Mechanism of the NCA polymerization. VI. Investigations on cocatalysts of the base-initiated NCA polymerization

The triethylamine-initiated polymerization of glycine-NCA [N-carboxylic acid anhydrides (oxazolidine-2,5-diones)], L -alanine-NCA, and sarcosine-NCA, as well as the pyridine-initiated polymerization of sarcosine-NCA, were carried out in the presence of potential cocatalysts. The 11 electrophilic reagents tested in this work can be divided into two classes: N-acyllactams and similar compounds, which are less reactive than the monomers and have no influence on the polymerization; and isocyanates and N-acyl-NCAs or -NTAs [N-thiocarboxylic acid anhydrides (thiazolldine-2,5-diones)], which are more electrophilic than the monomers and behave as cocatalysts in the case of glycine-NCA and alanine-NCA, since their base-initiated polymerization proceeds via the attack of NCA anions on the electrophilic N-acyl NCA chain and (“activated monomer mechanism”). In the case of sarcosine-NCA, however, the propagation involves a nucleophilic chain end (“carbamate mechanism”) and the strong electrophilic reagents behave as inhibitors.     

Photo and thermal polymerizations sensitized by donor–acceptor interaction. II. Photopolymerization and electronic spectroscopy of the isobutyl vinyl ether–acrylonitrile system

Photopolymerization of acrylonitrile (AN), an acceptor monomer, was found to be accelerated in the presence of isobutyl vinyl ether (IBVE), a donor monomer. The propagation is completed by a radical mechanism as judged by copolymer compositions; in contrast to the N-vinylcarbazole–AN system studied previously. This photopolymerization system is entirely stable if kept in the dark. The comparison of the relation between R_p and [IBVE]/[AN] ratio in the monomer feed found for the spontaneous photopolymerization with that for radical polymerization initiated by azobisisobutylonitrile in the dark leads to the conclusion that the rate of photoinitiation is enhanced by the interaction between AN and IBVE, whereas the propagation step by a radical mechanism is retarded by increasing concentration of IBVE. The contact charge-transfer complex between IBVE and AN was confirmed by electronic spectroscopy of the polymerization system, which showed photosensitization by charge-transfer interaction. The spectroscopic study of other weak donor–weak acceptor systems is also discussed.     

Cyclopolymerization. II. Mechanism of the free-radical polymerization of N-n-propyldimethacrylamide

The mechanism of cyclopolymerization was investigated by using N-n-propyldimethacrylamide (PDMA). Completely cyclized polymers were formed on polymerization of PDMA by a radical initiator. Moreover, those copolymers of PDMA and various monomers, such as styrene, methyl methacrylate, and vinyl acetate, obtained did not contain any detectable pendent double bonds. The kinetic investigation showed that the termination reaction proceeded between the cyclized radicals. The attempted polymerization of N-n-propyl-N-isobutyrylmethacrylamide, the monofunctional counterpart of PDMA, was failed. These results appear to confirm that cyclopolymerization of PDMA proceeds through a concerted mechanism which has been proposed for the mechanism of the cyclopolymerization of various difunctional monomers. Measurement of the ESR spectra of propagating radical has, however, revealed that the rate-determining step of the cyclopolymerization of PDMA is not intermolecular propagation but intramolecular cyclization, which indicates that the cyclization reaction proceeds in a stepwise way. This apparent contradiction was explained based upon thermodynamic considerations.     

Mechanism of N-vinylcarbazole polymerization on Co(II)–13X molecular sieve

Kinetics of polymerization of N-vinylcarbazole over Co(II)-13X molecular sieves in toluene have been studied. The rate of polymerization (R_p) has been found to be second order with respect to percent exchange level of Co(II) and also to the NVC concentration at all the reaction temperatures of 40, 50 and 60°C. The rate increases with decreasing pH of the original exchanging salt solution up to a pH of about 3.5, beyond which it falls. The overall activation energy of polymerization has been found to decrease with increase in monomer concentration, exchange level of Co(II), and the hydrogen ion concentration of the original exchange solution. Average degree of polymerization also follows a similar trend. A mechanism of polymerization involving simultaneous propagation on both metal ion Co(II) and proton on a zeolite surface has been suggested. The two propagation routes are characterized by an average activation energy of 10.36 kcal/mol and 5.40 kcal/mol on the metal ion and proton centers, respectively.     

Solid-state polymerization of acrylamide and its derivatives complexed with some lewis acids. II. Radiation-induced in-source polymerization

Radiation-induced solid-state polymerizations of complexes of N-tert-butylacrylamide, N-tert-amylacrylamide, and N-tert-hexylacrylamide with zinc chloride and zinc bromide have been studied. An accelerating effect of temperature and an inhibiting effect of oxygen on the polymerization process were observed. The activation energies have been established. The influence of monomer structure as well as the halide used on the polymerization rate have been discussed and some regularities have been pointed out. The polymers obtained show good solubilities in common solvents, which proves that they are not crosslinked.     

Anionic polymerization of N-ethyl-2-vinylcarbazole and N-ethyl-3-vinylcarbazole

The polymerization of N-ethyl-2-vinylcarbazole and N-ethyl-3-vinylcarbazole by an anionic mechanism has been demonstrated. Polymerization reactions were monitored by ultraviolet/visible spectroscopy and λ_max and ε values for the propagating carbanions determined. The 2-vinyl monomer exhibits all the features of a standard “living” polymer; the carbanion is stable at ambient temperatures and high molecular weight, M?_n ? 10⁶, narrow distribution polymers and block copolymers with styrene have been prepared. The carbanion from the 3-vinyl monomer is much less stable and a clean polymerization can only be conducted at temperatures below -60°C. A comparison of the anionic polymerization characteristics of the N-, 2-, and 3-vinyl carbazole monomer series is presented.     

Solid-state initiation of polymerization of N-vinylcarbazole by gases

Chlorine gas has been shown by previous investigators to initiate the polymerization of solid N-vinylcarbazole at room temperature, giving a maximum yield of 66% polymer after 18 hr. This initiation of polymerization of N-vinylcarbazole without the application of heat, by a gas, is the only solid-state initiation other than those that are radiation-induced known to us. This study was undertaken in order to determine both the scope and the mechanism of the room-temperature solid-state initiation of vinyl polymerization of N-vinylcarbazole by gases. The gases (HCl, Cl₂, and N₂O₄) were absorbed by solid N-vinylcarbazole, giving very rapid exothermic polymerization: HCl, 37% polymer yield, M?_n 2500; Cl₂, 50% polymer yield, M?_n 4703; N₂O₄, 91% polymer yield, M?_n 7073. The gases NOCl, BF₃, and HBr were not absorbed by N-vinylcarbazole and did not initiate polymerization. The N₂O₄-initiated polymerization, which gave a high yield of polymer, was complete within 5 min. after introduction of gas. This polymerization method and the resulting polymer compared favorably with conventional peroxide-initiated melt polymerization and the polymer obtained thereform. The mechanism of gas-initiated polymerization, which was studied with introduction of inhibitors, appears to be classically cationic in nature.     

Anionic polymerization of N-substituted maleimide. II. Polymerization of N-ethylmaleimide

Anionic polymerization of N-ethylmaleimide (N-EMI) was carried out with potassium t-butoxide, lithium t-butoxide, n-butyllithium, and ethylmagnesium bromide as initiators in THF and in toluene. An almost quantitative yield of poly(N-EMI) was obtained with potassium t-butoxide as initiator in THF in a wide range of polymerization temperatures. Initiators possessing lithium as counter cation produced poly(N-EMI) in slightly lower yields and ethylmagnesium bromide gave the polymer only in less than 35% yield in THF. As a polymerization reaction solvent, THF was preferable for the polymerization of N-EMI compared with toluene with respect to polymer yields. Poly(N-EMI) obtained with anionic initiators exerted unimodal molecular weight distribution. From ¹H- and ¹³C-NMR spectra of poly(N-EMI) anionic polymerization of N-EMI with potassium t-butoxide was revealed to proceed at carbon–carbon double bond. t-Butoxide system was found to have a “living” polymerization character, i.e., the observed average degree of polymerization was in good agreement with the one calculated from the initial molar ratio of N-EMI/initiator and the yield of polymer.     

Cationic polymerization of N-vinyl carbazole by trityl salts—II.: New considerations about the mechanism

The mechanism of initiation in the polymerization of N-vinyl carbazole by trityl salts in methylene dichloride has been investigated by spectroscopic studies of final reaction mixtures and molecular weight analysis. The proposed reaction mechanism involves regeneration of the Ph₃C⁺ cation and propagation through a dication as active species.     

Electrochemical and chemical polymerization of acrylamide

The electrochemical and chemical polymerization of acrylamide (AA) has been studied. The electrolysis of the monomer in N,N-dimethylformamide (DMF) containing (C₄H₉)₄NClO₄ as the supporting electrolyte leads to polymer formation in both anode and cathode compartments. The cathodic polymer dissolves in the reaction mixture and the anodic polymer precipitates during the course of polymerization. A plausible mechanism for the anodic and cathodic initiation reaction has been given. The chemical polymerization of acrylamide that has been initiated by HClO₄ is analogous to its anodic polymerization. The polymer yield increases with an increase in concentration of the monomer and HClO₄. Raising the reaction temperature also enhances the polymerization rate. The overall apparent activation energy of the polymerization was determined to be ca. 19 kcal/mole. The copolymerization of acrylamide was carried out with methyl methacrylate (MMA) in a solution of HClO₄ in DMF. The reactivity ratios are r₁ (AA) = 0.25 and r₂ = 2.50. The polymerization with HClO₄ appears to be by a free radical mechanism. When the polymerization of acrylamide is carried out with HClO₄ in H₂O, a crosslinked water-insoluble gel formation takes place.     

Radical polymerization behavior of ethyl ortho-formylphenyl fumarate involving intramolecular hydrogen abstraction

The radical polymerization behavior of ethyl ortho-formyl-phenyl fumarate (EFPF) using dimethyl 2,2′-azobisisobutyrate (MAIB) as initiator was studied in benzene kinetically and ESR spectroscopically. The polymerization rate (R_p) at 60°C was given by R_p = k[MAIB]^0.76[EFPF]^0.56. The number-average molecular weight of poly(EFPF) was in the range of 1600–2900. EFPF was also easily photopolymerized at room temperature without any photosensitizer probably because of the photosensitivity of the formyl group of monomer. Analysis of ¹H? and ¹³C-NMR spectra of the resulting polymer revealed that the radical polymerization of EFPF proceeds in a complicated manner involving vinyl addition and intramolecular hydrogen-abstraction. The polymerization system was found to involve ESR-observable poly(EFPF) radicals under the actual polymerization conditions. ESR-determined rate constant (2.4–4.0 L/mol s) of propagation at 60°C increased with decreasing monomer concentration, which is mainly responsible for the observed low de-pendency of R_p on the EFPF concentration. Copolymerizations of EFPF with some vinyl monomers were also examined. © 1995 John Wiley & Sons, Inc.     

Raman spectroscopic studies of the solid-state polymerization of diacetylenes. II. Thermal polymerization of 1,6-di-p-methoxybenzene-sulfonyloxy-2,4-hexadiyne

The Raman spectra of 1,6-di-p-methoxybenzene-sulfonyloxy-2,4-hexadiyne (MBS) have been recorded during thermal polymerization. The spectra are similar to those of the related p-toluene-sulfonyloxy monomer but indicate a higher strain in the initially formed MBS polymer chains. Despite this similarity, the polymerization kinetics for the two monomers are markedly different. The polymerization behavior of MBS shows that the polymer chain initiation and propagation are practically independent of lattice strain. Possible causes for this independence are discussed.     

Cationic polymerization of N-ethyl-3-vinylcarbazole

Highly purified samples of N-ethyl-3-vinylcarbazole are readily polymerized in CH₂Cl₂ by conventional initiators of cationic polymerization, including boron trifluoride etherate and tropylium hexachloroantimonate. Reaction rates measured calorimetrically yield an estimate for the free-cation propagation rate coefficient (k_p⁺ = 2 x 10⁺⁴ liter/mole-sec) at 0°C, which is some 20 times smaller than that for the closely related monomer N-vinylcarbazole. Distinguishing aspects of the cationic polymerization of N-ethyl-3-vinylcarbazole are the very high molecular weights obtained and the linear dependence of M?_n of the monomer/catalyst mole ratio, indicating that transfer and termination are comparatively unimportant. Polymerizations initiated by tropylium hexachloroantimonate exhibit a characteristic absorption band at 468 nm, tentatively assigned to the propagating cation, which undergoes rapid changes after all monomer has been consumed. The stability of the species responsible for the absorption band at 468 nm appears to be least in conditions where ion pairs are important.     

Free-ionic polymerization of isobutyl vinyl ether by radiation

Radiation-induced free-ionic polymerization of isobutyl vinyl ether in bulk system has been studied by dilatometry and electrical conductivity measurement. Some refinements in kinetic treatment of estimate the propagation rate constant k_p from the rate of polymerization and steady-state conductivity were attempted. Polymerization of superdried monomer which gave a half-power dose-rate dependence of R_p was carried out at 0, 25, and 50°C. The k_p value obtained at 25°C and an activation energy for propagation were estimated as 1.2 ± 0.4 × 10⁵ I./mole-sec and 9.6 ± 2.8 kcal/mole, respectively. In isobutyl vinyl ether, a propagation reaction in free-ionic mechanism was found to be characterized with a high frequency factor and presumably higher activation energy, compared with ion-pair mechanism. Discussions were also made as to several contrasting behaviors between the polymerization of alkyl vinyl ethers and other vinyl monomers as styrene both in free-ion and ion-pair mechanisms.     

Anionic polymerization of N-substituted maleimide. V. A study on the kinetic features of anionic polymerization of N-phenylmaleimide

The kinetic feature of the anionic polymerization of N-PMI was investigated in THF. The polymerization system initiated with lithium tert-butoxide was revealed to be so-called “slow-initiation” system. The rate constant of the initiation reaction, k_i, was obtained to be 4.2 × 10^?3 (L mol^?1 s^?1) at ?72°C. The apparent rate constants of the propagation reaction, k, at ?72°C were individually obtained from each slope of the first-order plots in the later stages of the polymerizations for four different initiator concentrations. Each k is fairly close to that of initiation rate around 10^?3. The propagation reaction was concluded to be dominated by ion-pair mechanism from the analysis of the kinetic data and the results of the addition effects of crown ether and common salt.     

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.     

Vinyl polymerization. 432. Initiation mechanism of uncatalyzed polymerization in the presence of copper(II) ion

The initiation mechanisms of the uncatalyzed polymerization of methyl acrylate and methyl methacrylate by the system of starch, α-amylase or water-soluble nylon 3/copper(II) ion/H₂O were investigated by spin trapping techniques. Using 2,4-dimethyl-3-nitrosobenzene sulfonate d₈ and 5,5-dimethyl-1-pyrroline-N-oxide as the water-soluble spin trapping agents, the simultaneous generation of both monomer and hydroxyl radicals has been observed, and the propriety of the initiation mechanism proposed previously was confirmed. Thus, the initiation of polymerization in the presence of copper(II) ion was concluded to take place through a hydrogen atom transfer from the complexed water to the β-carbon of the complexed monomer.     

Solid-state thermal polymerization of sodium acrylate

Initiation and propagation processes in thermally initiated solid-state polymerization of sodium acrylate have been studied. The kinetics of initiation, followed with the electron spin resonance technique, leads to an activation energy E of 28.8 kcal/mol, which is attributed to the formation of dimeric radicals. The activation energy of 16 ± 1 kcal/mol obtained for the solid-state polymerization of sodium acrylate by chemical analysis and differential scanning calorimetry has been attributed to the propagation process.     

Novel dialkylaminoalkyl- and dialkylaminoalcoxi-benzophenones as photoinitiators of polymerization. I. Photochemical characteristics and radical efficiencies

The behavior of the nonconjugated aminated benzophenones—4-[2′-N,N-(diethylamino)ethoxy]benzophenone (E4), 2-[2′-N,N-(diethylamino)ethoxy]-4-methoxybenzophenone (E2), and 4-N,N-dimethylaminomethylbenzophenone (DM)—as photoinitiators of MMA polymerization has been studied and the results compared with those obtained with the conjugated aminobenzophenone 4-N,N-dimethylamino-4′-isopropyl-benzophenone (CU—MI). Photoreduction behavior of these compounds in various solvents in the presence and absence of MMA has been also examined. The order of the polymerization reaction with respect to monomer and initiator concentrations has been investigated; values of initiation quantum yield (Φ_i), K_p/K^1/2_t and efficiencies of the different radicals have also been determined. Similar polymerization rates (R_p) of methyl methacrylate (MMA) were found when E4 and CU-MI were used as photoinitiators under the same range of absorbed irradiation intensity. This fact results from a compensation between the higher rate of E4 radical production (n-π* transition type) and the greater reactivity of the radicals generated from CU-MI.     

Chemical trapping experiments support a cation-radical mechanism for the oxidative polymerization of aniline

The oxidative polymerization of aniline in aqueous acidic solution was carried out in the presence of a variety of organic compounds as potential traps for postulated intermediates. The polymerization was inhibited by hindered phenols and electron-rich alkenes, traps for cation-radicals. However, polyaniline was still obtained in the presence of electron-rich arenes, such as 1,3-dimethoxybenzene and 1,4-dimethoxybenzene, known as excellent receptors of nitrenium ions. Polymerization of N-phenyl-1,4-phenylenediamine was similarly carried out in the presence of potential traps. Polyaniline containing an N-phenyl group was obtained in the presence of 1,3-dimethoxybenzene and 1,4-dimethoxybenzene. Hindered phenols and 4-methoxystyrene only slightly inhibited polymerization of N-phenyl-1,4-phenylenediamine which most probably proceeded by way of the stable diarylamino radical. Copolymerization of aniline with 10 wt % of N-phenyl-1,4-phenylenediamine in the presence of these traps gave similar results to the polymerization of pure aniline. These results have led to the proposed cation-radical polymerization mechanism of aniline, in which the polymerization is a chain growth reaction through the combination of a polymeric cation-radical and an anilinium cation-radical. Step growth character is also present when a polymeric aminium cation-radical end combines with a diarylaminoended polymer. The copolymerization of N-phenyl-p-phenylenediamine can also occur by reaction of aniline cation-radical with a polyarylamine radical. The nitrenium mechanism was further rejected by the fact that attempted polymerization of N-phenylhydroxylamine, which forms authentic nitrenium ions in acid, failed to give polymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2569–2579, 1999