Ring‐opening polymerization (ROP) of lactones or cyclic (di)esters is a powerful method to produce well‐defined, high‐molecular‐weight (bio)degradable aliphatic polyesters. While the ROP of lactones of various ring sizes has been extensively studied, the ROP of the simplest eight‐membered lactone, 7‐heptanolactone (7‐HL), has not been reported using metal‐based catalysts. Accordingly, this contribution reports the ROP of 7‐HL via metal‐catalyzed coordinative‐insertion polymerization to the corresponding high‐molecular‐weight polyester, poly(7‐hydroxyheptanoate) (P7HHp). The resulting P7HHp is a semi‐crystalline material, with a
- NSF-PAR ID:
- 10341372
- Date Published:
- Journal Name:
- Polymer Chemistry
- Volume:
- 12
- Issue:
- 37
- ISSN:
- 1759-9954
- Page Range / eLocation ID:
- 5271 to 5278
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract T mof 68 °C, which is ~10 °C higher than poly(ε ‐caprolactone) derived from the seven‐membered lactone. Mechanical testing showed that P7HHp is a hard and tough plastic, with elongation at break >670%. P7HHp‐based polyesters with higherT mvalues have been achieved through stereoselective copolymerization of 7‐HL with an eight‐membered cyclic diester, racemic dimethyl diolide (rac ‐8DLMe), known to lead to highT mpoly(3‐hydroxyburtyrate) (P3HB). Notably, catalyst's strong kinetic preference for polymerizingrac ‐8DLMeover 7‐HL in the 1/1 comonomer mixture rendered the formation of di‐block copolymer P3HB‐b ‐P7HHp, showing two crystalline domains withT m1 ~ 65 °C andT m2 ~ 160 °C. Semi‐crystalline random copolymers withT mup to 164 °C have also been obtained by adjusting copolymerization conditions. Mechanical testing showed that P3HB‐b ‐P7HHp can synergistically combine the high modulus of isotactic P3HB with the high ductility of P7HHp. -
How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer–Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF 3 ·Et 2 O/H 2 O 2 system. Although the primary effect (alignment of the migrating C–R m bond with the breaking O–O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C–R m bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol −1 Curtin–Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2–3 kcal mol −1 penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph 3 P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI – a ring-opening process.more » « less
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Abstract A polymer's properties and functionality are directly related to the constituent monomers from which it was synthesized, the order in which these monomers are assembled, and the degree to which monomers are enchained. Furthermore, a standing challenge in the field of polymer synthesis is to provide temporal polymerization control that can be leveraged to access a variety of advanced polymer architectures. Though many polymer classes are attractive for various applications, polyesters have drawn considerable recent interest due to the potential of these materials to provide biodegradable alternatives to other, often petroleum derived, polymeric materials that create concerning, long‐term environmental impacts. Many of these biodegradable polyesters can be produced via the transition‐metal catalyzed ring‐opening polymerization of cyclic ester and cyclic ether monomers. Through researchers' quest to access precise and well‐defined polyesters via ring‐opening polymerization, an intriguing class of stimuli‐responsive catalysts have emerged. More specifically, catalyst systems have been developed in which their electronic nature may be modulated via either ligand‐based or active metal site‐based redox‐switchability. These redox‐switchable catalysts have been shown to exhibit altered chemoselectivity and kinetic modulation as a function of catalyst redox‐state. Herein, we will discuss the beginnings, select recent advancements, and an outlook on the field of redox‐switchable ring‐opening polymerizations.
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Abstract The electrochemical reduction of several α,β ‐epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β ‐Epoxyketones undergo dissociative electron‐transfer reactions with C−O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the “sticky” concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring‐opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring‐closed radical anions are resonance‐stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 108 s−1, and not significantly affected by substituents on the 3‐membered ring (consistent with C−O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1PY00784J
