Vinyl carbazole cationic polymerization

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines, N-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

When the monomer is attached to the active centre in a manner leading to the formation of a weakly reactive ion, growth is terminated. A typical example of this kind of termination is the formation of an unreactive ion during cationic polymerization of vinyl carbazole. Instead of propagation... 

Cationic polymerizations are not only important commercial processes, but, in some cases, are attractive laboratory techniques for preparing well-defined polymers and copolymers. Polyacetal, poly(tetramethyl-ene glycol), poly(e-caprolactam), polyaziridine, polysiloxanes, as well as butyl rubber, poly(N-vinyl carbazol), polyindenes, and poly(vinyl ether)s are synthesized commercially by cationic polymerizations. Some of these important polymers can only be prepared cationically. Living cationic polymerizations recently have been developed in which polymers with controlled molecular weights and narrow polydispersity can be prepared. 

Other monomers that are suitable for cationic polymerization include cyclic ethers (like tetrahydrofuran), cyclic acetals (like irioxane), vinyl ethers, and N-vinyl carbazole. In these cases the hetero atom is bonded directly to the electron deficient carbon atom, and the respective carboxonium ion (9-13) and immonium ion (9-14) arc more stable than the corresponding carbocalions. 

There is a cationic analogue of Scheme 11 when TV-vinyl carbazole polymerization was induced by poly(vinyl chloride)-bound dimethylglyoxime complexes of Co11, Ni11, and even Cu11 (Scheme 12).390... 

Selenide-Based Nonsalt Photoinitiator In more recent years, diphenyldiselenide (DPDS) as nonsalt initiator was used for photosensitized cationic polymerization of /V-vinyl carbazole (NVC) [69]. Diselenide compounds in the presence of aromatic nitriles are well-known photosensitization system for in situ generation of electrophilic... 

The idea of two simultaneous homo-polymerizations was explored by Szwarc and his co-workers252. Pac and Plesch253 reported initiation of cationic polymerization of vinyl carbazole (VC) by tetranitromethane in nitrobenzene. They postulated the formation of a charge-transfer complex,... 

Reinvestigation of Pleseh s work confirmed their kinetic conclusions, viz. the first order kinetics in monomer, and led to a similar rate constant. However, addition of oxetane to the vinyl-carbazole-tetranitromethane system greatly retarded the vinyl-car-bazole polymerization, e.g. by a factor of 30, but neither the first order character of this reaction nor the molecular weight of the resulting polymer was affected provided that the concentration of oxetane was sufficiently large. At the same time, cationic polymerization of the added oxetane was observed. 

No true living cationic vinyl polymers are known at present, although the papers of Skorokhodov on 1,2-dimethoxyethylene 1011 and of Higashimura on the I2-initiated polymerization of vinyl ethers, p-methoxystyrene (Sect. 15.3.1.1.3) and of N-vinyl carbazole 102) approach living features. It is possible, however, to prepare a vinyl polymer with an end-group that is inactive towards its own monomer but is able to convert a heterocyclic monomer into the onium ion, in this way starting the polymerization of a second monomer. The application of this approach for the synthesis of block copolymers is described below. 

The polymerization of A -vinyl carbazole may be initiated by free radicals, by cations, by organometallic catalysts, by charge-transfer processes, or by electrochemical methods. Since the monomer is a solid at room temperature (m.p. 64-66°C, b.p. 154-155°C/3 mm), solid state polymerizations have been carried out both on powdered monomer and on the surfaces of larger crystals. In this process, the reactive species propagates into the interior of the solid. 

Examples of the different types of living cationic polymerization systems are listed in Table 3. All involve relatively fast initiation and optimal equilibria between a low concentration of active carbenium ions and a high concentration of dormant species (Scheme 9). Only hydroiodic acid initiated polymerizations of A-vinyl carbazol are controlled in the absence of a Lewis acid activator or a nulceophilic deactivator [115]. 

E ect of Additives m the Rate of Polymerization. The conventional cationic nature of the sieve catalyzed polymerbration of N-vinyl carbazole is endorsed by the... 

In (A), the reaction will predominate on the left-hand side. Exceptions appear to be olefins with strong electron-releasing substituents that confer thermodynamic stability to the newly formed cation, -CH2-CHR . This either results from a suitable charge delocalization over the 7r-electron system or from the presence of a heteroatom. Accordingly, only those olefins that possess relatively strong nucleophilic characteristics can be polymerized by stable carbon cations. Such olefins are alkyl vinyl ethers, A -vinyl carbazole, p-methoxystyrene, indene, and vinylnaphthalenes. Styrene and cf-methylstyrene, however, will not polymerize, because they are less reactive. 

Solvations of the charged species accelerate the transfer of electrons and the ionizations are enhanced by polar solvents.Charge transfer reaction studies with tetracyanoethylene, an acceptor, and A -vinyl carbazole, a donor, in benzene solution demonstrated that both cation radicals and anion radicals form. This can be used in a subsequent cationic polymerizations ... 

Solutions of maleic anhydride in ether will initiate cationic polymerizations of isobutyl vinyl ether or Af-vinyl carbazole, if subjected to attacks by free radicals. The same is true if the solutions are irradiated with ultraviolet light or gamma rays. Also, active species are generated from reactions of aldehydes or ketones with maleic anhydride when attacked by free radicals or irradiated by UV light, or gamma rays from These active species are presumed to be formed through... 

The initiator cation formed in this way can, in many cases then add directly on to the monomer to form the monomer cation. Examples of this are the polymerization of p-methoxystyrene or epoxides with trityl hexa-chloroantimonate, the polymerization of tetrahydrofuran with acetyl perchlorate, and the polymerization of vinyl ethers and A -vinyl carbazole with tropylium hexachloroantimonate, for example ... 

In addition to free radical polymerizations, however, cationic and anionic polymerizations may, depending on solvent used, also be observed. The anodic discharge of acetate ions into the homogeneous phase, for example, produces a free radical polymerization of styrene or acrylonitrile. The anodic discharge of perchlorate or boron tetrafluoride ions, on the other hand, leads to the cationic polymerization of styrene, iV-vinyl carbazole, and isobutyl vinyl ether. In contrast, the cathodic decomposition of tertaalkyl ammonium salts induce acrylonitrile to polymerize anionically. 

Chain growth addition polymerization based on active carbene (e.g., styrene, vinyl carbazole, isobutylene) is a fast reaction that is most difficult to control since termination by combination or disproportionation is not possible. The initiator for cationic polymerization is a proton from a donor such as water, in the presence of a Lewis acid or sulfuric acid. Polymerization tends to be uncontrollable when undiluted monomers are used, and so these reactions are carried out at low monomer levels, usually in chlorinated... 

Polymers from Novel Monomers.—Novel compounds that have been polymerized by cationic mechanisms usually incorporate the reactive functional groups of more conventional monomers, and several recently developed monomers can be classified as analogs of vinyl ethers, vinyl carbazole, p-methoxystyrene, or previously studied oxygen heterocycles. 

Vinyl Ethers and Vinyl Carbazoles.—With few exceptions, most recent kinetic studies of the cationic polymerizations of vinyl ethers and vinyl carbazoles have been conducted with the use of stable carbocation salt initiators, which facilitates analysis of the data. 

Rate constants of propagation for iV-vinyl carbazole were found to be only slightly influenced by the degree of dissociation of the active centres at low temperatures. Transfer and termination reactions appeared to be insignificant at a polymerization temperature of —70 °C. Well-defined living cationic polymerizations can be conducted with iV-ethyl-3-vinylcarbazole. ... 

Optically-active vinyl carbazole derivatives can be polymerized cationically to yield polymers whose optical rotation depends upon the initiator system employed. In contrast cationic polymerization of a series of 2-phenylvinylalkyl thioethers produced polymers of optical rotations identical to those of the corresponding mode compounds. ... 

Monomers ALkene monomers which contain an electron donating substituent that can stabilize a carbenium ion are well suited for cationic polymerizations. These include substituents based on nitrogen (A-vinyl carbazole),... 

The nitrogen atom attached to the vinyl group lowers the electron density in the vinyl group hy its inductive effect, but the mesomeric effect of the nitrogen electron pair overwhelms the inductive effect so that a conjugated 7r-electron system is formed. Therefore, NVK can be polymerized readily by cationic initiators. Anionic polymerization is not possible with NVK, hut related carbazole compounds with the vinyl group bonded to the aromatic kernel can undergo anionic polymerization. 

Results obtained in the many copolymerizations of carbazole-containing monomers with different chiral comonomers may be summarized as follows i) real copolymer macromolecules are obtained in the cationically and free radically initiated polymerization with alkyl vinyl ethers, acrylic and methacrylic derivatives, and butene-dioic acid diesters ii) homopolymer mixtures are obtained in copolymerization runs with a-olefms in the presence of Ziegler-Natta catalysts, indicating that the conventional anionic coordinate mechanism is not effective in the polymerization of carbazole monomers... 

Whereas free radically initiated polymerization yields polymers characterized by an essentially random distribution of monomeric units, block-like polymeric products are obtained in the cationic copolymerization of NVC with alkyl vinyl ethers. The possible formation of preferential aggregates of the vinyl aromatic monomer or a possible control of the homopropagation process connected with the bulkiness of the monomers has been suggested as responsible for such polymer structures . A random distribution of monomeric units is conceivable in copolymers based on the spaced-carbazole-containing monomers 11-14, whilst a quasi alternating distribution is observed in copolymers from NVC and fumaric acid diesters . ... 

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