Catalytic Behavior of Rare-Earth Borohydride Complexes in Polymerization of Polar Monomers

PMLABe Diol Synthesized by Ring‐Opening Polymerization of Racemic Benzyl β‐Malolactonate Initiated by Rare‐Earth Trisborohydride Complexes: An Experimental and DFT Study

“…From the characterizations of the isolated PHAs, we thus propose that α,ω‐dihydroxy‐telechelic PMLABes are being formed, through a now known typical pseudo‐anionic coordination–insertion mechanism, as established for the similar ROP of cyclic esters mediated by such rare‐earth borohydride derivatives 1a. 17a, 19a,b, 24 Remarkably, syndiotactic PMLABe diols were thus synthesized from the homoleptic rare‐earth trisborohydride complexes, a tacticity for which DFT investigations provided some rationalization. Furthermore, the present synthetic strategy directly affords PMLABe diols through the ROP of rac ‐MLABe, without requiring any chemical modification, a significant advantage over the previously reported route 24…”

“…Indeed, it is assumed that in ROP reactions carried out in THF, because of the high affinity of THF towards oxophilic rare‐earth metals, it competes with the polar lactone monomer for coordination onto the metal center, eventually slowing down the polymerization. Although this appears as a quite general trend, exceptions have been reported as for instance with the higher polymerization rate of ε‐caprolactone in THF than in toluene by using diaminobis(phenoxide)monoborohydride rare‐earth complexes 17a. 27 Thus, toluene was selected as a non‐polar and non‐coordinating reaction medium, whereas THF and CH 2 Cl 2 were chosen as polar, coordinating and non‐coordinating solvents, respectively.…”

“…As the mechanism under consideration is indeed a classical coordination–insertion one involving a borohydride complex (Figure 7),17a the main focus has been placed on the energetic of the reactions as well as on the selectivity. As observed for the ROP of BL mediated by complex 1 ,21a the free‐energy profile of the ROP is similar for all the considered cases.…”

“…The first stage of the process, as already mentioned in the literature,17a, 21a entails the coordination of MLABe‐R to complex 1 to produce the complex [La(BH 4 ) 3 (thf) 2 (MLABe‐R)] (A R ). The formation of A R is exergonic by 0.8 kcal mol −1 , which corresponds to the energy required for MLABe‐R to displace one THF molecule.…”

“…Numerous examples of the ROP of ε‐caprolactone,19 δ‐valerolactone,20 β‐butyrolactone (BL),21 lactides,20, 22 and trimethylene carbonate23 have been reported 17. The most significant advantage in using metallic borohydride derivatives to mediate the ROP of a cyclic ester relies on the inherent reactivity of the ${{\rm BH}{{- \hfill \atop 4\hfill}}}$ ligand itself, which eventually enables the formation of polyester diols, namely α,ω‐dihydroxy‐telechelic polyesters and polycarbonates 17a. 18, 20, 22, 24 We have previously established that the ROP of racemic β‐butyrolactone ( rac ‐BL) mediated by the homoleptic rare‐earth borohydride complexes [Ln(BH 4 ) 3 (thf) 3 ] (Ln=La ( 1 ), Nd ( 2 ), and Sm ( 3 ), Scheme ), successfully enables to isolate the corresponding PHB diols 21a.…”