PM-b-PCL diblock copolymers are effective compatibilizers for LDPE/PCL and LDPE/PC blends. Porous films were fabricated via the breath-figure method using different concentration of PM-b-PCL diblock copolymers in CH2Cl2 under a static humid condition. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 511-517, 2011