Microstructures of polybutadienes prepared by anionic polymerization in polar solvents. Ion-pair and solvent effects

The microstructure of polybutadiene produced by anionic initiation in diethyl ether and tetrahydrofuran with counterions Li⁺, Na⁺ and K⁺ was determined by ¹H- and ¹³C-NMR. Ionization suppressing salts were added in tetrahydrofuran to ensure that only the ion-pair reaction was studied. Results are compared with older published data. In general, the 1,4 content of the polymer increases with increasing counterion size but varies somewhat with solvent with a given counterion. The cis component of the 1,4 structures changes with temperature and counterion. It is suggested that this change reflects the proportion of cis and trans centers that carry the reaction.     

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.     

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.     

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.     

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.     

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.     

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.     

Anionic polymerization initiated by lithium diethylamide in organic solvents. III. Investigation of the polymerization of styrene

The polymerization of styrene, initiated by lithium diethylamide in mixtures of benzene and THF, has been investigated. Kinetic and molecular weight measurements are interpreted on the basis of simultaneous initiation and propagation steps, and the effect of solvation and coordination processes on these reactions is discussed. Initiation of polymerization is thought to involve addition of solvated lithium diethylamide ion-airs to styrene, giving species with diethylamide end groups. The possible influence of these end groups on the initiation is considered in terms of an intramolecular cyclization process. Propagation of polymerization is believed to involve polystyryllithium ion-pairs, solvated to varying extents by THF. No evidence has been found to suggest that chain transfer, or termination, reactions are an integral part of the polymerization process. The polymerization has a number of similarities to the alkyllithium-initiated polymerization of styrene, but also exhibits some interesting differences.     

Anionic polymerization initiated by lithium diethylamide in organic solvents. II. Investigation of the polymerization of isoprene

The polymerization of isoprene, initiated by lithium diethylamide has been investigated in the presence of a number of additives. Kinetic results are interpreted on the basis of simultaneous initiation and propagation reactions. The effect of additives, particularly diethyl either, has a profound effect on both the rate of initiation and propagation. The active centers are believed to be ion-pairs with the lithium counterions solvated by both ether and monomer molecules, and the actual propagation reaction is believed to involve a rearrangement of the monomer, complexed to the lithium, and the growing polymeric chain.     

Dual influence of lithium chloride on the anionic propagation of polystyryllithium in ethereal solvents

The addition of lithium chloride (LiCl) to a solution of polystyryllithium (PStLi) in tetrahydropyran (THP) reduces the rate of propagation of PStLi at a low concentration of the latter but accelerates it at higher concentrations of PStLi. Moreover, the addition of LiCl, which is dimeric in ethereal solutions, increases the conductance of PStLi solutions in tetrahydrofuran (THF) and THP to a much greater extent than expected from the separate conductances of PStLi and LiCl, which is itself even less dissociated than PStLi. These phenomena are fully explained by the dual action of LiCl. Below a certain concentration of PStLi, the dissociation, not of LiCl as such, as claimed before, but of its solvated dimer into free Li⁺ ions and ClLiCl⁻ triple ions provides Li⁺ ions that repress the ionic dissociation of PStLi by a common ion effect. This, in turn, diminishes the concentration of free polystyryl anions, which are the dominating species responsible for the propagation of PStLi, resulting in retardation. However, at higher concentrations of PStLi, Li⁺ ions produced by its dissociation are scavenged by the scavenging action of LiCl dimers, producing quintuple cations. This reduces the concentration of free Li⁺ ions and, therefore, increases the concentration of the reactive free polystyryl anions, resulting in an acceleration of the propagation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2148–2157, 2002     

Anionic polymerization. VI. Effect of polar modifiers on alkylsodium and alkylpotassium polymerization of butadiene

It was found that N,N,N′,N′-tetramethylethylene diamine and hexamethyl phosphorus triamide minimize chain transfer reactions in the polymerization of 1,3-butadiene in hydrocarbon solvent with alkylsodium or alkylpotassium initiators. The polymers obtained with alkylsodium initiators had a high molecular weight and high vinyl content at 90–95% conversion. The molecular weight of the polybutadiene made by alkylsodium and alkylpotassium initiators was dependent on the polymerization temperatures and modifier ratios, but the vinyl contents were independent of the modifier ratios. Vinyl contents of alkylpotassium-initiated polymers showed a slight dependency on polymerization temperature; the vinyl contents of alkylsodium-initiated polymers were independent of temperature. Addition of lithium tert-butoxide and potassium tert-amylate to these initiators in the presence of the modifiers affected the molecular weight but not the microstructure.     

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     

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.     

Studies on anionic polymerization of lactams. Part II. Effect of cocatalysts on the polymerization of pyrrolidone

The rates of addition of pyrrolidonate magnesium bromide (PyMgBr) to N-benzoyl-, N-acetyl-, and N-methylpyrrolidone were measured in solution in tetrahydrofuran (THF). The values found for the rate constants at 25°C. were 9.5 × 10^?2, 2.8 × 10^?2, and 5 × 10^?4 l./mole-sec., respectively. The rate constant for addition of PyMgBr to pyrrolidone was also measured and found to be 3 × 10^?9l./mole-sec. Possible causes for the large difference between the values of these constants are discussed.     

Active centre structure and the stereoregulation process in anionic polymerization

The mechanism of stereoregulation in the anionic polymerization of butadiene and isoprene is discussed in terms of what is known about the structure of the active centres. This includes the effect of the presence of cis and trans isomers and the dynamics of their interconversion and in the isoprene case, the effect of two distinct types having 1,4 and 4,1 structures.     

Physicochemical investigations of anionic–nonionic mixed surfactant microemulsions in nonaqueous polar solvents: I. Phase behavior

Phase behaviors of AOT/heptane (Hp)/formamide (FA), ethylene glycol (EG), propylene glycol (PG), triethylene glycol (TEG) and glycerol (GLY) have been investigated in the absence and presence of a nonionic surfactant, polyoxyethylene(2) cetyl ether (Brij-52) at 303 K. The phase characteristics of (AOT+Brij-52)/Hp/(EG or PG or TEG) have been found to be different from that of AOT/Hp/FA systems in respect of both the area of monophasic domain and the appearance of other mesophases. The area of monophasic domain of (AOT+Brij-52)/Hp/EG depends on the content of Brij-52 (X _Brij-52) and shows a maximum at X _Brij-52=0.4. A negligible effect on the area of the monophasic domain has been shown by more hydrophobic surfactants, polyoxyethylene(2) stearyl ether (Brij-72) and polyoxyethylene(2) oleyl ether (Brij-92). The effect of oils (dodecane and hexadecane) on the mixed systems stabilized by (AOT+Brij-52) in EG has been investigated. The area of monophasic domain has been found to be dependent on the type of nonaqueous solvents and follows the order GLY>EG>PG>TG. A systematic investigation on the measurement of phase volumes of mixed surfactant systems [AOT+nonionic surfactant(s)] stabilized in oils of different chain lengths (heptane, dodecane and hexadecane) and polar solvent (EG) has been carried out at different compositions of the ingredients to identify the phase transitions of these systems as a function of X _Brij-52. The threshold point of phase transition (both W I→W IV and W IV→W II transitions) has been found to be a function of the configuration of added nonionic surfactant, nature of the polar solvent and oil. The conversion of the initial oil/EG droplets into EG/oil droplets with increasing X _nonionic has been facilitated for hydrophobic surfactants polyoxyethylene(4) lauryl ether (Brij-30), Brij-52, and Brij-72 in comparison to the hydrophilic surfactants polyoxyethylene(10) cetyl ether (Brij-56) and polyoxyethylene(20) cetyl ether (Brij-58).     

Single-site anionic polymerization. Monomeric ester enolaluminate propagator synthesis, molecular structure, and polymerization mechanism

The synthesis and molecular structure of the first examples of monomeric lithium ester enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids, are reported. Reactions of isopropyl alpha-lithioisobutyrate (2) with suitable deaggregating and stabilizing organoaluminum compounds such as MeAl(BHT)2 (BHT = 2,6-di-tert-butyl-4-methylphenolate) in hydrocarbons cleanly generate lithium ester enolaluminate complexes such as Li+[Me2C=C(OiPr)OAlMe(BHT)2]- (3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and highly active polymerization can be achieved by using the 3/MeAl(BHT)2 propagator/catalyst pair, which is conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT)2. The structure of the added organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)2 pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator 2, and monomeric propagator 3 have been determined by X-ray diffraction studies.     

Hydroxypropyl cellulose (HPC)-stabilized dispersion polymerization of styrene in polar solvents: Effect of reaction parameters

Dispersion polymerization of styrene in polar solvents in the presence of hydroxypropyl cellulose (HPC) produces latex particles from ca. 1 to 26 μm depending on reaction parameters. Increasing the initiator concentration or temperature decreases the molecular weight, but increases the particle size and breadth of the size distribution. The decrease in molecular weight with increasing R_i, caused by larger initiator concentration or higher temperature, is expected based of fundamental kinetic relationships. The inverse correlation between size and rate of initiation is rationalized by polarity (stabilizing ability) of the grafted HPC-polystyrene formed in situ. High polar HPC-g-PS, which contains shorter graft polystyrene chain, stabilizes particles less effectively and this leads to larger particles. The primary influence of initial styrene concentration is a solvent effect: larger particles are obtained at high styrene concentration due to high solubility of polystyrene during the initial part of the reaction. The influence of the molecular weight of HPC is to change the polarity of the HPC-g-PS stabilizer. Comparison of particle growth of three critical polymerization systems suggests that the favorable continuous-phase solubility parameter for dispersion polymerization of styrene is around 11.6 (cal/mL)^1/2. Too high or too low polarity generates particles with broad size distribution because large particles are formed during the initial stage and nucleation continues as the polymerization proceeds. © 1992 John Wiley & Sons, Inc.     

Effect of an anionic emulsifier on the structure of butadiene-nitrile elastomers

X-ray diffraction and TMA studies show that surfactant sodium alkyl sulfonate (C₁₅) forms one of its two LC structures (distinguished by the smallest layer periodicity) in butadiene-nitrile elastomers containing different amounts of acrylonitrile units. In this case, the surfactant serves as a structural plasticizer and facilitates a more complete selective segregation of microblocks of trans-1,4-butadiene units and, especially, of sequences of alternating trans-1,4-butadiene and acrylonitrile units.