Polymerization of monolayers. VI. Influence of the nature of the exchangeable ion on the tacticity of insertion poly(methyl methacrylate)

The influence of the nature of exchangeable cations on the tacticity of poly(methyl methacrylate) (PMMA) prepared from montmorillonite–MMA adsorption complexes was investigated. The strength of ion-dipole interactions was estimated either by the value of the electrostatic interaction potential or by the value of the carbonyl-stretching frequency shift. It was shown that P_i, the percentage of isotactic triads in the polymer, increases with increasing strength of the ion-dipole interactions, while the persistence ratio and the mean length of closed isotactic (or syndiotactic) sequences remain essentially unchanged regardless of the nature of the cation. It was shown that values of P_i may vary from 0.15 for weakly interacting ions to 0.5 for strongly interacting ones, reaching at the upper limit the value calculated from the model proposed in a previous paper, thus confirming its validity.     

The stereoregularity of polyacrylonitrile and its dependence on polymerization temperature

The stereoregularity of polyacrylonitrile was studied by NMR spectroscopy. The methylene protons in isotactic configuration are equivalent in dimethylformamide-d₇ solution and are nonequivalent in dimethylsulfoxide-d₆ solution. In the case of the latter solution the difference in chemical shift between the isotactic methylene protons is 6.6 cps. The stereoregularity of polyacrylonitrile-α-d instead of polyacrylonitrile was determined on the dimethylformamide-d₇ solution. In the radical polymerization all the polyacrylonitrile-α-d's that polymerized at temperatures between 80 and ?78°C. have a configuration consisting of about 50% of isotactic diads and, accordingly, the stereoregularity of polyacrylonitrile does not depend upon the polymerization temperature. Analysis of the NMR spectra of isotactic polyacrylonitrile prepared from acrylonitrile–urea canal complex was also carried out. The NMR spectra of meso- and dl-2,4-dicyanopentanes, dimer models of isotactic and syndiotactic polyacrylonitriles, respectively, were analyzed by a computer program proposed by Bothner-By.     

Selective Substitution Reactions on PVC. Lability of Some “Normal” Structures

The nucleophilic substitution reaction on PVC with sodium thiophenate in cyclohexanone solution is studied. The reaction appears to be S_N2 substitution and it involves three different steps with decreasing reaction constants. By using ¹³C-NMR spectroscopy, the fast step is proposed to be due to the high reactivity of the central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads. The study of the thermal degradation of the modified samples shows that during the fast step of substitution a stabilizing effect occurs, and that as soon as the first step is over, the polymer stability decreases markedly. These effects are accompanied by changes in poly-ene distribution as proved by UV-VIS absorption spectroscopy study on degraded samples. The results, as discussed on the basis of the possible conformations of triads in PVC, suggest that some chlorine atoms in both the GTTG isotactic and the TTTG heterotactic triads should be considered as labile structures in PVC. It follows from the results obtained that on the one hand the initiation of degradation may occur by normal structures, and on the other the build-up of polyenes is favored by the presence of …TTTT… sequences. Both features contributing to a better understanding of the degradation of PVC.     

Configurational and conformational control of chemical modification and thermal degradation of poly(vinyl chloride)

In the light of some earlier works on nucleophilic substitution on poly(vinyl chloride) (PVC) in solution, a conformational mechanism is proposed. It considers the TT isotactic diad conformation to be the only reactive species and the reaction to be controlled by the conformational equilibria that make such conformation available. As a result all the isotactic and the heterotactic triads are capable of reacting provided that they adopt the GTTG⁻ and the GTTT conformation, respectively. Since the replacement of a definite fraction of isotactic triads, which are assumed to be of the GTTG⁻ conformation, results in an enhanced thermal and photochemical stability, the lability of some chlorines at such triads is proved. Further arguments in favour of the conformational mechanism are afforded through recent results of i) substitution studies in the melt and in aqueous suspension with phase transfer catalysts, ii) accurate ¹³C NMR measurements of triad variation with degree of substitution.     

Molecular conformation of optically active polyesters and their stereocomplexes

Crystal lattices for various polymorphs of poly(α-methly-α-ethyl-β-propiolactone) (PMEPL) are proposed on the basis of combined electron and x-ray diffraction data. Single crystal-like electron diffraction is observed from melt crystallized thin films of isotactic PMEPL and stereocomplex. Melt-crystallized isotactic PMEPL exhibits a monoclinic, pseudo orthorhombic lattice with the unit cell parameter c (4.75 Å) equivalent to the periodicity of the planar zigzag conformation typically obtained by sample elongation. The stereocomplex crystallizes in a distinctly different orthorhombic lattice with c = 7.1 Å, implying a 2₁ helical conformation. In addition, melt-crystallized mixtures of isotactic PMEPL and isotactic poly(α-methyl-α-n-propyl-β-propiolactone) (PMPPL) are examined by solid state ¹³C nuclear magnetic resonance spectroscopy and x-ray diffraction. These techniques reveal that, in such blends, the conformation and crystal structure found for isotactic PMEPL of a given chirality depends on the absolute configuration of the PMPPL present. In samples containing PMEPL and PMPPL of opposite absolute configuration, isotactic PMEPL crystallizes from the melt as a helix, in contrast to its behavior in the pure state, whereas PMPPL adopt the same helical conformation in the blend as when crystallized alone.     

Triad tacticity of polyacrylonitrile

The high-resolution NMR spectra of polyacrylonitrile-β,β-d₂ prepared by radical polymerization were determined, and the stereoregularity of the polymer was studied. The NMR spectra of methine protons of polyacrylonitrile-β,β-d₂ in dimethyl sulfoxide-d₆ and a mixture of nitromethane-d₃ and ethylene carbonate showed three partially resolved multiplets. The deuterium-decoupled spectra of the polymer were measured, and three well resolved peaks were observed in the two solvents and dimethylformamide-d₇. These three peaks were analyzed by comparison with the NMR spectra of model compounds and polyacrylonitrile-α-d, and they were assigned to isotactic, heterotactic, and syndiotactic triads with decreasing magnetic field. This order seems to be unchanged in other solvents. Triad stereoregularity of the polymer was determined according to the assignment. Polymerizations of acrylonitrile-β,β-d₂ by radical initiators between ?78°C and 60°C were explained by the Bernoulli trial propagation step. The polymers had an atactic structure, independent of polymerization temperature. This shows that in free-radical polymerization of acrylonitrile, the chain end is not represented as having any particular stereochemistry. Other stereochemical control is necessary to produce tactic polymers. The triad tacticity of isotactic polyacrylonitrile was also determined.     

Correlation between the tacticity and content of labile nondefect structures in poly(vinyl chloride)

The content of mm isotactic triads taking the GTTG^? conformation for samples of poly(vinyl chloride) (PVC) fractions of different tacticities was measured through a substitution reaction with sodium benzenethiolate. This quantity changed linearly with the ratio of rmmmrx to mmmmrx (x = m or r), as accurately determined by ¹³C NMR spectroscopy. In a comparison of this correlation and that obtained between the thermal degradation stability and overall isotactic content, as studied previously, some novel evidence for the GTTG^? conformation of a few mm triads, termini of isotactic sequences no shorter than one heptad as specific labile sites in PVC, was obtained. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3944–3949, 2002     

NMR study of poly-p-isopropyl-α-methylstyrene

220 MHz NMR spectra of sample of poly-p-isopropyl-α-methylstyrene prepared through anionic (A) and cationic (B) polymerizations are studied. Peaks at τ values of 9.07, 9.41, and 9.71 are assigned to isotactic (i), heterotactic (h), and syndiotactic (s) α-methyl triads, respectively. From the α-methyl triads and the β-proton tetrads it is found that polymer A deviates little from Bernoullian statistics and that first-order Markov configurational statistics applies to polymer B. Isopropyl methine and methyl proton resonances are also analyzed in terms of the configurational statistics of the polymer. Resonances of the phenyl protons are assigned with increasing field to s, h, i meta and ortho protons.     

Study on the chain conformation of polymers.: II. Conformational characteristics of 1,2-polybutadiene and its characteristic ratio

In this work, improvement was made for conformational analysis of 1,2-polybutadiene by means of the molecular mechanics force field program (MM2). Thus the first and the second order interaction energies obtained are much more reasonable. The characteristic ratio of isotactic 1,2-polybutadiene was emphasized properly, which varied with energy E (E = E_η + E_ω″). In the same time, the characteristic ratios for isotactic chain of polystyrene and so on were noticed, which varied with energy E according to the same way as that for isotactic 1,2-polybutadiene. It is shown that there is a general rule or a common feature for isotactic chains of poly -α-olefine and vinyl polymer.     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Stereoregularity of poly(2-vinylpyridine) determined by 1H-NMR and 13C-NMR spectroscopy

In order to determine the stereoregularity of poly(2-vinylpyridine), 2-vinylpyridine-β,β-d₂ was synthesized. The ¹H-NMR spectra of the deuterated polymer in D₂SO₄ and o-dichlorobenzene solutions showed three peaks, which were assigned to triad tacticities. Since the absorptions of heterotactic and syndiotactic triads of methine protons overlap those of methylene protons in nondeuterated polymers, only isotactic triad intensities can be obtained from the ¹H-NMR spectra of nondeuterated poly(2-vinylpyridine). The ¹³C-NMR spectra of poly(2-vinylpyridine) were obtained in methanol and sulfuric acid solutions. In methanol solution the absorption was split into three groups, which cannot be explained by triads, and in sulfuric acid solution several peaks were observed. These splittings may be due to pentad tacticity. The results show that poly(2-vinylpyridine) obtained by radical polymerization is an atactic polymer.     

Polymerization of monolayers. V. Tacticity of the insertion poly(methyl methacrylate)

Insertion poly(methyl methacrylate) (PMMA) formed within a monolayer of monomer adsorbed on montmorillonite was studied by means of NMR spectroscopy and shown to be composed of short stereosequences with a predominant isotactic component. The stereospecificity of the insertion PMMA can be understood in terms of the organization of the monomer adsorbed on the surface of the mineral. The monolayer of monomer is assumed to be composed of monads and isotactic diads due to dipole-ion interactions between the molecules of methyl methacrylate and exchangeable cations. Results of experiments relating factors such as temperature, density of ion population of the surface, and the nature of ions are discussed. It is shown that the populations of isotactic and syndiotactic triads P_i and P_s determined experimentally in the insertion PMMA are in a reasonable agreement with values calculated from the model. It is shown, furthermore, that P_i increases with increase in the density of the ion population, in agreement with the equations derived from the model.     

Stereoselective substitution on PVC using phase transfer catalysts

The nucleophilic substitution with sodium benzenethiolate in aqueous suspension in the presence of a phase-transfer catalyst has been studied for two samples of PVC of different tacticities. The kinetics shows two well-defined periods: one period is very fast and involves low conversions that are greater as the isotactic content of PVC increases, while another steady period is slow and its slope seems to depend on physical features, e.g., molecular weight and crystallinity, which would affect the accessibility of nucleophile. The evolution of unreacted syndio, iso, and heterotactic triad contents with degree of substitution has been followed by ¹³C-NMR spectroscopy. Since the content of syndiotactic triads does remain unreacted at the end of the reaction and that of isotactic and heterotactic triad decreases, but at a different rate, the reaction is proved to proceed by a stereospecifically controlled mechanism. The influence of degree of substitution on both the thermal degradation rate and the evolution of UV–Visible spectra of equally degraded samples has been also studied; the highest stability in terms of degradation rate and content of short polyenes is obtained when a specific fraction of isotactic triads has reacted. From the correlation between the results of this work and those previously obtained in solution, some enlightening conclusions about the reaction mechanism are drawn.     

13C nuclear magnetic resonance spectra. XV—the γ-anti substituent effect and its dependence on substitution at the α- and γ-carbon atom

It is shown that the γ_α effects of hetero substituents on the ¹³C chemical shift (γ_α SCS) are enlarged by +2 to +5 ppm by substituting the α-hydrogen atom by any group or atom (e.g. CH₃, OR, F, Cl, Br). The same is encountered when the axial γ-hydrogen is replaced by CH₃, OH or F. If, however, the substituting atom at the γ_α-carbon atom is a higher-row halogen (Cl or Br), diamagnetic γ_α SCS for this signal are observed which may even exceed those for unsubstituted γ_α-carbon atoms. The removal of a 1,3-diaxial hydrogen-hydrogen interaction and the existence of a still unspecified ‘heavy halogen effect’–both diamagnetic contributions to the γ_γ SCS of hetero substituents–are responsible for these findings. Methyl groups do not behave like hetero substituents with respect to the γ_α SCS.     

~(13)C-NMR研究高全同立构聚丙烯的立构缺陷

本文应用γ效应以及Suter-Flory的聚丙烯构象内旋转模型,计算了一组两端全部为全同2单元组(meso),中间为11单元组不同立构缺陷的高全同聚丙烯(IPP)分子链中心甲基碳的相对化学位移。将其与67.8MHz ~(13)C-NMR观测值比较,说明IPP中主要缺陷是孤立的rr,此外还有长间同序列的分子链混合,而不是孤立的rrrr缺陷分布在IPP分子链上。     

Stereoregular poly(methacrylic acids)

A convenient method is described for the preparation of isotactic and syndiotactic poly(trimethylsilyl methacrylates) by using the monomer trimethyl silyl methacrylate and butyllithium initiation in toluene and tetrahydrofuran, respectively. The structure of these polymers enables complete hydrolysis to the corresponding poly(methacrylic acids), which were characterized with respect to tacticity and molecular weight. The asymmetric induction in toluene produced 89% isotactic polymer, while that in tetrahydrofuran gave polymer <90% syndiotactic and heterotactic in terms of triads. A method of fractionation of the polyelectrolytes by gel-permeation chromatography on a preparative scale was shown to be applicable.     

The grafting of luminescent side groups onto poly(vinyl chloride) and the identification of local structural features

The phosphorescence and fluorescence emitted by both the chromophores existing in raw PVC (allylic and ketoallylic structures and polyenes) and luminescent probes introduced by substitution of chlorine atoms have been used to study local features of the chain microstructure, namely the formation of blocky sequences during the nucleophilic substitution of chlorine atoms and the length and nature of the chain segments relaxing below T_g, i.e., the β relaxation. The comparison with results obtained by dynamic mechanical analysis of some of the copolymers has been carried out. Blocky sequences are found to exist and all isotactic sequences over a diad together with their heterotactic associated triads are found to participate in the β motions.     

Local chain‐configuration dependence of the mechanisms of the chemical reaction of poly(vinyl chloride). IX. Novel results on stereoselectivity of chlorination reaction

A poly(vinyl chloride) (PVC) sample was chlorinated in solution in the presence of 2,2′‐azobisisobutyronitrile and by the fluid‐bed method. The aim was to evaluate the scope of the stereoselectivity of the chlorination reaction. The quantitative microstructural analysis of the residual PVC with a degree of chlorination was followed by ¹³C NMR spectroscopy. From the evolution of the content of isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads and of mmmm, mmmr, and rmmr isotactic pentads in the unchlorinated parts of the polymer, it was unambiguously inferred that the chlorination reaction proceeds by a stereoselective mechanism in that the mr heterotactic triads are the most reactive structures followed by the isotactic triad at mmmr and rmmr pentads. This conclusion was confirmed on the basis of the Fourier transform infrared results. The results provide valuable information regarding the effect of tacticity and related local conformations in the chemical reactions of PVC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 508–519, 2003     

Local chain configuration dependence of the mechanisms of analogous reactions of PVC. I. A conclusive study of the microstructure evolution in SN2 nucleophilic substitution

The nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium benzenethiolate has been studied in two kinds of solvent differing in the molecular structure in the vicinity of the carbonyl group. From the evolution of the content of isotactic ( mm ), heterotactic ( mr ), and syndiotactic ( rr ) triads; and of mmmm, mmmr and rmmr isotactic pentads, in the unmodified parts of the polymer, as followed by ¹³C-NMR, it is unambiguously inferred that any chlorine but the central one of either the isotactic triad at mmr tetrads or the heterotactic triad at rmrr pentads is unreactive. Only a small fraction of mmr tetrads reacts occasionally by the central chlorine of its mr triad instead of the mm . Of those structures, the mmr , especially when located at the end of long isotactic sequences, proves to be highly reactive compared to the rmrr structure. By comparing quantitatively the microstructural changes with degree of substitution and taking into account that the reaction is of S_N2 type, the mechanisms of substitution through the three foregoing reactive chlorines have been stated. They are found to be independent of the type of solvent and to account for all the changes in triad and pentad content as experimentally found. Instead, the solvent dependence of the ratio between the mmr - and rmrr -based processes of substitution is such that the depletion of mmr compared to that of rmrr structure may be controlled. The conformational sensitivity of this behavior is discussed on the basis of side work in our laboratory. As a whole, the results of the present work provide some original concepts as to the role of the tacticity dependent microstructure and the related local conformations in the chemical reactions of PVC. © 1996 John Wiley & Sons, Inc.     

Stereoregularity of poly(methyl acrylate)

The stereoregularity of poly(methyl acrylate) and poly(methyl acrylate-αd) was determined from the NMR spectra. A method of quantitative determination of stereoregularity of poly(methyl acrylate) proposed in this paper is based on the fact that in the 100 Mc./sec. NMR spectrum the absorption peaks due to methylene protons in syndiotactic configurations overlap absorptions due to only one of two methylene protons in isotactic configurations. The stereostructure of poly(methy1 acrylates) polymerized with anionic catalysts such as Grignard reagents, n-butyllithium, and LiAlH₄ is generally richer in isotactic diads than in syndiotactic diads. For example, poly(methyl acrylate) polymerized with phenylmagnesium bromide as catalyst at ?20°C. consists of 99% isotactic and 1% syndiotactic diads. In radical polymerization, the isotacticity of poly(methyl acrylate) is independent of polymerization temperature. Poly(methyl acrylates) polymerized with a Ziegler-Natta catalyst consisting of Al(C₂H₅)₂Cl and VCl₄ have configurations similar to those polymerized by radical initiators. The stereoregularity of poly(methyl acrylate-α-d) resembled that of poly(methyl acrylate) polymerized under the same conditions.