Vinyl polymerization. 179. Effects of substituents on chain transfer reaction of substituted cumenes toward poly(methyl methacrylate) radical

In order to study the effects of substituents on the chain transfer reaction to cumenes, the polymerization of methyl methacrylate in a series of nuclear-substituted cumenes with α,α′-azobisisobutyronitrile as initiator was carried out at 60°C. and the chain transfer constants C were determined. The C values obtained for all substituted cumenes were greater than that (C₀) for unsubstituted cumene, regardless of the electronattracting or -repelling nature of the substituents. Hence the plot of log (C/C₀) against the Hammett σ constants gave no linear relationship. When the plots were made by the modified Hammett equation including resonance effect of the substituents: log (C/C) = ρσ + γE_R, however, a straight line with ρ = 0.03 and γ = 0.9 was obtained. From these results, it may be concluded that the effect of the substitutents on the chain transfer reactivities of cumenes toward attack of a poly(methyl methacrylate) radical is attributable mainly to the resonance contribution in the transition state. These results are also compared with those for polystyryl radical reported previously and discussed.     

Vinyl polymerization. 178. Copolymerizations of p-substituted phenyl vinyl sulfides

The copolymerizations of p-substituted phenyl vinyl sulfides (M₂) having OCH₃, CH₃, H, Cl, and Br substituents with styrene and methyl methacrylate (M₁) and their intercopolymerizations at 60°C. were studied. From the results of copolymerizations with styrene and methyl methacrylate, the monomer reactivity ratios and the Q₂e₂ values were determined. For example, the Q and e values for unsubstituted phenyl vinyl sulfide were 0.45 and ?1.26 in the copolymerization with methyl methacrylate. This result indicated the importance of the 3d orbital resonance between the sulfur atom and the adjacent carbon atom in the transition state of copolymerizations. The relative reactivities of these monomers toward the polymer radicals were found to be correlated with the Hammett σ constants of the substituents. In the intercopolymerizations of these monomers, it was also found that the relative reactivities followed the Hammett equation approximately.     

The Reactivities of α -Substituted Acrylonitriles and Acrylic Esters in Radical Copolymerizations

Abstract

In order to clarify the effect of the substituents in a-substituted acrylonitriles and acrylic esters on their relative reactivities toward a polystyryl radical, the radical copolymerizations of diethyl methylenemalonate, ethyl a-chloroacrylate, ethyl a-bromoacrylate, a-chloroacrylonitrile, methyl a-methoxyacrylate, and a-methoxyacrylonitrile with styrene (M₂) were investigated at 60°C. From the copolymerization parameters obtained in this study and those reported in the literature, it was confirmed that the a substituents additively contributed to the values of log Q and e of the monomers. Hence, the reactivities of a-substituted acrylonitriles and acrylic esters relative to unsubstituted acrylonitrile and methyl acrylate toward polystyryl radical, respectively, were expressed by the following equation: log (rel. react.) = δlog Q_X + 0.83 σp where δ log Q_x and σp are the resonance and polar substituent constants of α substituents, respectively. The values of δ log Q_x were determined for CH₃0, CH₃, C₆H₅, Cl, Br, OCOCH₃, COOCH₃, and CN substituents, and these values were closely related to the other resonance substituent constants such as ER and R of corresponding substituents, except for OCH₃ substituent. This relationship could also be applied to monosubstituted ethylenic monomers by using the same parameters. However, the reactivities of a-substituted styrenes deviated from this relationship because of the low ceiling temperatures of these monomers.     

Copolymerization Reactivities of α-Substituted Crotonyl Monomers

The radical copolymerizations of various α- substituted crotonyl monomers with styrene (St) and acrylonitrile (AN) were investigated, and the copolymerization parameters were determined by a least-squares procedure reported previously. The relative reactivities of the α-substituted crotonyl monomers toward polymer radicals of St and AN were found to correlate with the equation: log (relative reactivity of CH₃CH[dbnd]CXY) = ρ (σ + σ_Y) + A(Δlog Q_x + Δlog Q_Y), where Σ and Δlog Q are the polar Hammett and resonance substituent constants, respectively, and p and A are reaction constants. From the observed straight line relationships, the values of p and A were obtained to be as follows: ρ = 0.66, A = 0.75 for attack of poly-St radical, and ρ = -3.20, A = 1.3 for attack of poly-AN radical.     

Retardation of radical polymerization by phenylacetylene and its p-substituted derivatives

Radical polymerizations of styrene and methyl methacrylate in the presence of phenylacetylene and five of its p-substituted derivatives were carried out with the use of 2,2′-azobisisobutyronitrile as the initiator at 60°C. The initial overall rates of the polymerizations of styrene and methyl methacrylate in the presence of phenylacetylene were not proportional to the square root of the initiator concentration under the experimental conditions employed. The relationship between the overall polymerization rate and the concentration of the phenylacetylenes could be expressed by the Kice equation for the rate of a radical polymerization in the presence of a terminator. From this relationship the rate constant (k_s) of the reaction of a growing polymer radical with the phenylacetylenes and the constant C_s = (k_s/k_p), where k_p is the propagation rate constant of vinyl monomers, were determined. The C_s value thus obtained agree well with that derived from the relationship between the number-average degree of polymerization and the molar ratio of the phenylacetylenes to the vinyl monomer. Therefore the mechanism of the reaction may be considered as being one in which the growing radical reacts with the ethynyl group of the phenylacetylenes to yield a comparatively stable radical which terminates mainly by reaction with the growing radical, and so apparently the phenylacetylenes retard the vinyl polymerization. The substituent effects on the reaction were discussed on the basis of the following modified Hammett equation proposed by Yamamoto and Otsu: log [C_s(p-sub. PA)/C_s(PA)] = ρσ + γE_R where PA represents phenylacetylene, σ and E_R are the Hammett polar substituent constant and resonance substituent constant, respectively, and both ρ and γ are reaction constants. The γ value for the polymerization of both styrene and methyl methacrylate was 1.7. The ρ value was 1.0 for the polymerization of styrene and approximately zero for that of methyl methacrylate. These results demonstrate that the reactivity of the phenylacetylenes with the growing chain is influenced by both polar and resonance effects of their p-substituents in the degradative copolymerization of styrene and only by the resonance effect in that of methyl methacrylate.     

Interaction of poly(vinylpyrrolidone) with p-substituted phenols in aqueous solution

The interaction of poly(vinylpyrrolidone) (PVP) with various p-X-substituted phenols (X = CH₃O, CH₃, C₂H₅, H, Cl, Br, and NO₂) in aqueous solution was investigated by means of equilibrium dialysis at 30°C. By applying the Klotz equation for the results obtained, the bonding constants K between PVP and p-substituted phenols were determined. It was found that the K values were approximately correlated with the Hammett σ values of p-substituents in phenols, indicating that the bonding forces due to electrostatic and hydrogen-bonding interactions were significant, and they increased with increasing electron-withdrawing nature of the substituents. Therefore, in addition to the hydrophobic interaction which has been accepted, it was assumed that the above interaction forces also played an important role in the interactions between PVP and p-substituted phenols in aqueous solution.     

Determination of Absolute Rate Constants for Radical Polymerization and Copolymerization of Ethyl α-Chloroacrylate: Effects of Substituents on Reaction Rates of Monomer and Polymer Radical

The propagation and termination rate constants (k_p and k_t) for the radical polymerization of ethyl a-chloroacrylate (ECA) were determined by the rotating sector method k_p = 1660 and k_t = 3.33 × 10⁸ L/mol?s at 30° C. The absolute rate constants for cross-propagations in copolymerization were evaluated from the k_p determined for ECA or those for common monomers and the monomer reactivity ratios. The reactivities of ECA and poly-(ECA) radicals estimated as the rate constants of cross-propagations were accounted for by using equations relating these rate constants to the polar and resonance effects of the substituents. ECA was highly reactive toward various polymer radicals as expected from the resonance effects of the carbethoxy and chloro substituents. The poly(ECA) radical was found to be more reactive than common polymer radicals. The reactivity of a polymer radical in cross-propagation seemed to increase with increasing electron-accepting power by facilitating electron transfer from a monomer required for the new C-C bond formation.     

Copolymerization reactivities and conductivities of some living polymers of p-substituted styrenes

A kinetic study in tetrahydrofuran of the addition of 1,1-diphenylethylene on para-substituted styrylcarbanions (with Cs⁺ as counterion) has given the reactivities of free ions and ion-pairs. For both species, the addition rate constant increased in the following sequence for the substituent of the phenyl ring: pH < p-CH₃ <p-CH₃O. It was also shown that in the same solvent, the dissociation constants of the polystyryl ion-pairs decreased with the electron-donating power of the substituent.     

Use of two-parameter correlations for studying the radical reactivity of aromatic compounds

Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para-substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta- and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (σ or π-complex) in free-radical reactions.     

Effect of the substituents on radical copolymerization of dialkyl fumarates with some vinyl monomers

Radical copolymerization of dialkyl fumarates (DRF) with various vinyl monomers was carried out in benzene at 60°C. The monomer reactivity ratios, r₁ and r₂, were determined from the comonomer-copolymer composition curves. The relative reactivity of DRFs with various ester substituents toward a polystyryl radical was revealed to depend on both steric and polar effects of the ester groups. It has also been clarified that α-substituents of the polymer radical have a significant role in addition of DRF, from the comparison of the monomer reactivity ratios determined in copolymerizations with monosubstituted and 1,1-disubstituted ethylenes. The absolute cross-propagation rate constants were also evaluated and discussed. © 1992 John Wiley & Sons, Inc.     

THE APPLICATION OF THE HAMMETT EQUATION TO CYCLOPHOSPHAZENES

Abstract

The applicability of the extended Hammett equation to substituted cyclophosphazene pK _a values taken from the literature is demonstrated. Steric effects were shown to be absent or constant by correlation with the LDS form of the extended Hammett equation. The magnitude and composition of the electrical effects in the cyclotriphosphatriazenes is generally comparable to that observed for 2-substituted pyrimidines and 6-substituted-2,4-diamino-1,3,5-triazines, and differs significantly from that observed for 2- and 4-substituted pyridines. The data were also correlated with the Kabachnik equation. The results that were obtained with the extended Hammett equation were at least as good as those obtained with the Kabachnik equation.     

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ⁺) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.     

Rh-Catalyzed Oxidation of Anthracenes with tert-Butyl Hydroperoxide. Kinetic Aspects

The polar effects of substituents on reactivity in oxidation of 2-substituted anthracenes with tert-butyl hydroperoxide (TBHP)/Rh(PPh₃)₃Cl have been investigated and compared with those obtained with TBHP/VO(acac)₂ and chromic acid. The anthracene reactivities obtained from competition experiments are correlated with Hammett's σ_p-constants. The P-values are -2.60 for chromic acid and 0.72 for TBHP/VO(acac)₂. A poor correlation with p = ?0.17 (r = 0.756) was obtained for TBHP/Rh (PPh₃)₃Cl. It is concluded that the Rh-catalyzed reaction does not consist in electrophilic oxygen transfer to the anthracene.     

Solvent effects on the proton magnetic resonance spectra of p-substituted phenyltin chlorides

A PMR study of solvent effects on some p-substituted phenyltin chlorides (substituents = CH₃, (CH₃)₃C, CH₃O) is reported. Co-ordination of solvent molecules to the tin atom leads to an unusual low field shift of the o-ring protons. The results for p-tolytin trichloride have been compared with those for p-tolylsilyl trichloride and discussed. In o-, m- and p-tolyltin trichlorides long range spin-spin couplings between the ring methyl protons and the tin atom have been observed. The order of magnitude is o- > p- > m-methyl protons.     

Reactivities of methacrylate derivatives in radical polymerization as evaluated by mercury method

Reactivities of methyl methacrylate derivatives bearing substituents on the ester methyl group have been investigated by competitively adding a cyclohexyl radical generated by a reaction of cyclohexylmercuric chloride with sodium borohydride (mercury method) to these substituted methacrylates and methyl methacrylate or styrene. The relative rate constants of the cyclohexyl radical addition have been found to be nicely correlated with parameters such as Traft α* constants of alkyl esters, Q–e values, lowest unoccupied molecular orbital energies, β-carbon chemical shifts, and relative reactivities toward a polystyryl radical, indicating that the mercury method is a simple and useful technique for evaluation of the relationship between structure and reactivity of vinyl monomers in their radical polymerization, even when the structural modification is small.     

Through-Space Polar-π Interactions in 2,6-Diarylthiophenols

Molecular recognition between polar groups and aromatic molecules is fundamentally important to rational drug design. Although it has been well established that many polar functionalities interact with electron-rich aromatic residues through energetically favorable polar-π interactions, there is a limited understanding of the association between thiols and aromatic systems. Herein we report physical-organic chemistry studies on 2,6-diarylthiophenols that possess the central thiophenol ring and two flanking aromatic rings with tunable electronic properties caused by substituents at distant para position. Hammett analysis revealed that pK_a values and proton affinities correlate well with Hammett sigma values of substituents. Additional energy decomposition analysis supported the conclusion that both through-space SH-π interactions and S⁻-π interactions contribute to intramolecular stabilization of 2,6-diarylthiophenols.     

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

Stabilisation of 2,6‐Diarylpyridinium Cation by Through‐Space Polar–π Interactions

The through‐space polar–π interactions between pyridinium ion and the adjacent aromatic rings in 2,6‐diarylpyridines affect the pK_a values. Hammett analysis illustrates that the basicity of pyridines correlates well with the sigma values of the substituents at the para position of the flanking aryl rings.     

Interaction of several polymers with p-substituted phenols in aqueous solution

The interactions between several water-soluble polymers, i.e., polyethyleneimine (PEI), poly-(ethylene glycol) (PEG), polyacrylamide (PAM), and poly(methacrylic acid) (PMAA), and p-X-substituted phenols (X = CH₃O, CH₃ C₂H₅, H, Cl, Br and NO₂) in aqueous solution at 30°C were investigated by means of equilibrium dialysis. By applying the Klotz equation to the results obtained, the bonding constants between these polymers, including poly(vinylpyrrolidone) (PVP) which was reported previously, and phenol were determined; they decreased in the following order: PVP > PMAA > PEI > PAM > PEG. This order was in agreement with that reported from the solubility of naphthalene, biphenyl, and alkyl halides in aqueous solution of these polymers caused by a hydrophobic bonding interaction. In this case, therefore, a hydrophobic bonding seemed to be significant. The bonding constants of these polymers with p-substituted phenols were also determined, and they were found to be approximately correlated with Hamett σ constants of the para substituents in phenols. Therefore, it was concluded that the interactions due to electrostatic and hydrogen-bonding forces were also important.     

Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first‐principles methods, we investigated the spin‐density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron p_z orbital, that is, for three‐coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol‐2‐ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron‐donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.