Copolymerization anionic

For the copolymerization of two monomers by an anionic mechanism, the copolymer composition equation (7.11) or (7.17), derived in Chapter 7, is applicable with the monomer reactivity ratios defined in the same way as before, namely. 

The reactivity ratios ri and V2 can be determined from the composition of the copolymer product. However, a difficulty arises from the fact that the propagation rate constants, kij, are composite rate constants (Allcock and Lampe, 1990) having contributions from both ion pairs and free ions. Consequently, the reactivity ratios also will be composite quantities composed of free-ion and ion-pair contributions, which are strongly dependent on the reaction conditions. The reactivity ratios can, therefore, be applied only to systems identical to those for which they were determined. This greatly limits the utility of such ratios. 

Because of the complicating effects of counterion and solvent associated with anionic polymerization, relatively few reactivity ratios have been determined for anionic systems. Typical reactivity ratios for the anionic copolymerization of styrene and a few other monomers are shown in Table 8.3. Most of the values were determined from the copolymer composition equation [Eq. (7.11) or (7.18)]. A dramatic effect of solvent is seen with styrene-butadiene copolymerization, where a change from the nonpolar hexane to the highly solvating THF reverses the order of reactivity. Again in the case of hydrocarbon solvent, the reaction temperature shows a minimal in uence on reactivity ratios, while in the case of polar solvents, such as THF, the reactivity ratios vary considerably, which has been rationalized by considering the solvation of carbon-lithium bond. Thus as the temperature is increased (from -78°C to 25°C), the extent of solvation by THF is expected to decrease, resulting in more covalent carbon-lithium bond. 

Monomer reactivities in anionic copolymerization are the opposite of those in cationic copolymerization. Reactivity is enhanced by electron-withdrawing substituents that decrease the electron density on the double bond and resonance stabilize the carbanion formed. Although the available data are rather limited [Bywater, 1976 Morton, 1983 Szwarc, 1968], reactivity is generally increased by substituents in the order 

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 

The general characteristics of anionic copolymerization are very similar to those of cationic copolymerization. There is a tendency toward ideal behavior in most anionic copolymerizations. Steric effects give rise to an alternating tendency for certain comonomer pairs. Thus the styrene-p-methylstyrene pair shows ideal behavior with t = 5.3, fy = 0.18, r fy = 0.95, while the styrene-a-methylstyrene pair shows a tendency toward alternation with t — 35, r% = 0.003, i ii 2 — 0.11 [Bhattacharyya et al., 1963 Shima et al., 1962]. The steric effect of the additional substituent in the a-position hinders the addition of a-methylstyrene to a-methylstyrene anion. The tendency toward alternation is essentially complete in the copolymerizations of the sterically hindered monomers 1,1-diphenylethylene and trans-, 2-diphe-nylethylene with 1,3-butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene [Yuki et al., 1964]. 

The composition of the copolymer formed at the beginning of polymerization is generally different from the composition of the equilibrium polymerization product [167, 173, 174]. Whereas kinetic factors are decisive for the former, the heat and entropy of polymerization determine the composition of the equilibrium copolymer. 

In the copolymerization of lactams of different ring size, the relative rate of incorporation of the two lactams is not necessarily determined by the reaction in which the lactam ring is cleaved. Vofsi et al. [167] showed that in the anionic copolymerization of caprolactam and pyrrolidone (Table 9), the acylation of lactam anions with the exocyclic carbonyl of the growing acyllactam structure (i.e. exchange of monomer units) occurs faster than acylation with the cyclic carbonyl (propagation), viz. 

Therefore, the copolymer composition is determined by the relative acidities of both lactams and by the nucleophilicities of the corresponding anions in the transacylation reactions. The distribution of lactam anions is given by the equilibrium constant 

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Anionic Gopolymerization. Typical copolymers prepared by anionic copolymerization are shown ia equations 21—24 (30—32). 

Lustoh, J. and Vass, F. Anionic Copolymerization of Cyclic Ethers with Cyclic Anhydrides. Vol. 56, pp. 91 —133. 

In the anionic copolymerization of lactams, this exchange reaction is faster than the propagation reaction and the copolymer composition is determined by this reaction and not by the propagation reaction127. A general solution of the copolymerization problem considering this equilibrium has not as yet been obtained. 

Anionic copolymerization of cyclic monomers occurs only between similar monomer pairs. Random copolymers are not formed between vinyl monomers and epoxides or lactones198 because the propagating species are very different. Thus, the success of the copolymerization of cyclic disulfide and nitropropylene was an exceptional case... 

Anionic copolymerization involving similar monomer pairs which propagate through a similar chain end occurs rather easily. Examples are the copolymerization involving aldehydes, isocyanate and ketenes, although these are not always random... 

Anionic copolymerization of lactams presents an interesting example of copolymerization. Studies of the copolymerization of a-pyrrolidone and e-caprolactam showed that a-pyrrolidone was several times more reactive than e-caprolactam at 70 °C, but became less reactive at higher temperatures due to depropagation210 2U. By analyzing the elementary reactions Vofsi et al.I27 concluded that transacylation at the chain end occurred faster than propagation and that the copolymer composition was essentially determined by the transacylation equilibrium and the acid-base equilibrium of the monomer anion together with the usual four elementary reactions of the copolymerization. 

Anionic copolymerization of e-caprolactam and cj-caprylolactam was also reported212,213. Organosiloxane copolymers can be prepared from two different cyclics by using acid or base catalysts214. ... 

Anionic copolymerization of monomers which do not polymerize by themselves sometimes yields alternating copolymers. 

Pioneering work in living anionic copolymerization of siloxanes was reported by Morton and co-workers 139 140, who synthesized isoprene-dimethylsiloxane block copolymers utilizing D4 as the siloxane monomer. The use of D3 in the synthesis of siloxane block copolymers with controlled structures was demonstrated by Bostick and others. Excellent reviews of these earlier studies and subsequent developments are available in the literature 22 137 13S). 

The reactivities of substituted monomers are different from those of the unsubstituted ones. For example, in crosspropagation an electron donating methyl group introduced to the C = C bond of a vinyl monomer makes it less reactive in anionic copolymerization, while it increases its reactivity in a cationic process. Thus, in THF at 25 °C the reactivity of isoprene towards polystyrene anion is lower by about a factor of 2 than that of butadiene (only one end of this bivalent monomer is affected),... 

These results may be subject to considerable error resulting from excessive conversions reached in rapid anionic copolymerization. 

Under current treatment of statistical method a set of the states of the Markovian stochastic process describing the ensemble of macromolecules with labeled units can be not only discrete but also continuous. So, for instance, when the description of the products of living anionic copolymerization is performed within the framework of a terminal model the role of the label characterizing the state of a monomeric unit is played by the moment when this unit forms in the course of a macroradical growth [25]. 

The ethylene-1-butene block cannot be obtained directly since the two monomers do not undergo anionic copolymerization. 

A totally different route based on dehydrogenation of a saturated polymer precursor was introduced by Francois et al. [49] (Scheme 2.9). The method is based on anionic copolymerization of cyclohexadiene with styrene, followed by oxidation with chloranil. Due to possible coupling of two styrene (or two cyclohexadiene) molecules, a block copolymer, containing oligo(phenylene vinylene) units separated by oligo(phenylacetylene) and oligo(phenylene) blocks, is obtained. To the best of our knowledge, it was, so far, used only in the synthesis of phenyl-substituted PPV 10. 

Anionic complexes, tungsten, 25 386-387 Anionic copolymerization, 7 624-626 block copolymers, 7 645 Anionic emulsifiers, in VDC emulsion polymerization, 25 722 Anionic extractants, 10 750 Anionic flotation, 24 497 Anionic gels, 9 59-60... 

Fortunately this approach, illustrated mainly in anionic copolymerization, has been extended significantly along two different lines of achievements. (4)... 

The core first method has been applied to prepare four-arm star PMMA. In this case selective degradation of the core allowed unambiguous proof of the star structure. However, the MWD is a little too large to claim that only four-arm star polymers are present [81], Comb PMMAs with randomly placed branches have been prepared by anionic copolymerization of MMA and monodisperse PMMA macromonomers [82], A thorough dilute solution characterization revealed monodisperse samples with 2 to 13 branches. A certain polydispersity of the number of branches has to be expected. This was not detected because the branch length was very short relative to the length of the backbone [83]. Recently, PMMA stars (with 6 and 12 arms) have been prepared from dendritic... 

What is the composition of the first copolymer chains produced by the copol5Tnerization of equimolar quantities of styrene and methyl methacrylate in (a) free radical, (b) cationic, and (c) anionic copolymerization ... 

The effect of a substituent on the reactivity of a monomer in cationic copolymerization depends on the extent to which it increases the electron density on the double bond and on its ability to resonance stabilize the carbocation that is formed. However, the order of monomer reactivities in cationic copolymerization (as in anionic copolymerization) is not nearly as well defined as in radical copolymerization. Reactivity is often influenced to a larger degree by the reaction conditions (solvent, counterion, temperature) than by the structure of the monomer. There are relatively few reports in the literature in which monomer reactivity has been studied for a wide range of different monomers under conditions of the same solvent, counterion, and reaction temperature. 

Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

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