The Mechanism of the Self-Initiated Thermal Polymerization of Styrene. Theoretical Solution of a Classic Problem

Khuong, Kelli S.; Jones, W. Jeremy; Pryor, William A.; Houk, K. N.

doi:10.1021/ja0448667

Cited by 198 publications

(203 citation statements)

References 49 publications

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“…Rather than relying upon thermally induced homolytic cleavage of Si-H bond, [ 44 ] or the presence of an external radical initiator, [ 60 ] styrene can thermally self-initiate producing radicals via a Diels-Alder cycloaddition. [ 66 ] Besides the initiation of styrene polymerization in solution, these radicals can abstract hydrogen atoms from the Si-H terminated SiNC surface and the resulting silyl radicals are free to react with solution-phase styrene. This would form a Si-C bonds and new radicals localized on the beta-carbon of the surface bonded moiety which can subsequently react with more styrene to produce surface bonded oligomers/polymers or react in a surface chain-hydrosilylation reaction.…”

Section: Resultsmentioning

confidence: 99%

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

Yang

Dasog

Dobbie

et al. 2013

Adv Funct Materials

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Silicon nanocrystals (SiNCs) have received much attention because of their exquisitely tunable photoluminescent response, biocompatibility, and the promise that they may supplant their CdSe quantum dot counterparts in many practical applications. One attractive strategy that promises to extend and even enhance the utility of SiNCs is their incorporation into NC/polymer hybrids. Unfortunately, methods employed to prepare hybrid materials of this type from traditional compound semiconductor (e.g., CdSe) quantum dots are not directly transferable to SiNCs because of stark differences in surface chemistry. Herein, the preparation of chemically resistant SiNC/polystyrene hybrids exhibiting exquisitely tunable photoluminescence is reported and material processability is demonstrated by preparing micro and nanoscale architectures.

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Section: Resultsmentioning

confidence: 99%

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

Yang

Dasog

Dobbie

et al. 2013

Adv Funct Materials

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“…The opposite is observed here for the B3LYP trajectories, that is, a standard theoretical analysis would describe the mechanism as stepwise, but around half of the reactive trajectories act concerted. 65 The lesson from these observations, as well as other studies, 6,7,8,12,28,66,67 is that the classical division of multibond reactions into stepwise versus concerted mechanisms is often an oversimplification. A majority of cycloadditions and related reactions are likely to be mechanistically simple, but for reactions approaching a stepwise/concerted boundary -just those cases where the question of concert is most interesting -the consideration of trajectories will often be essential to understanding the mechanism.…”

Section: Bypassing Intermediates and The Mechanism Of Cycloadditionsmentioning

confidence: 94%

Dynamic Effects on the Periselectivity, Rate, Isotope Effects, and Mechanism of Cycloadditions of Ketenes with Cyclopentadiene

2006

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The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4 + 2] and [2 + 2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4 + 2] and [2 + 2] products. The dichloroketene reaction is finely balanced between [4 + 2] and [2 + 2] cycloaddition modes in mPW1K calculations, as the minimum-energy path (MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4 + 2] and [2 + 2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The 13 C isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6−31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that non-statistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.Selectivity in cycloadditions may take many forms, e.g., endo/exo stereoselectivity, regioselectivity, facial stereoselectivity, and diene/dienophile role selectivity. When two distinct formally allowed processes are possible, as in the [4 + 2] versus [6 + 4] cycloadditions of cyclopentadiene with tropone, 1 their differentiation is referred to as periselectivity. The underlying framework within which chemists usually understand any of these forms of selectivity is transition state theory (TST). The preferred product would be that involving the lowest-energy transition state, and the degree of selectivity would be determined by the relative energies for separate transition states. Even when there is no enthalpic barrier, reactivity and selectivity can be discussed in terms of free-energy barriers. 2 Qualitative theories of selectivity such as FMO theory may be thought of as a simplified surrogate for TST, easing the task of predicting which cycloaddition barrier is lowest in energy.singleton@mail.chem.tamu.edu. Publisher's Disclaimer: This PDF receipt will only be used as the basis for generating PubMed Central (PMC) documents. PMC ...

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“…The evidence of the occurrence of the Mayo mechanism in spontaneous thermal polymerization of styrene has been provided via laboratory experiments, [15][16][17][18] and theoretical studies using density functional calculations. 20 By contrast, Flory's mechanism 13 of self-initiation proposes two monomers to combine and form a diradical, which either undergoes ring closure to form a cyclobutane derivative (reversible reaction) or undergoes hydrogen transfer or abstraction with a third monomer to form monoradical species. However, whether the diradical in its ground (singlet) state can undergo hydrogen transfer or abstraction reactions is questionable.…”

Section: Introductionmentioning

confidence: 99%

Self-Initiation Mechanism in Spontaneous Thermal Polymerization of Ethyl and n-Butyl Acrylate: A Theoretical Study

et al. 2010

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In this study, the mechanism of self-initiation in spontaneous thermal polymerization of ethyl and n-butyl acrylate is explored using first-principles calculations. Density functional theory (with B3LYP functional and 6-31G* basis set) was used to study [4 + 2] and [2 + 2] cycloaddition reactions on the singlet and triplet potential energy surfaces. Diels-Alder (DA) dimers of ethyl acrylate [6-ethoxy-2-ethoxycarbonyl-3,4-dihydro-2H-pyran (EDP)] and of n-butyl acrylate [6-butoxy-2-butoxycarbonyl-3,4-dihydro-2H-pyran (BDP)] were found to form on the singlet surface via the concerted pathway proposed by Mayo. The formation of diethyl cyclobutane-1,2-dicarboxylate (DECD) and dibutyl cyclobutane-1,2-dicarboxylate (DBCD) via a nonconcerted pathway was identified on the singlet surface of ethyl and n-butyl acrylate, respectively. The presence of a diradical transition state for the formation of the DECD and DBCD was predicted. Triplet potential energy surfaces for the formation of diradical dimer of ethyl and n-butyl acrylate were computed, and the presence of a triplet diradical intermediate was identified for each of the monomers. A low energy monoradical generation mechanism was identified to be involving hydrogen abstraction by a third acrylate monomer from the triplet diradical species. The molecular structure of the computed monoradical species was found to correlate with chain initiating species of the dominant series of peaks in previously reported electrospray ionization-Fourier transform mass spectra of spontaneously polymerized samples of ethyl and n-butyl acrylate. In view of these observations, it is concluded that this self-initiation mechanism is most likely the initiating mechanism in spontaneous thermal polymerization of alkyl acrylates.

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The Mechanism of the Self-Initiated Thermal Polymerization of Styrene. Theoretical Solution of a Classic Problem

Cited by 198 publications

References 49 publications

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

Highly Luminescent Covalently Linked Silicon Nanocrystal/Polystyrene Hybrid Functional Materials: Synthesis, Properties, and Processability

Dynamic Effects on the Periselectivity, Rate, Isotope Effects, and Mechanism of Cycloadditions of Ketenes with Cyclopentadiene

Self-Initiation Mechanism in Spontaneous Thermal Polymerization of Ethyl and n-Butyl Acrylate: A Theoretical Study

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