Some effects of the initiator on the cationic polymerization of alkenes

The content of this article is indicated by what could be its full title: “An Explanation of the dependence of the rate of the cationic polymerizations of alkenes and of the DP of their products, on the reaction variables, especially the size of the anionic moiety of the initiator.” We continue here the discussion started in 1965 and show mathematically how the theory of dieidic polymerizations by unpaired and paired cations can explain why some of these polymerizations become faster with falling temperature, why the Arrhenius plot of the DP of the polymers obtained from most such systems shows a discontinuity or kink, and also how the temperature of minimum rate, T_M, and that at which the kink occurs, T_K, depend on the reaction variables, namely the concentrations of monomer, m, and of initiator, c, and the a, D, and T (interionic distance in the ion‐pair, dielectric constant of the reaction mixture and temperature). Our treatment explains why the most effective way of achieving the economically desirable aim, to make the longest polymers at the highest possible temperatures, is by maximizing the product a.D, so as to increase the T_K, preferably by the use of polar solvents and initiators with large anions. The choice of such combinations by several investigators, but for other, vaguer, reasons, is given here a theoretical basis. Our argument is illustrated by Literature examples and is presented in the form of a new diagram (the Plesch‐Austin plot) which shows the T_K as a function of a.D for several systems. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4265–4284, 2008     

Some new developments in vinyl ether polymerization

Different combinations of acetals with trimethylsilyl iodide have been explored as new initiating systems for the vinyl ether polymerization. The resulting polymers are characterized by controlled molecular weights and narrow molecular weight distributions, confirming the living polymerization mechanism. Acetals can also be used as transfer agents in the polymerization of vinyl ethers. When using 1,1-diethoxyethane (DEE) as transfer agent and isobutyl vinyl ether (IBVE) as monomer, a transfer constant of 0.2 was obtained (at −40°C in toluene). This method, transposed to functional acetals, provides a new way to prepare polyvinyl ethers with one or two functional end groups. The cationic polymerization of isobutyl vinyl ether initiated with the combination triflic acid/thietane, where thietane acts as electron donating moderator, leads to star-shaped polyvinylether-polythietane block-copolymers (at −40°C in dichloromethane). The block-copolymer structure is obtained because the vinyl ether polymerization is stopped when the α-alkoxy thietanium ion (active species) is attacked by a thietane molecule, which is at the same time an initiation reaction for the thietane polymerization. The star-shaped structure of the block-polymer is the result of the intermolecular termination in the cationic polymerization of thietane. When using a bifunctional initiator system, a polymer network is obtained consisting of linear polyIBVE-segments interconnected by branched polythietane segments. These findings support the sulfonium ion structure of the active species in the cationic polymerization of vinyl ethers initiated by the acid-sulfide system.     

Redox initiated cationic polymerization

A novel catalytic method for carrying out the cationic polymerizations has been developed based on a redox initiator system in which the reducing component is delivered to the reaction mixture in the vapor state. The redox couple consists of a diaryliodonium salt that is dissolved in the monomer and a noble metal catalyst is added. The silane reducing agent is introduced to the reaction mixture in the vapor state using air as the carrier gas. Reduction of the diaryliodonium salt by the silane results in the liberation of a Brønsted superacid that initiates cationic polymerizations. A study of the effects of variations in the structures of the diaryliodonium salt, the silane, and the type of noble metal catalyst was carried out. In principle, the initiator system is applicable to all types of cationically polymerizable monomers and oligomers including: the ring‐opening polymerizations of such heterocyclic monomers as epoxides and oxetanes and, in addition, the polymerization of vinyl ether monomers such as vinyl ethers. The use of this initiator system for carrying out commercially attractive cross‐linking polymerizations for coatings, composites, and encapsulations is discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1825–1835, 2009     

Dye-sensitized photoinitiated cationic polymerization

Several dyes have been discovered which sensitize the photolysis of diaryliodonium salts at visible wavelengths. By using the combination of a dye and a diaryliodonium salt, visible light can be used to photoinitiate cationic polymerization. In this article, the dye-sensitized photopolymerizations of cyclohexene oxide, epichlorohydrin, and 2-chloroethyl vinyl ether are described.     

Electroinitiated cationic polymerization of styrene

A constant controlled current was passed through a solution of styrene in methylene chloride containing a tetraalkylammonium salt as supporting electrolyte. Reproducible rates of polymerization were initiated by the electrochemical techniques employed and the kinetics of the reaction were investigated. Sigmoidal curves of conversion versus time were observed. A kinetic relationship of the form In ([M₀]/[M]) = ½ Kt² was derived on the basis of simple assumptions regarding the mechanism and fitted the data accurately. The rate constants obtained were compared to others reported, and the influences of ion association on the values of the rate constants obtained are discussed. The reactions were decreased in rate by a reversal of polarity of the electrodes. However, the stoichiometry of the production of active centers and of their destruction was not ideal, in that each electron did not result in the initiation of a polymer chain.     

Some new initiators and inhibitors in vinyl polymerization

This work examines some discrepancies between claimed effects in polymerization of certain metal acetylacetonates. Cupric acetylacetonate inhibits the thermal polymerization of styrene but acts only as a retarder in the sensitized polymerization of styrene and in the thermal polymerization of methyl methacrylate. Aluminium acetylacetonate initiates weakly the polymerization of styrene, but ferric acetylacetonate does not influence the rates of polymerization of the above mentioned monomers.     

Modelling propagation in cationic polymerization

Propagation in cationic polymerization is modelled by ethene homopolymerization. Cationization and three propagation steps are investigated by the MINDO/3 method employing complete and partial optimization of geometry. A potential energy surface is calculated describing the first propagation step, the nucleophilic attack of ethene on an ethyl cation. The results indicate a reactant-like activated complex and three energetic minima representing structures of the products for both the first and the second propagation step. The thermodynamic foundation of polyreactions is well reflected both with and without consideration of statistic-thermodynamic calculations.     

Living cationic polymerization of vinyl monomers: Mechanism and synthesis of new polymers

This paper reviews the recent progress in our research on the living cationic polymerization of vinyl compounds by the hydrogen iodide/iodine (HI/I₂) initiating system, with emphasis on its scope, mechanism, and applications to new polymer synthesis. The scope of the living cationic polymerization has been expanded to include vinyl ethers, propenyl ethers, unsaturated cyclic ethers, and styrene derivatives as monomers. The initiation/propagation mechanism was discussed on the basis of recent direct analysis on the living system by NMR and UV/visible spectroscopy. The proposed mechanism involves a quantitative formation of Hl-vinyl ether adduct [CH₃-CH(OR)-I; l] that is by itself incapable of initiating polymerization. In the presence of iodine, however, the CH-I bond of l is electrophilically activated by iodine and living propagation occurs via the insertion of vinyl ether to the activated CH-I bond. Such living polymerizations were found to proceed in not only nonpolar but polar solvents (CH₂Cl₂) as well. Quenching the living end with amines gave polymers capped with an amino group that in turn enabled us to determine the living end concentration. Applications of the HI/I₂-initiated living process to the synthesis of new bifunctional and block polymers were also described.     

A new multifunctional initiator system for the living cationic polymerization of vinyl ethers

Combination of hexa(chloromethyl)melamine (HCMM) and zinc chloride was found to be a multifunctional initiator system for the living cationic polymerization of isobutyl vinyl ether. HCMM was synthesized by reaction of hexa(methoxymethyl)melamine and boron trichloride. Characterization of the polymers by means of GPC and ¹H NMR showed that initiation was rapid and quantitative and that the initiator is hexafunctional, leading to six‐armed star‐shaped polymers.     

A new visible light sensitive photoinitiator system for the cationic polymerization of epoxides

This communication reports the development of an efficient three‐component visible light sensitive photoinitiator system for the cationic ring‐opening photopolymerization of epoxide monomers and epoxide functional oligomers. The photoinitiator system consists of camphorquinone in combination with a benzyl alcohol to generate free radicals by the absorption of visible light. Subsequently, the radicals participate in the free radical chain induced decomposition of a diaryliodonium salt. The resulting strong Brønsted acid derived from this process catalyzes the cationic ring‐opening polymerization of a variety of epoxide substrates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 866–875, 2009     

Redox initiated cationic polymerization of oxetanes

3,3‐Disubstituted oxetane monomers were found to undergo rapid, exothermic redox initiated cationic ring‐opening polymerization in the presence of a diaryliodonium or triarylsulfonium salt oxidizing agent and a hydrosilane reducing agent. The redox reaction requires a noble metal complex as a catalyst and several potential catalysts were evaluated. The palladium complex, Cl₂(COD)Pd^II, was observed to provide good shelf life stability while, at the same time, affording high reactivity in the presence of a variety of hydrosilane reducing agents. A range of structurally diverse oxetane monomers undergo polymerization under redox cationic conditions. When a small amount of an alkylated epoxide was added as a “kick‐start” accelerator to these same oxetanes, the redox initiated cationic polymerizations were extraordinarily rapid owing to the marked reduction in the induction period. A mechanistic interpretation of these results is offered. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1854–1861     

Living cationic polymerization of vinylnaphthalene derivatives

The living cationic polymerization of 6‐tert‐butoxy‐2‐vinylnaphthalene (tBOVN), a vinylnaphthalene derivative with an electron‐donating group, was achieved with a TiCl₄/SnCl₄ combined initiating system in the presence of ethyl acetate as an added base at –30 °C. The absence of side reactions at low temperature was confirmed by ¹H NMR analysis of the resulting polymer. In contrast to this controlled reaction at –30 °C, reactions performed at higher temperature, such as 0 °C, frequently involved unwanted intramolecular or intermolecular Friedel–Crafts reactions of naphthalene rings due to the high electron density of these rings. The cationic polymerization of 6‐acetoxy‐2‐vinylnaphthalene, a derivative with an acetoxy group, was also controlled under similar conditions, but chain transfer reactions were not completely suppressed during the polymerization of 2‐vinylnaphthalene. The glass transition temperature (T_g) of the obtained poly(tBOVN) was 157 °C, a value higher by 94 °C than that of the corresponding styrene derivative. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4828–4834     

Living cationic polymerization of vinyl ethers

The cationic polymerization of vinyl ethers initiated by CH₃-CH(OR)(I) / R₄N⁺A⁻ (R = Alkyl, A⁻ = ClO₄⁻, BF₄⁻, PF₆⁻, I⁻, NO₃⁻) shows the characteristics of a living polymerization. The rate of polymerization is a function of the solvent polarity, the temperature, the type and concentration of the ammonium salt. The experimental data can be explained on the basis of the secondary salt effect overlapped by some dipol-dipol interactions of the chain end and the added salt. Functionalization of the chain end with thermolabile azo functions yields polymeric initiator which was applied for the synthesis of blockcopolymers. Vinyl ethers functionalized with furylacrylic ester groups were polymerized and crosslinked via [2+2] cycloaddition.     

Reactivity of fluorostyrenes in cationic polymerization

Reactivity ratios relative for cationic copolymerization of three fluorostyrenes and styrene were studied. The values of r₁ and r₂ for various experimental conditions were determined. The influence of the nature of the solvent and of the polymerization temperature were studied in particular. Relative activation entropies and enthalpies were determined, and an isokinetic relationship was found for 2-, 3-, and 4-fluorostyrenes. There is a fairly linear correlation between the C8 chemical shift and the values of 1/r₂. All the experimental reactivities were correlated with the quantum chemistry parameters. From this correlation, interaction with C8 and also with C7 and F, was found to be possible, depending on the nature of the monomer.     

Photoinitiated cationic polymerization of monofunctional benzoxazine

The photoinitiated ring‐opening cationic polymerization of a monofunctional benzoxazine, 3‐phenyl‐3,4‐dihydro‐2H‐1,3‐benzoxazine, with onium salts such as diphenyliodonium hexafluorophosphate and triphenylsulfonium hexafluorophosphate as initiators was examined. The structures of the polymers thus formed were complex and related to the ring‐opening process of the protonated monomer either at the oxygen or nitrogen atoms. The phenolic mechanism also contributed, but its influence decreased with decreasing monomer concentration. Thermal properties of the polymers were also investigated by differential scanning calorimetry and thermogravimetric analysis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3320–3328, 2003     

Design of new polymer architectures by combination of anionic and cationic living polymerization techniques

The grafting of polystyryl lithium onto poly(chloroethyl vinyl ether) chains has been investigated. The reaction proceeds cleanly and quantitatively thus allowing the synthesis of comblike polymers. Since the dimensions of the polystyrene branches and of the poly(chloroethyl vinyl ether) backbone can be controlled by living polymerizations, both the length and the number of branches of the graft copolymers can be tuned. The latter behave as star polymers. The possibility to initiate a new cationic polymerization of chloroethyl vinyl ether from polystyrene branches bearing acetal termini in order to prepare the corresponding stars with poly(chloroethyl vinyl ether-b- styrene) branches is also examined. Finally access to hyperbranched polymers of controlled architecture and dimensions by deactivation of a second amount of polystyryl lithium onto the last blocks of poly(chloroethyl vinyl ether) is also reported.     

Metal-free multicomponent polymerization toward cationic polyamidines

Cationic polymers, also known as polycations, are considered to be the most potential non-viral gene carriers due to their unique advantages such as the ability to bind the negative charge of nucleic acid molecules. Multicomponent polymerization（MCP） is a one-step, tandem strategy to construct complex structures based on multicomponent reactions. Herein, we developed a metal-free MCP method based on three monomers of p-dinitrovinylbenzene（p-DNVB）, 1,1-dimethylethyl N,N-dibromocarbamate（BocNBr     

Emulsifier-free emulsion polymerization with cationic comonomer

An earlier article¹ described the emulsion polymerization of styrene and various anionic comonomers, together with an anionic initiator, to give uniform latices at ca. 35% solids content. This article extends the work to cationic systems. Cationic comonomers 1,2-dimethyl 5-vinylpyridinium methylsulfate and 1-ethyl 2-methyl 5-vinylpyridinium bromide were synthesized and used with azobis(isobutyramidine hydrocholoride) initiator in the emulsifier- free emulsion polymerization of styrene. Recipes and results were generally comparable to those of the anionic systems, excepts for the dependence of particle diameter on comonomers concentration. Here the initial decrease was followed by an increase in particle diameter at higher comonomer content. The surface charge increased sharply with comonomer content.