Electron-attracting monomers

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. 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

To be eligible to living anionic polymerization a vinylic monomer should carry an electron attracting substituent to induce polarization of the unsaturation. But it should contain neither acidic hydrogen, nor strongly electrophilic function which could induce deactivation or side reactions. Typical examples of such monomers are p-aminostyrene, acrylic esters, chloroprene, hydroxyethyl methacrylate (HEMA), phenylacetylene, and many others. 

The donor ability of the monomer is decreased by electron-attracting substituents. 

The acceptor ability of the cation is increased by electron-attracting substituents. The task was to seperate the substituent influences on the reactivity of the monomer from that on the cation and to find a relationship between these influences and the brutto rate constant of the cationic polymerization 76). 

The deviation of riV2 from unity has already been cited as a measure both of alternating tendency and of specificity in the radical-monomer reactions. This product of the reactivity ratios approaches unity only in those cases in which the monomer substituents are similar to one another in their electron-attracting or releasing capacities. Devia-... 

Styrene is a unique monomer since the aromatic ring is both electron releasing and electron attracting. It has been polymerized with a second type of catalyst. 

Reaction medium. When lignosulfonate was subjected to graft copolymerization with vinyl monomers, the extent of copolymerization due to the effect of medium varied from one monomer to another. In a LS-styrene system (16), it was found that methanol was a better medium than water under certain given conditions while in a LS-acrylonitrile system (17),the contrary was true, i.e., water better than methanol. This contradiction was thought due to the fact that styrene has electropositive (i.e., electronreleasing) substituent while acrylonitrile has electronegative (i.e., electron-attracting) substituent. In the present study,... 

The necessary electron delocalization is possible by the existence of free d orbitals on the transition metals, with small energy level differences it is aided by a suitable modification of the electron configuration on the transition metal atom by its oxidation state and the presence of electron-attracting or electron-donating ligands. An excess of electrons is manifested by a reduced tendency of the active centre to interact with the n electrons of the monomer, the transition complex is formed only with difficulty or not at all a lack of electrons results in the formation of a relatively stable complex of the active centre with the monomer with little tendency to decompose at the necessary rate in the required way. 

In summary the results observed in these studies [160] of poly(Sty-co-DVB) swelling in aromatic liquids serve to show that the method of measuring a is so sensitive that it can detect an effect caused by even the smallest modification in the molecular geometry of attached substituents, and that these differences correlate qualitatively with expectation based on the known principles of physico-organic chemistry of aromatic compounds. Since the observed a is the net effect of electronic attraction and steric hindrance between the sorbed molecule and the adsorption site, i.e. the monomer unit of the polymer, it would be impossible to separate quantitatively the electronic and steric contributions of a particular substituent. The ability to make such a differentiation, however, appears to be more promising with liquids that comprise homologous series of the type Z(CH2)nH (where Z is a phenyl, chloro, bromo or iodo substituent), since the added electronic contribution to Z by each additional methylene group is well known to be extremely small when n becomes >3 [165],... 

In addition, with these monomers the substituent not only preferentially complexes the electrophile but may even reduce the nucleophilicity of the double bond by electron attraction. Acrylates (and similarly vinyl acetate) thus do not polymerize cationically. (It may be noted that vinyl acetate is also not polymerized by anionic initiators as they attack the acetate linkage. Vinyl acetate is polymerized only by free radicals.)... 

The order of occurrence of monomers in Table 8.5 is obviously a reflection of the polarity of the double bond. Observe that the product rit2 approaches unity only in those cases where the substituents on the monomers are either both electron-donating or electron-withdrawing substituents. Expressed differently, alternation tendency increases if the substituents on both monomers exhibit different electron-donating or electron-attracting characteristics. Alternation tendency is enhanced by an increased difference in the polarity of monomer pairs. 

The values of e are also informative for instance, maleic anhydride with two strong electron-attracting side groups has e = -1-1.5, indicating an electropositive double bond. This leads to a repulsion of other maleic anhydride molecules, and so no homopolymerization takes place. Similarly, isobutylene has e = —l.l, and repnl-sion of like monomers is again a strong possibility. Copolymerization of oppositely charged monomers, however, should take place readily. 

The monomer pairs in free radical polymerizations can be arranged in a series according to the products of the copolymerization parameters (Table 22-3). On the left-hand side in this series are monomers with electron-donating groups, such as butadiene, styrene, or vinyl acetate and on the right are monomers with electron-attracting substituents, such as maleic anhydride, acrylonitrile, vinylidene chloride, etc. The product r rg decreases from one to zero in the vertical series, whereas in the horizontal series it increases from low values (left) to values up to unity (right). 

The influence of polarity is especially noticeable when both monomers produce resonance-stabilized polymer free radicals. The resonance-stabilized styrene with the electrodonating phenyl group always has a copolymerization parameter of less than unity when copolymerized with resonance-stabilized comonomers with electron-attracting groups (i.e., acrylic compounds) (see Table 22-10). The unlike monomer is then preferentially added on, which is easily understandable for the copolymerization of two monomers with opposed polarities. The relationships are also similar in the copolymerization... 

Usually electropolymerization of these monomers occurs without any problems, and the properties of the subsequent polymers are detailed in Section 18.3.3. However, in very few occasions, the classical electropolymerization reaction does not work. This has been especially demonstrated in the case of tetrazines [121], where probably the too strong electron attracting character of the tetrazine impedes the classical cation-radical coupling, favoring another type of decomposition reaction, e.g., a proton loss. However, while even EDOT functionalized tetrazine does not polymerize, the same tetrazine ring can be incorporated into an ECP with the help of a bithienyl functionalized moiety [97]. 

In another way, ct-amino acid N-carboxyanhydrides are attractive monomers for the synthesis of controlled architectures (14AML378). Poly-phosphonohomoalanine is a pH-responsive polymer which undergoes consequent conformational modifications when pH goes from 7.4 to 1.0. At pH 1, circular dichroism experiments showed that the fuUy protonated polyacid was mainly present on a ct-helical form whereas at pH 7.4, electronic repulsions of the polyanionic system induced a disordered chain conformation (Figure 3). 

Ionic copolymerizations are only possible under three conditions. The first is that the ionic chain end can induce the polymerization of the other monomer at any given time. For example, in the anionic copolymerization of ethylene oxide with vinyl compounds CH2=CHR, alkoxide ions—CH2—CH2—O" may be formed. These alkoxide ions can only add monomers with electrophilic double bonds, i.e., monomer containing electron-attracting substituents R (e.g., acrylonitrile, vinylidene cyanide). 

The development of polythiophenes since the early 1980s has been extensive. Processible conducting polymers are available and monomer derivathation has extended the range of electronic and electrochemical properties associated with such materials. Problem areas include the need for improved conductivity by monomer manipulation, involving more extensive research using stmcture—activity relationships, and improved synthetic methods for monomers and polymers alike, which are needed to bring the attractive properties of polythiophenes to fmition on the commercial scale. 

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