Highly Stereocontrolled Ring‐Opening Polymerization of Racemic Alkyl β‐Malolactonates Mediated by Yttrium [Amino‐alkoxy‐bis(phenolate)] Complexes

Abstract

Yttrium [amino-alkoxy-bis(phenolate)]amido complexes have been used for the ring-opening polymerization (ROP) of racemic alkyl β-malolactonates (4-alkoxycarbonyl-2-oxetanones, rac-MLA(R) s) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho-substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)-disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA(R) s (Pr ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho-substituted ligands proved largely ineffective. All polymers have been characterized by (1) H and (13) C{(1) H} NMR spectroscopy, MALDI-ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain-end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first-order (Mk1) model for the ROP of rac-MLA(Bz) and rac-MLA(All) , the ROP of rac-MLA(Me) led to a chain end-control of Markov second-order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl- and Br-substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.

Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand‐Controlled Yttrium‐Catalyzed Stereoselective Ring‐Opening Polymerization of Functional Racemic β‐Lactones

“…[7] We have shown that yttrium catalysts stabilized by nonchiral tetradentate amino-alkoxy-bisphenolate ligands {ONOO R'2 } 2 offer highly syndiotactic poly(3-hydroxybutyrate) (PHB) with a probability of syndio enchainment (Ps) up to 0.96, provided sterically bulky aryl-containing substituents are installed on the ortho-site of phenolate ligands. [8] This ROP catalysis has been extended to different racemic -lactones, [9] in particular to valuable functional ones such as alkyl -malolactonates (i.e., 4alkoxycarbonyl--lactones, MLA R ; R = allyl, benzyl); [10] in this latter case, surprisingly, only yttrium catalysts bearing an ortho,para (o,p)-dichloro-substituted {ONOO Cl2 } 2 ligand enabled achieving highly syndiotactic polymerizations (Ps > 0.95). Regular catalysts incorporating sterically crowded R' alkyl/aryl o,p-substituents on the {ONOO R'2 } 2 platform afforded only modest syndioselectivity.…”

“…ROP reactions were performed on three racemic BPL OR monomers (R = allyl, benzyl, methyl) (Scheme 1), which were readily prepared by carbonylation of the corresponding racemic epoxides following Coates' Co-catalyzed protocol ( Figure S1 in the Supporting Information). [13] As for previous studies, [8,10] yttriumisopropoxide catalysts/initiators were conveniently generated in situ from the action of 1 equiv. of iPrOH onto the respective amido precursors 1a-g that were selected with ligands differing by the nature of the R' phenolate substituents.…”

“…Reactivity of the catalyst systems in THF was significantly lower, [14] most likely as the result of competitive coordination of THF vs. monomer onto the metallic center; such a behavior has been previously observed in the ROP of alike polar cyclic ester monomers. [10,15] The reactions proceeded with a fair control over the PHB OR s molar masses. This is evidenced by (i) the quite good agreement between experimental molar mass values (Mn,NMR) and theoretical ones (Mn,theo), as determined by 1 H NMR analyses (from the isopropoxycarbonyl end-group and main-chain resonances), and monomer-to-initiator along with BPL OR conversion, respectively, (ii) the linear growth of experimental molar masses (e.g., Mn,NMR) as a function of monomer conversion as illustrated with BPL OAll (Figure S3), as well as (iii) the rather narrow dispersity values (1.07 < ÐM < 1.23) as determined by SEC analysis.…”

Advances in drug delivery systems based on synthetic poly(hydroxybutyrate) (co)polymers

“…The tacticity of these copolymers, as determined by 13 C{ 1 H} NMR analyses, was tuned according to the catalyst used. Polymerization of a racemic mixture of the monomers using the syndiospecific yttrium(III) catalyst led to syndiotactic copolymers (probability of racemic linkage Pr = 0.82-0.84) [122][123][124][125], whereas polymerization of such a mixture with the nonstereospecific -diketiminate zinc catalyst [126] …”

Across the Board: Michael North

“…Ring‐opening polymerisation (ROP) of strained lactones (Scheme ) is a well‐established methodology for the synthesis of polyesters . The methodology has been extensively studied for the polymerisation of 4‐, 6‐ and 7‐membered ring lactones, including malonic‐acid‐derived β‐lactones 1 , lactides 2 , and ϵ‐caprolactone 3 ,. In contrast, 5‐membered ring‐containing γ‐lactones were found not to undergo ROP except under extremely harsh conditions.…”