Monomers as electron donors

In a broader sense, each cationically polymerizing monomer has a nucleophilic character. To the best of the author s knowledge, monomers have not so far been ordered according to their electron-donating ability. 

The following monomers have been classified as donors styrene [78, 79], vinyl ethers and vinyl sulphides [78], vinyl acetate [80] vinylcarbazole [81, aziridine [82], 2-oxazoline [83], 2-benzyliminotetrahydrofuran [84], five-and six-membered cyclic iminoethers, cyclic amines, cyclic phospholanes and Schifif bases [85]. Trialkoxyethylenes [86] 

The ability to donate electrons is a very important property of monomers, especially in copolymerizations. It may be expected that monomer characterization from this point of view by a suitable quantitative method under various conditions would considerably extend our knowledge of the addition mechanism in polymerizations. 

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In cationic polymerisation monomer molecule acts as electron donor and higher activity is shown by vinyl derivatives which have an increased electron density at the double bond. 

The group of Professor Nazario Martin developed the synthesis of several donor-acceptor arrays containing -conjugated oPPEs of different length linking -extended tetrathiafulvalene (exTTF) as electron donor and C o as electron acceptor. Thereby, they systematically increased the length of the molecular wire from the monomer to the trimer. A detailed description of the corresponding synthesis is described elsewhere [8]. 

A similar reaction scheme which accounts for the coordination mode of diene monomers was already suggested by Witte in 1981 [49]. Though DIBAH or TIBA are known as Lewis acids, in an anhydrous hydrocarbon environment aluminum alkyls can act as electron donors which are capable of coordinating to vacant Nd sites. This is evidenced as DIBAH forms oligomers [461]. Thus, DIBAH acts as an electron donor as well as an electron acceptor. In addition, complexes of aluminum alkyls with various Nd precursors were isolated and characterized, e.g. [184-186,234]. 

Further examples for electron acceptor monomers are acrylonitrile [37], diethyl fumarate [39], fumaronitrile [29,30, 38], maleonitrile [38], N-carbethoxymaleimide [29], N,N-diethylaminoethyl methacrylate [39], nitroethylene [10] and iV-ethyl-maleimide [40], As electron donor monomem also are used vinyl alkyl ethers [38, 40], alkyl methacrylate [40], JV-vinyl pyrrolidone [40] and cyclohexene oxide [10]. 

These monomers usually polymerize by classical methods, i.e. radical or ionic, more readily than ethylene. On the other hand, they are too good as electron donors in coordination polymerizations they act as catalytic poisons. The group of polar vinyl monomers is very large. Mostly these compounds are of only theoretical interest. Many of them are, however, technically and socially important, and the exploitation of others is anticipated. 

Indeed, copolymers containing 2-(A, A-dimethylaminoethyl) methacrylate units as electron donor moieties, when allowed to interact with photoexcited fluorenone in the presence of vinyl monomers, are effective in causing rapid crosslinking and gelation through a graft copolymerization process [30,38] (Scheme 9). 

Taking into account polarity, steric factors, and resonance stabilization, T. Alfrey and C. C. Price (7) developed a Q-e scheme and predicted monomer reactivity. The effect of polarity on vinyl monomer copolymerization was recognized by F. R. Mayo and coworkers (8), who distinguished between monomers of average activity and those acting as electron donors or acceptors. By combining these theories with experimental data, calculation of product probabilities of various monomer combinations and determination of monomer reactivity parameters were possible. 

The chemical structures of the monomers also determine their reactivity toward cationic polymerizations. Electron-donating groups enhance the electron densities of the double bonds. Because the monomers must act as nucleophiles or as electron donors in the course of propagation, increased electron densities at the double bonds increase the reaction rates. It follows, therefore, that electron-withdrawing substituents on olefins will hinder cationic polymerizations. They will, instead, enhance the ability for anionic polymerization. The polarity of the substituents, however, is not the only determining factor in monomer reactivity. Steric effects can also exert considerable controls over the rates of propagation and the modes of addition to the active centers. Polymerizations... 

Cytochrome c peroxidase (CcP) [215] isolated from yeast mitochondria uses cytochrome c(//) as electron donor. Its crystal structure has been determined [216]. It is a monomer of molecular mass 34000 containing a single h-type heme group that is largely buried within the molecule. The Fe is coordinated on one side by a histidine residue while the sixth position is occupied by a weakly bound HjO in a channel suitable for entry and exit of small substrates. 

Homo and copolymerization of maleonitrile and fumaronitrile have been achieved by using medium-high temperature free-radical initiators, such as di-rerr-butyl peroxide.Copolymerization of the two monomers, with other vinyl monomers, particularly electron-donor monomers, is readily achieved with typical free-radical initiators.Black, non fusible but soluble homopolymers are obtained. Spectroscopic and chemical evidence shows that polymerization occurs through the nitrile moieties to give polymers with 1-pyrroline rings. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

Acrylonitrile copolymeri2es readily with many electron-donor monomers other than styrene. Hundreds of acrylonitrile copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymeri2ations is readily available (34,102). Copolymeri2ation mitigates the undesirable properties of acrylonitrile homopolymer, such as poor thermal stabiUty and poor processabiUty. At the same time, desirable attributes such as rigidity, chemical resistance, and excellent barrier properties are iacorporated iato melt-processable resias. 

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. 

Let us consider the conditions which favor the formation and survival of the dimeric and polymeric radical ions. This might be achieved by keeping the concentration of monomer high, the concentration of monomer" ions low and by removing the radical ions as rapidly as possible from the zone containing the primary electron donors. Moreover, since the radical ions dimerize, their average life time increases as their concentration decreases. The following experiment should probably produce the best results. 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

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