Radical/cation transformation polymerization and its application to the preparation of block copolymers

p-Methoxystyrene (MOS), butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) were polymerized in high yield by azoinitiators such as 2, 2'-azodiisobutyronitrile (AIBN) in the presence of electron acceptors such as Ph₂I⁺PF₆⁻. An electron paramagnetic resonance (ESR) study of the model radicals of the propagating radical showed the transformation of the radical to the corresponding cation in the presence of the electron acceptors. In the case of BVE, the polymer formation was caused by cationic species produced by the transformation of the initiating radical. The polymerizations of MOS and VCZ were ascribed to the transformation of the growing radical to the corresponding cation during the propagation step which was classified as the radical/cation transformation polymerization. Block copolymers of MOS/cyclohexene oxide (1, 2-epoxycyclohexane) (CHO) and VCZ/CHO were effectively prepared by the radical/cation transformation polymerization of the appropriate monomers in the presence of AIBN, electron acceptor and CHO. The formation of block copolymers was characterized by turbidimetry, thin-layer chromatography, and solubility tests.     

Radical/Cation Transformation Polymerization and Its Application to the Preparation of Block Copolymers

Propagating radicals could be transformed into corresponding cations in the radical polymerization process of vinyl monomers in the presence of an electron acceptor. The electron transfer reaction from the propagating radicals to the acceptor was confirmed by an electron spin resonance study and by the model compound reaction. The radical/cation transformation polymerization was effectively applied to the preparation of a new type of block copolymer compound of radically polymerized segments and cationically polymerized segments by the one shot method. Thus, a block copolymer of p-methoxystyrene (MOS) and cyclohexene oxide (CHO) was prepared by the radical polymerization of MOS in the presence of Ph₂I⁺PF⁻₆ and CHO. The formation of the block copolymer was confirmed by extraction separation, ¹H-NMR (nuclear magnetic resonance), thin layer chromatography and turbidimetric titration. © 1997 John Wiley & Sons, Ltd.     

自由基/正离子转化聚合法合成对甲氧基苯乙烯与环己烯氧化物/1,2,5,6-二环氧基环辛烷的嵌段共聚物

用自由基 /正离子转化聚合法 ,由对甲氧基苯乙烯、环己烯氧化物、1,2 ,5 ,6 二环氧基环辛烷体系一步法合成了它们的嵌段共聚物 .其中 ,聚对甲氧基苯乙烯构成自由基聚合链段 ,而环己烯氧化物与 1,2 ,5 ,6 二环氧基环辛烷的共聚物构成正离子聚合链段 .实验结果初步表明 ,1,2 ,5 ,6 二环氧基环辛烷的加入可有效地消除由正离子聚合时链转移产生的均聚物     

Photoinitiated cationic polymerization using o-phthaldehyde and pyridinium salt

The cationic polymerization of cyclohexene oxide (CHO), n-butyl vinylether (BVE), N-vinylcarbazole (NVC), and 4-vinyl cyclohexenedioxide (4-VCHD) is initiated upon UV irradiation (Λ_inc. = 350 nm) of dichloromethane solution containing N-ethoxy-2-methyl-pyridinium hexafluorophosphate (EMP⁺PF₆) and o-phthaldehyde. A feasible initiation mechanism involves formation of biradical by intramolecular hydrogen abstraction of triplet o-phthaldehyde. Oxidation of these radicals by EMP⁺ ions yields protons capable of initiating the cationic polymerization. © 1995 John Wiley & Sons, Inc.     

Photo and thermal polymerization sensitized by donor–acceptor interaction. I. N-vinyl-carbazole–acrylonitrile and related systems

Spontaneous photo and thermal polymerization of N-vinylcarbazole (VCZ)–acrylonitrile (AN), VCZ–acetonitrile, AN-N-ethylcarbazole, and AN-ferrocene were studied. These combinations of electron donor with acceptor were thermally rather stable but showed prominent photopolymerizability when the systems were irradiated by near ultraviolet light. The VCZ–AN system showed multireactivity producing VCZ polymer and a copolymer of VCZ with AN. The composition of copolymer was approximately the same as that of polymer produced in radical copolymerization. The effects of additives (DPPH, NH₃, H₂O, air) indicated simultaneous occurrence of cationic and radical polymerization in the AN–VCZ and acetonitrile–VCZ systems. The results were interpreted on the assumption of initial formation of a cation radical–anion radical pair. The ratio of cationic to radical polymerization differed for photo and thermal polymerization. In no case was anionic polymerization detected.     

Photochemical reactions of N-vinylcarbazole in the binary solvent of benzonitrile and nitrobenzene

Photochemical reactions of N-vinylcarbazole (VCZ) in the binary solvent of benzonitrile (?CN) and nitrobenzene (?NO₂) were investigated. Both solvent and oxygen effects on the final products were examined. Benzonitrile and nitrobenzene behaved differently in the photochemical reaction of VCZ. At higher concentrations of benzonitrile in the aerated system, cyclodimerization was favored and it was inhibited by a cation scavenger and retarded by a radical scavenger. Polymerization occurred in the deaerated system and was inhibited by a radical scavenger and not by a cation scavenger. Using picosecond laser photolysis it was concluded that cyclodimerization occurs through the diffusion-controlled encounter collision of the excited singlet state of VCZ with an oxygen molecule, producing the VCZ cation radical and oxygen anion radical, and that this oxygen anion radical plays a very important role in the cyclodimerization of VCZ. It was also suggested that radical polymerization in the deaerated system is initiated by the excited triplet state of VCZ. On the other hand, at higher concentrations of nitrobenzene, only cationic polymerization took place irrespective of the presence of oxygen, and it was suggested that a contact charge-transfer complex is produced by the mixing of VCZ with ?NO₂ producing VCZ cation radical and NO₂ anion radical by an excited-state electron transfer.     

Copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms

Butyl vinyl ether (BVE) and methyl methacrylate (MMA) mixtures were polymerized by using free radical initiators in conjunction with a cationic initiator such as diphenyl iodonium salt. Polymerization mechanism involves free radical polymerization of MMA which is switched to cationic polymerization of BVE by addition of growing poly(MMA) radicals to BVE and subsequent oxidation of electron donating polymeric radicals to the corresponding cations by iodonium ions. Two representative bifunctional monomers, ethylene glycol divinyl ether (EGDVE) and ethylene glycol dimethacrylate (EGDMA) were also used together with MMA and BVE, respectively, in photo and thermal crosslinking polymerizations. Vinyl ether and methacrylate type monomers can successfully be copolymerized by this double-mode polymerization under photochemical conditions.     

Mechanism of charge-transfer polymerization. V. Photopolymerization and photocyclodimerization of N-vinylcarbazole in the N-vinylcarbazole–oxygen–solvent system

Photochemical reactions of N-vinylcarbazole (VCZ), studied in various solvents, were profoundly influenced by the atmosphere. In the deaerated system radical polymerization of VCZ occurred in various solvents, e.g., tetrahydrofuran, acetone, ethyl methyl ketone, acetonitrile, methanol, sulfolane, N,N-dimethylformamide (DMF), or dimethyl sulfoxide (DMSO). By contrast, when dissolved oxygen was present, cyclodimerization of VCZ occurred exclusively to give trans-1,2-dicarbazole-9-yl-cyclobutane in such polar, basic solvents as acetone, ethyl methyl ketone, acetonitrile or methanol. In stronger basic solvents, i.e., sulfolane, DMF, or DMSO, simultaneous radical polymerization and cyclodimerization of VCZ proceeded, the ratio of the cyclodimerization to the radical polymerization decreasing in the order, sulfolane > DMF > DMSO. In dichloromethane, on the other hand, cationic polymerization of VCZ occurred irrespective of the atmosphere. It is suggested that oxygen acts as an electron acceptor to the excited VCZ, electron transfer occurring in polar solvents from the excited VCZ to oxygen to give transient VCZ cation radical. The effect of solvent basicity on the photocyclodimerization of VCZ is discussed.     

Visible light induced free radical promoted cationic polymerization using thioxanthone derivatives

The free radical promoted cationic polymerization cyclohexene oxide (CHO), was achieved by visible light irradiation (λ_inc = 430–490 nm) of methylene chloride solutions containing thioxanthone‐fluorene carboxylic acid (TX‐FLCOOH) or thioxanthone‐carbazole (TX‐C) and cationic salts, such as diphenyliodonium hexafluorophosphate (Ph₂I⁺PF) or silver hexafluorophosphate (Ag⁺PF) in the presence of hydrogen donors. A feasible initiation mechanism involves the photogeneration of ketyl radicals by hydrogen abstraction in the first step. Subsequent oxidation of ketyl radicals by the oxidizing salts yields Bronsted acids capable of initiating the polymerization of CHO. In agreement with the proposed mechanism, the polymerization was completely inhibited by 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy and di‐2,6‐di‐tert‐butylpyridine as radical and acid scavengers, respectively. Additionally polymerization efficiency was directly related to the reduction potential of the cationic salts, that is, Ag⁺PF (E = +0.8 V) was found to be more efficient than Ph₂I⁺PF (E = ?0.2 V). In addition to CHO, vinyl monomers such as isobutyl vinyl ether and N‐vinyl carbazole, and a bisepoxide such as 3,4‐epoxycyclohexyl‐3′,4′‐epoxycyclohexene carboxylate, were polymerized in the presence of TX‐FLCOOH or TX‐C and iodonium salt with high efficiency. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

SET and HAT/PCET acid-mediated oxidation processes in helical shaped fused bis-phenothiazines

Helical shaped fused bis-phenothiazines 1 – 9 have been prepared and their red-ox behaviour quantitatively studied. Helicene radical cations (Hel^.+) can be obtained either by UV-irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5 – 9 with peroxyl radicals (ROO^.) occurs with a ‘classical’ HAT process giving HelO^. radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium-promoted proton-coupled electron transfer (PCET) process takes place with formation of HelO^. radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1 – 4 , lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium-promoted PCET mechanism with formation of the radical cations Hel^.+. Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel-OH group (BDE_OH), and kinetic constants (k_inh) of the reactions with ROO^. species of helicenes 1 – 9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.     

Reverse ATRP Process of Methacrylonitrile in [C4mim][PF6]

In this original experiment, an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]), was used as the reaction media for reverse atom transfer radical polymerization of methacrylonitrile (MAN) initiated by azobisisobutyronitrile (AIBN) with FeCl₃ and isophthalic acid (IA) as catalyst and ligand. The polymerization in [C₄mim][PF₆] proceeded in a well-controlled manner as evidenced by kinetic studies. Compared with the polymerization in N, N-dimethylformamide (DMF), the polymerization in [C₄mim][PF₆] not only showed better control of molecular weight and narrower molecular weight distribution but also provided more rapid reaction rate with the ratio of [MAN]:[AIBN]:[FeCl₃]:[IA] at 300:1:2:4. The block copolymer PMAN-b-PSt was obtained via a conventional ATRP process in [C₄mim][PF₆] by using the resulting PMAN as macroinitiator. [C₄mim][PF₆] and FeCl₃/IA could be easily recycled and reused and had no effect on the living nature of reverse atom transfer radical polymerization of MAN.     

Long-lived polymer radicals. VI. Polymerization of N-methylmethacrylamide with formation of living propagation radicals

The polymerization of N-methylmethacrylamide (NMMAm) with azobisisobutyronitrile (AIBN) was investigated kinetically in benzene. This polymerization proceeded heterogeously with formation of the very stable poly(NMMAm) radicals. The overall activation energy of this polymerization was calculated to be 23 kcal/mol. The polymerization rate (R_p) was expressed by: R_p = k[AIBN]^0.63-0.68[NMMAm]^1?2.5. Dependence of R_p on the monomer concentration increased with increasing NMMAm concentration. From an ESR study, cyanopropyl radicals escaping the solvent cage were found to be converted to the living propagating radicals of NMMAm in very high yields (ca. 90%). Formation mechanism of the living polymer radicals was discussed on the basis of kinetic, ESR spectroscopic, and electron microscopic results.     

Structure,spectroscopy and magnetism of 1,4-dihydro-1,4-diazine radical cations: Exceptionally stable intermediates related to methylviologen and flavosemiquinone radicals

-1,4-Dialkyl-1,4-dihydro-1,4-diazine radical cations 1–3 have been established in recent years as unusually stable intermediates of corresponding two-step redox systems. The stability is evident from large comproportionation constants K_c > 10¹² and from the isolability of persistent radical cation salts with counter anions such as Br^-, I^-, I₃ ^-, PF₆ ^-, BPh4-, or (TCNQ₂)^-. The structures of several crystalline derivatives have been determined, showing planar π systems and, in one instance, an anion-dependent tendency to form π/π dimers. Effects of dimerization are also evident from comparative magnetic susceptibility measurements of 1,4-diethyl-1,4-dihydroquinoxalinium iodide and tetraphenylborate. UV/Vis absorption spectra of the radical cations have been determined and interpreted with the help of molecular orbital calculations. The most simple member of the series, 1,4-diethyl-1,4-dihydropyrazinum radical cation 1, exhibits a long wavelength forbidden band (²B_1u → ²A_u) with a conspicuous vibrational fine structure. The results obtained for the small but very stable new radical cations 1 and 2 provide clues to the stability of flavosemiquinone oxidation states in pertinent oxidoreductase enzymes and show ways to new components for the design of materials with anisotropic physical properties.     

Formation of Dimer Radical Cations of Selenourea on Oxidation: Pulse Radiolysis Studies

Abstract

Reactions of oxidizing radicals like hydroxyl (·OH) radical, specific electron transfer agents like N ₃ ·, and I ₂ ^?. radicals were studied with selenourea (SeU) and compared with thiourea (ThU) using pulse radiolysis technique in microsecond time scales. Both the compounds efficiently react with ·OH radicals, however, SeU undergoes easier oxidation by secondary oxidants as compared to ThU. The results were supported by cyclic voltammetry studies. The radical cations of both SeU and ThU formed on oxidation undergo dimerization with the parent molecule to form two-centered three-electron-hemi bonded radical cations absorbing at 410 and 400 nm respectively with the stabilization energies of 21.1 and 20.5 kcal/mol for SeU and ThU, respectively. Preliminary studies indicated that at low concentration of SeU, the dimerization is prevented and the oxidation reaction produced metallic Se nanoparticles.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

π-dimerization of pleiadiene radical cations at low temperatures revealed by UV–vis spectroelectrochemistry and quantum theory

One-electron oxidation of the non-alternant polycyclic aromatic hydrocarbon pleiadiene and related cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene in THF produces corresponding radical cations detectable in the temperature range of 293–263 K only on the subsecond time scale of cyclic voltammetry. Although the EPR-active red-coloured pleiadiene radical cation is stable according to the literature in concentrated sulfuric acid, spectroelectrochemical measurements reported in this study provide convincing evidence for its facile conversion into the green-coloured, formally closed shell and, hence, EPR-silent π-bound dimer dication stable in THF at 253 K. The unexpected formation of the thermally unstable dimeric product featuring a characteristic intense low-energy absorption band at 673 nm (1.84 eV; logε _max = 4.0) is substantiated by ab initio calculations on the parent pleiadiene molecule and the PF₆⁻ salts of the corresponding radical cation and dimer dication. The latter is stabilized with respect to the radical cation by 14.40 kcal mol⁻¹ (DFT B3LYP) [37.64 kcal mol⁻¹ (CASPT2/DFT B3LYP)]. An excellent match has been obtained between the experimental and TD-DFT-calculated UV–vis spectra of the PF₆⁻ salt of the pleiadiene dimer dication, considering solvent (THF) effects.     

Synthesis and reactions of polymers with photoactive terminal groups—3. The use of radical promoted cationic polymerization for the synthesis of poly(n-butyl vinylether) with n-acyl dibenz(b,f)azepine terminal units

The free radical promoted cationic polymerization of n-butyl vinyl-ether (BVE) was achieved by the thermal decomposition of N-[4,4-azobis-(4-cyanopentanoyl)]-bisdibenz(b, f)azepine (ADBA) in the presence of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF₆⁻) or silver hexafluorophosphate (AgPF₆). Polymer samples prepared in the presence of AgPF₆ contained a significant number of terminal dibenzazepine units. Chain extension via dimerization of the dibenzazepine units occurred upon irradiating these polymers with u.v. light (λ = 366 nm) in CH₂Cl₂ solution containing a triplet sensitizer (benzophenone or benzil). Polymer samples prepared in the presence of the iodonium salt contained only a very small fraction of macromolecules having dibenzazepine end-groups and were, therefore, not appropriate for the chain extension procedure.     

N-alkoxy-pyridinium and N-alkoxy-quinolinium salts as initiators for cationic photopolymerizations

The polymerization of n-butyl vinyl ether (BVE), cyclohexene oxide (CHO) and 3,4-epoxycyclohexyl(methyl)-3′,4′-epoxycyclohexane carboxylate (EEC) was initiated upon UV irradiation (λ_inc > 300 nm) of dichloromethane solutions containing N-ethoxy-2-methylpyridinium ( V ), N-ethoxy-4-phenyl-pyridinium ( VI ) or N-ethoxy-isoquinolinium hexafluorophosphate ( VII ). Whereas the bifunctional EEC was converted into an insoluble gel, BVE and CHO formed polymers of molar mass: M_w = 2 X 10⁴?2 X 10⁵ (PCHO) and M_w ≈ 2 X 10⁴ (PBVE). Protons are formed with a rather high quantum yield [ø(H+) = 0.48 on irradiating VII in dichloromethane; titration with sodium p-nitrophenolate] and it is, therefore, assumed that the polymerization is initiated by photochemically generated protons. © 1992 John Wiley & Sons, Inc.     

Initiation of cationic polymerization of n-butyl vinyl ether by stable cation radicals of substituted phenothiazines

3,7-Diethyl- 10-phenylphenothiazine (DEPPT), a phenothiazine derivative whose 3,7- and 10-positions are blocked, was synthesized. Potentiostatic electrolysis of DEPPT in acetonitrile (ACN) in the presence of 0.1M of LiClO₄ at 0.7 V (vs. Ag/Ag/Cl) yielded the stable cation radical of DEPPT (DEPPT^+·) which was characterized by cyclic voltammetry, UV-visible spectroscopy, and ESR spectrometry. Stable cation radicals of 10-phenylphenothiazine and 3,7-diethyl-10-methylphenothiazine were also prepared. The cationic polymerization of n-butyl vinyl ether was initiated by these cation radicals, including DEPPT^·+. The electron transfer mechanism for the initiation step, which we proposed previously, was supported by the fact that DEPPT^·+ was capable of initiating the polymerization; dimerization of DEPPT^·+ by releasing protons is precluded because 3,7- and 10-positions are all blocked. © 1994 John Wiley & Sons, Inc.     

Sequential photodecomposition of bisacylgermane type photoinitiator: Synthesis of block copolymers by combination of free radical promoted cationic and free radical polymerization mechanisms

A block copolymer of cyclohexene oxide (CHO) and styrene (St) was prepared by using bifunctional visible light photoinitiator dibenzoyldiethylgermane (DBDEG) via a two‐step procedure. The bifunctionality of the photoinitiator pertains to the sequential photodecomposition of DBDEG through acyl germane bonds. In the first step, photoinitiated free radical promoted cationic polymerization of CHO using DBDEG in the presence of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF) was carried out to yield polymers with photoactive monobenzoyl germane end groups. These poly(cyclohexene oxide) (PCHO) prepolymers were used to induce photoinitiated free radical polymerization of styrene (St) resulting in the formation of poly(cyclohexene oxide‐block‐styrene) (P(CHO‐b‐St)). Successful blocking has been confirmed by a strong change in the molecular weight of the prepolymer and the block copolymer as well as NMR, IR, and DSC spectral measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4793–4799, 2009