PMLABe Diol Synthesized by Ring-Opening Polymerization of Racemic Benzyl [beta]-Malolactonate Initiated by Rare-Earth Trisborohydride Complexes: An Experimental and DFT Study

Polymer diols are a class of polymeric building blocks of high interest for the synthesis of complex macromolecular edifices. Rare-earth borohydride complexes are known as efficient initiators for the ring-opening polymerization (ROP) of cyclic esters, directly affording [alpha],[omega]-dihydroxy-telechelic polyesters. Here, were report the direct synthesis of poly(benzyl [beta]-malolactonate) (PMLABe) diols, from the ROP of racemic (benzyl [beta]-malolactonate) (rac-MLABe), a valuable and renewable monomer, initiated by the homoleptic [Ln(BH4)3(thf)3] (Ln=La, Nd, and Sm) complexes. These initiators enabled the controlled ROP of this [beta]-lactone, affording well-defined syndiotactic-enriched (Pr[asymptotically =]0.83) PMLABes (Mn up to 21300gmol-1, ÐM[asymptotically =]1.5) as evidenced by size exclusion chromatography, 1H and 13CNMR spectroscopy, and MALDI-ToF mass spectrometry analyses. The first and second insertions of rac-MLABe, as assessed by DFT calculations, revealed more favorable stationary front-side than migratory back-side insertions, the thermodynamically and kinetically competitive ROP on two distinct arms with that on a one arm-only, and the thermodynamically slightly favored formation of syndiotactic-enriched PMLABes.