Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of δ-Valerolactone with N-Heterocyclic Carbenes

Acharya, Ashwin; Chang, Young A.; Jones, Gavin O.; Rice, Julia E.; Hedrick, James L.; Horn, Hans W.; Waymouth, Robert M.

doi:10.1021/jp500200b

Cited by 55 publications

(71 citation statements)

References 48 publications

(91 reference statements)

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“…18,22,27 The data in Fig. 2a and 3 implicate that initiation is more efficient in the presence of LiCl.…”

Section: Introductionmentioning

confidence: 79%

“…18,22 Computational studies imply that the ringopening of the initial NHC-monomer complex has a high energy barrier, which may account for the slow initiation. 27 The SEC traces of polymer samples obtained by the ZROP of valerolactone with NHC 1 (Fig. 1) revealed the presence of a high molecular weight shoulder which is evident throughout the polymerization.…”

Section: Introductionmentioning

confidence: 93%

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Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

Chang

Waymouth

2015

Polym. Chem.

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The ring-opening polymerization (ZROP) of δ-valerolactone (VL) by N-heterocyclic carbenes (NHC) in the absence of alcohol initiators generates cyclic poly(valerolactones). A zwitterionic mechanism has been proposed where the propagating alkoxide anion undergoes ion-pairing with the acyl-imidazolium polymer chain end. To evaluate the consequences of zwitterionic ion-pairing, the influence of added LiCl to the NHC-mediated ZROP of δ-valerolactone (VL) in THF was investigated. In THF in the absence of LiCl, the molecular weights of the resultant p(VL) are higher than that predicted from the initial ratio of [VL] 0 /[NHC] 0 and the molecular weight distributions are bimodal and broad (M w /M n = 1.35). In the presence of 1.0 M LiCl, the molecular weights of the p(VL) match that predicted from [VL] 0 /[NHC] 0 and the molecular weight distributions are monomodal and narrow (M w /M n = 1.08-1.26). The presence of LiCl also influences both the rate of polymerization and the topology of the resultant poly(valerolactones). In the absence of LiCl, cyclic poly(valerolactones) are isolated upon aqueous methanol work-up, but at high LiCl concentrations, only linear chains are produced. These results imply that zwitterionic ion-pairs can be readily broken up by added LiCl.

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“…18,22,27 The data in Fig. 2a and 3 implicate that initiation is more efficient in the presence of LiCl.…”

Section: Introductionmentioning

confidence: 79%

Section: Introductionmentioning

confidence: 93%

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

Chang

Waymouth

2015

Polym. Chem.

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“…The simple structure of ZnEt 2 /DMAP Lewis pair gives us an opportunity to gain insights into the reaction mechanism of this unique catalytic system by DFT calculations. All calculations were performed at the B3LYP level of theory, which has been widely used in the investigation of the ROP mechanism . Two possible reaction mechanisms involving single molecular initiation and biomolecular initiation were proposed in scheme .…”

Section: Resultsmentioning

confidence: 99%

From Zn(C₆F₅)₂ to ZnEt₂‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

Wang

et al. 2018

ChemCatChem

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In this contribution, the structure‐reactivity relationships of ZnR2/DMAP (R=C6F5, C6H5 and C2H5) Lewis pairs in ring‐opening polymerization (ROP) of lactones were investigated by crystallographic analysis, density functional theory (DFT) calculations and kinetic studies. With decrease of ZnR2 Lewis acidity, the interaction between ZnR2 and DMAP weakens and the dissociation of ZnR2⋅2DMAP Lewis adducts become easier, which facilitates the activation of cyclic ester monomers. Thus, the ZnEt2/DMAP Lewis pair, bearing weakest interaction between Lewis acid and Lewis base, exhibits high catalytic activity and broad monomer adaptability. ZnEt2⋅2DMAP can convert lactide (LA) rapidly into polylactide (PLA) even at room temperature. Furthermore, a wide range of cyclic esters can also be polymerized using this dual catalyst, from small lactones such as β‐butyrolactone (β‐BL), δ‐valerolactone (δ‐VL) and ϵ‐caprolactone (ϵ‐CL) to the strainless macrolactone. The optimal reaction pathway, key species and active species in the ZnEt2⋅2DMAP catalytic ROP of LA were figured out by DFT calculations. The results clearly indicated that a DMAP‐LA exchange was necessary for activation of monomer, and the single molecular initiation is preferred. Meanwhile, the cyclic active species is more stable than the linear analogues, in which DMAP and ZnEt2 bond with each polymer chain end respectively and ZnEt2 interact with DMAP. The DFT calculation gives an account for the formation of the cyclic polyester in the ROP of LA by ZnR2‐based Lewis pairs.

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“…Out-of-plane dihedral angles (oop) formed by the C-N bonds involving the nitrogen atom in the carbonate ring as an indicator of hydrogen bond acceptor strength (for the definition of the oop angle see Figure S12). If the rate-limiting step of the ROP was the opening of the ring (TS2, Figure 3) as observed in other systems, 43 Information) 46 was not attempted, instead structures were simply reoptimized. 46 These calculations reveal that the calculated enthalpy of ring-opening (∆H ro ) for the three cyclic carbonates differ significantly ( Table 1) 12…”

Section: Figurementioning

confidence: 99%

A Simple and Facile Approach to Aliphatic N-Substituted Functional Eight-Membered Cyclic Carbonates and Their Organocatalytic Polymerization

Venkataraman

Coady

et al. 2015

J. Am. Chem. Soc.

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Aliphatic N-substituted functional eight-membered cyclic carbonates were synthesized from N-substituted diethanolamines by intramolecular cyclization. On the basis of the N-substituent, three major subclasses of carbonate monomers were synthesized (N-aryl, N-alkyl and N-carbamate). Organocatalytic ring opening polymerization (ROP) of eight-membered cyclic carbonates was explored as a route to access narrowly dispersed polymers of predictable molecular weights. Polymerization kinetics was highly dependent on the substituent on the nitrogen atom and the catalyst used for the reaction. The use of triazabicyclodecene (TBD), instead of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), as the catalyst for the N-alkyl substituted monomers significantly enhanced the rate of polymerizations. Computational studies were performed to rationalize the observed trends for TBD catalyzed polymerizations. With the optimal organocatalyst all monomers could be polymerized generating well-defined polymers within a timespan of ≤2 h with relatively high monomer conversion (≥80%) and low molar-mass dispersity (Đ(M) ≤ 1.3). Both the glass transition temperatures (T(g)) and onset of degradation temperatures (T(onset)) of these polymers were found to be N-substituent dependent and were in the range of about -45 to 35 °C and 230 to 333 °C, respectively. The copolymerization of the eight membered monomers with 6-membered cyclic comonomers including commercially available l-lactide and trimethylene carbonate produced novel copolymers. The combination of inexpensive starting materials, ease of ring-closure and subsequent polymerization makes this an attractive route to functional polycarbontes.

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Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of δ-Valerolactone with N-Heterocyclic Carbenes

Cited by 55 publications

References 48 publications

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

From Zn(C₆F₅)₂ to ZnEt₂‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

A Simple and Facile Approach to Aliphatic N-Substituted Functional Eight-Membered Cyclic Carbonates and Their Organocatalytic Polymerization

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Experimental and Computational Studies on the Mechanism of Zwitterionic Ring-Opening Polymerization of δ-Valerolactone with N-Heterocyclic Carbenes

Cited by 55 publications

References 48 publications

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

Ion pairing effects in the zwitterionic ring opening polymerization of δ-valerolactone

From Zn(C6F5)2 to ZnEt2‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

A Simple and Facile Approach to Aliphatic N-Substituted Functional Eight-Membered Cyclic Carbonates and Their Organocatalytic Polymerization

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From Zn(C₆F₅)₂ to ZnEt₂‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones