Ion-pair structure in anionic polymerization. Effect of temperature and association on an allylic lithium compound in two polar solvents

¹³C NMR studies are reported on enriched samples of a one-unit model of the active centres present in butadiene polymerization. Concentrations between 2 × 10³ M and 1M, and temperatures between −120° and 50° were investigated. The effect of ion-pair association on charge distributions in diethyl ether solutions is described. The effect of temperature on the overall structure in tetrahydrofuran solutions is discussed.     

Solvation effects in anionic polymerization of polar vinyl monomers

The anionic polymerization (AP) of polar vinyl monomers (PVM) is strongly affected by additives with electron donor and electron acceptor properties. For the purpose of estimating the interactions of the active centers (AC) of chain growth with different ligands, the approach of studying a reference system was adopted, viz. lithium picrate (LiPi) in dioxane (DO). Three types of interaction were distinguished: with electron pair donors (EPD), with Li salts and alkoxides, and with Lewis acids. Spectral evidence was found of charge transfer complexation with Li alkoxides and enolates, leading to modified dianionic Meisenheimer adducts. The experimental procedure was employed in studies of competitive solvation of LiPi by two ligands. The results were related to reviewed literature data on the kinetics and stereochemistry of methyl methacrylate (MMA) and 2-vinylpyridine (2VP) polymerizations by lithium initiators.     

Effects of mercaptides on anionic polymerization. III. Polymerization of methyl methacrylate by thiophenoxides in polar aprotic solvents

Sodium thiophenoxide initiated the polymerization of methyl methacrylate in polar aprotic solvents (DMF, DMSO, HMPA). The active species that initiated the polymerization of the monomer was found by spectrophotometric measurements and by the sodium fusion method to be sodium thiophenoxide itself. The activation energy for the polymerization of the monomer in DMF solvent obtained was E = 3.4 kcal/mole below 30°C, and E = ?3.3 kcal/mole above the temperature. The phenomena were reasoned as the result of the formation of two active species: a solvent-separated ion pair and a contact ion pair. The effects of counterions on the reactivity of thiophenoxide increased with increasing electropositivity of the metals: Li < Na < K. Sodium phenoxide, the oxygen analog of thiophenoxide, was also found to initiate the polymerization of the monomer in the solvents. The relative reactivity of thiophenoxide to phenoxide for the monomer in HMPA at 30°C was thus determined: phenyl-SNa > phenyl-ONa. The relative effect of the polar aprotic solvents on the reactivity of thiophenoxide was also as follows: HMPA > DMF > DMSO. The kinetic studies were made by the graphical evaluation of rate constants. The following results were obtained for the monomer at 20°C in DMF solvent: K_p = 3.5 × 10² 1./mole-hr and K_t = 9.8 × 10^?2/hr.     

Dispersion polymerization in polar solvents

The effect of the nitrogen purge, monomer purification, type of agitation, and presence of costabilizer on the particle size distribution (PSD) was investigated in the dispersion po-lymerization of styrene in ethanol and in the dispersion copolymerization of styrene and butyl acrylate in a water–ethanol mixture. Purging with nitrogen and, to a lesser extent, monomer purification, were of paramount importance to achieve monodispersity. The type of agitation had a week effect on the PSD, whereas the presence of costabilizer had no effect on the PSD. © 1995 John Wiley & Sons, Inc.     

The influence of polar solvents on ions and ion pairs in the anionic polymerization of styrene

In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurements. Both the equilibrium between the two kinds of ion pairs and the dissociation of solvent-separated ion pairs to free anions are followed over a wide range of temperature. Thereby, conditions can be found under which the polymerization takes place almost exclusively via one of the two types of ion pairs.The thermodynamic parameters of the equilibria and the Arrhenius parameters of the propagation rate constants of the different kinds of propagating chain ends are reported. The equilibria between these species are strongly influenced by the solvent whereas the individual propagation rate constants are scarcely affected by the solvent. The “effective charge distance” in the solvent-separated ion pair can be estimated from the corresponding dissociation constant. The mobility of the polymer carbanion is discussed.     

On the termination reaction in the anionic polymerization of methyl methacrylate in polar solvents—I. Kinetic studies

The kinetics of the termination reaction in the anionic polymerization of methyl methacrylate, using monofunctional initiators with sodium as the counterion in the presence of excess NaB(C₆H₅)₄ have been studied. For monitoring the living-end concentration, a labelling technique was used. Tetrahydrofuran (THF) and tetrahydropyran (THP) were used as solvents. The rate constants for the propagation reaction in THF are smaller than those in THP by a factor of two. The kinetic investigation of the termination shows that only a fraction of chains becomes terminated. This fraction, as well as the rate of termination, is dependent on the initial monomer concentration. None of the reaction mechanisms discussed in the literature (i.e. termination by ester groups of the polymer or of the monomer) is able to explain these results. A new mechanism for the termination is proposed and verified; it is based on the assumption that a deactivating species is formed in the initial step of the polymerization. This species reacts with the living polymers during polymerization, leading to termination in a second-order reaction. The Arrhenius plot of the termination rate constant is linear (activation energy E_u = 48 kJ/mol; frequency exponent A = 13) in both solvents. The initial concentration of the deactivating species shows a linear dependence on the initial concentration of both the monomer and the living ends.     

Homopolymer and block copolymer brushes on gold by living anionic surface‐initiated polymerization in a polar solvent

Living anionic surface‐initiated polymerization on flat gold substrates has been conducted to create uniform homopolymer and diblock copolymer brushes. A 1,1‐diphenylethylene (DPE) self‐assembled monolayer was used as the immobilized precursor initiator. n‐BuLi was used to activate the DPE in tetrahydrofuran at –78 °C to initiate the polymerization of different monomers (styrene, isoprene, ethylene oxide, and methyl methacrylate). Poly(styrene) (PS) and poly(ethylene oxide) (PEO) in particular were first investigated as grafted homopolymers, followed by their copolymers, including poly(isoprene)‐b‐poly(methylmethacrylate) (PI‐b‐PMMA). A combined approach of spectroscopic (Fourier transform infrared spectroscopy, surface plasmon spectroscopy, ellipsometry, X‐ray photoelectron spectroscopy) and microscopic (atomic force microscopy) surface analysis was used to investigate the formation of the polymer brushes in polar solvent media. The chemical nature of the outermost layer of these brushes was studied by water contact angle measurements. The effect of the experimental conditions (solvent, temperature, initiator concentration) on the surface properties of the polymer brushes was also investigated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 769–782, 2006     

Chemical structure of polycaproamide obtained by anionic polymerization of caprolactam in solvent

Chemical structure of polycaproamide (nylon-6), obtained by low temperature anionic polymerization of caprolactam in the presence of the sodium salt of caprolactam and carbon dioxide in solvent has been investigated. The polymer formed in a heterogeneous system, depending upon its final treatment, is terminated with amino and cyclic lactam groups or, after hydrolysis, with amino and carboxyl group. i.r. Spectra and acid-base titrations have shown the presence of amino and carboxyl groups in macromolecules. The contents of basic and acidic groups in the final product and its fractions were determined by potentiometric titrations. Number-average molecular weights of samples based on the results of titrations were compared with the results of osmotic measurements. Equal numbers of basic and acid groups in the polymer suggests that macromolecules are terminated at one end by an amino group and at the other by a carboxyl group. The results indicate a linear and regular structure for the polycaproamide.     

The active centres' nature in anionic polymerization of polar monomers

In the anionic polymerization of polar monomers the nature of active centres is considerably influenced by the reaction conditions. In a survey, the present state of knowledge is represented and discussed for the polymerization of acrylonitrile. Analogous problems of methyl methacrylate and vinylpyridine polymerization are included in the considerations.     

Synthesis and properties of polymeric materials prepared by anionic polymerization of epoxy oligomers

The kinetics of anionic polymerization of epoxy oligomers of different functionalities in the presence of imidazoles was studied. Polymers based on the chosen systems, exhibiting high elastic and deformation characteristics at compression in a wide temperature range, and also resistance to heat and water, were synthesized. The epoxy compounds exhibit high adhesion to aluminum, preserved at elevated temperatures (150°C).     

Nonsteady-state kinetics for anionic polymerization of polar monomer initiated instantaneously by electron transfer

With the help of the nonsteady-state method, the kinetic differential equations for the anionic polymerization of polar monomer initiated instantaneously by electron transfer with monomer termination are treated rigorously. The expressions for the molecular weight distribution, the number-average and weight-average degrees of polymerization, and the functionality distribution are derived in closed form. A theoretical means is established by which all the molecular parameters of the polymer can be calculated from polymerization conditions, such as the reaction constants, the concentrations of initiator and monomer, and the monomer conversion.     

A study of molecular weight of polycaproamide obtained by anionic polymerization of caprolactam in solvent

Molecular weight of polycaproamide obtained by anionic polymerization of caprolactam in solution in the presence of sodium caprolactam and carbon dioxide has been investigated by absolute methods. Number average molecular weights were determined by osmometry. Weight average molecular weights were found by light scattering. Schulz's number of heterogeneity of unfractionated polycaproamide was 0–83. Polycaproamide obtained by anionic polymerization in solution is more homogeneous than products of hydrolytic or anionic polymerization of caprolactam in the bulk. This result corroborated our previous report based on fractionation data.     

Effect of tertiary diamines on anionic polymerization of polar vinyl monomers

Tertiary diamines that exhibit chelating capability to lithium are shown to decrease the reactivity of growing species when used in classical anionic polymerization of methacrylic monomers. Out of the different diamines investigated, sparteine turns out to be the most efficient chelating agent. The beneficial effect of the latter additives on the overall reactivity of methacrylic anions was exploited to prepare poly(MMA) samples of controlled size at room temperature. The decrease in reactivity was also demonstrated from a kinetic point of view, upon measuring the actual rate constant of propagation using vinyl pyridine as model monomer.     

Theoretical study of chain transfer in anionic polymerization, 2. Copolymerization accompanied by chain transfer to solvent

The main factors determining molar mass characteristics of copolymers formed in nonterminating copolymerization under the conditions of chain transfer to solvent are studied theoretically. The dependences of the mean polymerization degrees on conversion and monomer feed composition for various values of reactivity ratios are obtained. The results obtained greately differ from those for homopolymerization. This is explained by the contribution of cross-propagation reactions. In particular, it is shown that at ≪ and ≪ the azeotropic copolymerization proceeds like the living one even if homopolymerization of each monomer is accompained by extensive chain transfer to solvent.     

Preparation of polymethyl methacrylate with well-controlled stereoregularity by anionic polymerization in an ionic liquid solvent

This study investigates the effect of ionic liquids (ILs) on the anionic polymerization of methyl methacrylate (MMA). Polymethyl methacrylate (PMMA), an isotactic polymer, is prepared by anionic polymerization at a high reaction temperature with an IL that acts as both solvent and additive. The most plausible reaction mechanism is determined using ¹H NMR and Fourier-transform infrared spectroscopy. The electrostatic interaction between MMA and the IL increases the apparent steric hindrance in MMA, resulting in the isotactic PMMA.     

Vesicle formation of 1:1 cationic and anionic surfactant mixtures in nonaqueous polar solvents

The vesicle formations of 1:1 cationic-anionic surfactants in various nonaqueous polar solvents and their aqueous mixtures were investigated. Outstanding vesicle-forming capability and stability of cationic and anionic surfactants were found in nonaqueous polar solvents and their aqueous mixtures except for in formamide. A small amount of formamide destroys the vesicles formed by cationic-anionic surfactants in aqueous solutions. These results could be very well explained based on the effect of the medium dielectric constant. Received: 26 May 1998 Accepted in revised form: 24 November 1998     

Living polymerization of isobutyl vinyl ether by HI/I2 initiator in polar solvents

This study has shown a nearly perfect living polymerization of isobutyl vinyl ether (IBVE) to proceed not only in nonpolar media (e.g., n-hexane) but in relatively polar CH₂Cl₂ solvent, provided that the HI concentration is sufficiently high. The produced polymers had a nearly monodisperse molecular weight distribution (M _w/M _n ≤ 1.1); the number-average molecular weight (M _n) increased in direct proportion to IBVE conversion and its increase continued on the addition of a fresh feed of the monomer at the end of the polymerization. The use of a more polar medium (PhNO₂/CH₂Cl₂) or a lower HI concentration leads to chain transfer reactions, by promoting the ionic dissociation of the “nondissociated” living propagating species. The successful living polymerization by HI/I₂ in CH₂Cl₂ indicates a very strong interaction between the iodide anion and the growing end.     

A comparison of thermoreactive water-soluble poly-N,N-diethylacrylamide prepared by anionic and by group transfer polymerization

Several batches of poly-N,N-diethylacrylamide were synthesized by anionic and by group transfer polymerization (GTP). A radical poly-N,N-diethylacrylamide prepared from the same monomer was also included in the comparison. According to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) both types of living polymerization resulted in narrow molecular weight distributions with M_w/M_n values below 1.5. Average molecular weights (M_n) between 888 and 4678 g/mol were calculated in these cases. The radical polymer had an average molecular weight (M_n) of approximately 130,000 g/mol. The dry anionic and GTP polymers were investigated by differential scanning calorimetry (DSC) and x-ray diffraction spectrometry. Evidence for partial crystallinity in the solid state was found. The conformation of all polymers was examined by high resolution (600 MHz) NMR. According to these measurements, 75% of the ? CHR? groups of the anionic poly-N,N-diethylacrylamide were located in an isotactic triade. The remaining 25% had heterotactic structure, while no indication for the presence of syndiotactic protons was found. Poly-N,N-diethylacrylamide prepared by GTP, on the other hand, had mainly syndiotactic structure. The aqueous solutions of the polymers showed phase separation upon heating. Whereas the lower critical solution temperature (LCST) was approximately 30°C in the case of the poly-N,N-diethylacrylamide prepared by GTP and by radical polymerization, uncommonly high LCSTs of more than 40°C were observed for the anionic poly-N,N-diethylacrylamide. © 1994 John Wiley & Sons, Inc.     

Electroconductivity study of active centers of polar vinyl monomers in the anionic polymerization

Ion equilibria in THF solutions of organolithium compounds are studied at ?30°C. The extremely low values of the dissociation constants (from 10^?11 to 10^?13M) and the formation of ion triples of the types A^?, B⁺, A^? prove that active centers exist in THF mainly as contact ion pairs with very small interionic distance (～ 2.5 Å) and tend to form higher associates. A comparison is made between the physicochemical characteristics of active centers of vinyl nitriles, ketones, and esters. A planar structure is proposed for the anion of methyl methacrylate. The results indicate that the intramolecular chelate complexation between the polar group and the counterion of the active center decreases in the order . A greater participation of ion triples in the propagation reaction is proposed.