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Controlled Polymerization
Published in Timothy P. Lodge, Paul C. Hiemenz, Polymer Chemistry, 2020
Timothy P. Lodge, Paul C. Hiemenz
The preceding example illustrates one of the potential complications encountered in cationic polymerization, but it is not in itself an impediment to living polymerization. There are several other potential transfer reactions, however, that collectively do impede a living cationic polymerization. Four of these are the following: β-Proton transfer. This is exemplified by the case of polyisobutylene. Protons on carbons adjacent (β) to the carbocation are electropositive, due to a phenomenon known as hyperconjugation; we can view this as partial electron delocalization through σ bonds, in contrast to resonance, which is delocalization through π bonds. Consequently there is a tendency for β-protons to react with any base present, such as a vinyl monomer: The activated monomer can now participate in propagation reactions, whereas the previous chain is terminated. Note that in isobutylene there are two distinct β-protons, and thus two possible structures for the terminal unsaturation of the chain. There is also a possibility that these double bonds can react subsequently.Hydride transfer from monomer. In this case, the transfer proceeds in the opposite direction, but has the same detrimental net effect from the point of view of achieving a living polymerization: In the particular case of isobutylene the resulting primary carbocation is less stable than the tertiary one on the chain, so Reaction (4.T) is less of an issue than Reaction (4.S).Intermolecular hydride transfer. This is an example of transfer to polymer, and can be written generally as Spontaneous termination. This process, also known as chain transfer to counterion, is essentially a reversal of the initiation step, as a β-proton is transferred back to the anion. Refer back to Reaction (4.P), but with a growing chain rather than the first monomer.
Influence of acetylation on anomeric effect in methyl glycosides
Published in Molecular Physics, 2019
Tomasz Gubica, Andrzej Zimniak, Łukasz Szeleszczuk, Kinga Dąbrowska, Michał K. Cyrański, Marianna Kańska
Most of the carbohydrate molecules are six-membered cycles called pyranosidic rings. The name of these rings comes from pyran, the derivative of cyclohexane with an endocyclic oxygen atom. The resemblance between cyclohexane and carbohydrates manifests in the preferable chair conformation of both systems. However, in 1955 Edward undermined the similarity of cyclohexane and carbohydrate derivatives in molecular structure [1]. He discovered an unexpected preference of the axial position exhibited by electronegative substituents at anomeric carbon atom. This phenomenon was later called ‘anomeric effect’ by Lemieux [2-4]. As it was revealed subsequently, the anomeric effect also causes shortening (strengthening) of the O5–C1 bond and lengthening (weakening) of the C1–O1 bond in α anomers in opposite to β anomers [5-12]. These characteristic differentiations in bond lengths are easily understandable when hyperconjugation model is employed as a possible explanation of anomeric effect [13-17]. According to this theory, lone electron pairs of an endocyclic oxygen atom (O5) are the most efficiently delocalised to the vacant antibonding orbital in the case of axial position of O1 (Figure 1(a)). Another plausible explanation of anomeric effect stems from the unfavourable electrostatic interactions of two dipoles in β anomers, hence the stability privilege of α anomers (Figure 1(b)) [1,18].
Weak and strong hydrogen bonds conducting the supramolecular framework of 1-butyl-3-(1-naphthoyl)thiourea: crystal structure, vibrational studies, DFT methods, Pixel energies and Hirshfeld surface analysis
Published in Molecular Physics, 2018
E. Contreras Aguilar, G. A. Echeverría, O. E. Piro, S. E. Ulic, J. L. Jios, M. E. Tuttolomondo, H. Pérez
The role of hyperconjugative interactions in the conformers stabilisation has been assessed using NBO analysis, where the hyperconjugation represents the transfer of an electron between a lone pair (LP) or bonding orbital and an antibonding orbital. As afore mentioned, the rotational barrier around the C–N bond is solvent dependent. The effect is greater on thioamides since for them the polarity changes between the basal and the rotational transition state is more pronounced [23]. For this reason, the calculations include also the evaluation in acetonitrile as solvent. It was chosen as a polar aprotic solvent (3.92 D) in order to detect the major differences and avoiding the rupture of intramolecular hydrogen bond interactions.
Regium bonds formed by MX (M═Cu, Ag, Au; X═F, Cl, Br) with phosphine-oxide/phosphinous acid: comparisons between oxygen-shared and phosphine-shared complexes
Published in Molecular Physics, 2019
Baishu Zheng, Yi Liu, Zhaoxu Wang, Fengxiang Zhou, Yuan Liu, XunLei Ding, Tian Lu
For example, Papadopoulos et al. [24] first predicted the existence of the gold bond as a typical regium bond between HF/H2O and AuOH by the theoretical study. They suggested that a strong bonded HF/H2O···AuOH was formed from the lone pair HF/H2O donors to AuOH due to the significant charge transfer. Despite certain similarity to the hydrogen-bonded species, the Au-bonded complexes should be considered as Lewis acid–base pairs. Recently, Zhang et al. [25] performed a natural resonance theory study of the structures of H2O/H2S/CO···MX (M═Cu, Ag, Au; X═F, Cl, Br, CH3, CF3) complexes. They found that strongly hyperconjugation interactions B: M-X↔ B+- M:X- resonance hybrid resulted in strong 3c/4e hyperbonding for H2S/CO···MX complexes, while there is a weakly resonance hybrid in H2O···MX complex. In our previous papers [12] we have computed MY complexes involving coinage metals, Cu, Ag and Au, interacting with XH2P (X═H, CH3, F, CN, NO2), to investigate the effect of substituent groups on the P···M-Y bonding strength for the XH2P···MY series of complexes. However, we noticed that the covalent interaction was significantly higher in all F-containing FH2P···MY complexes than for H, CH3, CN, NO2 containing ligand. The results revealed that the competition between noncovalent and covalent interactions mainly arises from the weightings of hyperconjugation interactions P:M-Y ↔ P–M: Y resonance structures. Although the nature of metal–ligand interactions has been studied extensively, discussions of complexes involving coinage metal complexes and some lone-pair donating ligand molecules are still insufficient.