Cationic polymerization of α,β-disubstituted olefins. Part II. Cationic polymerization of propenyl N-butyl ether

The cationic polymerization of propenyl n-butyl ether (PBE) in methylene chloride with boron fluoride etherate at ?78°C. has been studied. The copolymerization of PBE with vinyl n-butyl ether (VBE) showed that both the isomers are more reactive than VBE, and their monomer reactivity ratios were found to be:     

Cationic polymerization of α,β-disubstituted olefins. XV. Stereospecific polymerization of butenyl ethers by cationic catalysts

Methyl, ethyl, and isopropyl butenyl ethers, CH₃CH₂CH?CHOR, were polymerized with homogeneous catalysts at ?78°C. Toluene, methylene chloride, and nitroethane were used as solvents, and BF₃O(C₂H₅)₂ and SnCl₄·CCl₃CO₂H were used as catalysts. The stereoregularity of the polymers were compared by x-ray diagrams and infrared absorption ratios. The stereoregularity of polymers increased with increasing content of the trans isomer in the monomer and with increasing polarity of the solvent. In the polymerization of methyl and ethyl butenyl ethers, crystalline polymers were obtained from both the trans and cis isomers. The crystalline polymer prepared from the trans isomer and that from the cis isomer had the same steric structure. This behavior is quite different from that observed in the polymerization of propenyl ethers. It is concluded that the bulkiness of the group on the olefinic β-carbon plays an important role in the stereospecific polymerization of α,β-disubstituted olefins.     

Cationic polymerization of α,β-disubstituted olefins. VI. Steric structure of poly(methyl propenyl ether) obtained by cationic catalysts

The steric structure of poly(methyl propenyl ether) obtained by cationic polymerization was studied by NMR spectra. From the analysis of β-methyl and α-methoxyal spectra, it was found that the tacticities of the α-carbon were different from those of the β-carbon in all polymers obtained. In the crystalline polymers obtained from the trans isomer by homogeneous catalysts, BF₃·O(C₂H₅)₂ or Al(C₂H₅)Cl₂, and from the cis isomer by a heterogeneous catalyst, Al₂(SO₄)₃–H₂SO₄ complex, the structure of polymers was threo-di-isotactic. Though the configurations of all α-carbons were isotactic, a small amount of syndiotactic structure was observed in the β-carbon. On the other hand, in the amorphous polymer obtained from cis isomer by the homogeneous catalyst, the configuration of the α-carbon was isotactic, but that of the β-carbon was atactic. These facts suggest that the type of opening of a monomeric double bond is complicated, or that carbon–carbon double bond in an incoming monomer rotates in the transition state. From these experimental results, a probability treatment was proposed from the diad tacticity of α,β-disubstituted polymers. It shows that the tacticity is decided by a polymerization mechanism different from that proposed by Bovey.     

Cationic polymerization of α,β-disubstituted olefins. Part 17. Effect of polymerization conditions on the reactivity of alkenyl ethers relative to vinyl ethers

In order to clarify the propagation reaction, vinyl ether was copolymerized with the corresponding alkenyl ether under various conditions. cis-Propenyl ether (cis-PE) was several times more reactive than trans-PE and the corresponding vinyl ether in the copolymerization catalyzed by BF₃ · O(C₂H₅)₂ in toluene. However, the reactivity of cis-PE relative to trans-PE and the vinyl ether was found to be greatly decreased with increasing polarity of the solvent and to be very close to unity in such polar solvents as nitroethane. On the other hand, the reactivity of trans-IBPE relative to IBVE was scarcely changed by polymerization conditions. Also, the nature of the initiator and polymerization temperature affect the reactivity of cis-PE relative to the vinyl ether. These phenomena were explained by the relative stability of the bridged and open car bonium ions based on the polarity of the solvent and steric hindrance due to substituents in the trans isomer.     

Cationic polymerization of α,β-disubstituted olefins. IV. Stereospecific polymerization of propenyl alkyl ethers catalyzed by BF3·O(C2H5)2 and Al2(SO4)3–H2SO4 complex

The cis- and trans-propenyl alkyl ethers were polymerized by a homogeneous catalyst [BF₃·O(C₂H₅)₂] and a heterogeneous catalyst [Al₂(SO₄)₃–H₂SO₄ complex]. Methyl, ethyl, isopropyl, n-butyl and tert-butyl propenyl ethers were used as monomers. The steric structure of the polymers formed depended on the geometric structures of monomer and the polymerization conditions. In polymerizations with BF₃·O(C₂H₅)₂ at ?78°C., trans isomers produced crystalline polymers, but cis isomers formed amorphous ones except for tert-butyl propenyl ether. On the other hand, highly crystalline polymers were formed from cis isomers, but not from the trans isomers in the polymerization by Al₂(SO₄)₃–H₂SO₄ complex at 0°C. The x-ray diffraction patterns of the crystalline polymers obtained from the trans isomers were different from those produced from the cis isomers, except for poly(methyl propenyl ether). The reaction mechanism was discussed briefly on these basis of these results.     

Structure and reactivity α,β-unsaturated ethers. II. Cationic copolymerizations of propenyl isobutyl ether

cis- and trans-Propenyl isobutyl ethers were copolymerized with each other and each with vinyl isobutyl ether separately under various conditions. In homogeneous polymerizations a cis-β-methyl substitution on vinyl isobutyl ether apparently enhanced the reactivity, whereas the trans substitution tended to reduce it slightly. In heterogeneous catalysis, on the other hand, a β-methyl group on the vinyl ether, whether cis or trans, greatly reduced the reactivity, probably because of the steric hindrance toward the adsorption of monomers on the catalyst surface. The relative reactivities of cis- and trans-propenyl isobutyl ethers ranged from 2 to 20, depending on the polymerization conditions. The polymer end formed from the cis monomer exhibited special steric effects. It was concluded that even in homogeneous media the rotation of the polymer end around the terminal carbon–carbon bond is restricted.     

Structure and reactivity of α,β-unsaturated ethers. III. Cationic copolymerizations of alkenyl alkyl ethers

The relative cationic polymerizabilities of the geometrical isomers of various alkenyl alkyl ethers were studied both in copolymerizations with each other and in their respective copolymerizations with vinyl isobutyl ether as standard. Copolymerizations were carried out in methylene dichloride at ?78°C. with boron trifluoride etherate as catalyst. The cis isomers have been found to be more reactive than the corresponding trans isomers. A primary alkyl substituent on the β-cis position of vinyl ethyl ether enhances the reactivity. Yet the steric effect is noticeable when the substituents are bulky. Compounds substituted with cis-β-isobutyl and with β-dimethyl showed little tendency to homopolymerization. It was proved that the polymer ends derived from cis and from trans monomers are respectively different in character because of the restricted rotation of the end unit around the terminal carbon–carbon bond. The alternation tendency, remarkable in the copolymerization of cis monomers with vinyl ether, was explained in terms of the cis-opening mechanism.     

Cationic copolymerization of anethole: Reactivity of geometric isomers of α,β-disubstituted ethylenes

Cationic copolymerizations of anethole were carried out under various conditions in order to confirm the relative reactivities of its geometric isomers. trans-Anethole was more reactive than cis-anethole in copolymerizations with p-methoxystyrene or styrene, but less reactive in the mutual copolymerization of cis- and trans-anethole; i.e., the trans isomer was more reactive to a growing chain end with little steric hindrance. Thus the intrinsic reactivity of an olefinic double bond to carbonium ion is greater for the trans isomer than for the cis isomer. This idea is supported by ¹³C NMR spectra, since the signal of the olefinic β-carbon of the trans isomer is at higher field than that of the cis isomer. The behavior of anethole was compared with the results observed in vinyl ethers, where the cis isomer was always more reactive irrrspective of the structure of the growing chain end. In addition, the dependence of monomer reactivity ratios on polymerization conditions is discussed.     

Structure and reactivity of α,β-unsaturated ethers. VIII. Cationic copolymerizations and acetal additions of propenyl and isobutenyl ethyl ethers

Cationic copolymerizations of cis- and trans-propenyl ethyl ethers (PEE) with isobutenyl ethyl ether (IBEE) were carried out in methylene chloride at ?78°C with the use of boron trifluoride etherate as catalyst. Monomer reactivity ratios were r₁ = 24.0 ± 2.4 and r₂ = 0.02 ± 0.02 for the cis-PEE (M₁)–IBEE (M₂) system and r₁ = 19.1 ± 1.8 and r₂ = 0.04 ± 0.02 for the trans-PEE (M₁)–IBEE (M₂) system, indicative of the reactivity order: cis-PEE > trans-PEE ? IBEE. In separate experiments, these β-methyl-substituted vinyl ethers were allowed to react with various acetals in the presence of boron trifluoride etherate. The relative reactivities of these ethers were generally found to decrease in the order: cis-β-monomethylvinyl > vinyl > trans-β-monomethylvinyl > β,β-dimethylvinyl. Comparisons of these results with previously published copolymerization data have permitted the conclusion that, in both the copolymerizations and acetal additions, the single β-methyl substitution on vinyl ethers exerts little steric effect against their additions toward any alkoxycarbonium ion, whereas the β,β-dimethyl substitution results in a large adverse steric effect toward both β-monomethyl- and β,β-dimethyl-substituted alkoxycarbonium ions.     

Structure and reactivity of α,β-unsaturated ethers. V. Cationic copolymerizations of ring-substituted phenyl vinyl ethers

Phenyl vinyl ether (M₁) has been copolymerized with its various ring-substituted derivatives (M₂) in toluene at ?78°C with stannic tetrachloride as catalyst. The substituents investigated include p-CH₃O, m-CH₃O, p-CH₃, m-CH₃, p-Cl, and m-Cl. The course of copolymerization was followed by gas chromatographic determinations of residual monomers, and the monomer reactivity ratios were evaluated by use of the integral form of the Mayo-Lewis copolymerization equation. Except for the unusual case of the m-CH₃O derivative, the observed values of log (1/r₁) were found to be linearly correlated with Hammett's σ constants, the reaction constant being ρ = ?1.76 with the correlation coefficient r = 0.990. Comparisons of these results with the existing data for the styrene copolymerizations have enlightened the behavior of the oxygen atom in transmitting the electronic effects of ring substituents onto the reaction center.     

Ring-opening polymerization of α,β-disubstituted oxiranes by α-methoxyphenylmethylium hexachloroantimonate

α-Methoxyphenylmethylium hexachloroantimonate was used as a novel initiator for the polymerization of α,β-disubstituted oxiranes such as cyclohexene oxide (CHO) and 2-butene oxide (trans and cis) (2-BO) at ?78°C with dichloromethane or dichloromethane-toluene mixtures as solvents. The CHO polymerization mixture became turbid and the polymer precipitated in dichloromethane. The CHO polymerization proceed quantitatively in dichloromethane–toluene mixtures. The molecular weight distribution of polyCHO obtained was bimodal regardless of the solvent used. The polymerization of trans-2-BO was heterogeneous in both dichloromethane and dichloromethane–toluene mixture. The polymerization mixtures of cis-2-BO were transparent but reached a limit yield which was less than the polymer yield of trans-2-BO. Furthermore, the microstructure of the poly2-BOs were analyzed by Vandenberg's method and the results confirmed Vandenberg's finding that inversion of configuration occurs in the propagation step.     

Living cationic polymerization of isobutyl propenyl ether as β-substituted vinyl ether

Isobutyl propenyl ether [IBPE; CH₃CH=CH? OCH₂CH(CH₃)₂] was polymerized with a mixture of hydrogen iodide and iodine (HI/I₂ initiator) in n-hexane at ?40°C to yield living polymers with a nearly monodisperse molecular weight distribution (MWD) (M?_w/M?_n ≈ 1.1). The number-average molecular weight (M?_n) of the polymers increased proportionally to IBPE conversion and further increased when a new monomer feed was added to a completely polymerized solution. The M?_n was controlled by the initial concentration of hydrogen iodide if the acid was charged in excess over iodine. In polymerization by iodine alone the M?_n of the polymers obtained in nonpolar solvents (n-hexane and toluene) also increased with conversion, but their MWD was broader (M?_w/M?_n = 1.3–1.4) than in the HI/I₂-initiated systems under similar conditions. The iodine-initiated polymerization in polar CH₂Cl₂ solvent, in contrast, led to nonliving polymers with a broad MWD (M?_n/M?_n = 1.6–1.8) and M?_n, independent of conversion. The living polymerization of IBPE was also compared with that of the corresponding isobutyl vinyl ether, to determine the effect of the β-methyl group in IBPE.     

Lipophilic Functionalized Cobyrinic-Acid Derivatives. Part 1. Cobester α-monoacids (α,α,α,β,β,β-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates)

The four α,α,α, β,β,β,-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates 2b, d–f , with a free b-, d-, e-, and f-propionic-acid function, respectively, were prepared by partial hydrolysis of heptamethyl Coα, Coβ-dicyanocobyrinate (cobester; 1 ) in aqueous sulfuric acid. The cobester monoacids 2b, d–f were obtained as a ca. 1:1:1:1 mixture which was separated. The monoacids were purified by chromatography and isolated in crystalline form. The position of the free propionic-acid function was determined by an extensive analysis of 2b, d–f using 2D-NMR techniques; an analysis of the C,H-coupling network topology resulted in an alternative assignment strategy for cobyrinic-acid derivatives, based on pattern recognition. Additional information on the structure of the most polar of the four hexamethyl cobyrinates, of the b-isomer 2b , was also obtained in the solid state from a single-crystal X-ray analysis. Earlier structural assignments based on 1D-NMR spectra of the corresponding regioisomeric monoamides 3b, d–f (obtained from crystalline samples of the monoacids 2b, d–f ) were confirmed by the present investigations.     

Structure and reactivity of α,β-unsaturated ethers. XIII. Cationic copolymerization and acid-catalyzed hydrolysis of 1,2-dialkoxyethylenes

The cationic copolymerizations of geometrical isomers of 1,2-dimethoxy- and 1,2-diethoxyethylenes with vinyl isobutyl ether as a reference monomer have been carried out in methylene chloride at ?70° using boron trifluoride etherate as catalyst. The kinetics of the acid-catalyzed hydrolysis of these ethers has also been investigated in 80% aqueous dioxane, in order to compare the results with the polymerizabilities. It has been found that the cis ethers are ca. four times as reactive as their trans isomers in both reactions. On the other hand, it has been proved that a β-alkoxyl substitution reduces the hydrolysis reactivity of vinyl alkyl ethers by a factor of ca. 10^?3 while it even enchances the cationic polymerizability. These contrasting results are interpretable from the nature of the transition states which are different for the two reactions.     

Cationic polymerization of β-pinene

β-Pinene was polymerized with cationic initiators in reactions that were carried out to study the solvent, temperature, initiator, and monomer-to-solvent ratio. Polymers obtained under different conditions were characterized by intrinsic viscosity, softening point, and gel permeation chromatography.     

Carbon Magnetic Resonance Spectra of α, β, γ, δ- and α, β, α′, β′- Unsaturated Ketones

Carbon-13 spectra of a series of 26 unsaturated ketones (ortho- and para-cyclo-hexadienones and corresponding open-chain analogues) have been measured by Fourier-transform. Pulse spectroscopy. A complete analysis has been achieved by means of double resonance experiments using noise-modulated and coherent off-resonance proton irradiation and with the aid of non-decoupled spectra. Chemical shifts are interpreted in terms of charge distribution in the dienone system and of methyl substituent effects. Carbon chemical shifts were also obtained for O-protonated ortho- and para-cyclohexadienones. One-bond and long-range carbon-proton and carbon-fluorine spin coupling constants are reported for several compounds.