Influence of Solvent on Radical Trap-Assisted Dimerization and Cyclization of Polystyrene Radicals

Arce, Maya M.; Pan, Ching W.; Thursby, Madalyn M; Wu, Jessica P.; Carnicom, Elizabeth M.; Tillman, Eric S.

doi:10.1021/acs.macromol.6b01794

Cited by 21 publications

(19 citation statements)

References 45 publications

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“…The results presented in this paper are focused not on assessing the yields or purity of the cyclic polymers, as the methods used have already been reported and proven capable of producing macrocycles [5,12]. Rather, the focus is on elucidating the identify of a low-molecular-weight species whose presence cannot be accounted for by the chemistry used to close the polymer chains into rings.…”

Section: Resultsmentioning

confidence: 99%

“…This approach is conceptually simple, with the overall process involving end-group transformations of one or both of the chain ends to prepare the linear precursors capable of cyclization. Examples of reactive chain ends for cyclization include, but are not limited to: (i) alkynes and azides for “click” reactions [3,4,5]; (ii) dienes and dienophiles for Diels–Alder chemistry [6]; (iii) alkenes and thiols for thiol-ene reactions [7]; (iv) anthracenes for photodimerization reactions [8]; (v) carbanions for reactions with external electrophiles (by consecutive S N 2 reactions between a polymer anion and bifunctional electrophile) [9,10]; and (vi) C–Br bonds capable of activation in a radical transfer coupling sequence [11,12].…”

Section: Introductionmentioning

confidence: 99%

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Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF

et al. 2018

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Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by 1 H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF

et al. 2018

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“…While these results are promising, the addition of a radical trap assist (RTA) to the coupling sequence, such as 2‐methyl‐2‐nitrosopropane (MNP), may be incorporated to further increase the rate of coupling under certain conditions by altering the kinetic order of the reaction (Scheme ) . Prior work has supported the mechanistic role of the radical trap using both thermolysis and MALDI‐TOF .…”

Section: Introductionmentioning

confidence: 99%

Atom transfer coupling reactions performed with benign reducing agents and radical traps

Xia

Rubaie²,

Johnson³

et al. 2019

J. Polym. Sci. Part A: Polym. Chem.

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Monobrominated polystyrene (PSBr) was prepared by ATRP, and the resulting chain ends were activated in the presence of radical traps to induce chain end-coupling. In atom transfer radical coupling (ATRC) with radical trap assistance, to achieve significant coupling requires excess metal catalyst, ligand, and a reducing agent that is often additional metal. In this work, activators generated by electron transfer (AGET) and radical trap assistance are used in the ATRC sequence to successfully lead to chain-end coupling without the need for the oxidatively unstable copper (I) and with environmentally friendlier agents in place of copper metal. High extents of coupling (Xc) were achieved using ascorbic acid (AA) as the reducing agent and copper(II) bromide as the oxidized version of the catalyst, and when combined with AGET ATRP to prepare the PSBr precursor, only a fraction of the total metal was required compared to traditional atom transfer reactions, while still retaining similar Xc values.

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“…This is consistent with the reactivity of the radical trap itself being a dominant factor in RTA‐ATRC reactions, with its reactivity higher in benzene than in HFB. The solvent's effect on the radical trap has previously been shown to influence the rate and extent of coupling …”

Section: Resultsmentioning

confidence: 99%

Effect of Aromatic Co‐Solvents on the Efficiency of Atom Transfer Radical Coupling Reactions of Fluorinated and Non‐Fluorinated Vinyl Aromatic Polymers

Ching

Lee

et al. 2019

Macro Chemistry & Physics

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Monobrominated versions of poly(pentafluorostyrene) (PPFSBr), polystyrene (PSBr), and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization (ATRP) and employed in a variety of atom transfer radical coupling (ATRC)‐type reactions to observe the impact of external aromatic faces on the extent of coupling (Xc). In ATRC reactions assisted with the radical trap 2‐methyl‐2‐nitrosopropane (MNP), Xc is nearly unchanged when the electron‐rich benzene co‐solvent (50% v/v with THF) is replaced with the electron‐poor hexafluorobenzene (HFB) for PSBr and PMABr. In the case of PPFSBr, the addition of benzene to the reaction mixture results in far lower extents of coupling (Xc < 0.2). 1H NMR spectra of the radical trap MNP in HFB show greater aggregation to the inactive form, compared to the spectra obtained in benzene. To remove the effect of the radical trap interacting with the aromatic co‐solvent and altering the rate of coupling, traditional ATRC reactions are performed with the same co‐solvent systems and, in this case, HFB results in higher Xc values across all polymer types. This is consistent with HFB pushing the position of the KATRP further toward the active radical, while benzene increases the reactivity of the MNP radical trap.

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Influence of Solvent on Radical Trap-Assisted Dimerization and Cyclization of Polystyrene Radicals

Cited by 21 publications

References 45 publications

Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF

Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF

Atom transfer coupling reactions performed with benign reducing agents and radical traps

Effect of Aromatic Co‐Solvents on the Efficiency of Atom Transfer Radical Coupling Reactions of Fluorinated and Non‐Fluorinated Vinyl Aromatic Polymers

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