Reactivity of monomers

This is a relative value derived from measurements of the rate of monomer addition to a specific active centre. 

Many attempts have been made to define monomer reactivity by some numerical value, and to find relations suitable for relating this value to monomer structure. These attempts have always resulted in only short series, based almost always on experience. They are very important and must be regarded as an indispensable quantitative basis for a future general theory of monomer reactivity. 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

Such a series naturally calls for an explanation of why a specific monomer assumes the experimentally found position in the sequence. Many authors are attempting to find a relation between reactivity and some well-defined physico-chemical properties of monomers. The most valuable are the correlations indicating the type of electron distribution around the atoms on the double bond. 

Hatada et. al [104] made use of the finding that the 13C NMR spectrum of vinyl groups is controlled by re-electron density [105, 106]. Therefore they 

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The reciprocal of a radical reactivity ratio is sometimes used to quantitatively express the reactivity of monomer M2 by comparing its rate of addition to radical Mi - relative to the rate of Mi adding to Mi-. 

Polymer formed using radiolabeled initiators may be isolated and analyzed to determine the concentration of initiator-derived residues and calculate the initiator efficiency. Radiolabeled initiators have also been used extensively to establish the relati ve reactivity of monomers towards radicals. 107,5 -5 2... 

In this section, we consider the kinetics of propagation and the features of the propagating radical (Pn ) and the monomer (M) structure that render the monomer polymerizable by radical homopolymerization (Section 4.5.1). The reactivities of monomers towards initiator-derived species (Section 3.3) and in copolymerizalion (Chapter 6) arc considered elsewhere. 

When one compares the brutto polymerization rate constants, a measure of the reactivity of monomers during cationic homopolymerizations is obtained. It was found for p-substituted styrenes that lg kBr increased parallel to the reactivity, which the monomers show versus a constant acceptor 93). The reactivity graduation of the cationic chain ends is apparently overcomed by the structural influence on the monomers during the entire process of the cationic polymerization. The quantitative treatment of the substituent influences with the assistance of the LFE principle leads to the following Hammett-type equations for the brutto polymerization rate constants ... 

The effects of 1-substituents in increasing the reactivity of monomers towards attacking radicals are in the order, —Cells >—CH=CH2>... 

Almost linear polymers with pendant vinyl groups are formed as intermediates in the anionic polymerization of 1,4-DVB due to the different reactivities of monomers and pendant vinyl groups. 1,4-DVB microgels are formed towards the end of monomer conversion. In the anionic polymerization of EGDM or 1,3-DVB, reactive microgels are formed already at the beginning of the polymerization. 

The above observations with respect to the reactivities of monomers IV-VI can be explained by postulating a direct interaction of the carbonyl and the ether functional groups with the propagating cationic center. This can occur by either an inter- or intramolecular process. As shown in equation 5, intramolecular backside attack by the ester carbonyl group of the d,l-trans IV isomers at either carbon of the protonated or alkylated epoxy group gives rise to bicyclic dioxacarbenium ions IX and X. 

A high concentration of B will help to compensate for its lesser reactivity, since the growing chain has a greater chance of meeting B molecules more often than A molecules. The relative reactivity of monomers depends on polar and steric effects of the substituents on the monomers. 

This very efficient, time-honoured pathway suffers however a very severe limitation, i.e. the relative reactivity of the living centre C must be adapted to the structure and reactivity of monomer M2, a requirement which is not very often met. 

Although the mechanism of copolymerization is similar to that discussed for the polymerization of one reactant (homopolymerization), the reactivities of monomers may differ when more than one is present in the feed, i.e., reaction mixture. Copolymers may be produced by step-reaction or by chain reaction polymerization. It is important to note that if the reactant species are Mi and M2, then the composition of the copolymer is not a physical mixture or blend, though the topic of blends will be dealt with in this chapter. 

Because of a difference in the reactivity of monomers, expressed as reactivity ratios (r), the composition of the copolymer (k) may be different from that of the reactant mixture or feed (x). When x equals n, the product is said to be an azeotropic copolymer. 

Table 5-3 shows the order of reactivity of monomers in propagation. It is not a simple matter to explain the order of propagation rate constants for a set of monomers because there are variables—the reactivity of the monomer and the reactivity of the carbocation. For example, carbocation stability is apparently the more important feature for isopropyl vinyl ether and results in decreasing its propagation reactivity compared to isobutylene. 

Chain copolymerization is important from several considerations. Much of our knowledge of the reactivities of monomers, free radicals, carbocations, and carbanions in chain polymerization comes from copolymerization studies. The behavior of monomers in copolymerization reactions is especially useful for studying the effect of chemical structure on reactivity. Copolymerization is also very important from the technological viewpoint. It greatly increases the ability of the polymer scientist to tailor-make a polymer product with specifically desired properties. Polymerization of a single monomer is relatively limited as to the number of different products that are possible. The term homopolymerization is often used to distinguish the polymerization of a single monomer from the copolymerization process. 

The monomer reactivity ratios for many of the most common monomers in radical copolymerization are shown in Table 6-2. These data are useful for a study of the relation between structure and reactivity in radical addition reactions. The reactivity of a monomer toward a radical depends on the reactivities of both the monomer and the radical. The relative reactivities of monomers and their corresponding radicals can be obtained from an analysis of the monomer reactivity ratios [Walling, 1957]. The reactivity of a monomer can be seen by considering the inverse of the monomer reactivity ratio (1 jf). The inverse of the monomer reactivity ratio gives the ratio of the rate of reaction of a radical with another monomer to its rate of reaction with its own monomer... 

The reactivity of monomers with electron-releasing substituents in anionic copolymerization is nil. Correlation of reactivity in copolymerization with structure has been achieved in some studies [Favier et al., 1977 Shima et al., 1962]. The reactivities of various substituted styrenes and methacrylates in anionic polymerization, as well as the reactivities of various vinyl... 

Statistical copolymerization occurs among ethylene and various a-olefins [Baldwin and Ver Strate, 1972 Cooper, 1976 Pasquon et al., 1967 Randall, 1978]. The reactivities of monomers in copolymerization generally parallel their homopolymerization behavior ethylene > propene > 1-butene > 1-hexene [Soga et al., 1989]. Table 8-7 shows monomer reactivity ratios for several comonomer pairs. 

The Q,e system of describing the reactivity of monomers in copolymerization gives the rate constant for the addition of monomer 2 to the radical of monomer 1 as... 

Reactivity Fluorine substitution changes -the reactivity of monomers— often dramatically, if adjacent to reactive sites and other times only moderately or minimally if reactive sites are separated from fluorine by unconjugated bonds. Reactivity is generally reduced for fluorine-contain-ins amines and often increased for fluorine-containina dianhydrides. 

Bevington has continued his studies of the initiation reaction and of the reactivities of monomers towards reference radicals (69—71). A study of the polymerization of substituted styrenes was recorded (72). In methyl methacrylate polymerization by ammonium trichloroacetate in the presence of copper derivatives, the complexities of the initiation and termination reactions were elegantly unravelled by Bamford and Robinson using two differently labelled trichloroacetates (73). Apparently cyclic processes involving alternate oxidation and reduction of copper may arise. 

The effect of changes in reactivity of monomers due to substitution on the process of polymerization, mostly on the position of gel point was studied by using Monte Carlo calculations applying essentially the same approach as that presented in this section [50-52]. 

Even if the reactivity of monomers is described in Sect. 5, it is worth comparing those of CF3CH = CF2 and CF3-CF=CH2 about CF3I. Only this latter olefin leads to telomers, useful for hydraulic fluids, lubricants and for heat transfer media [293 ]. Similarly, C2F5CF = CF2 has been telomerised thermally by Paciorek et al. [294] who suggested a valuable mechanism. 

Co+3, Mn+2 and Fe+2 have been found to be effective in producing free radical sites on the polymer backbone through the alcohol groups present on them [75]. In an alternative method, free radical initiators like BPO and AIBN are thermo-chemically activated to give rise to macro-radical sites on polymer backbone to initiate grafting of desired vinylic monomer. The efficiency of these initiators was found to be predominantly dependent on the nature of monomer while the course of reaction depended on the relative reactivity of monomer versus that of the macro-radical. 

Other things that are particularly controlled in condensation polymerization are chain topology and cascade reactivity of monomers. In general condensation polymerization, not only linear polymers but also cyclic polymers can be produced because both ends of the polymer are reactive during polymerization. When cascade reactions selectively occur on a monomer, condensation polymerization proceeds in a way different from general polymerization behavior based on the principle of Carothers and Flory. Thus, in condensation polymerization of AA and BB monomers, if AA monomers successively react with both the functional groups on a BB monomer, a poly-... 

The polymerization rate depends on both the reactivity of monomers and the nature of the counter anion of the initiator salt. In the fastest case (3-vinylcyclohexene oxide), a quantitative conversion was attained at 25 °C within 1.5 minutes whereas in the slowest case (e-caprolactone), irradiation at 60 °C for 60 min was required. 

In this synthesis, the reactivity of monomers M decreases in the classic order methyl acrylate > MMA acrylonitrile vinyl acetate > Sty, and DPn values increase linearly with the monomer conversion. Moreover, molecular weights range from 9000 (styrene) to 70000 (methyl acrylate) and the yields are quite high after only 3 h (ca. 90%) [230,231]. In addition, the authors observed that the initial value of DPn decreases with increase in the concentration of xanthogen disulfide [TX], an observation in agreement with the following kinetic equation ... 

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