Studies on propagating species in cationic polymerization of styrene derivatives by acetyl perchlorate or iodine. IV. Common-ion salt effect on the polymerization rate and the steric structure of the polymer obtained by acetyl perchlorate

Effects of a common-ion salt, n-Bu₄NClO₄, on the cationic polymerization of styrene and p-chlorostyrene by acetyl perchlorate were studied in a variety of solvents at 0°C. In polymerization (in CH₂Cl₂) which yielded polymers with a bimodal molecular weight distribution (MWD), addition of the salt suppressed the formation of higher polymers, but affected neither the molecular weight nor the steric structure of the lower polymers. The polymerization rate decreased with increasing salt concentration and became constant at or above a certain concentration. In nitrobenzene, on the other hand, the MWD of the polymers was unimodal and steric structure was unchanged even in the presence of salt at a concentration 50 times that of the catalyst. However, the polymerization rate and the polymer molecular weight decreased monotonically as salt concentration increased. On the basis of these results, it was concluded that the ion pair in methylene chloride differs from that in nitrobenzene, and that the species in the latter solvent is similar in nature to free ions. The fractional contribution of the dissociated and nondissociated propagating species to polymer formation was determined from the rate depression caused by addition of the salt.     

Modification of properties of ion-exchange membranes. IV. Change of transport properties of cation-exchange membranes by various polyelectrolytes

A change in electrodialytic transport properties of various cation exchange membranes was observed after the membranes had been adsorbed or ion-exchanged with various cationic polyelectrolytes. Transport properties measured in this report were the relative transport number of calcium ion to sodium ion P^Ca_Na, the current efficiency of cations, and the electric resistance of the membrane during the electrodialysis. Weakly basic and strongly basic cationic polyelectrolytes of various molecular weights were used. Though P^Ca_Na of any cation exchange membrane decreased by the adsorption or ion exchange of any cationic polyelectrolyte, the degree of the decrease in P^Ca_Na changed with species and molecular weight of polyelectrolytes and species of cation-exchange membranes. Weakly basic polyelectrolytes and low molecular weight, strongly basic polyelectrolytes were effective in producing a marked decrease in P^Na_Ca of any cation exchange membrane. The effect of strongly basic polyelectrolytes of high molecular weight on P^Ca_Na was weak in most cases. However, if it was possible to make the polyelectrolyte adhere to the surface of the membrane to form a compact coiled structure, any cationic polyelectrolyte was effective in producing a remarkable decrease in P^Ca_Na of any cation-exchange membrane.     

Effect of polarity of the medium on the stereospecific polymerization of propylene by ansa-zirconocene catalyst

Propylene was polymerized with rac-ethylene-bis (1-η⁵-indenyl)dichlorozirconium/methylaluminoxane in solvents of different polarity. The poly (propylene) formed was separated by solvent extraction; ¹³C-NMR and DSC measurements were made on the polymer fractions. The poly(propylene) in each solvent fraction has its characteristic molecular weight steric pentad distributions, melting transition temperature, and enthalpy for fusion irrespective of the polymerization medium. The results suggest that the medium dielectric constant does not affect the polymerization rate or the intrinsic stereoselectivity, propagation and chain transfer rates a given catalytic species but can alter the occurrence of steric insertion errors through shifting of distributions of the propagating species producing poly(propylenes) of different stereoregularities. © 1994 John Wiley & Sons, Inc.     

Use of gel-permeation chromatography in the determination of the composition of two-polymer mixtures

The possibility of evaluating with acceptable accuracy the composition of a two-polymer mixture which is well separated by GPC, was studied by using mixtures of high molecular weight polybutadiene (M?_w = 4.5 × 10⁵) and low molecular weight polyiso-butylene (M?_n in the range of 10³). It was concluded that a satisfactory evaluation of the composition of a polymer mixture can be achieved, provided that the variations of the refractive index with the molecular weight are taken into account for the low molecular weight polymer (the polyisobutylene).     

Adsorption behavior of end-functionalized polymers modified by iminium ion

In order to clarify the end-functionalization effect of polymers modified by iminium ion, a model compound with a modified chain end was prepared by directly reactingn-butyllithium withN-methyl-2-pyrrolidone and then quenching by the addition of methanol. It is shown that the model compound includes the iminium ion group and that the adsorption bands in the IR-spectra of the chemically modified polymer were similar to those of model compoundThe adsorption behavior of end-functionalized polystyrene(PS-X) and diblock copolymer of PS and polybutadiene(PB) which were modified by iminium ion(PS-PB-X), were examined over a wide range of molecular weights of these polymers. It is evident that the amount of adsorption increased significantly by the end-functionalization and this trend was especially strong in the low molecular weight polymers (M _w<-10⁵). The preference for the end-functionalized polymer was also examined by competitive and sequential adsorption experiments between different molecular weight species. It is shown that a prefered adsorption of PS-X with low molecular mass is pronouced over the adsorption of PS or PS-X with high molecular weight and in the combination of high and low molecular weight species of PS-X, a special enhancement of total adsorption can be observed. This enhancement is based on a combination of different adsorption layers formed by these two molecular weight species.     

Influence de la solvatation par le tert-butanol sur la structure et la reactivite de l'enolate de potassium de l'acetylacetate d'ethyle en presence de couronne ou de cryptand. etude par spectrometrie infra-rouge et cinetique

The structure and the nucleophilic reactivity of crowned (18-crown-6) or cryptated {cryptand (2.2.2)} potassium ethyl acetoacetate enolate have been compared in tert-butanol and in DME (or THF). In the protic solvent tert-butanol, the crowned and the cryptated potassium enolate species both exist as loose ion pairs in which the enolate anion, strongly hydrogen-bonded to the solvent, is in a “transoid” (non chelating) conformation. Both species show similar reactivities towards alkylating agents but completely different reactivities are observed in aprotic weakly dissociating media (THF, DME). In contrast to what is observed in tert-butanol, the cryptated species and the crowned species have very different nucleophilic reactivities in THF or DME; in those solvents only the cryptated species retains a loose ion pair structure; the crowned species is a contact ion pair in which the enolate anion chelates the potassium cation. The solvation of this crowned chelate species by tert-butanol has been demonstrated in binary mixtures of solvents (C₆D₆-t-BuOH, THF-t-BuOH). The oxygen basicity of the enolate anion is very different in the crowned chelated ion pair compared with the cryptand separated ion pair.     

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) of polyisoprenes

Polyisoprenes (PIPs) with average molecular weights from 650 to 800,000 Da have been studied by time-of-flight secondary ion mass Spectrometry (TOF-SIMS) in the static mode. Polymer samples were bombarded by argon primary ions, and positive SIMS spectra were collected. Effects of branching and unsaturation in the polymer structure on ion formation were studied. The pendant methyl group showed little tendency to fracture as a cation. In the low mass region, C_nH _2n–1 ⁺ appeared to be more intense than C_nH _2n+1 ⁺ , attributed to the double bond structure of polyisoprene. Additionally, ion formation varied as a function of polymer molecular weight. Cationized intact oligomers and fragments dominate the high mass region. Oligomer distributions were used to calculate average molecular weights for polyisoprenes. A statistical chain scission mechanism was used to qualitatively explain the formation of five clusters within a unique fragmentation pattern. Detailed studies of the cluster structure pointed out that each cluster contained several species having varied degrees of unsaturation. It is believed that double bond rearrangements occur.     

Polymerization of Dioxolane by Triethyloxonium Hexafluorophosphate

Abstract

The polymerization of dioxolane by triethyloxonium hexafluorophosphate in methylene chloride has been studied with a view to determine the nature of the active center. NMR studies of solutions of the initiator with low ratios of monomer led to little reaction of the initiator over long times. Analysis of normal reaction mixtures showed that only a small amount of the initiator was consumed during the reaction. The polymer was studied by GPC, UV spectroscopy, and NMR. The polymer appears to consist largely of high molecular weight material with M_w/M_n less than 2, and also low molecular weight material perhaps formed by a different mechanism. The high molecular weight material appears not to have been formed by a simple linear trialkyloxonium ion, from end-group studies, and it is suggested the active center is a secondary oxonium ion on a large cyclic polymer.     

Single-center vs. multi-center post-metallocene catalysts for propylene polymerization

The structural characteristics of polypropylene samples prepared with two post-metallocene catalysts based on complexes bis-{M-(3,5-di-tert-butyl-salicylidene)-4-[bis-(5-methyl-2-furyl)methyl]aniline}titanium dichloride and [(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(pentafluorophenyl)-1,3-dioxalan-4,4-dimethanol)titanium dichloride are investigated by GPC, ¹³C NMR, IR, DSC, and XRD methods. A combination of the first complex and MAO forms a single-center catalyst which polymerizes propylene to a nearly perfectly atactic polymer. A combination of the second complex and MAO forms a multi-center catalyst system producing polymer mixtures with broad molecular weight distributions containing five to six Flory components with different average molecular weights. Relative contents of the Flory components strongly depend on the type of solvent in the polymerization reactions. Some of the active centers produce high molecular weight, highly isotactic crystalline material with the melting point over 154 °C. The nature of steric errors in these polymer fractions (determined by ¹³C NMR) can be explained by a variant of stereocontrol similar to that exerted by metallocene catalysts of the C₁ symmetry.     

A DFT Quantum‐Chemical Study of Ion‐Pair Formation for the Catalyst Cp2ZrMe2 /MAO

A process of ion‐pair formation in the system Cp₂ZrMe₂/methylaluminoxane (MAO) has been studied by means of density functional theory quantum‐chemical calculations for MAOs with different structures and reactive sites. An interaction of Cp₂ZrMe₂ with a MAO of the composition (AlMeO)₆ results in the formation of a stable molecular complex of the type Al₅Me₆O₅Al(Me)O–Zr(Me)Cp₂ with an equilibrium distance r(Zr–O) of 2.15 Å. The interaction of Cp₂ZrMe₂ with “true” MAO of the composition (Al₈Me₁₂O₆) proceeds with a tri‐coordinated aluminum atom in the active site (OAlMe₂) and yields the strongly polarized molecular complex or the μ‐Me‐bridged contact ion pair ( d ) [Cp₂(Me)Zr(μMe)Al≡MAO] with the distances r(Zr–μMe) = 2.38 Å and r(Al–μMe) = 2.28 Å. The following interaction of the μ‐Me contact ion pair ( d ) with AlMe₃ results in a formation of the trimethylaluminum (TMA)‐separated ion pair ( e ) [Cp₂Zr(μMe)₂AlMe₂]⁺–[MeMAO]^– with r[Zr–(MeMAO)] equal to 4.58 Å. The calculated composition and structure of ion pairs ( d ) and ( e ) are consistent with the ¹³C NMR data for the species detected in the Cp₂ZrMe₂/MAO system. An interaction of the TMA‐separated ion pair ( e ) with ethylene results in the substitution of AlMe₃ by C₂H₄ in a cationic part of the ion pair ( e ), and the following ethylene insertion into the Zr–Me bond. This reaction leads to formation of ion pair ( f ) of the composition [Cp₂ZrCH₂CH₂CH₃]⁺–[Me‐MAO]^– named as the propyl‐separated ion pair. Ion pair ( f ) exhibits distance r[Zr–(MeMAO)] = 3.88 Å and strong C_γ‐agostic interaction of the propyl group with the Zr atom. We suppose this propyl‐separated ion pair ( f ) to be an active center for olefin polymerization.     

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.     

Radiation-Induced Polymerization of Urea Canal Complex. Part 4. Radiation-Induced Polymerization of Acrylonitrile in Urea-Ethylene Glycol Supercooled Liquid System

Abstract

Using elementary analysis, NMR on ^{3 1}P and ¹H nuclei, and electroconductivity methods, the acrylonitrile, methacrylonitrile, formaldehyde, and β-propiolactone anionic polymerization in the presence of triethylphosphine is shown to follow the macrozwitterion mechanism: quartary phosphonium being on one end of a polymer chain and the growing anion on the other. The number of covalent bonds through the whole polymer chain between charges forming the active center increases with the propagation reaction. The active centers stationary concentration in the system is low when connected with both the slow initiation reaction and with the fast active centers termination reaction. Thus the ion interaction of different growing polymer chains can be ignored. The active centers parts occurring in the form of ion pairs (the ends are near and form the “cyclic”) and of free ions (the ends are separated) are determined by the monomolecular equilibrium, and its constant depends upon the macro-zwitterion polymerization degree K_d ⁽ⁿ⁾ = K_d ^(I)n^3/2. Such constant depends upon the chain length affords the macrozwitterion self-accelerated propagation with its length, as the free ion reactivity is more than that of ion pairs. The self-accelerated chain propagation effect shows up as an increase of polymerization initial rate order and polymer molecular weight in the monomer concentration. This effect can be avoided by the introduction of electrolyte into the system, which dissociates into ions and transforms all cyclic ion pairs into the linear form, the latter dissociating independently of chain length. The strict mathematical analysis of stationary and nonstationary polymerization kinetics made it possible to determine all the elementary constants separately: K_i = 5.6 × 10^?4 liters/ (mole) (min); K_- = 2.5 × 10⁴ liter/ (mole) (min); K_± = 2.0 liters/ (mole) (min); K_t = 0.84/min; K_t ¹ = 4/min; K_d ^(I) = 10^?4; K₃ = 0.07 × 10^?4 mole/liter.     

Mechanism of nucleophilic fluorination promoted by bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene

Theoretical calculations were performed to elucidate the ability of the recently reported bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) molecule to promote nucleophilic fluorination of alkyl mesylates with cesium fluoride reagent. It was found that a similar structure, named BACCAt, can separate the cesium fluoride ion pair in tert‐butanol solution. This separation has a free energy cost, even considering the double hydrogen bonds with the fluoride ion. The solvent has an important effect on the stabilization of this complex, due to interaction with the high dipole moment of the separated ion pair. The observed rate acceleration effect involves a structure with double hydrogen bonds between the BACCAt and the centers of negative charges of the S_N2 transition state. The predicted free energy barrier of 27.3 kcal mol⁻¹ is in excellent agreement with the estimated experimental value of 26.2 kcal mol⁻¹.     

Characterization of acrylic dental polymers

The chemical structure and the molecular parameters of four dental acrylic polymer materials (samples P-1 to P-4) and two polyacrylic acids of different molecular weight (relative molecular mass) used as model compounds (samples Paa-1 and Paa-2) were studied and correlated with polymer structure and molecular weight. All polymer samples show low molecular weights, MW, and broad poly-dispersity as obtained by GPC. Samples P-3 and P-4 show the lower MW and bi-modal distribution, one peak corresponding to the polymer and the other to a low molecular weight compound at a lower concentration. The other polymer samples show unimodal distribution. Initially, all samples were soluble in water and dioxane above 99.8%. However, after lyophilization at −50 °C they showed different degrees of solubility because of partial gelation. The FTIR and, ¹H and ¹³C-NMR spectra of Paa-1, Paa-2 in D₂O show the pattern characteristic of poly(acrylic acid). The polymers of P-1 and P-2 are mainly poly(acrylic acid). The P-3 spectra show the peak pattern for an (acrylic acid/methyl acrylate) copolymer of about 2:1 composition as calculated from the NMR spectra. The P-4 is an oligomer derived from 2-hydroxyethyl methacrylate. Solid ¹³C-NMR spectra confirm the above structures and evidence anhydride formation after lyophilization. The MW and the linear expansion coefficient, α, were derived from intrinsic viscosity in theta and perturbed conditions. From this, the steric hindrance parameter, A, the molecular stiffness, σ, and the second virial coefficient, A₂, were calculated using different thermodynamic models. The Flory-Fox-Shafgagen and the Stockmayer-Fixman models fit better the experimental data and can be used to describe the molecular parameters of the acrylic polymers. Light scattering was used to compare results.     

Polymerization of methyl methacrylate with butyllithium–diethylzinc complex

The low-temperature polymerization of methyl methacrylate initiated with butyllithium–diethylzinc has been studied in toluene and in toluene–tetrahydrofuran and toluene–dioxane mixtures in various proportions. The polymerization process is typically anionic; it is characterized by a very rapid initiation reaction, and the absence of termination and chain transfer reactions, the molecular weight increasing proportionally with the degree of conversion. With toluene as a solvent, the polymer chains are associated, as is shown by viscometric measurements; moreover the polymers produced are highly polydisperse (M_v/M_n = 5.4). The kinetics are very complicated and vary with the range of the catalyst and monomer concentrations. In pure toluene in the presence of the organometallic complex, butyllithium–diethylzinc, the monomer addition is more stereospecific than when butyllithium alone is used as catalyst. By adding tetrahydrofuran to the reaction mixture, the polymer chain association disappears; concomitantly the stereochemical structure of the polymer changes from an isotactic to a mainly syndiotactic configuration. In toluene–tetrahydrofuran mixtures containing from 1 to 10 vol.-% tetrahydrofuran, the kinetics of polymerization can easily be interpreted by assuming the presence of two propagating reactive species which are in equilibrium with each other: the ion pair and the THF-solvated ion pair. The energy of activation of propagation for the free ion pair is equal to 7.5 kcal./mole; for the solvated ion pair a value of 5.5 kcal./mole was found, including the solvation enthalpy of the organometal with tetrahydrofuran. The existence of any relation between the reactivity of the propagating species and the tactic incorporation of the monomeric units has been discussed. The polymerization in mixtures of toluene–dioxane is intermediate between that in pure toluene and that in toluene–HF mixtures; the reaction mechanism however cannot be interpreted with the usual kinetic scheme. The experimental data concerning the rate dependence on catalyst and monomer concentrations are briefly summarized.     

The determination of the transformation products of epoxides used in the heat stabilization of poly(vinyl chloride)

Methyl 9, 10-epoxyoctadecanoate has been used as a model compound to study the mode of action of epoxides when employed in the stabilization of poly(vinyl chloride) (PVC). Sheets of PVC containing 2% radioactively labelled methyl (1-[¹⁴C]) 9,10-epoxyoctadecanoate (both with and without calcium/zinc stearate) were prepared on a hydraulic press and subsequently heated in an oven for various times to simulate heat processing. The polymer was separated from the low molecular weight components by steric exclusion chromatography, the extent of bonding of epoxide to the polymer being monitored by the [¹⁴C] activity of the high molecular weight fraction, and the level of residual methyl 9,10-epoxyoctadecanoate being determined colorimetrically on the basis of the epoxide concentration. The main transformation product was identified by combined gas chromatography-mass spectrometry as the methyl 9,10-chlorohydroxyoctadecanoate which was subsequently quantified after suitable derivatization by electron capture gas chromatography. A direct correlation was shown between the loss of epoxide and the extent of heat processing but this loss could not be fully accounted for on the basis of the estimated levels of chlorohydrins.     

Plastic deformation of polytetramethylene oxide. I. Influence of molecular weight distribution,crystallinity, and structure

Polytetramethylene oxide with a planar zig-zag structure similar to polyethylene can be obtained with narrow molecular weight distributions. The plastic deformation of samples differing in molecular weight, molecular weight distribution, crystallinity, and structure has been studied. The degree of crystallinity of the undeformed annealed samples, as studied by NMR and DSC, leads to a value of the enthalpy of melting ΔH_m = 167 J g^?1, supporting the lower of two previously reported values. The low natural draw ratios and low Young's modulus of the drawn samples, together with the effect of blending a small amount of high molecular weight material into a low molecular weight sample, highlight the role of the flexibility and of the high molecular weight tail of the distribution in the plastic deformation process. The stress required for the propagation of the neck and the tensile strength are found to be linear functions of, respectively, the natural and maximum draw ratio.     

Studies in ring-opening polymerization. I. 5,5-diethyl-1,3,2-dioxathiolan-4-one-2-oxide

The polymerization of 5,5-diethyl-1,3,2-dioxathiolan-4-one-2-oxide has been examined in various solvents at 60–100°C. Kinetic studies have shown that steric hindrance by the C₅ ethyl substituents prevents the occurrence of a bimolecular propagation reaction involving direct attack by a terminal hydroxyl group on the ring. In dry, nonhydroxylic solvents, the first-order rate-determining step in the sequence of reactions leading to polymer formation is a primary ring scission reaction in which a reactive intermediate is formed and sulfur dioxide eliminated. This intermediate, which is formally depicted as an α-lactone, then takes part in a very rapid chain-propagation process, the individual steps of which govern the molecular weight distribution of the polymer. The values of the activation energy (25–30 kcal/mole) and frequency factor (10¹¹?10¹³ sec^?1) for this polymerization reaction are, therefore, those associated with monomer decomposition and not the chain growth process. The molecular weight of the resultant polymer, poly-(3-pentylidene carboxylate) is controlled by adventitious traces of water which produce one carboxyl and one hydroxyl group per chain. Polymers having M?_n ～ 20,000 are readily obtained; these are materials of moderately high melting point (T_m ? 200°C) which crystallize from the melt into a banded spherulitic structure.     

The correlation between free volume and gas separation properties in high molecular weight poly(methyl methacrylate) membranes

Poly(methyl methacrylate) membranes of different fractional free volume (FFV) were prepared by dry casting from different solvents. Free volume data were determined by means of Bondi method and positron annihilation lifetime spectroscopy (PALS). It was found that both the boiling point and the solubility parameter of casting solvent affect the membrane’s free volume. It was believed that the difference in free volume was arisen from the difference in polymer packing.The gas permeability is higher when membranes are cast from higher molecular weight PMMA. But the plasticizing effect of CO₂ is less serious compared with the low molecular weight one. The high molecular weight PMMA membrane also has an extremely high O₂/N₂ selectivity, indicating its high structure uniformity. These results indicate that membranes made from polymer of higher molecular weight have the advantages of high permeability, gas selectivity and are less sensitive to CO₂ plasticization. The intrinsic gas transport properties such as the permeability, solubility and diffusivity of O₂, N₂, and CO₂ are measured or calculated. The effects of fractional free volume on membrane gas separation properties were investigated. It was found that the fractional free volume had no definite effects on gas solubility, but the gas permeability and diffusivity increased accordingly to the measured free volume.     

Dependence of Polymer Chain Entanglements on the Solution Concentration

For the viscometric determination of molecular weights of polymers, sufficiently dilute solutions have to be used so that entanglements of the polymer chain are absent. The concentration of the polymer should be such that the relative viscosity (η_r) lies in the range 1.1–1.5 [1]. Similarly, for molecular weight determination by light scattering, the suggested concentration for polymer with weight-average molecular weight ( M _w ) > 10⁵ is 0.5 wt%; for those with M _w < 10⁵, up to 1% may be used [2].

The limits of polymer concentration for such measurements are not clearly known. On dissolution, the polymer molecules adopt a more or less extended configuration whose shape depends on the structure and molecular weight of the polymer, the properties of the solvent, and the temperature

[3]. The molecules of flexible linear polymers acquire a coiled configuration due to free rotation about the C-C bonds. When a dilute solution satisfies theta conditions, the polymer molecules are free from all kinds of interaction and move freely. Then their solution properties could possibly be related to their end-to-end distance. Based on this concept, our attempt to establish the permissible limits of polymer concentration for dilute solutions of several polymers of different molecular weights is reported here.