Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, [Sm(BH4)(3)(thf)(3)]. This initiator shows a high activity to give high-molar-mass PTMCs with molar-mass distributions ranging from 1.2 to 1.4, and with a regular structure void of ether linkages. ne polymers were characterized by H-1 and C-13 NMR spectroscopy, H-1-H-1 COSY, H-1-C-13 HMQC NMR spectroscopy, size-exclusion chromatography (SEC), viscosimetry, and MALDI-TOF MS analyses. A coordination-insertion mechanism was established based on detailed NMR characterizations, especially of the polymer chain end-functions. The monomer initially coordinates the samarium to give [Sm(BH4)(3-)(tMc)(3)], 1. The monomer then opens up through cleavage of the cyclic ester oxygen-acyl bond and inserts into the Sm-HBH3 bond resulting in an alkoxide complex, [Sm{O(CH2)(3)OC(O)HBH3}(3)], 2, or [Sm{O(CH2)(3)OC(O)H)(3)], 2', which then propagates the polymerization of TMC to give the active polymer [Sm({O(CH2)(3)OC(O)(n)-O(CH2)(3)OC(O)HBH3)(3)], 3 or [Sm(O(CH2)(3)OC(O){O(CH2)(3)OC(O)}(n)O- (CH2)(3)OC(O)H)(3)], 3'. Finally, acidic hydrolysis of 3 or 3' gives HO-(CH2)(3)OC(O)[O(CH2)(3)OC(O)](n)O-(CH2)(3)OC(O)H, 4. This novel alpha-hydroxy w-formatetelechelic PTMC represents the first example of a formate-terminated polycarbonate. TMC and epsilon-caprolactone (CL) were copolymerized to afford both random PTMC-co-PCL and block PTMC-b-PCL copolymers that were characterized by H-1 NMR spectroscopy, SEC, and differential scanning calorimetry (DSC). The structure of the block copolymers depends on the order of addition of monomers: if CL is introduced first, dihydroxytelechelic HO-PTMC-b-PCL-OH polymers are formed, whereas introduction of TMC first or simultaneous addition of comonomers leads to hydroxyformatetelechelic HC(O)O-PTMC-b-PCL-OH analoguesRead less <