Trivalent rare earth metal borohydride complexes Ln(BH4)3(THF)3 (Ln = La, Nd, Sm) initiate the ring-opening polymerization of ε-caprolactone (ε-CL) to give, in quantitative yields and in a few minutes, α,ω-dihydroxytelechelic poly(ε-caprolactone) displaying a relatively narrow molar mass distribution (≤1.3). The polymer features depend on both the metal and the solvent. A good agreement between M̄ n(theo) and M̄ n(exp) is observed for low [monomer]0/[initiator]0 values whereas a deviation (M̄ n(exp) < M̄ n(theo)) is obtained at higher ratios; this behavior has been rationalized by the occurrence of some transfer reactions. The gel formed during the polymerization arises from the involvement of the −(OBH2) end groups of the active polymer chains in several inter- and intramolecular van der Waals interactions, as revealed by in-depth infrared investigations of the various intermediates implicated in the whole polymerization mechanism. Finally, the polymers synthesized display only traces of residual nontoxic rare earth metals, thus making them suitable for biomedical applications.

The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, epsilon-CL opening (through cleavage of the cyclic ester oxygen-acyl bond) and insertion into the Sm--HBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2[O(CH2)6O(BH2)]] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)5C(O)[O(CH2)5C(O)]nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.

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.

The polymerization of ε‐caprolactone initiated by trivalent rare earth (Ln) derivatives, namely the isopropoxide ‘Ln(OiPr)3’ opposed to the borohydride Ln(BH4)3(THF)3 (Ln = La, Nd, Sm) and (Cp*)2Sm(BH4)(THF) (Cp* = (η‐C5Me5)), has been evaluated. The comparison, based on the structure of the initiator and on the polymerization features (molar mass, molar mass distribution, side reactions) reveals some differences especially in the number of active polymer chains per metal and in the occurrence of some transfer reactions with Ln(BH4)3(THF)3. Polymerizations performed with borohydride derivatives lead to a gel as a result of van der Waals interactions, while the reaction medium remains fluid with the alkoxide species. A typical coordination‐insertion mechanism with oxygen‐acyl bond cleavage of the monomer prevails with both types of initiators. However, while the alkoxide complexes lead to α,ω‐hydroxyalkoxypoly(ε‐caprolactone), the borohydride compounds allow the synthesis of α,ω‐dihydroxypoly(ε‐caprolactone). Both polymerization processes are well controlled under specific conditions. Copyright © 2006 Society of Chemical Industry

A series of pentamethylcyclopentadienyl bis(phosphinimino)methanide complexes of yttrium and the lanthanides, [{CH(PPh 2 NSiMe 3 ) 2 }Ln(η 5 -C 5 Me 5 )Cl] (Ln ) Y (1a), Sm (1b), Yb (1c)), were prepared by two different synthetic approaches. The compounds can be obtained either from [{CH(PPh 2 NSiMe 3 ) 2 }-LnCl 2 ] 2 (Ln ) Y, Sm, Yb) and K(C 5 Me 5 ) or in a one-pot reaction when K{CH(PPh 2 NSiMe 3 ) 2 } is reacted with anhydrous rare earth trichlorides in the presence of K(C 5 Me 5 ). When 1a-c were combined in situ with 1 equiv of 2-propanol, an active [Ln]-OiPr initiator was formed that enabled the pseudo-living ring-opening polymerization of -caprolactone to polymers with controlled molecular features (end groups, M h n ) and very narrow molar mass distributions.