Controllable synthesis of bio-based polylactide (PLA) diols was realized by the ring-opening polymerization (ROP) of lactide (LA) in the presence of 1,4-butanediol (BDO) using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an organocatalyst in solvent-free conditions. The catalytic activity and conversion of LA could reach approximate to 1 kg g(-1) DBU and >97%, respectively, and the polymerization yielded polymers with narrow polydispersity index (PDI) (1.15-1.29). Interestingly, the number average molecular weight (M-n) of the obtained PLA diol was in excellent linear relation with the molar ratio of LA and BDO, and hence can be precisely controlled. The structure of the diol was clearly confirmed by H-1 and C-13 NMR, FTIR, and MALDI-TOF mass spectra, proving BDO as an initiation-transfer agent to participate in the polymerization. Kinetic study of the ROP demonstrates a pseudo-first-order kinetic model and a controlled living nature. Notably, it is found that the glass transition temperature (T-g) of the diol significantly depends on the M-n. Furthermore, various chain transfer agents and organocatalysts can also be used to successfully synthesize well-defined PLA diols. Especially, functional bio-based dihydric alcohols such as 2,5-furandimethanol (FDMO)-initiated ROP in this system could result in fully bio-based PLA diols with functionality. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 968-976