Unprecedented polymerization of trimethylene carbonate initiated by a samarium borohydride complex: mechanistic insights and copolymerization with epsilon-caprolactone

Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, Sm(BH(4))(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. The polymers were characterized by (1)H and (13)C NMR spectroscopy, (1)H-(1)H COSY, (1)H-(13)C 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(BH(4))(3)(tmc)(3)], 1. The monomer then opens up through cleavage of the cyclic ester oxygen--acyl bond and inserts into the Sm--HBH(3) bond resulting in an alkoxide complex, Sm{O(CH(2))(3)OC(O)HBH(3)}(3)], 2, or Sm{O(CH(2))(3)OC(O)H}(3)], 2', which then propagates the polymerization of TMC to give the active polymer Sm({O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)HBH(3))(3)], 3 or Sm(O(CH(2))(3)OC(O){O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)H)(3)], 3'. Finally, acidic hydrolysis of 3 or 3' gives HO(CH(2))(3)OC(O)O(CH(2))(3)OC(O)](n)O(CH(2))(3)OC(O)H, 4. This novel alpha-hydroxy,omega-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 (1)H 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 analogues.