Charge transfer initiation systems

Reaction Mechanism of Vinyl Polymerization with Amine in Redox and Photo-Induced Charge-Transfer Initiation Systems... 

A substantial number of photo-induced charge transfer polymerizations have been known to proceed through N-vinylcarbazole (VCZ) as an electron-donor monomer, but much less attention was paid to the polymerization of acrylic monomer as an electron receptor in the presence of amine as donor. The photo-induced charge-transfer polymerization of electron-attracting monomers, such as methyl acrylate(MA) and acrylonitrile (AN), have been recently studied [4]. In this paper, some results of our research on the reaction mechanism of vinyl polymerization with amine in redox and photo-induced charge transfer initiation systems are reviewed. 

Charge-transfer initiator systems are attracting increased interest, as is cationic photopolymerization. Photopolymers of acetylenes and polyacetylenes are of potential importance as conductors and semiconductors, for example in solar cell applications (Fouassier et al., Yee, Day, et al., inter alia). Fouassier et al. report that vinyl polymerization photosensitized by aromatic ketones can proceed faster in a micellar system through enhanced initiation rather than propagation. 

Spontaneous polymerizations of monomers without the addition of initiator or catalyst are relatively rare. Examples are the thermal polymerization of styrene (Section 20.1.3) and the charge-transfer-initiated polymerization of monomers with opposite polarities (Section 18.1.2.2). It is often difficult to distinguish these genuine spontaneous reactions from nongenuine ones where polymerization is induced by some unrecognized admixture to the system. 

Furan and maleic anhydride undergo the Diels-Alder reaction to form the tricycHc 1 1 adduct, 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyHc anhydride (4) in exceUent yield. Other strong dienophiles also add to furan (88). Although both endo and exo isomers are formed initially, the former rapidly isomerize to the latter in solution, even at room temperature. The existence of a charge-transfer complex in the system has been demonstrated (89,90). 

The diacyl peroxide-amine system, especially BPO-DMT or BPO-DMA, has been used and studied for a long time but still no sound initiation mechanism was proposed. Some controversy existed in the first step, i.e., whether there is formation of a charge-transfer complex of a rate-controlling step of nucleophilic displacement as Walling 1] suggested ... 

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

The conspicuous separation between the cathodic and anodic peak potentials was initially interpreted in terms of the simple theory for redox polymers as a kinetic effect of slow heterogeneous charge transfer the thermodynamic redox potential of the whole systems was calculated from the mean value between Ep and Ep ... 

Adsorption of Reaction Components In many cases, adsorption of a reactant is one of the hrst steps in the electrochemical reaction, and precedes charge transfer and/or other steps of the reaction. In many cases, intermediate reaction products are also adsorbed on the electrode s snrface. Equally, the adsorption of reaction products is possible. The example of the adsorption of molecular hydrogen on platinum had been given earlier. Hydrogen adsorption is possible on the platinum electrode in aqueons solntions even when there is no molecular hydrogen in the initial system at potentials more negative than 0.3 V (RHE), the electrochemical reaction... 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

The second section focuses on emerging classes of photopolymerizations that are being developed as alternatives to acrylates. Three types of polymerization systems are included cationic photopolymerizations, initiator-free charge-transfer polymerizations, and a thiol-ene reaction system. The last section covers four interesting emerging applications of photopolymerization technology. 

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

Photochemical ET reactions can be classified in at least three categories (which can co-exist), namely (i) simple homolysis of bonds of neutral molecules to give radicals of low redox reactivity (ii) excitation of a species D to produce an excited state D which initiates a second-order ET reaction involving another component of acceptor type, A, with formation of the radical pair D + A (iii) direct excitation of a charge transfer (CT) complex formed between two reaction components D and A to form the same radical pair D + A -. The first case is obviously an ideal situation if it can be realized, but this is seldom the case. The incursion or predominance of situations (ii) and/or (iii) in almost any system is possible, and precautions must be taken to avoid these complications. Much can be done by controlling the wavelength of the light source, but it is also possible to affect the chemistry in a predictable manner. 

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