Monomers in radical polymerization

In a radical polymerization, the reactive centers are free radicals and the process is a typical chain reaction. The monomers in radical polymerizations normally contain carbon-carbon double bonds in their molecules styrene is typical. Usually radical polymerization is performed in the liquid phase. The chain reaction can be divided inlo the following steps ... 

All the above factors controlling monomer and radical reactivities contribute to the rate of polymerization, but in a manner which makes it difficult to distinguish the magnitude of each effect. Attempts to correlate copolymerization tendencies based on these factors are thus mainly of a semiempirical nature and can, at best, be treated as useful approximations rather than rigorous relations. However, a generally useful scheme was proposed by Alfrey and Price [23] to provide a quantitative description of the behavior of diferent monomers in radical polymerization, with the aid of two parameters, for each monomer rather than for a monomer pair. These parameters are denoted by Q and e and the method has been called the Q — e scheme. It allows calculation of monomer reactivity ratios r and T2 from properties of monomers irrespective of which pair is used. The scheme assumes that each radical or monomer can be classified according to its reactivity or resonance effect and its polarity so that the rate constant... 

Otsu, T. Structure and Reactivity of Vinyl Monomers in Radical Polymerization. In ... 

Compilations of reactivity ratios for various pairs of monomers in radical polymerization have been provided by Eastmond [131] and Odian [132], The reactivity ratios for pairs of given monomers can be very different for the different types of chain-growth copolymerization radical, anionic, cationic, and coordination copolymerization. Although the copolymer equation is valid for each of them, the copolymer composition can depend strongly on the mode of initiation (see Figure 11.8). 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

Head-to-Head (H-H) and tail-to-tail (T-T) linkages may also be present in polymer structures. They have been identified in addition polymers prepared by radical or coordination polymerization (polypropylene). Introduction of H-H linkages into addition polymers by radical polymerization can proceed by two mechanisms (a) terminations of radical polymerization (recombination or disproportionation of the polymer radicals) (3). The recombination introduces one H-H linkage, disproportionation of a vinyl and a saturated polymer end. Recombination and disproportionations are not always equally important termination reactions what type of termination depends on the type of monomer and polymerization conditions, (b) H-H linkages are also introduced into polymers by reverse addition of the monomer in radical polymerization. This behavior is particularly noticeable in radical polymerizations of hal-... 

The growth center in this class of ionic polymerizations is eationic in nature. The polymer cation adds on the monomer moleeules to it sequentially, just as the polymer radical adds on the monomer in radical polymerization. The initiation of the polymerization is accomplished by catalysts that are proton donors (e.g., protonic acids such as H2SO4). The monomer molecules act like electron donors and react with the catalyst, giving rise to polymer ions. The successive addition of the monomer to the polymer ion is the propagation reaction. These two elementary reactions are expressed schematically as follows ... 

The elastomer produced in greatest amount is styrene-butadiene rubber (SBR) Annually just under 10 lb of SBR IS produced in the United States and al most all of it IS used in automobile tires As its name suggests SBR is prepared from styrene and 1 3 buta diene It is an example of a copolymer a polymer as sembled from two or more different monomers Free radical polymerization of a mixture of styrene and 1 3 butadiene gives SBR... 

Polymerization Kinetics of Mass and Suspension PVC. The polymerization kinetics of mass and suspension PVC are considered together because a droplet of monomer in suspension polymerization can be considered to be a mass polymerization in a very tiny reactor. During polymerization, the polymer precipitates from the monomer when the chain size reaches 10—20 monomer units. The precipitated polymer remains swollen with monomer, but has a reduced radical termination rate. This leads to a higher concentration of radicals in the polymer gel and an increased polymerization rate at higher polymerization conversion. 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

The relative propensity of radicals to abstract hydrogen or add to double bonds is extremely important. In radical polymerization, this factor determines the significance of transfer to monomer, solvent, etc. and hence the molecular weight and end group functionality (Chapter 6). It also provides one basis for initiator selection (Section 3.2.1). 

The initiator in radical polymerization is often regarded simply as a source of radicals. Little attention is paid to the various pathways available for radical generation or to the side reactions that may accompany initiation. The preceding discussion (see 3.2) demonstrated that in selecting initiators (whether thermal, photochemical, redox, etc.) for polymerization, they must be considered in terms of the types of radicals formed, their suitability for use with the particular monomers, solvent, and the other agents present in the polymerization medium, and for the properties they convey to the polymer produced. 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

Table 5.1 Parameters Characterizing Chain Length Dependence of Termination Rate Coefficients in Radical Polymerization of Common Monomers 1...

Traditionally thiols or mercaptans are perhaps the most commonly used transfer agents in radical polymerization. They undergo facile reaction with propagating (and other) radicals with transfer of a hydrogen atom and form a saturated chain end and a thiyl radical (Scheme 6.6). Some typical transfer constants are presented in Table 6.2. The values of the transfer constants depend markedly on the particular monomer and can depend on reaction conditions.4"1 44... 

Propagation reactions in radical polymerization and copolymerization arc generally highly exothermic and can be assumed to be irreversible. Exceptions to this general rule arc those involving monomers with low ceiling temperatures (Section 4.5.1). The thermodynamics of copolymerization has been reviewed by Sawada.85... 

The data in Table I are not directly comparable, since the viscosity of the 3-isomer was determined in benzene while the others were measured in DMSO. In addition, the first two polymers were prepared in bulk polymerizations, while the polymerization of methyl 3-vinylsalicylate was carried out with the monomer diluted 1 1 with benzene. Thus no certain conclusion can be drawn the data are, however, an indication of possible difficulty in radical polymerization of substituted styrenes bearing a phenol ortho to the vinyl group. 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

Since PTFE was first synthesized more than 50 years ago, fluoropolymers have been produced by radical polymerization and copolymerizaton processes, but without any functional groups, for several reasons. First, the synthesis of functional vinyl compounds suitable for radical polymerization is much more complicated and expensive in comparison with common fluoroolefins. In radical polymerization of one of the simplest possible candidates—perfluorovinyl sulfonic acid (or sulfonyl fluoride—there was not enough reactivity to provide high-molecular-weight polymers or even perfluorinated copolymers with considerable functional comonomer content. Several methods for the synthesis of the other simplest monomer—trifluoroacrylic acid or its esters—were reported,1 but convenient improved synthesis of these compounds as well as radical copolymerization with TFE induced by y-irradiation were not described until 1980.2... 

Anionic polymerization represents a powerful technique for synthesizing polymers with low PDI values, thus providing good control over the chain length. This method leads to less side reactions than radical polymerizations. For instance, unlike in radical polymerization, there is no termination by the combination of two active chains. However, the mechanism is more sensitive to impurities and functional groups, and therefore applicable for only a limited class of monomers. 

Sonication, the application of high-intensity ultrasound at frequencies beyond the range of human hearing (16 kHz), of a monomer results in radical polymerization. Initiation results from the effects of cavitation—the formation and collapse of cavities in the liquid. The collapse (implosion) of the cavities generates very high local temperatures and pressures. This results in the formation of excited states that leads to bond breakage and the formation of... 

The low reactivity of a-olefins such as propylene or of 1,1-dialkyl olefins such as isobutylene toward radical polymerization is probably a consequence of degradative chain transfer with the allylic hydrogens. It should be pointed out, however, that other monomers such as methyl methacrylate and methacrylonitrile, which also contain allylic C—H bonds, do not undergo extensive degradative chain transfer. This is due to the lowered reactivity of the propagating radicals in these monomers. The ester and nitrile substituents stabilize the radicals and decrease their reactivity toward transfer. Simultaneously the reactivity of the monomer toward propagation is enhanced. These monomers, unlike the a-olefins and 1,1-dialkyl olefins, yield high polymers in radical polymerizations. 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

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