For the synthesis of aromatic polyesters with defined molecular weights and narrow molecular weight distributions (MWDs), we investigated the chain-growth polycondensation of active amide derivatives of 4-hydroxybenzoic acid, la and 1b, having an octyl or 4,7-dioxaoctyl side chain, respectively. To suppress the transesterification of the polymer backbone with the monomer, the polymerization of I was carried out in tetrahydrofuran (THF) at -30 degrees C in the presence of initiator 2 and Et3SiH/CsF/18-crown-6, which generated a hydride ion as a base in situ. The number-average molecular weight (Mu) of poly1a was controlled, and narrow MWDs were maintained, until the [1a](0)/[2](0) feed ratio was 14.3 (M-n <= 3500), whereas that of poly1b was controlled until the feed ratio was 30 (M-n <= 7250). The difference stemmed from the higher solubility of poly1b in THF. This chain-growth polycondensation was applied to the synthesis of a diblock copolyester of la and 1b of a defined architecture. (c) 2005 Wiley Periodicals, Inc.