Vinyl and vinylidene monomers

This group has been best investigated theoretically, and is the most used in large-scale production. The basic members are ethylene and its derivatives. According to the type of substituent, the group contains two subgroups, alkenes (olefins) and polar vinyl monomers. 

The number and position of substituents are important. One bulky group, e.g. naphthyl, or two substituents on a single olefinic carbon do not usually hinder polymerization. On the other hand, an ethylene derivative with a substituent on each of the two carbons does not undergo radical polymerization at conventional pressures and temperatures. Usually, however, it can be copolymerized with a suitable monomer or it can be polymerized by an ionic mechanism. With a larger number of substituents, the tendency to polymerization is further limited. The small fluorine atom represents an exception, as do the cases discussed in Sects. 1.3 and 5.2. 

The most important representatives are the lowest 1-alkenes, ethylene and propene. Ethylene is not particularly easily polymerized by radical or ionic mechanisms. Its importance as a monomer was greatly enhanced by the discovery of coordination polymerizations. Propene is oligomerized by radical and ionic initiators. This explains the importance of Natta s modification [1] of Ziegler [2,3] catalysts, enabling inferior raw materials to yield high-quality polymers. 

Of great importance are the ethylene derivatives with aromatic substituents. Styrene (vinylbenzene) is one of the monomers produced industrially in large volume. Polystyrene and styrene copolymers still belong to the important representatives of modern plastics and rubbers. Styrene can be polymerized by any of the known procedures. It has suitable physical properties, and therefore it is one of the most frequently studied monomers. It also 

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. 

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Vinyl and vinylidene monomers are basically unsymmetrical because the iwo ends of the double bond are distinguishable (ethylene and tetrafluorethylene are exceptions). One C of the double bond can be arbitrarily labeled the head and the other the tail of the monomer, as shown in the formula for vinyl fluoride (4-3). 

Radical reactions with the n bond of vinyl monomers are not nearly as selective as ionic attack, and free-radical initiators cause the polymerization of nearly all vinyl and vinylidene monomers. (Some of these polymerizations are not elTicient because of side reactions. Propylene is acase in point as described in Section 6.8.5.) Resonance stabilization occurs to some extent with most vinyl monomers but it is important in radical polymerizations only when the monomers contain conjugated C—C double bonds as in styrene, 1,3-butadiene, and similar molecules ... 

Two types of monomers A and B, have been distinguished on the basis of the relationship between their structure and polymer chirality. To the former type belong monomers, such as vinyl and vinylidene monomers, which need to be chiral in order to give optically active polymers, while monomers of the latter type such as suitably substituted dienes suffice to be prochiral. 

Whilst the glass transition is of very great importance in adhesives, crystalline melting is not. This is because the polymers in most adhesives are totally amorphous, which in turn is due to a lack of molecular regularity. Stereoregular polymers can be made from vinyl and vinylidene monomers with the employment of organometallic initiators, but these are not used when such monomers are converted into polymers which are used in adhesives. The latter are atactic and examples are MMA copolymers in structural acrylic adhesives, amorphous polypropylene, polyacrylic pressure-sensitive adhesives, and polycyanoacrylates. 

In order to be useful for structure analysis, the observed resonances must be assigned to chemical structures. Eor copolymer analysis, the enormity of the problem of assigning the observed resonances can be recognized by the following simple considerations. For the 21 most common vinyl and vinylidene monomers, there are a total of 210 possible binary copolymers and 1330 ternary copolymers [29]. These copolymer combinations can also have an alternating, random, or block sequence structure so that the total number of possible copolymers is quite large. Experimentally, only about 30 of these copolymers have been studied. This extremely complex situation suggests that computer simulation of the spectra of the copolymers is helpful in the interpretation of the NMR spectra [30]. 

Macromolecules having identical constitutional repeating units can nevertheless differ as a result of isomerism. For example, linear, branched, and crosslinked polymers of the same monomer are considered as structural isomers. Another type of structural isomerism occurs in the chain polymerization of vinyl or vinylidene monomers. Here, there are two possible orientations of the monomers when they add to the growing chain end. Therefore, two possible arrangements of the constitutional repeating units may occur ... 

The polar monomers cited include acrylic acid, acrylic esters, meth-acrylic acid, methacrylic esters, acrylonitrile, methacrylonitrile, acrolein, and vinyl acetate. While this list is reasonable, it also includes vinyl halides and vinylidene halides, although no examples with the latter are given. In view of the fact that the vinyl and vinylidene halides do not form complexes with Friedel-Crafts catalysts, these monomers would not be expected to be operable, as demonstrated by the results of Imoto (30). 

Steric hindrances prevent the polymerization of most 1,2-disubstituted ethylenes by any mechanism. However, 1,1 -disubstituted monomers and vinylidene monomers usually polymerize more readily than the corresponding vinyl analogs. 

Data on the significant role of the transfer reaction of the polymer chains with a monomer are in agreement with the number of vinyl and vinylidene groups in polyolefins 29.n8.n9)... 

Each of the polymers listed above is made from one monomer only. These polymers are called homopolymers. Any number of coreactive vinyl or vinylidene monomers can be polymerized together and the product is called a random copolymer. In a copolymer molecule, the different monomer units are combined together in a random way. As the polymer free radical grows, the next unit to be added is decided by chance it depends upon which monomer molecule encounters the right part of the chain first... 

Premium grade SLS for emulsion polymerization applications including vinyl and vinylidene chlorides, styrene and acrylic monomers. Choice surfactant for carboxylated SBR and acrylic froth applications. 

The C-H stretch first overtone of terminal methylene groups of vinyl and vinylidene structures is isolated enough that it can be used in traditional quantitative analysis. Figure 3.1 provides one example, and Table 3.1 provides some typical peak locations. Goddu provides tables of absorp-tivities for the first overtone absorption of the terminal methylene group in a variety of compounds and solvents. Molar absorptivities are about 0.2-0.5 1/mol-cm. Put another way, a 100-ppm amount of methylene gives an absorbance of 0.01 in a 10-cm cell. Analyses using this peak to measure the vinyl content of acrylate monomers, butadienes, and edible oils" have been reported. 

It is known that vinyl aromatic monomers when inserted in copolymer macromolecules with optically active vinyl or vinylidenic monomers become optically active and may or may not contribute to the optical rotation of the whole polymer measured at the sodium D line In particular, by mvestigating the electronic transitions of the aromatic chromophores, it is general found that both symmetry forbidden and... 

Spontaneous termination by IJ-hydrogen abstraction is less typical with Ziegler-Natla catalysts than with Phillips catalysts, because of the low temperature of operation at 60°-80°C. It can be induced at a higher temperature, however, and results in the formation of vinyl and vinylidene end groups. The polymer chain can also exchange with the monomer to form a new active center, with the release of free polymer, in a process known as chain transfer. 

Shelton, L.G, Hamilton, D.E., Fisackerly, R.H. Vinyl and Vinylidene Chloride, in Vinyl and Diene Monomers, Part 3, 1205-1289, E. Leonard (ed ), Wiley-Interscience, New York (1971)... 

In addition to homopolymers of varying molecular and particle structure, copolymers are also available commercially in which vinyl chloride is the principal monomer. Comonomers used eommercially include vinyl acetate, vinylidene chloride, propylene, acrylonitrile, vinyl isobutyl ether, and maleic, fumaric and acrylic esters. Of these the first three only are of importance to the plastics industry. The main function of introducing comonomer is to reduce the regularity of the polymer structure and thus lower the interchain forces. The polymers may therefore be proeessed at much lower temperatures and are useful in the manufacture of gramophone records and flooring compositions. 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

List C contains peroxidisable monomers, where the presence of peroxide may initiate exothermic polymerisation of the bulk of material. Precautions and procedures for storage and use of monomers with or without the presence of inhibitors are discussed in detail. Examples cited are acrylic acid, acrylonitrile, butadiene, 2-chlorobutadiene, chlorotrifluoroethylene, methyl methacrylate, styrene, tetraflu-oroethylene, vinyl acetate, vinylacetylene, vinyl chloride, vinylidene chloride and vinylpyridine [1]. 

Adding plasticizer, like dioctyl phthalate, is generally accomplished by mechanical methods. Permanent or chemical plasticization can be done by copolymerization of VCM with monomers such as vinyl acetate, vinylidene chloride, methyl acrylate, or methyl rhethacrylate. Comonomer levels vary from 5-40%. The purpose of the co-polymers, of course, is to change the properties such as softening point, thermal stability, flexibility, tensile strength, and solubility. 

Acrylic fibers. Acrylic fibers are polymers of acrylonitrile and another chemical. When acrylonitrile is 85% or more of the polymer, the fiber is called acrylic. If there s more copolymer so the percentage of acrylonitrile decreases to 35-85%, the fiber is called modacrylic. Some of the popular monomers used as copolymers are methyl acrylate and methacrylate, acrylamide, vinyl acetate, vinylidene chloride, and vinyl chloride, Dynel is 40% acrylo and 60% vinyl chloride. 

The effect of pressure on polymerization, although not extensively studied, is important from the practical viewpoint since several monomers are polymerized at pressures above atmospheric. Pressure affects polymerization through changes in concentrations, rate constants, and equilibrium constants [Ogo, 1984 Weale, 1974 Zutty and Burkhart, 1962], The commercial polymerizations of most gaseous monomers (e.g., vinyl chloride, vinylidene chloride, tetrafluoroethylene, vinyl fluoride) are carried out at very moderate pressures of about 5-10 MPa (1 MPa = 145 psi), where the primary effect is one of increased... 

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