Studies on propagating species in cationic polymerization of styrene derivatives by acetyl perchlorate or iodine. II. Salt effect on the relative reactivity in the cationic copolymerization of styrene derivatives with 2-chloroethyl vinyl ether initiated by iodine

To determine the effect of the dissociation of propagating species on the relative reactivity of monomers, 2-chloroethyl vinyl ether was copolymerized with p-methoxystyrene or with p-methylstyrene by using iodine in various solvents at 0°C. A common-ion salt (tetra-n-butylammonium iodide or tetra-n-butylammonium triiodide) was added to these copolymerization systems in a polar solvent to depress the dissociation of the propagating species. The addition of a common-ion salt increased the vinyl ether content in the copolymer. The more the dissociation of propagating species was depressed, the more the vinyl ether content in the copolymer increased. This effect of common-ion salt was in agreement with that of decreasing solvent polarity which yielded vinyl ether-rich copolymer as well. Therefore, the change of the monomer reactivity ratio by the solvent polarity, which used to be explained in terms of a selective solvation, must be reconsidered from the viewpoint of varying degrees of the dissociation of propagating species.     

Studies on propagating species in cationic polymerization of styrene derivatives by acetyl perchlorate or iodine. I. Bimodal molecular weight distribution of polymers produced by acetyl perchlorate or iodine

To investigate the nature of the propagating species in cationic polymerization of para-substituted styrenes, p-chlorostyrene (pCIS), p-methylstyrene (pMS), and p-methoxystyrene (pMOS), were polymerized with acetyl perchlorate or iodine in various solvents at 0°C, and the molecular weight distribution (MWD) of the polymers was measured by means of gel-permeation chromatography. When ClO_4? was a counterion, poly(pCIS) having a bimodal MWD was produced, while polymers of pMOS and pMS possessed a unimodal MWD, regardless of the solvent polarity. When more nucleophilic I^? (or I₃^?) was a counterion, however, polymers having a bimodal MWD were produced from pMOS and pMS. These results showed that either dissociated or nondissociated propagating species existed in the cationic polymerization of styrene derivatives with acetyl perchlorate or iodine, and that the type of MWD was strongly dependent on the stability of the growing cation and the nucleophilicity of the counterion.     

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.     

Cationic Copolymerization of 2-Chloroethyl Vinyl Ether with Styrene Derivatives. III. Solvent and Catalyst Effects on Relative Reactivity

Abstract

The change in relative reactivity in the cationic copolymerization of 2-chloroethyl vinyl ether and styrene derivatives was investigated with various catalysts and solvents. p-Methoxystyrene, p-methylstyrene, and a-methyl-styrene were used as styrene derivatives. The styrene content in the co-polymer increased when a polar solvent and/or a strong catalyst was used. The change of relative reactivity in the copolymerization of 2-chloroethyl vinyl ether with styrene derivatives was much greater than that in the copolymerization between vinyl ethers or styrene derivatives. When nitro-ethane was used as a solvent, not only the polarity but also the nucleophilicity influenced the copolymer composition. The results were discussed by two energies, E^π and E_rs, which are measures of complex formation between monomer and carbonium ion, and stabilization energy in the transition state, respectively.     

Cationic polymerization of p-methylstyrene initiated by acetyl perchlorate: An approach to living polymerization

The cationic polymerization of p-methylstyrene initiated by acetyl perchlorate at ?78°C led to long-lived (living-like) polymers with a narrow molecular weight distribution (M?_w/M?_n = 1.1–1.4) in methylene chloride containing a common ion salt (n-Bu₄NClO₄) or in a less polar solvent (CH₂Cl₂/toluene, 1/4v/v). Under these conditions, the number-average molecular weight (M?_n) of the polymers increased in proportion to monomer conversion and was regulated by the monomer-to-initiator ratio. When fresh feeds of the monomer were repeatedly added to a completely polymerized solution, the polymerization ensued at the same rate as before and the linear increase in M?_n with monomer conversion continued. The effects of solvent polarity and the common ion salt on the polymerization showed the suppression of the ionic dissociation of the propagating species, resulting in a “nondissociated species,” to be the key factor for the formation of the long-lived polymers.     

Sulfur-containing vinyl monomers. XIII. Cationic copolymerizations of vinyl sulfides with some vinyl monomers

Cationic copolymerizations of vinyl sulfides (VS) with some vinyl monomers with boron tri-fluoride-diethyl etherate catalyst were investigated to evaluate their monomer reactivities. The effects of VS on the copolymer yield and viscosity of the resulting copolymers revealed the inhibition or retardation mechanism which was explained in terms of the formation of a stable vinylsulfonium salt by the reaction between a propagating carbonium ion and VS monomer. From the results of copolymerizations of phenyl vinyl sulfide (PVS) with isobutyl vinyl ether (IBVE), β-chloroethyl vinyl ether (CEVE), α-methylstyrene (α-MeSt), and styrene (St), the relative reactivities of these monomers were found to be in the following order: IBVE > CEVE > PVS > α-MeSt > St. The relatively higher reactivity of PVS than St derivatives was explained on the basis of the conjugative and electron-donating nature of the VS monomer. The effects of alkyl and para-substituted phenyl groups in vinyl sulfides on their reactivities toward the propagating carbonium ion were correlated with polar factors and compared with those of the hydrolysis of α-mercaptomethyl chlorides. The transition state for the propagation reaction in cationic polymerization of VS was proposed to be a π-complex type structure.     

Cationic polymerization of styrene by acetyl perchlorate: Molecular weight distribution of polystyrene and nature of the propagating species

To clarify the nature of the propagating species in cationic polymerization of styrene catalyzed by acetyl perchlorate, the molecular weight distribution of the polymer was investigated under various conditions. The molecular weight distribution curve for the polymer obtained in methylene chloride at 0°C showed a double peak phenomenon. This suggests that two or more kinds of propagating species participate simultaneously in the propagation reaction. The weight fraction W(H) of the polymer corresponding to the higher molecular weight peak increased with increasing polarity of the solvent. W(H) decreased when the concentration of the ionic species was increased either by an increase of the catalyst concentration or by the addition of the common salt such as tetra-n-butylammonium perchlorate. On the other hand, the position of the peak in the molecular weight distribution curve was independent of polymerization conditions. It was concluded that the higher molecular weight part of the polymer was produced under conditions for conductive to dissociation of the propagating species and the less dissociated propagating species was responsible for the lower molecular weight part of the polymer.     

Alternating copolymerization of 2,3-dichloro-5,6-dicyano-p-benzoquinone with styrene and polymerization behavior with vinyloxy compounds

2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ) was found to copolymerize alternatingly with styrene (St). DDQ–isobutyl vinyl ether and DDQ–2-chloroethyl vinyl ether systems gave homopolymers of vinyl ethers, while DDQ–phenyl vinyl ether and DDQ–vinyl acetate systems gave oligomers containing both monomer units. In the terpolymerization of DDQ, p-chloranil (pCA), and St, terpolymers obtained were found to have about 50 mole % of St units regardless of monomer feed ratio and DDQ was incorporated much more rapidly into the terpolymer than pCA. The difference in the reactivity of the acceptor monomers could be attributed to that in their electron-accepting character.     

Cationic polymerization of vinyl compounds in the presence of tetra-n-butylammonium salts

The effects of salts were examined in cationic polymerization of vinyl compounds. Cationic polymerization of styrene was carried out at 0°C, with acetyl perchlorate, stannic chloride, stannic chloride–trichloroacetic acid and boron trifluoride etherate as catalysts. Tetra-n-butylammonium perchlorate, fluoroborate and iodide were used as salts. The presence of small amounts of the salts changed both the polymerization rate and the molecular weight of polymer considerably. The consideration of various effects led to the conclusion that the results are explicable principally on the basis of counterion exchange. To confirm this, the copolymerization of 2-chloroethyl vinyl ether with γ-methylstyrene was investigated at ?78°C. The copolymer composition curve when stannic chloride was used as catalyst was changed and coincided with that of polymer obtained with acetyl perchlorate catalysis when the perchlorate salt was added. This supports the concept of counterion exchange.     

Selective dimerization of styrene to 1,3-diphenyl-1-butene by acetyl perchlorate

The linear unsaturated dimer of styrene, 1,3-diphenyl-1-butene, was obtained exclusively in the oligomerization of styrene by acetyl perchlorate in various solvents. In benzene, the linear dimer was produced in more than 90% yield at 50°C. In n-hexane and cyclohexane, the yield of the linear dimer was lower. The yield of the linear dimer was strongly dependent on the nature of solvent. When an increasing amount of 1,2-dichloroethane was added to benzene, the yield of the linear dimer gradually decreased. On the other hand, when a small amount of 1,2-dichloroethane was added to n-hexane or cyclohexane, the yield of the linear dimer increased. The yield of the linear dimer was almost independent of the reaction temperature and the initiator concentration. For comparison, the dimerization of α-methylstyrene was carried out, and the effects of the initiator and the solvent on the structure of dimers were investigated. On the basis of these results, the mechanism of the dimerization of styrene initiated by acetyl perchlorate is discussed.     

Effect of reaction medium on the ionizing radiation-initiated cationic copolymerization of styrene derivatives

The influence of reaction medium polarity on the ionizing radiation-initiated copolymer-ization of styrene derivatives involving unpaired carbocations is examined. In the copo-lymerization of nonpolar monomers such as p-CH3styrene/styrene, a small effect consistent with that predicted by Laidler-Eyring theory is found. In the copolymerization of nonpolar/polar monomer pairs such as p-CH3styrene/p-Clstyrene and styrene/p-Clstyrene, any such effect is masked by specific solvation phenomena. A competition between such phenomena appears to exist, in which the different cations are dominated by different interactions. The p-Clstyryl cation appears to undergo strong intramolecular complexation with the penultimate aromatic ring in nonpolar conditions; thus, this cation displays increased se-lectivity toward monomers best able to disrupt the complex. The p-CH₃styryl and styryl cations do not appear to be subject to such strong complexation; thus, in nonpolar solvent, their selectivity tends towards monomers with the highest cation-solvating ability. The differing copolymerization behavior of the p-Clstyryl cation is consistent with the findings of previous investigations of the effect of reaction medium on chain transfer with these monomers. © 1995 John Wiley & Sons, Inc.     

Dimerization of methylstyrenes by acetyl perchlorate and trifluoromethanesulfonic acid

Ring-substituted methylstyrenes (p-, m-, and o-methylstyrenes) in conjunction with acetyl perchlorate (AcClO₄) or trifluoromethanesulfonic acid as catalysts gave their linear unsaturated dimer in high yield in benzene at temperatures from 50 to 70°C. In particular, the yield of o-methylstyrene dimer was as high as 90% in the AcClO₄ catalysis at 50°C. The dimer yield depended on solvent and catalyst. The terminal structures of the dimers and higher oligomers were analyzed by NMR spectroscopy. Oligomers with a cyclic terminal structure increased in the products at higher temperature. The dimer yield was improved by codimerizing p-methylstyrene with less reactive m-methylstyrene or styrene with AcClO₄ catalyst. The dimers obtained partly consisted of linear unsaturated codimers.     

Anionic copolymerization of α-methylstyrene and styrene: Kinetics and mechanism

A kinetic study of copolymerization of styrene and α-methylstyrene accompanied with depropagation, initiated by n-butyilithium in cyclohexane with tetrahydrofuran as an additive polar solvent, has been performed. The various propagation rate constants of active species and the complexation equilibrium constants between different kinds of active species were determined. Furthermore, the reactivity ratios of two monomers with regard to monomeric, monoetherated and dietherated active species were obtained.     

Alternating copolymerization of polar vinyl monomers in the presence of zinc chloride. I. Propagation and initiation of the copolymerization of acrylonitrile with styrene copolymer

Copolymerization of acrylonitrile with styrene spontaneously occurred on addition of zinc chloride without addition of any other radical initiator. The composition of the copolymer approached that of strictly alternating copolymer as zinc chloride added to the copolymerization system increased. The significance of the apparent monomer reactivity ratios of this copolymerization system was studied from a kinetic point of view, and it was shown that the monomer sequence distribution is indicated by the apparent monomer reactivity ratios. Further, equations which represent the relation between the apparent monomer reactivity ratios and Q,e values at a given salt concentration were derived. These equations reasonably accounted for the decrease of the apparent monomer reactivity ratios of the copolymerization of acrylonitrile with styrene in the presence of zinc chloride and the behavior of the other acrylonitrile copolymerization systems in the presence of zinc chloride. The initiation step of the spontaneous radical copolymerization of acrylonitrile with styrene in the presence of zinc chloride was explained by a cross-initiation mechanism.     

Selective vinyl cationic polymerization of monomers with two cationically polymerizable groups. II. p-vinylphenyl glycidyl ether: An epoxy-functionalized styrene

p-Vinylphenyl glycidyl ether (VPGE), a styrene derivative with an epoxy pendant, was polymerized by various cationic initiators, and its selective vinyl polymerization was investigated at low temperatures below ?15°C. BF₃OEt₂ (a metal halide) and CF₃SO₃H (a strong protonic acid) polymerized both vinyl and epoxy groups of VPGE, and produced cross-linked insoluble polymers. The HI/I₂ initiating system and iodine, in contrast, polymerized its vinyl group in polar solvents (CH₂Cl₂ and nitroethane) highly selectively in the temperature range of ?15 to ?40°C to give soluble polymers with a polystyrene backbone and epoxy pendants; however, under these conditions, 10–15% of the epoxy groups of the polymers were consumed during the polymerization by the reaction with the growing species. The polymerization by HI/I₂ in CH₂CI₂ involved a long-lived propagating species, as indicated by a progressive increase in the molecular weight (M?_n) of the polymers with monomer conversion and their fairly narrow molecular weight distributions (M?_w/M?_n ～ 1.6). The differences between the polymerizations of VPGE and p-isopropenylphenyl glycidyl ether, an α-methylstyrene-type counterpart of VPGE, were also discussed with an emphasis on the effects of the α-methyl group in the latter monomer.     

Kinetics of free-radical copolymerization of α-methylstyrene and styrene

The free-radical copolymerization of α-methylstyrene and styrene has been studied in toluene and dimethyl phthalate solutions at 60°C. Gas chromatography was used to monitor the rate of consumption of monomers. For styrene alone, the measured rate of polymerization R_p and M?_n of the polymer coincided with values expected from previous studies by other workers. Solution viscosity η affected R_p and M?_n of styrene homopolymers and copolymers as expected on the basis of an inverse proportionality between η^1/2 and termination rate. The rate of initiation by azobisisobutyronitrile appears to be independent of monomer feed composition in this system. Molecular weights of copolymers can be accounted for by considering combinative termination only. The effects of radical chain transfer are not significant. A theory is proposed in which the rate of termination of copolymer radicals is derived statistically from an ideal free-radical polymerization model. This simple theory accounts quantitatively for R_p and M?_n data reported here and for the results of other workers who have favored more complicated reaction models because of the apparent failure of simple copolymer reactivity ratios to predict polymer composition. This deficiency results from systematic losses of low molecular weight copolymer species in some analyses. Copolymer reactivity ratios derived with the assumption of a simple copolymer model and based on rates of monomer loss can be used to predict R_p values measured in other laboratories without necessity for consideration of depropagation or penultimate unit effects. The 60°C rate constants for propagation and termination in styrene homopolymerization were taken to be 176 and 2.7 × 10⁷ mole/l.-sec, respectively. The corresponding figures for α-methylstyrene are 26 and 8.1 × 10⁸ mole/l.-sec. These constants account for the sluggish copolymerization behavior of the latter monomer and the low molecular weights of its copolymers. The simple reaction scheme proposed here suggests that high molecular weight styrene–α-methylstyrene copolymers can be produced at reasonable rates at 60°C by emulsion polymerization. This is shown to be the case.     

Solution copolymerization of styrene and 2-hydroxyethyl mechacrylate.

The rate of solution copolymerization of styrene (M₁) and 2-hydroxyethyl methacrylate (M₂) was investigated by dilatometry. N,N-dimethyl formamide, toluene, isopropyl alcohol, and butyl alcohol were used as solvents. Polymerization was initiated by α,α′-azobisisobutyronitrile at 60°C. The initial copolymerization rate increased nonlinearly with increasing 2-hydroxyethyl methacrylate (HEMA)/styrene ratio. The copolymerization rate was promoted by solvents containing hydroxyl groups. Two different approaches were used for the prediction of copolymerization rates. The relationships proposed for the copolymerization rates calculation involve the effects of the total monomer concentration, mole fraction of HEMA, and of the solvent type. Different reactivity ratios were found in polar and nonpolar solvents: r₁ = 0.53, r₂ = 0.59 in N,N-dimethyl formamide, isopropyl alcohol and n-butyl alcohol; r₁ = 0.50, r₂ = 1.65 in toluene. The usability of these reactivity ratios was confirmed by batch experiments.     

Effects of solvent as an electron-pair acceptor on propagation reactions during radical polymerization and copolymerization of polar vinyl monomers

We carried out radical homopolymerization and copolymerization in various kinds of solvents at 60°C by using diisopropyl fumarate (DiPF) and methyl methacrylate (MMA) as electron-accepting polar monomers and styrene (St) and vinyl benzoate (VB) as electron-donating monomers. The highest polymerization rate was observed in the polar and electron-pair accepting solvents, such as 2,2,2-trifluoroethanol for the homopolymerization and copolymerization of these monomers. It has been revealed that the polymerization rate is correlated to the electron-pair–accepting property of the solvent used, rather than the polarity in the linear free energy relationship. We have demonstrated the validity of the acceptor number as the index for interpreting the interaction of the solvent with the monomer and the propagating chain end. The monomer reactivity ratios were determined for the St–DiPF, VB–DiPF, and St–MMA copolymerizations. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2803–2814, 1999     

Investigation of cationic copolymerization of styrene with p-benzoquinone

The kinetics of copolymerization of styrene with p-benzoquinone in the presence of BF₃OEt₂ is investigated. The rate constants and activation energy of the copolymerization process are determined. The reaction orders for monomer and catalyst are estimated. It is found that the rate of styrene and quinone copolymerization increase and the induction period decreases owing to addition of the latter. It is shown that the copolymerization rate achieves its maximum at an equimolal ratio of monomers. This phenomenon is explained by formation of active molecular complex between styrene and p-benzoquinone. On the basis of obtained data the course of copolymerization is interpreted and the cationic mechanism of copolymerization process is proposed.     

Phenolic resins for solid-phase peptide synthesis: Copolymerization of styrene and p-acetoxystyrene

Details are given of the synthesis and purification of p-acetoxystyrene and its solution and suspension copolymerization with styrene. Reactivity ratios, evaluated by the Tidwell-Mortimer method, were r₁ (p-acetoxystyrene) = 1.18, and r₂ (styrene) = 0.88 for (bulk) solution copolymerization. Corresponding values of the reactivity ratios for suspension copolymerization were, within experimental error, indistinguishable from unity. Thus the copolymer composition is governed simply by the monomer feed composition. Use of a specially designed reactor vessel permits convenient suspension copolymerization of styrene, p-acetoxystyrene, and divinylbenzene to give crosslinked resins having comparatively narrow particle size distributions. Acetoxy groups in the crosslinked resin are cleaved by hydrazine hydrate under very mild conditions to give crosslinked polystyrenes having phenolic groups which, in turn, provide a useful alternative to the more usual chloromethylated polystyrene resins for solid-phase peptide synthesis.