Stereochemistry in cationic polymerization of alkenyl ethers. II. Formation of poly(ß-substituted vinyl ether)s with erythro-meso structures

The formation of polymers with erythro-meso structures, which could not be obtained from propenyl ethers with BF₃O(C₂H₅)₂, was studied by ¹³C-NMR spectroscopy on poly(ß-substituted vinyl ether)s obtained under a variety of conditions of polymerization. It was established that poly(cis-ethyl propenyl ether) obtained with Al₂(SO₄)₃–H₂SO₄ complex in toluene at 0°C was a highly stereoregular polymer with an erythro-meso structure. Cis-2-chlorovinyl ethyl ether and cis-methyl and ethyl butenyl ethers also yielded polymers with erythro-meso structures under the same conditions. In addition, with BF₃O(C₂H₅)₂ at ?78°C these three cis isomers produced amorphous polymers with threo-meso, racemic, and, in a few cases, erythro-meso structures, whereas cis-ethyl propenyl ether produced polymers with only threo-meso and racemic structures by the same catalyst. On the other hand, all trans isomers produced stereoregular polymers with threo-meso structures with BF₃O(C₂H₅)₂ at ?78°C, regardless of their ß-substituents; no erythro-meso structures were found in the polymers obtained.     

Stereoelective cationic polymerization of propenyl ether by asymmetric catalysts

In order to elucidate the possibility of stereoelective cationic polymerization (asymmetric selective polymerization) of olefinic monomers, racemic cis- and trans-1-methylpropyl propenyl ether and racemic 1-methylpropyl vinyl ether were polymerized by asymmetric alkoxyaluminum dichlorides. In the polymerization of racemic cis-1-methylpropyl propenyl ether with (?)-menthoxyaluminum dichloride in toluene at ?78°C, the polymer obtained showed a positive optical activity, and the residual monomers were converted by BF₃OEt₂ into a polymer having a negative optical activity. Thus, the stereoelective polymerization of racemic cis-1-methylpropyl propenyl ether was beyond any doubt attained in homogeneous cationic polymerization. In the polymerization of the trans isomer by the same catalyst, an optically active polymer was hardly formed. In the polymerization of racemic 1-methylpropyl vinyl ether which has no β-methyl group, stereoelectivity was not observed at all. The cis-1-methylpropyl propenyl ether did not produce an optical active polymer in the polymerization catalyzed by (S)-1-methylpropoxyaluminum dichloride or (S)-2-methylbutoxyaluminum dichloride under the same polymerization conditions.     

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.     

Stereochemistry in cationic polymerization of alkenyl ethers. I. 1H- and 13C-NMR spectra of poly(propenyl ethers)

Benzyl, ethyl, and deuterated ethyl propenyl ethers (BzPE, EPE, and EPE-d₅, respectively) with different cis/trans isomer ratios were polymerized by BF₃O(C₂H₅)₂ at ?78°C in toluene, methylene chloride, and nitroethane solvents. Poly(propenyl alcohol) [poly(PA)] was also prepared by treating poly(BzPE) with HBr gas. The steric structure of these polymers was studied by ¹H- and ¹³C-NMR spectroscopy. The poly(EPE-d₅) obtained from a trans-rich monomer mixture was crystalline, while one from a cis-rich monomer mixture was amorphous. Poly(BzPE) was always amorphous. Poly(BzPE), poly(PA), and poly(EPE-d₅) gave ¹H-NMR spectra with two β-methine peaks, relative intensities of which depended on the geometric structure of the monomers. Their spectra of β-methyl protons decoupled from β-methine proton were also split into two peaks, relative intensities of which corresponded to those of the two α-methine peaks. ¹³C-NMR spectra of α-OCH₂ and β-CH₃ of poly(EPE), respectively, consisted of two peaks. These observations show that the poly(propenyl ethers) examined, whether crystalline or amorphous, have only two dyad structures (probably threo or erythro-meso and racemic), and that their α- and β-carbons have the same dyad structure, irrespective of the kind of the alkoxyl groups.     

Cationic polymerization of α,β-disubstituted olefins. V. Reactivity of propenyl alkyl ethers in cationic polymerization

To elucidate the effect of the introduction of a methyl group in the β-position of a vinyl monomer, propenyl alkyl ethers were copolymerized with vinyl ethers having the same alkoxy group. Propenyl alkyl ethers with an unbranched alkoxy group (ethyl or n-butyl propenyl ether) were more reactive than the corresponding vinyl ethers. This behavior is quite different from that of β-methylstyrene derivatives. However, propenyl alkyl ethers with branched alkoxy groups at the α carbon atom (isopropyl or tert-butyl propenyl ether) were less reactive than the corresponding vinyl ethers. Also, cis- isomers were more reactive than the trans isomers, regardless of the kind of alkoxy group and the polarity of the solvent.     

Stereochemistry in cationic polymerization of alkenyl ethers. V. Stereochemistry of acetal additions as model reactions for the polymerization of alkenyl ethers

Acetal additions to β-substituted vinyl ethers having a variety of substituents (alkenyl ethers) were stereochemically investigated as model reactions for their cationic polymerization. The reactions catalyzed by BF₃O(C₂H₅)₂ in CH₂Cl₂ at O°C gave 1:1 adducts, the steric structure of which was determined by means of ¹³C-NMR spectroscopy. trans-Alkenyl ethers always gave adducts with a single structure stereospecifically, indicating that the intermediate carbocation attacks a trans-alkenyl ether from a definite direction independent of the bulkiness of substituents. On the other hand, cis-alkenyl ethers formed adducts with two steric structures, and the direction of cation addition was found to depend on the bulkiness of the alkoxy group involved. The above trends were in agreement with the results for poly(alkenyl ether)s and allowed detailed discussion of the stereochemistry of the propagation processes in alkenyl ether polymerizations.     

Cationic copolymerization of phenyl propenyl ethers

Ring-substituted phenyl propenyl ethers were found to form homopolymers without any rearrangement by metal halides. Phenyl propenyl ethers were less reactive than the corresponding phenyl vinyl ethers in cationic polymerization. In order to study the electronic effect of a substituent on the reactivity, cis-p-Cl,p-CH₃, and p-CH₃O-phenyl propenyl ethers were copolymerized with phenyl propenyl ether in methylene chloride at ?78°C with stannic chloride–trichloroacetic acid, and their ¹H- and ¹³C-NMR spectra were measured. The reaction constant ρ against Hammett σ_p was ?2.1. The cis-phenyl propenyl ethers were slightly more reactive than the corresponding trans isomers. On the other hand, an o-methyl group decreased the reactivity of phenyl propenyl ether. The low reactivity of o-methyl phenyl propenyl ether was attributed to the steric hindrance between the propagating carbocation and the monomer.     

Cationic polymerization of styrene by acetyl perchlorate. II. Steric structure of the polymer and nature of the propagating species

Cationic polymerization of styrene initiated by acetyl perchlorate in CH₂Cl₂ yields a polymer having a bimodal molecular weight distribution. The high molecular weight and the low molecular weight portions of the polymer were separated by thin-layer chromatography, and the steric structure of these separated polymers was investigated by ¹³C NMR spectra. The high molecular weight polymer had a larger racemic dyad content than the low molecular weight material. From the dependence of the steric structure of the polymer on the polarity of a solvent, it was estimated that the propagating species producing the high molecular weight material was a loose ion pair or a free ion, and that producing the high molecular weight material was a loose ion pair or a free ion, and that producing the low one was a nondissociated species.     

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.     

Stereospecific polymerization of α-methylstyrene by friedel-crafts catalysts

The relationship between stereoregularity and polymerization conditions of α-methylstyrene has been studied by means of NMR spectra. The effects of solvents and various Freidel-Crafts catalysts have been investigated. The stereoregularity of poly-α-methylstyrene increased with increased polymer solubility in the solvent used and with decreasing polymerization temperature. This behavior is completely different from the stereospecific polymerization of vinyl ethers and methyl methacrylate in homogeneous systems. This may be due to the strong steric repulsion exerted by the two substituents in the α-position of α-methylstyrene. For example, with BF₃ · O(C₂H₅)₂ as catalyst at ?78°C., atactic polymer is obtained in n-hexane, a nonsolvent for α-methylstyrene, whereas highly stereoregular polymer is produced in toluene or methylene chloride, good solvents for the polymer. However, the polarity of the solvent and the nature of the catalyst hardly affect the stereoregularity of the polymer.     

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.     

A proposal on the steric course of propagation in the homogeneous cationic polymerization of vinyl and related monomers

A stereochemical scheme of propagation was proposed for polymerizations of vinyl and related monomers by Friedel-Crafts catalysts. For the cationic propagation proceeding via the simple carbonium ion pair, the following two factors were considered to be of primary importance in determining the steric course of propagation: (1) the conformation of the last two units of the propagating polymer segment and the direction of approach of the incoming monomer; (2) the tightness of the growing ion pair. Thus, the front-side (less hindered site) attack to the carbonium ion gives rise to a syndiotactic placement and the back-side attack an isotactic placement. The present model can satisfactorily explain the effects of substituents, catalysts, polymerization media, and polymerization temperature on the steric structure of polymers in cationic polymerization of vinyl ethers. Extension of the scheme to polymerization of the β-substituted vinyl ethers in nonpolar solvents predicts formation of the diisotactic structures consistent with the experimental result. The influences of the polymerization condition on the steric structure of polymer were studied for cationic polymerizations of α-methylstyrene at low temperatures. Highly syndiotactic polymers were obtained for homogeneous reactions in toluene-rich media. The isotactic unit increased by increasing the content of methylcyclohexane in the solvent mixture. The effect of catalysts, though insignificant in toluene-rich media, was clearly noted in methylcyclohexane-rich media, less active catalysts (e.g., SnCl₄) yielding higher amounts of the isotactic unit than more active catalysts (e.g., AlCl₃). These results can be readily accommodated in the present model.     

Living polymerization of aryl substituted acetylenes by MoCl5 and WCl6 based initiators: The ortho phenyl substituent effect

The polymerization of phenylacetylene initiated by MoCl₅ and WCl₆ based initiators was monitored directly in the NMR sample tube and demonstrated the presence of backbiting and intramolecular cyclization reactions. It was shown that the ratio of cis to trans structural units obtained by isomerization prior to double bond formation dictates the degree of backbiting and intramolecular cyclization reactions. This cis–trans ratio determines the length of cis–transoidal sequences present in the polymer backbone which are available for both backbiting and intrachain cyclization reactions. The cyclic trimers obtained in the metathesis polymerization of phenylacetylene are formed only through the cis–cisoidal-induced backbiting and/or intramolecular reactions. The o-trimethylsilylphenylacetylene follows a living mechanism of polymerization. This is due to the fact that the size of the ortho substituent suppresses the cis–transoidal to cis–cisoidal isomerization reactions and therefore eliminates the backbiting reactions. The steric hindrance provided by the size of the ortho substituent also eliminates interchain and intrachain reactions.     

Stereochemie des Übergangszustandes aliphatischer CLAISEN-Umlagerungen. Vorläufige Mitteilung

The stereochemistry of the transition state in the CLAISEN rearrangement of crotyl propenyl ethers ( 2 ) has been established. By heating trans, cis-, cis, cis- and trans, trans-crotyl propenyl ether at 142,5°, erythro and threo 2, 3-dimethylpent-4-en-1-al ( 3 and 4 ) were obtained. From the ratio 3/4 it was shown that the rearrangement of the three ethers largely involves (97–98%) a chair-like transition state.     

Allyl chloride/AIC13/ether cationic initiating systems for polymerization of 1,3-pentadiene

Various ethers were used to mediate the polymerization of 1,3-pentadiene (PD) initiated by AlCl₃ and by allyl chloride (AllyCl)/AlCl₃. The introduction of the ethers exert considerable effects on polymer yield and molecular weight due to its interaction with the propagating carbocation. The carbocation reactivity is reduced by this interaction which is subject to the ether's nucleophilicity determined by the steric hindrance of groups adjacent to oxygen. The reduction of carbocation reactivity gives rise to a decrease of polymer yield owing to inhibition of propagation but results in an augmentation of molecular weight due to suppression of various side reactions such as terminations. By using suitably nucleophilic ethers such as diphenyl ether, the polymerization can be mediated to give an high molecular weight polymer in high yield.     

Highly active artificial potassium channels having record-high K+/Na+ selectivity of 20.1

Replicating extraordinarily high membrane transport selectivity of protein channels in artificial channel is a challenging task. In this work, we demonstrate that a strategic application of steric code-based social self-sorting offers a novel means to enhance ion transport selectivities of artificial ion channels, alongside with boosted ion transport activities. More specifically, two types of mutually compatible sterically bulky groups (benzo-crown ether and tert-butyl group) were appended onto a monopeptide-based scaffold, which can order the bulky groups onto the same side of a one-dimensionally aligned H-bonded structure. Strong steric repulsions among the same type of bulky groups (either benzo-crown ethers or tert-butyl groups), which are forced into proximity by H-bonds, favor the formation of hetero-oligomeric ensembles that carry an alternative arrangement of sterically compatible benzo-crown ethers and tert-butyl groups, rather than homo-oligomeric ensembles containing a single type of either benzo-crown ethers or tert-butyl groups. Coupled with side chain tuning, this social self-sorting strategy delivers highly active hetero-oligomeric K⁺-selective ion channel (5F12‧BF12)_n, displaying the highest K⁺/Na⁺ selectivity of 20.1 among artificial potassium channels and an excellent EC₅₀ value of 0.50 μmol/L (0.62 mol% relative to lipids) in terms of single channel concentration     

Polymerization of Cyclohexene Oxide By Organoaluminum Compounds

Abstract

The polymerization of cyclohexene oxide (CHO) by various organoaluminum compounds such as R₃Al (R = Me, Et, i-Bu), Et₂AlCl, and EtAlCl₂ is reported. Ethyl-substituted aluminum compounds were found to be very effective for this polymerization. As more chlorine atom is substituted on the aluminum atom, the polymer yield was increased though the molecular weight was decreased. The polymer yields at varying monomer-to-catalyst mole ratios (M/C) were similar (80–93%). The temperature and solvent effect for the present polymerization were also studied. The present poly(CHO) was a less stereoregular (atactic) isomer, regardless of catalysts and polymerization conditions. The resulting poly(CHO) was a white powder ad was soluble in aromatic and halogenated hydrocarbon solvents such as benzene, chlorobenzene, CCl₄, chloroform, etc.     

Living cationic polymerization of α‐methyl vinyl ethers using SnCl4

Cationic polymerization of α‐methyl vinyl ethers was examined using an IBEA‐Et_1.5AlCl_1.5/SnCl₄ initiating system in toluene in the presence of ethyl acetate at 0 ～ ?78 °C. 2‐Ethylhexyl 2‐propenyl ether (EHPE) had a higher reactivity, compared to corresponding vinyl ethers. But the resulting polymers had low molecular weights at 0 or ?50 °C. In contrast, the polymerization of EHPE at ?78 °C almost quantitatively proceeded, and the number‐average molecular weight (M_n) of the obtained polymers increased in direct proportion to the EHPE conversion with quite narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight ≤ 1.05). In monomer‐addition experiments, the M_n of the polymers shifted higher with low polydispersity as the polymerization proceeded, indicative of living polymerization. In the polymerization of methyl 2‐propenyl ether (MPE), the living‐like propagation also occurred under the reaction conditions similar to those for EHPE, but the elimination of the pendant methoxy groups was observed. The introduction of a more stable terminal group, quenched with sodium diethyl malonate, suppressed this decomposition, and the living polymerization proceeded. The glass transition temperature of the obtained poly(MPE) was 34 °C, which is much higher than that of the corresponding poly(vinyl ether). This poly(MPE) had solubility characteristics that differed from those of poly(vinyl ethers). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2202–2211, 2008     

Synthesis and polymerization of (E)‐p‐[(p‐methoxyphenyl)‐2‐ethenyl]phenylacetylene with Ziegler–Natta,rhodium, and palladium complexes

The synthesis and polymerization of (E)‐p‐[(p‐methoxyphenyl)‐2‐ethenyl]phenylacetylene was carried out with a homogeneous vanadium acetylacetonate/aluminum triethyl catalyst system, a bis(rhodium chloride cycloocta‐1,5‐diene) complex, and a palladium/trimethylsilyl complex. In all cases, the main fraction was a polymer with a stereoregular structure. The polymerization with the vanadium catalyst gave a polymer fraction in a low yield. The polymerization of (E)‐p‐[(p‐methoxyphenyl)‐2‐ethenyl]phenylacetylene with the soluble rhodium complex gave a polymer in a high yield. The soluble palladium/chlorotrimethylsilane complex gave a polymer in a good yield. On the basis of the spectroscopic data, the poly{(E)‐p‐[(p‐methoxyphenyl)‐2‐ethenyl]phenylacetylene)} obtained, in all cases, showed a cis–transoidal stereoregular structure. The molecular mass of poly{(E)‐p‐[(p‐methoxyphenyl)‐2‐ethenyl]phenylacetylene)} was determined by the matrix‐assisted laser desorption/ionization time‐of‐flight technique. The kinetics of the reaction were analyzed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6438–6444, 2005     

Stereochemistry of photoinitiated emulsion polymerization

The tacticity of poly(methyl methacrylate) (PMMA) samples made by emulsion polymerization (EP) has been determined by NMR spectroscopy at higher resolution than any previous study. When photoinitiation is employed at room temperature, the polymer obtained is highly syndiotactic and more stereoregular than that obtained in homogeneous radical polymerization under the same conditions. The percentage of racemic dyads and the length of racemic sequences vary significantly with the degree of conversion and the temperature, but not with variation of the hydrophobic or the hydrophilic part of the detergent molecules. A magnetic field of 5 kG or less accelerates the polymerization reaction and increases the polymer molecular weight when dibenzyl ketone is employed as oil-soluble photoinitiator, and has an indirect influence on the polymer tacticity. From measurements of the temperature dependence of the polymer steric composition, the difference of activation enthalphy and entropy of the meso and racemic additions have been calculated. The values do not follow the correlations from precedent literature for radical polymerization in solution if the reaction is considered a first-order Markov process. The persistence ratio p depends on the detergent and the temperature of the EP; in some cases its difference from unity exceeds the experimental error. These results are interpreted as a conformational effect of the locus of the polymerization in the first stage of the EP (when micelles are present) that favors the racemic addition more than is expected in a homologous solvent such as an alkane, e.g., at the same temperature.