Stereo-electronic contributions in yttrium-mediated stereoselective ring-opening polymerization of functional racemic β-lactones Ali Ahmad Dhaini 1 , Rama Shakaroun, Ali Alaeddine 2 , Jean-François Carpentier 1 , Sophie M. Guillaume 1 1 Université de Rennes 1, France, 2 EDST Lebanese University, Lebanon Poly(hydroxyalkanoate)s (PHAs) are a family of linear aliphatic biopolyesters that have in the recent years gained increasing interest in academia and industrial domains due to their eco-friendly character. PHAs, initially identified in the early twenties by M. Lemoigne as small granules of isotactic polyesters synthesized by bacteria, show a low thermal stability which limits their industrial use. 1,2 A convenient synthetic route to enhance the properties of PHAs is the ring-opening polymerization (ROP) of four-membered β-lactones using metal-catalyzed polymerizations. 3 The achiral yttrium complexes with diamino-bis(phenolate) ligands {ONXO R’ } 2 − (X = NMe 2 ), in the presence of an initiator (isopropanol), are regarded as a “privileged” class of catalysts due to their ability to efficiently promote the highly stereocontrolled ROP of 4-alkoxymethylene-β propiolactones ( rac -BPL CH2OR ) into their corresponding poly(BPL CH2OR )s. Depending on the substituents flanked onto the yttrium ancillary (R’ = Me, Cumyl, t Bu, halogen), the corresponding PHAs can exhibit different microstructures. Highly syndiotactic polymers are obtained when crowded substituted ligands are performing. However, the simple modification of ortho -substituents from crowded to dimethyl-substituted yttrium complex, gives atactic polyesters. Unexpectedly, only in the specific combination of a BPL FG monomer containing two methylene hydrogens apart the central oxygen on the methylene alkoxy side- functionality along with complexes bearing halogenated ligands induces the formation of highly isotactic PHAs. The objective of our study is to explore the contribution of the “outer” methylene group in the pendant arm of BPL CH2OCH2X monomers in governing isotactic PHA with the halogenated yttrium catalyst. To achieve our goal, three b-propiolactones displaying an alkoxy tertiary or quaternary carbon/silicon, namely rac -BPL CH2OiPr , rac - BPL CH2OtBu , and rac -BPL CH2OTBDMS deprived of an “outer” methylene within the alkoxide moiety, were explored in the yttrium-mediated ROP, and compared with BPL FG containing two sets of methylene hydrogens within the side-functionality, i.e. with FG = CH 2 OCH 2 X with X = H, CH=CH 2 , C 6 H 5 as in BPL CH2OMe , BPL CH2OAllyl , BPL CH2OBn , respectively. Our most recent advances in the understanding of the mechanism at play in the stereoselective ROP of BPL FG will be presented. 4 References
1. B. Laycock, et al. , Prog. Polym. Sci. , 2013 , 38 , 536 2. S. K. Gulrez, et al. , Polym. Compos. 2014 , 35 , 900 3. C. G. Jaffredo, et al. , Chem. Eur. J. , 2016 , 22 , 7629 4. R. Shakaroun et al. Polym. Chem. 2023, in press, DOI: 10.1039/d2py01573k
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