Living Polymerization of Conjugated Polar Alkenes Catalyzed by <i>N</i>-Heterocyclic Olefin-Based Frustrated Lewis Pairs

Wang, Qianyi; Zhao, Wuchao; Zhang, Sutao; He, Julong; Zhang, Yuetao; Chen, Eugene Y.‐X.

doi:10.1021/acscatal.8b00333

Cited by 110 publications

(105 citation statements)

References 56 publications

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“…Lewis pair polymerization (LPP), [33,34] as recently reviewed by Chen, [35] for which also NHO-derived examples exist. [36,37] Hence,atriple role is proposed for Mg(HMDS) 2 .F irstly, activation of the epoxide enables zwitterion formation and efficient propagation. Secondly,c omplexation of the NHO entails ahigh effective ratio of monomer to initiator.…”

Section: Angewandte Chemiementioning

confidence: 99%

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

2019

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Ad ual catalytic setup based on N-heterocyclic olefins (NHOs) and magnesium bis(hexamethyldisilazide) (Mg(HMDS) 2 )w as used to prepare poly(propylene oxide) with amolar mass (M n ) > 500 000 gmol À1 ,insome cases even > 10 6 gmol À1 ,a sd etermined by GPC/light scattering.T his is achieved by combining the rapid polymerization characteristics of az witterionic,L ewis pair type mechanism with the efficient epoxidea ctivation by the Mg II species.T ransfer-tomonomer,t raditionally frustrating attempts at synthesizing polyethers with ahigh degree of polymerization, is practically removed as al imiting factor by this approach.N MR and MALDI-ToF MS experiments reveal key aspects of the proposed mechanism, wherebyt he polymerization is initiated via nucleophilic attackbythe NHO on the activated monomer, generating az witterionic species.T his strategy can also be extended to other epoxides,i ncluding functionalized monomers.Itremains am ajor challenge to prepare high-molar-mass polyethers via ring-opening polymerization (ROP) of substituted epoxides such as propylene oxide (PO). [1] Transfer reactions (Scheme 1) pose the main difficulty in this context, most blatantly so for simple metal alkoxide catalysts. [2] Here, the anionic polymerization is not only impractically slow,but molecular weights are also severely capped (typically < 10 000 gmol À1 )a nd control over the end groups is limited. Application of crown ethers or phosphazene-type bases accelerates the reaction, but this still entails significant or even increased transfer rates. [3,4] In this regard, the application of organocatalysts was as ignificant improvement;e mployment of N-heterocyclic carbenes (NHCs) as demonstrated by Taton [5] and, later,ofN-heterocyclic olefins (NHOs) by some of us [6,7] enabled the well-controlled preparation of poly-(propylene oxide) (PPO) with designed end groups and highly defined molecular weight distribution (down to M 1.03). [7] While convenient as well as metal-and solvent-free,t hese methods still displayed limitations;P PO with M n > 12.000 gmol À1 was not reported in these cases.P erhaps tellingly,t he few studies describing the preparation of truly high-molecular-weight PPO resort to dual catalytic,c ooperative setups.E legant work by Carlotti, Deffieux, [8,9] and Coates [10] can be understood as early examples for this concept (M n up to 150 000 gmol À1 ). Very recently ad ual catalytic approach using phosphazenes and triethylborane was used to prepare PPO with M n up to 200 000 gmol À1 . [11,12] In the following,i tw ill be detailed how zwitterionic polymerization, am echanism well established in organopolymerization chemistry, [13][14][15][16] and the monomer-activating Lewis acidity of Mg II combine to provide apowerful tool for the generation of polyethers with exceptionally high molecular weight.NHOs (Scheme 1), which are identical to the so-called deoxy-Breslow intermediates frequently encountered in NHC catalysis, [17] possess an electron-rich, polar olefinic bond. [18][19][20] Them esomeric structure suggests...

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Section: Angewandte Chemiementioning

confidence: 99%

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

2019

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“…Recently, cooperative catalytic systems, especially involving a combination of metal‐based Lewis acid with nucleophilic organic base (Lewis base), were widely used in the polymerization of lactones and polar olefins . In this strategy, ROP rate will be accelerated significantly, because the coordination of metal‐center to lactone increase the reactivity of the carbonyl group and thus facilitate the ring‐opening reaction.…”

Section: Introductionmentioning

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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“…In 2010, we reported FLPs derived from π‐basic enamines or pyrroles and B(C 6 F 5 ) 3 that underwent FLP addition reactions with alkynes . More recently, the group of Zhang showed N‐heterocyclic olefins (NHOs) based FLP‐catalyzed living polymerization of conjugated polar alkenes …”

Section: Figurementioning

confidence: 99%

“…[4] More recently,t he group of Zhangs howedN -heterocyclic olefins( NHOs) based FLP-catalyzed living polymerization of conjugated polar alkenes. [5] The use of p-electron-rich substituents (i.e. amino,i mino, or phosphino group) at the Lewis basic center is knownt ob oost the basicity of the lone pair.…”

mentioning

confidence: 99%

N‐Heterocyclic Carbene Derived 3‐Azabutadiene as a π‐Base in Classic and Frustrated Lewis Pair Chemistry

Kim

Liu

Stephan

2019

Chemistry A European J

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N‐Heterocyclic carbene (NHC) derived 3‐azabutadienes 1 and 2 have been prepared by a single‐step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π‐basic, delocalized nucleophilic sites over the 3‐azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl3, GaCl3, and Me2SAuCl, to form diverse classic Lewis adducts 3–5. Combination of 1 with B(C6F5)3 or [Ph3C][B(C6F5)4] resulted in single‐electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π‐basic 1 and BPh3 effects the splitting of the O−H bond of phenol and the N−H bond of imidazole to give 7 and 8, respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π‐electrons of 1 to a newly formed C−H localized σ‐bond.

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Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs

Cited by 110 publications

References 56 publications

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

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

N‐Heterocyclic Carbene Derived 3‐Azabutadiene as a π‐Base in Classic and Frustrated Lewis Pair Chemistry

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Living Polymerization of Conjugated Polar Alkenes Catalyzed by N-Heterocyclic Olefin-Based Frustrated Lewis Pairs

Cited by 110 publications

References 56 publications

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High‐Molar‐Mass Polyethers

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

N‐Heterocyclic Carbene Derived 3‐Azabutadiene as a π‐Base in Classic and Frustrated Lewis Pair Chemistry

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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