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.     

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.     

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

Abstract

ESR study on the primary radicals obtained by decomposition of azo-compounds showed that primary radicals with electron donating substituents were transformed to the corresponding cations in the presence of electron acceptors such as ph₂I⁺PF^? ₆. Accordingly, propagating radicals are transformed to the corresponding cations in the polymerization of p-methoxy-styrene (MOS), n-butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) with azoinitiators such as AIBN in the presence of electron acceptors such as Ph₂I⁺PF^? ₆. 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 (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

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.     

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.     

Dilute solution properties of styrene–acrylonitrile and styrene–methyl methacrylate copolymers. I. Intrinsic viscosity–molecular weight relations

Styrene–acrylonitrile (St–AN) copolymers of three compositions—27.4 mole-% (SA₁); 38.5 mole-% (SA₂); and 47.5 mole-% (SA₃) acrylonitrile—and styrene–methyl methacrylate (St–MMA) copolymer (SM) of 46.5 mole-% methyl methacrylate were prepared by bulk polymerization at 60°C with benzoyl peroxide as the initiator, and were then fractionated. The molecular weights of unfractionated and fractionated samples were determined by light scattering in a number of solvents. The [η] versus M?_w relations at 30°C were established for SA₁, SA₂, SM, and polystyrene (PSt) in ethyl acetate (EAc), dimethyl formamide (DMF), and γ-butyrolactone (γ-BL), and for SA₃ in methyl ethyl ketone (MEK), DMF, and γ-BL. Second virial coefficients A₂ and the Huggins constant were determined. From values of A₂ and the exponent a of the Mark–Houwink relation it is seen that the solvent power for samples SA₁, SA₂, and PSt is in the order EAc < γ-BL < DMF, while for sample SA₃ the solvent power is in the order MEK < γ-BL < DMF. The solvent power decreases with an increase in AN content. The solvent power of the three solvents used for SM copolymer sample is practically the same within experimental errors. From the a values it is concluded that in a given solvent the copolymer chains are more extended than the corresponding homopolymers.     

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.     

Complex‐radical terpolymerization of acceptor–donor–acceptor systems: Maleic anhydride (n‐butyl methacrylate)–styrene–acrylonitrile

Some features of radical ternary copolymerization of maleic anhydride (MA)–styrene (St)–acrylonitrile (AN) and n‐butyl methacrylate (BMA)–St–AN acceptor–donor–acceptor monomer systems have been revealed. The terpolymer compositions and kinetics of copolymerizations were studied in the initial and high conversion stages. The considerable divergence in the copolymer compositions was observed when a strong acceptor MA monomer was substituted with BMA having comparatively low acceptor character in the ternary system studied. Obtained results show that terpolymerization proceeded mainly through “complex” mechanism in the state of near binary copolymerization of St…MA (or BMA) and AN…St complexes only in the chosen ratios of complexed monomers. The terpolymers synthesized have high thermal stabilities (295–325 °C), which is explained by possible intermolecular fragmentation of AN‐units through cyclization and crosslinking reactions during thermotreatment in the isothermal heating conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2652–2662, 2000     

Random copolymer of butadiene and propylene prepared with TiCl4–Et3Al–phosgene catalyst

The copolymerization of butadiene and propylene was investigated. It was found that the catalyst system of TiCl₄–Et₃Al–COCl₂ yields a random copolymer of high molecular weight with a small amount of gel polymer above room temperature. Tetrachloroethylene was a good solvent for the production of high polymer containing a high proportion of propylene units in high yield. The fractionation and the analysis of degradation experiments of copolymer indicate that the copolymer is of random distribution of propylene units in the copolymer. However, the monomer reactivity ratios, r_BD = 6.36 and r_Pr = 0.42, suggest some degree of blocked character. The properties of the copolymer were superior to those of cis-1,4–polybutadiene, especially in resistance to thermal aging.     

A comparison of the absolute reactivity of vinyl monomers. I. Kinetic studies on radical polymerization of vinyl monomers initiated by a Mn3+–diglycolic acid redox system

The kinetics of polymerization of acrylamide (AM), acrylic acid (AA), and acrylonitrile (AN) initiated by the redox system Mn³⁺–diglycolic acid (DGA) was studied. All three systems followed the same mechanism; namely, initiation by an organic free radical arising from the oxidation of diglycolic acid and termination by the interaction of polymer radicals with Mn³⁺ ion. The rate coefficients k_i/k₀ and k_p/k_t were related to monomer and polymer radical reactivity, respectively. An inverse relation between monomer and polymer radical reactivity was observed. Monomers with higher Q values gave higher k_i/k₀ values but lower k_p/k_t values. The e values of the monomers were important in determining the reactivities of monomers with nearly the same Q values.     

“Living” radical polymerization of methyl methacrylate with diethyl 2,3-dicyano-2,3-diphenylsuccinate as a thermal iniferter

The bulk polymerization of methyl methacrylate (MMA) initiated with diethyl 2,3-dicyano-2,3-diphenylsuccinate (DCDPS) was studied. This polymerization showed some “living” characteristics; that is, both the yield and the molecular weight of the resulting polymers increased with reaction time, and the resultant polymer can be extended by adding MMA. The molecular weight distribution of PMMA obtained at high conversion is fairly narrow (M_w/M_n = 1.24≈1.34). It was confirmed that DCDPS can serve as a thermal iniferter for MMA polymerization by a “living” radical mechanism. Furthermore, the PMMA obtained can act as a macroinitiator for radical polymerization of styrene (St) to give a block copolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4610–4615, 1999     

Silyl enol ethers as monomer. I. Radical copolymerizability of α-trimethylsilyloxystyrene

α-Trimethylsilyloxystyrene (TMSST), the silyl enol ether of acetophenone, was not homopolymerized either by a radical or a cationic initiator. Radical copolymerization of TMSST with styrene (ST) and acrylonitrile (AN) in bulk and the terpolymerization of TMSST, ST, and maleic anhydride (MA) in dioxane were studied at 60°C and the polymerization parameters of TMSST were estimated. The rate of copolymerization decreased with increased amounts of TMSST for both systems. Monomer reactivity ratios were found as follows: r₁ = 1.48 and r₂ = 0 for the ST (M₁)–TMSST (M₂) system and r₁ = 0.050 and r₂ = 0 for the AN (M₁)–TMSST (M₂) system. The terpolymerization of ST (M₁), TMSST (M₂), and MA (M₃) gave a terpolymer containing ca. 50 mol % of MA units with a varying ratio of TMSST to ST units and the ratio of rate constants of propagation, k₃₂/k₃₁, was found to be 0.39. Q and e values of TMSST were determined using the values shown above to be 0.88 and ?1.13, respectively. Attempted desilylation by an acid catalyst for the copolymer of TMSST with ST afforded polystyrene partially substituted with hydroxyl groups at the α-position.     

A minimal amount of acrylonitrile turns the nitroxide‐mediated polymerization of methyl methacrylate into an almost ideal controlled/living system

Nitroxide‐mediated controlled/living free‐radical polymerization of methyl methacrylate initiated by the SG1‐based alkoxyamine BlocBuilder was successfully performed in bulk at 80–99 °C with the help of a very small amount of acrylonitrile (AN, 2.2–8.8 mol %) as a comonomer. Well‐defined PMMA‐rich P(MMA‐co‐AN) copolymers were prepared with the number‐average molar mass, M_n, in the 6.1–32 kg mol^?1 range and polydispersity indexes as low as 1.24. Incorporation of AN in the copolymers was demonstrated by ¹H and ¹³C NMR spectroscopy, and its effect on the chain thermal properties was evaluated by DSC and TGA analyses. Investigation of chain‐end functionalization by an alkoxyamine group was performed by means of ³¹P NMR spectroscopy and chain extensions from a P(MMA‐co‐AN)‐SG1 macroinitiator. It demonstrated the very high proportion of SG1‐terminated polymer chains, which opened the door to block copolymer synthesis with a high quality of control. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 34–47, 2010     

Thermal and radiation-induced polymerizations of N-tert-butylacrylamide and (N-tert-butylacrylamide)2–ZnCl2 complex

The thermal and radiation-induced in-source and postirradiation polymerizations of N-tert-butylacrylamide and (N-tert-butylacrylamide)₂–ZnCl₂ complex of this monomer were studied at various temperatures. In in-source, solid-state polymerizations of monomer and complex the conversion was about 95% at 21°C in about eight days. Their postirradiation polymerizations were also studied in solid state. The conversion-time curves of these two systems show an autoacceleration as in-source polymerization. In both types of polymerization the overall rate of polymerization of complex was higher than that of pure monomer at the same polymerization temperature. In investigations of the thermal polymerization of N-tert-butylacrylamide and ZnCl₂-complex it was observed that the ZnCl₂-complex system can be polymerized in air in the molten and solid state. The conversion of monomer to polymer reaches limiting values in solid state in about 1 hr. The thermal polymerization of ZnCl₂-complex in the molten state was also studied and 100% conversion was obtained in 30 min. The thermal polymerization of pure monomer was studied in vacuum and an appreciable amount of polymer was obtained in the molten state; however, the thermal polymerization of this monomer is negligible in solid state. In this work rates of polymerization for N-tert-butylacrylamide and (N-tert-butylacrylamide)₂–ZnCl₂ are compared under various experimental conditions and overall activation energies are calculated.     

Polymerization of methyl methacrylate initiated by Mn(III)–glycerol redox system

The kinetics of thermal polymerization of methyl methacrylate initiated by the redox system Mn(III)–glycerol was studied in aqueous sulfuric acid in the temperature range of 30–40°C, and the rates of polymerization, R_p, and Mn³⁺ disappearance, etc., were measured. The effect of certain water-miscible organic solvents and certain cationic and anionic surfactants on the rates of polymerization has been investigated. A mechanism involving the formation of a complex between Mn³⁺ and glycerol whose decomposition yields the initiating free radical with the polymerization being terminated by the metal ion has been suggested.     

Divinylsiloxane‐bisbenzocyclobutene‐based polymer modified with polystyrene–polybutadiene–polystyrene triblock copolymers

Divinylsiloxane‐bisbenzocyclobutene (DVS‐bisBCB) polymer has very low dielectric constant and dissipation factor, good thermal stability, and high chemical resistance. The fracture toughness of the thermoset polymer is moderate due to its high crosslink density. A thermoplastic elastomer, polystyrene–polybutadiene–polystyrene triblock copolymer, was incorporated into the matrix to enhance its toughness. The cured thermoset matrix showed different morphology when the elastomer was added to the B‐staged prepolymer or when the elastomer was B‐staged with the DVS‐bisBCB monomer. Small and uniformly distributed elastomer domains were detected by transmission electron micrographs (TEM) in the former case, but TEM did not detect a separate domain in the latter case. A high percentage of the polystyrene–polybutadiene–polystyrene triblock copolymer could be incorporated into the DVS‐bisBCB thermoset matrix by B‐staging the triblock copolymer with the BCB monomer. The elastomer increased the fracture toughness of DVS‐bisBCB polymer as indicated by enhanced elongation at break and increased K1c values obtained by the modified edge‐lift‐off test. Elastomer modified DVS‐bisBCB maintained excellent electrical properties, high T_g and good thermal stability, but showed higher coefficient of linear thermal expansion values. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1591–1599, 2006     

Direct through anionic,cationic, and radical active species: Terminal carbon–halogen bond for “controlled”/living polymerizations of styrene

In this work, we examined the synthesis of novel block (co)polymers by mechanistic transformation through anionic, cationic, and radical living polymerizations using terminal carbon–halogen bond as the dormant species. First, the direct halogenation of growing species in the living anionic polymerization of styrene was examined with CCl₄ to form a carbon–halogen terminal, which can be employed as the dormant species for either living cationic or radical polymerization. The mechanistic transformation was then performed from living anionic polymerization into living cationic or radical polymerization using the obtained polymers as the macroinitiator with the SnCl₄/n‐Bu₄NCl or RuCp^*Cl(PPh₃)/Et₃N initiating system, respectively. Finally, the combination of all the polymerizations allowed the synthesis block copolymers including unprecedented gradient block copolymers composed of styrene and p‐methylstyrene. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 465–473     

Polymerization of o‐quinodimethanes. IV. Radical polymerization of 1‐cyano‐o‐quinodimethane in the presence of TEMPO

The radical polymerization behavior of 1‐cyano‐o‐quinodimethane generated by thermal isomerization of 1‐cyanobenzocyclobutene in the presence of 2,2,6,6‐tetramethylpiperidine‐N‐oxide (TEMPO) and the block copolymerization of the obtained polymer with styrene are described. The radical polymerization of 1‐cyanobenzocyclobutene was carried out in a sealed tube at temperatures ranging from 100 to 150 °C for 24 h in the presence of di‐tert‐butyl peroxide (DTBP) as a radical initiator and two equivalents of TEMPO as a trapping agent of the propagation end radical to obtain hexane‐insoluble polymer above 130 °C. Polymerization at 150 °C with 5 mol % of DTBP in the presence of TEMPO resulted in the polymer having a number‐average molecular weight (M_n ) of 2900 in 63% yield. The structure of the obtained polymer was confirmed as the ring‐opened polymer having a TEMPO unit at the terminal end by ¹H NMR, ¹³C NMR, and IR analyses. Then, block copolymerization of the obtained polymer with styrene was carried out at 140 °C for 72 h to give the corresponding block copolymer in 82% yield, in which the unimodal GPC curve was shifted to a higher molecular weight region. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3434–3439, 2000     

Synthesis and characterization of cationic copolymers of butylacrylate and [3‐(methacryloylamino)‐propyl]trimethylammonium chloride

Cationic copolymers of butylacrylate (BA) and [3‐(methacryloylamino)‐propyl]trimethylammonium chloride (MAPTAC) were synthesized by free‐radical‐solution polymerization in methanol and ethanol. An FT‐Raman Spectrometer and NMR were used to monitor the polymerization process. The copolymers were characterized by light scattering, NMR, DSC, and thermogravimetric analysis. It was found that random copolymers could be prepared, and the molar fractions of BA and cationic monomers in the copolymers were close to the feed ratios. The copolymer prepared in methanol had a higher molecular weight than that prepared in ethanol. As the cationic monomer content increased, the glass‐transition temperature (T_g) of the copolymer also increased, whereas the thermal stability decreased. The reactivity ratios for the monomers were evaluated. The copolymerization of BA (M₁) with MAPTAC (M₂) gave reactivity ratios such as r₁ = 0.92 and r₂ = 2.61 in ethanol as well as r₁ = 0.79 and r₂ = 0.90 in methanol. This study indicated that a random copolymer containing a hydrophobic monomer (BA) and a cationic hydrophilic monomer (MAPTAC) could be prepared in a proper polar solvent such as methanol or ethanol. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1031–1039, 2001     

Polymerization of α-methyleneindane: A cyclic analog of α-methylstyrene

α-Methyleniedane (MI), a cyclic analog of α-methylstyrene which does not undergo radical homopolymerization under standard conditions, was synthesized and subjected to radical, cationic, and anionic polymerizations. MI undergoes radical polymerization with α,α′-azobis(isobutyronitrile) in contrast to α-methylstyrene, owing to its reduced steric hindrance, though the polymerization is slow even in bulk. Cationic and anionic polymerization of MI with BF₃OEt₂ and n-butyllithium, respectively, proceed rapidly. The thermal degradation behavior of the polymer depends on the polymerization conditions. The anionic and radical polymers are heteortactic-rich. Reactivity ratios in bulk radical copolymerization on MI (M₂) with methacrylate (MMA, M₁) were determined at 60°C (r₁ = 0.129 and r₂ = 1.07). In order to clarify the copolymerization mechanism, radical copolymerization of MI with MMA was investigated in bulk at temperatures ranging from 50 to 80°C. The Mayo–Lewis equation has been found to be inadequate to describe the result due to depolymerization of MI sequences above 70°C.