ANIONIC POLYMERIZATION OF THE CARBON CARBON DOUBLE BOND in .NET

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ANIONIC POLYMERIZATION OF THE CARBON CARBON DOUBLE BOND
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butyllithium in benzene is orders of magnitude slower compared to the corresponding sodium naphthalene polymerization. When butyllithium polymerizations are carried out in polar solvents such as tetrahydrofuran, the association of initiator and propagating species vanishes completely and the polymerizations are much more rapid. The association phenomena can also be disrupted by adding a Lewis base, which can coordinate with the initiator (Alev et al., 1980; Cheminat et al., 1979; Dumas et al., 1978; Fontanille, 1989; Muller, 1989]. Polyamines such as N, N, N 0 , N 0 -tetramethylethylene diamine (TMEDA) and polyethers such as tetraglyme, CH3 O-(CH2 CH2 O)4 CH3 , are more effective in breaking up the association than are simple ethers and amines such as diethyl ether or triethyl amine. The effectiveness of a Lewis base in promoting initiation and polymerization decreases if the base : initiator ratio is too high. One observes the polymerization rate to increase with increasing [base]/[RLi] and then level off at a maximum rate. With many Lewis bases, the polymerization rate decreases after reaching the maximum as [base]/[RLi] is increased further [Hay et al., 1976]. Complexation of base with initiator becomes so extensive as to decrease the reactivity of initiator. The macrocyclic crown ethers such as 18-crown-6 (XXIXa) and cryptands such as 2.2.2cryptand (XXIXb) are extremely powerful for breaking up association of organolithium compounds. These macrocyclic ligands complex lithium and also other counterions so strongly as to greatly increase the concentration of free-ion propagating species, resulting in very large rate increases (Cheng and Halasa, 1976; Def eux and Boileau, 1976].
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O O O O XXIXa XXIXb O N O O O O O O O N
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The need for solvation in anionic polymerization manifests itself in some instances by other deviations from the normal reaction rate expressions. Thus the butyllithium polymerization of methyl methacrylate in toluene at 60 C shows a second-order dependence of Rp on monomer concentration [L Abbe and Smets, 1967]. In the nonpolar toulene, monomer is involved in solvating the propagating species [Busson and Van Beylen, 1978]. When polymerization is carried out in the mixed solvent dioxane toluene (a more polar solvent than toluene), the normal rst-order dependence of Rp on [M] is observed. The lithium diethylamide, LiN(C2 H5 )2 , polymerization of styrene at 25 C in THF benzene similarly shows an increased order of dependence of Rp on [M] as the amount of tetrahydrofuran is decreased [Hurley and Tait, 1976]. Propagation of two-ended (bifunctional) propagating species often proceeds at a rate lower than that of the corresponding monoanion species as a result of triple-ion formation [Bhattacharyya et al., 1964; Smid, 2002]. For example, ionic dissociation of the counterion from one end of the cesium salt of a two-ended propagating species XXX yields XXXI in which the newly dissociated anionic center remains near the ion pair at the other end of
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IONIC CHAIN POLYMERIZATION
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XXXI. The result is XXXII, referred to as a triple ion. The triple ion propagates faster than a simple ion pair but slower than a free ion [Muller, 1989]. The major effect of triple ion formation is a lowering of the concentration of free ions and the overall result is a decrease
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in polymerization rate. Similar results are found for divalent counterions such a barium and strontium [de Groof et al., 1975, 1977; Mathis and Francois, 1978; Mathis et al., 1978]. Triple ion formation decreases with increasing molecular weight of the two-ended dicarbanion; thus, Rp increases with conversion. As the length of the intramolecular chain between the two anion centers increases, the probability of cyclization to the triple ion decreases. The higher reactivity of 2-vinylpyridine relative to styrene has been attributed to a combination of intramolecular solvation and triple-ion formation [Sigwalt, 1975; Soum et al., 1977].
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5-4 BLOCK AND OTHER POLYMER ARCHITECTURES Anionic and cationic living polymerizations offer routes to block copolymers, star polymers, telechelic polymers, and other polymers [Charleux and Faust, 1999; Hadjichristidis et al., 2002]. 5-4a Sequential Monomer Addition
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Sequential addition of monomer works well in anionic polymerization for producing wellde ned block copolymers [Morton, 1983; Morton and Fetters, 1977; Quirk, 1998; Rempp et al., 1988]. An AB diblock copolymer is produced by polymerization of monomer A to completion using an initiator such as butyllithium. Monomer B is then added to the living polyA carbanions. When B has reacted completely a terminating agent such as water or
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