Cationic polymerization induced by metal salts—I. Heterogeneous systems

After a critical review of the literature, results are reported on the use of several baked metal perchlorates and trifluoromethanesulphonates as heterogeneous initiators of the cationic polymerization of various monomers, particularly isobutene. Lithium and silver salts were found to be inactive. Of the group Ila metals, the magnesium salts displayed the most pronounced activity. The aluminium salts, Co(ClO₄)₂, Ni(ClO₄)₂ and Ga(CF₃SO₃)₃ exhibited very high efficiencies. Poor reproducibility did not allow a quantitative study, but several specific experiments were conducted to ascertain the nature of the initiation mechanism. The evidence strongly suggests that metal cations on the salt surface are the Lewis sites responsible for the electrophilic attack on the monomer double bond, and that moisture is detrimental to this process.     

Effects of metal salts on polymerization. Part III. Radical polymerizabilities and infrared spectra of vinylpyridines complexed with zinc and cadmium salts

Radical polymerization of 4-vinylpyridine (4-VP), 2-vinylpyridine (2-VP), and 2-methyl-5-vinylpyridine (MVP) was studied in concentrated DMF solutions of ZnCl₂, ZnBr₂, ZnI₂, Zn(CH₃COO)₂, and Cd(CH₂COO)₂ at 50°C. Polymerization of 4-VP and MVP was accelerated by the addition of the metal salts, while the polymerization of 2-VP was greatly retarded. The sequence of the accelerating effect of metal salts for 4-VP was in the following order: Cd(CH₃COO)₂ > ZnCl₂ > Zn(CH₃COO)₂ > ZnBr₂ > ZnI₂. This sequence is almost the same as that reported in a previous report for MVP. However, the order was reversed for the retarding effect on the polymerization of 2-VP. At the intermediate concentration of metal salts, polymerization of 4-VP proceeded heterogeneously, which was explained by considering crosslinking of poly-4-VP by the metal ion. Since a linear correlation between the rate R_p and the degree of polymerization was observed for the 4-VP–Zn(CH₃COO)₂ system, the accelerating effect was postulated to be due to the enhancement in k_p. Results of copolymerization of VP with styrene as M₂ in a concentrated solution of Zn(CH₃COO)₂ indicated the strong activation of 4-VP by complex formation (r₁ = 2.7 ± 0.5, r₂ = 0.08 ± 0.03), whereas the change in the monomer reactivity of MVP is smaller (r₁ = 2.0 ± 0.2, r₂ = 0.35 ± 0.05). The behavior of 2-VP was abnormal (r₁ = 3.35 ± 0.3, r₂ = 0.55 ± 0.15, then r₁r₂ > 1), which was attributed to the steric effect by complex formation. Solid complexes formed between pyridine, 4-VP, 2-VP, or MVP and zinc salts were prepared as samples for infrared spectroscopy. The shifts in infrared absorption bands of these amines were studied by comparing the infrared spectra of the amines before and after the complex formation, and the results were interpreted in terms of electronic as well as steric interactions of metal salts with ligands. Conjugation of the metal salt with the ligand π-orbitals was necessary to explain both infrared spectra and polymerization results.     

Effects of metal salts on polymerization. Part II. Polymerization of vinylpyridine complexed with the group IIb metal salts

The following stoichiometric vinylpyridine complexes have been prepared: (4-VP)₂—Zn(SCN)₂, (2-VP)₂—Zn(SCN)₂, (MVP)₂—Zn(SCN)₂, (MVP)₂—ZnCl₂, (MVP)₂—ZnBr₂, (MVP)₂—ZnI₂, and (MVP)₂—HgCl₂, where 4-VP, 2-VP, and MVP denote 4-vinylpyridine, 2-vinylpyridine, and 2-methyl-5-vinylpyridine, respectively. Results of radical polymerization initiated by azobisisobutyronitrile indicate that the effect of complex formation between the monomers and the metal salts is to enhance the rate of polymerization with the exception of the 2-VP complex. The R_p for the solution polymerization in dimethylformamide increases in the following order: (1) (MVP)₂—Zn-(SCN)₂ > (MVP)₂-ZnCl₂ > (MVP)₂—ZnBr₂ > (MVP)₂—ZnI₂ > free MVP; (2) (4-VP)₂—Zn(SCN)₂ > (MVP)₂—Zn(SCN)₂ > free MVP > (2-VP)₂—Zn(SCN)₂; and (3) MVP + Zn(CH₃COO)₂ < MVP + Cd(CH₃COO)₂. When ethanol, acetone, or tetrahydrofuran is used as solvent, the change in R_p is more marked, partly due to insolubility of the PMVP complexed with the metal salts. The increase in R_p would be attributed to the change in k_p since the molecular weights of PMVP are nearly proportional to R_p when (MVP)₂—ZnX₂ where X is Cl^?, Br^?, I^?, or SCN^? is polymerized in DMF under fixed conditions. Copolymerizations of MVP—ZnX₂ complexes (where X is Cl^?, Br^?, I^?, or CH₃COO^?) with styrene indicate that the e values of complexed MVP are more positive than that of free vinylpyridine, and the amounts of the positive shift in e values increase with decreasing polarizability of the halide anions. These results are discussed in terms of the charge-transfer properties of anions, the nature of coordination bonds, and the structures of vinylpyridines. The complexed monomers are hardly polymerized by a cationic or an anionic mechanism. Radiation-induced solid-state polymerization gives polymers in low yields.     

Vinyl polymerization with Fe(III)-thiourea as initiator system. Part I. General features and kinetics of Fe(ClO4)3-thiourea reaction

Initiation of polymerization of methyl methacrylate, styrene, and acrylonitrile with the redox system Fe(III)—thiourea has been examined. For the heterophase polymerization any of the ferric salts, such as FeCl₃, Fe₂(SO₄)₃, and Fe(ClO₄)₃ can be used as oxidant, but there is no polymerization in the homogeneous phase when FeCl₃ is used as oxidant. It was also observed that Fe(ClO₄)₃ retards the radical polymerization of styrene, though this salt has hardly any effect on the radical polymerization of methyl methacrylate. Further, the reaction between Fe(ClO₄)₃ and thiourea was found to be kinetically of second order. The rate is largely influenced by the nature of the solvent. It is concluded that apart from the dielectric constant of the solvents, specific effects like complex formation of Fe(III) with solvents should have a marked influence on the rate of this reaction.     

Photoinitiated cationic polymerization with triarylsulfonium salts

Triarylsulfonium salts Ar₃S⁺MX_n⁻ with complex metal halide anions such as BF₄⁻, AsF₆⁻, PF₆⁻, and SbF₆⁻ are a new class of highly efficient photoinitiators for cationic polymerization. In this article we describe several synthetic routes to the preparation of these compounds along with their physical and spectroscopic properties. Mechanistic studies have shown that when these compounds are irradiated at wavelengths of 190–365 nm carbon-sulfur bond cleavage occurs to form radical fragments. At the same time the strong Brϕnsted acid HMX_n, which is the active initiator of cationic polymerization that takes place in subsequent “dark” steps, is also produced. A study of the parameters that affect the photolysis of triarylsulfonium salts is reported with a measurement of the absolute quantum yields. The cationic polymerizations of four typical monomers—styrene oxide, cyclohexene oxide, tetrahydrofuran, and 2-chloroethyl vinyl ether—with triarylsulfonium salt photoinitiators are described.     

Polymerization of N-vinylcarbazole by transition metal salts

The polymerization of N-vinylcarbazole (NVC) in the presence of transition metal salts such as WCI₆, MoCI₅, TaCl₅ and NbCl₅ under different reaction conditions was studied. In general, aromatic solvents were found to be superior to aliphatic solvents in the polymerization of NVC, i. e., both conversion and molecular weight were higher in aromatic solvents. It was observed that the polymerization reaction proceeds rapidly and almost quantitatively, even at low monomer concentration (< 5 × 10^?2M) and at low catalyst to monomer mole ratio (10^?5) in aromatic solvents. The copolymerization of NVC with acenaphthylene (ACN) was also investigated in solution at room temperature. The resulting homo- and copolymer were characterized by IR, NMR, x-ray diffraction, and elemental analysis. Thermal and photophysical properties are also reported. From the spectral data, the polymerization solvent was found to have a strong influence upon the polymer stereoregularity.     

Phase diagrams of non-ionic microemulsions containing reducing agents and metal salts as bases for the synthesis of bimetallic nanoparticles

Phase diagrams of microemulsions containing metal salt(s) and reducing agent, respectively, were studied in detail. The microemulsions were based on non-ionic surfactants, namely pure tetraethyleneglycol monododecylether, C₁₂E₄, and technical grade Brij30. We studied the influence of the metal salts H₂PtCl₆, Pb(NO₃)₂, Bi(NO₃)₃, H₂PtCl₆ + Pb(NO₃)₂ (1:1 mixture), and H₂PtCl₆ + Bi(NO₃)₃ (1:1 mixture) as well as of the reducing agent NaBH₄ on the location of the phase boundaries. The focus was on the water emulsification failure boundary (wefb) where the aqueous phase forms spherical droplets. The temperature shifts of the wefb, which were caused by the presence of the salt(s), are directly related with the shift of the clouding points of the corresponding oil-free systems. The location of the wefb is affected in a complex manner by the pH (the lower the pH the higher the temperature at which the wefb occurred), the ionic strength and by specific salting-in or salting-out effects of the electrolyte ions. The desired overlap of the wefb of the microemulsions containing the metal salt(s) and the reducing agent, respectively, could be achieved by adding NaOH to the C₁₂E₄-based microemulsions and by titrating 1-octanol to the Brij30-based microemulsions, respectively.     

Syndiotactic polymerization of styrene in the presence of CpTiCl2(OC6H4X)/MAO catalytic systems

CpTiCl₂(OC₆H₄X‐p) complexes (where X =­CH₃, Cl, NO₂,; Cp = cyclopentadienyl) activated with methylaluminoxane (MAO) were used in syndiotactic polymerization of styrene. High activity and selectivity for all catalysts were found. The styrene conversion and reaction selectivity depend on the catalyst ageing time and temperature, polymerization temperature and the nature of the substituent in the phenoxy ring. Copyright © 2001 John Wiley & Sons, Ltd.     

Acidic peroxo salts: A new class of initiators for vinyl polymerization. III. Kinetics of polymerization of methyl methacrylate initiated by potassium monopersulfate catalyzed by Co(II)

Kinetics of the polymerization of methyl methacrylate by an acidic peroxo salt like potassium monopersulfate catalyzed by Co(II) have been investigated in aqueous medium, over the range of 35–50°C. The rates of polymerization (R_p) have been studied at various concentrations of monomer and initiator. The efficiencies of various metal salts in catalyzing the polymerization have been evaluated from the observed value of R_p. The effects of catalyst (CoSO₄), initiator, monomer, and various concentrations of FeCl₃ on R_p and percentage conversion have been studied. The end groups of the recovered polymers have been studied using standard methods. From the observed end groups and kinetic results, a reaction scheme has been proposed involving initiation by S O₄· or OH· radicals, generated by the interaction of the initiator with cobalt sulfate and termination of the polymer chains by mutual combination.     

Synthesis and addition polymerization of vinylbenzyl transition metal compounds

Summary The synthesis of three vinylbenzyl complexes (n⁵-C₅H₅)-M(CO)_n(CH₂C₆H₄CH=CH₂), [1), M=Mo, n=3; (2), M=W, n=3; (3), M=Fe, n=2] is reported. Complexes (1) and (2) have been copolymerized with monomers, such as styrene, methyl methacrylate, andN-vinyl-2-pyrrolidone, in benzene using azoisobutyronitrile as initiator. The rates of incorporation of (2) into copolymers with styrene and methyl methacrylate were the same as the rate of incorporation of the organic monomer. The homopolymerization of (2) was also carried out. Polymerizations occurred satisfactorily except for the copolymerization of (1) with styrene, where little incorporation of the organometallic monomer occurred.     

Catalytic action of metallic salts in autoxidation and polymerization. IX. Polymerization of methyl methacrylate with sodium hexanitrocobaltate(III) in mixed solvent of acetone and water

Polymerization of methyl methacrylate with some cobalt (III) complexes was carried out in various solvents and in mixed solvents of acetone and water or alcohols. Sodium hexanitrocobaltate(III) was found to be an effective initiator in mixed solvent of water and acetone. The kinetic study on the polymerization of methyl methacrylate with Na₃[Co(NO₂)₆] in a water-acetone mixed solvent gave the following over-all rate equation: R_p = 8.04 × 10⁴ exp{ ?13,500/RT} [I]^1/2[M]² (mol/1.?sec). The effects of various additives on polymerization rate and the copolymerization curve with styrene suggest that polymerization proceeds via a radical mechanism. The dependence of the polymerization rate on the square of monomer concentration and the spectroscopic data were indicative of the formation of a complex between initiator and monomer.     

A study of polypyrrole synthesized with oxidative transition metal ions

Polypyrrole powder and films were chemically synthesized by the reaction of AgNO₃, FeCl₃, Fe(NO₃)₃, Cu(NO₃)₂, or Cu(NO₃)₂-AlCl₃ with pyrrole in an aqueous solution or a water—toluene two-phase system. Products were characterized by elemental analysis, IR, scanning electron microscopy with energy dispersive x-ray analysis (SEM with EDAX), and conductivity measurements. The polypyrrole synthesized from pyrrole with FeCl₃ had a composition of C_4.00H_3.05N_0.99Cl_0.25. The pressed powder had a conductivity of 2.7 × 10^?2 S/cm and the film 2.8 S/cm. All the other metal salts produced films that had the same organic backbone, morphology, and conductivity as the polymer synthesized using Fe(III) salts, regardless of the considerable differences in the reduction potentials of the metal ions. The nature of the anions of the transition metal salts had no effect on the reaction. Anions, however, were retained as the counterions of the cationic polypyrrole backbone and could be easily exchanged with other anions.     

Synthesis, characterisation and anion exchange properties of copper, magnesium, zinc and nickel hydroxy nitrates

Anion exchange reactions of four structurally related hydroxy salts, Cu₂(OH)₃NO₃, Mg₂(OH)₃NO₃, Ni₂(OH)₃NO₃ and Zn₃(OH)₄(NO₃)₂ are compared and trends rationalised in terms of the strength of the covalent bond between the nitrate group and the matrix cation. Powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and elemental analysis are used to characterise the materials. Replacement of the nitrate anions in the zinc and copper salts with benzoate anions is possible although exchange of the zinc salt is accompanied by modification of the layer structure from one where zinc is exclusively six-fold coordinated to a structure where there is both six- and four-fold zinc coordination. Magnesium and nickel hydroxy nitrates, on the other hand, hydrolyse to their respective metal hydroxides.     

Quinone transfer radical polymerization (QTRP) of styrene: Catalysis by different metal complexes

Styrene has been polymerized by a Quinone Transfer Radical Polymerization (QTRP) based on the redox reaction of an ortho‐quinone and a metal catalyst. Several metal acetylacetonates have been tested in this work. The radical polymerization of styrene is largely controlled when phenanthrenequinone (PhQ) is used with catalytic amounts of Co(acac)₂, Ni(acac)₂, Mn(acac)_{2 or 3}, and Al(acac)₃. As a rule, in the presence of all these metallic complexes, the polystyrene molar mass increases with the monomer conversion, and polydispersity (M_w/M_n) is in the 1.3–1.6 range (at least until 40% monomer conversion). Styrene polymerization has also been resumed by polystyrene chains prepared by QTRP. In the specific case of manganese acetylacetonates, an amine or phosphine ligand has to be added for the control to be effective. Finally, two mechanistic hypotheses have been proposed, depending on whether the oxidation state of the metal can be easily changed or not. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2723‐2733, 2005     

A comparative study of coordination architectures constructed by di[4-(pyridin-3-yl)pyrimidinyl]disulfide and different metal salts: crystal structures and luminescence properties

Assembly of di[4-(pyridin-3-yl)pyrimidinyl]disulfide (3-ppds) with different metal salts resulted in a variety of coordination polymers that were structurally elucidated. For MnCl₂, a 1-D repeated rhomboidal chain structure [MnCl₂(3-ppds)₂] _n (1) was obtained, whereas a 1-D helical chain structure [Zn(NO₃)₂(3-ppds)] _n (2) was built from Zn(NO₃)₂. A 1-D zigzag chain structure [Cu₂(OAc)₄(3-ppds)] _n (3) was produced from Cu(OAc)₂. In all three complexes, the 3-ppds ligand plays the same role as a bis(monodentate) bridging linker but with variations in both C–S–S–C torsion angles and dihedral angles defined by its conjugated heteroaromatic rings (pyrimidine and pyridine). The luminescence properties of the complexes have been evaluated in the solid state.     

Thermodynamic properties and solution equilibria of aqueous bivalent transition metal nitrates and magnesium nitrate

Osmotic coefficients for Mn(NO₃)₂, Co(NO₃)₂, Ni(NO₃)₂, Cu(NO₃)₂, Zn(NO₃)₂, and Mg(NO₃)₂ in aqueous solution have been determined by the isopiestic method at 25°C, and activity coefficients have been derived. The results agree with the literature data for Zn(NO₃)₂, while they are significantly different for Co(NO₃)₂, Cu(NO₃)₂, and Mg(NO₃)₂, and those for Mn(NO₃)₂ and Ni(NO₃)₂ are new. The concentration dependence of the osmotic coefficients for the bivalent metal nitrates is similar to that for the trifluoroacetates, while it differs from those for the other salts of the same series of metals. The results are discussed in terms of the inner-sphere and outer-sphere association of ions, auxiliary information being derived from the concentration effects in the visible spectra of the coloured metal nitrates.     

Bulk polymerization of methyl methacrylate electroinitiated by tetrabutylammonium salts

The radical electroinitiated polymerization of methyl methacrylate has been performed in solutions of tetrabutylammonium salts. TBANO₃ is by far the most efficient, because of the relatively low anodic potential required for the oxidation to the radical. The influence of factors such as temperature, current density, and salt concentration on yields and molecular weights has been examined. The absence of a gel effect allows the onset of a steady state in which the polymerization rate is first-order with respect to monomer.     

Cationic rare earth metal alkyls as novel catalysts for olefin polymerization and copolymerization

Cationic rare earth metal alkyl species, generated by the treatment of mono(cyclopentadienyl) bis(alkyl) rare earth metal complexes with 1 equiv. of a borate compound such as [Ph₃C][B(C₆F₅)₄], act as an excellent catalyst for the polymerization and copolymerization of various olefins such as ethylene, 1-hexene, styrene, norbornene, dicyclopentadiene, and isoprene. These catalysts show unprecedented activity and regio- and stereo-selectivity and afford a series of new polymers which are difficult to be prepared previously.     

Effects of metal salts on polymerization. Part IV. Polymerization of N-vinylcarbazole initiated by oxidizing metal nitrates

The polymerization of N-vinylcarbazole (VCZ) in ethylene dichloride, acetone, benzene, and dioxane with cupric nitrate, ferric nitrate, and ceric ammonium nitrate as catalyst was studied. In all cases the polymerization seemed to be of a cationic nature, judged by copolymerization with styrene. Electron spin resonance (ESR) spectroscopy was made for the polymerization system and also for a system containing N-ethylcarbazole instead of VCZ. Singlet ESR spectra were observed for all systems containing ceric salt and for some systems containing ferric salt but not for systems containing cupric salt. The ESR spectra indicated the formation of an ion radical by electron transfer between the oxidizing metal salt and the carbazole derivatives. Mechanisms of initiation other than electron transfer were less likely, and it was concluded that the initiation process was most likely to be of the electron transfer type.     

Thermally induced cationic polymerization of divinyl ethers using iodonium and sulfonium salts

Cationic polymerization of diethyleneglycoldivinyl ether was thermally induced by diphenyliodonium, alkylbenzylsulfonium, and phenacyltetramethylenesulfonium salts. The reactivity was enhanced by using free radical sources in combination with diphenyliodonium or phenacyltetramethylenesulfonium salts. Even at low onium salt concentrations extremely reactive formulations could be obtained, e.g., the polymerization was complete within 1 minute at 100°C when using the most reactive salts. Polymerizations were induced at temperatures ranging from 50 to 180°C. The Counterion, Supplied by the onium salts, strongly influenced the appearance of the resulting crosslinked polymer: salts containing SbF⁻₆ usually gave highly discolored samples due to the large heat evolution during polymerization whereas polymerization with PF⁻₆ proceeded smoother resulting in transparent, uncolored polymers. The purity of the monomer greatly affected the initiation by the diphenyliodonium and phenacyltetramethylenesulfonium salts. Adventitious radical sources, e. g., hydroperoxides on oxidized monomer, lowered the activation temperature but also led to poor storage stability of these formulations.