Free radical vinyl polymerization chemistry

It is interesting to note that due to their industrial importance, free radical polymerizations are the most studied reactions in chemistry. Furthermore, the kinetic approaches taken in this chapter are experimentally verified for essentially all typical free radical vinyl polymerizations. 

As was shown above, the interaction of high-energy rays with monomers results in the generation of free radicals. In radiation chemistry, the yield of a reaction is generally expressed in terms G values, that is, the number of radiolytically produced or consumed species per 100 eV absorbed. As far as radical vinyl polymerization is concerned, G(radical) values depend on the proneness of a monomer to form radicals. Thus, for styrene, G(radical) values of 0.7 are found for vinyl acetate, the G(radical) value amounts to —12 (see Table 1). 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

Free radical polymerization is a key method used by the polymer industry to produce a wide range of polymers [37]. It is used for the addition polymerization of vinyl monomers including styrene, vinyl acetate, tetrafluoroethylene, methacrylates, acrylates, (meth)acrylonitrile, (meth)acrylamides, etc. in bulk, solution, and aqueous processes. The chemistry is easy to exploit and is tolerant to many functional groups and impurities. 

Chemistry. Vinyl acetate is polymerized commercially using free-radical polymerization in either methanol or, in some circumstances, ethanol. Suitable thermal initiators include organic peroxides such as butyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, butyl peroxyneodecanoate, benzoyl peroxide, and lauroyl peroxide, and diazo compounds such as 2,2 -azobisisobutyronitrile (205—215). The temperatures of commercial interest range from... 

The fundamental chemistry of the structural adhesives described here can change very little. Vinyl and acrylic monomers polymerize by chain growth polymerization initiated by free radicals or ions. Isocyanate and epoxy compounds react with compounds containing active hydrogen in step growth polymeriza-... 

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. 

Numerous examples exist of combining CRP methods with other polymerization techniques for preparation of block copolymers. Non-living polymerization methods like condensation, free-radical, and redox processes can easily be combined with CRP to produce novel materials. Transformation chemistry may be the only route to incorporate polymers like polysulfones (as described above), polyesters, or polyamides that are prepared solely through condensation processes into subsequent CRP to form block copolymers with vinyl monomers. The same can be said of polymers prepared through coupling techniques, like po-ly(phenylenevinylene) and poly(methylphenylsilylene), which can maintain their conductive or photoluminescence properties, but become easier to process... 

A detailed study of mechanisms both of photodecomposition of triarylsul-fonium salts to yield Bronsted acids and of catalysis of cationic polymerization of representative monomers—styrene oxide, cyclohexene oxide, tetrahydrofuran (THF), and 2-chloroethyl vinyl ether—was reported in 1979 by Crivello and Lam [14]. Crivello [15] and Green et al. [16] provided further reviews shortly thereafter. The mechanisms of photodecomposition of a variety of initiators for free radical photopolymerization, including onium salts, were compared by Vesley [17] in 1986. A review, similar in scope, but providing more mechanistic detail was also published in 1986 by Timpe [10a]. An updated coverage of aspects of this chemistry has been provided by the same author in his review of photoinduced electron transfer polymerization [10b]. 

From elementary organic chemistry, we know that the positions and hence reactivities of the electrons in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents on the double bond. As a result of these influences, the double bond reacts well with a free radical for compounds of the types CHj = CHY and CHj = CXY. These compounds constitute the so-called vinyl monomers where X and Y may be halogen, ally l, ester, phenyl, or other groups. It must, however, be noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds (e.g., 1,2 disubstituted ethylenes) and sterically hindered compounds of the type CHj = CXY, polymerization, if it occurs at all, proceeds slowly. 

Two major types of materials have until now been applied to molecular imprinting, either organic polystyrenes/polyacrylates, or inorganic polysiloxanes. Variations are abundant but these have nevertheless been the most popular, sometimes also used in combination. In this chapter, a survey of building blocks is given and basic synthetic schemes presented. Since organic matrices are by far the more important, organic polymeric systems based on free-radical polymerization of vinyl monomers are exclusively covered. An account on polysiloxane chemistry is covered by Chapter 11 in this volume. 

The concept of copolymerizing with a functional comonomer that is soluble in the continuous phase can virtually be extended to any vinyl functional monomer, provided that under such conditions the copolymerization parameters will allow a copolymerization to occur. The functionalities available using hydrophobic monomers with functional monomers in direct miniemulsions are summarized in Table 15.1. Latexes with a double functionality were prepared via a free-radical polymerization of divinylbenzene in miniemulsion [49, 50] after polymerization, the remaining vinyl bond might be reacted with a thiol-functionalized PEG via the thiol-ene chemistry [49]. 

Basic acrylate chemistry. The basic acrylic monomers or oligomers contain unsaturated double bonds (vinyl groups), and consequently cure by addition polymerization involving a free-radical reaction. Free-radical-producing compounds such as peroxides, peracetic acids, and sulfones are added to acrylic resins to initiate polymerization. Free-radical polymerization of acrylics may also be induced by exposure to U V or visible light. These UV-curing adhesives, most of which are based on acrylic or modified acrylic... 

Some commercially available protein-inert polymers commonly used in microfluidic applications, all of which require permanent surface modification, are polyacrylamide, poly (N-hydroxyethyl acrylamide), poly(N,N -dimethyl acrylamide) (PDMA), polyvinylpyrrolidone (PVP), poly(vinyl alcohol) (PVA), hydroxyethyl cellulose (HEC), and hydroxypropyl methylceUulose (HPMC). To permanently attach protein-resistant materials to the channel sinface, high-energy sources, special chemistries, or even strrMig physical adsorption have been employed to introduce reactive functionalities. After activatirm, protein-resistant polymers can be anchored via UV-initiated free-radical polymerization. Polymeric materials... 

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