Polymers, anionic

Anionic polymers Anionic starches Anionic surfactants... 

The flocculation responses studied are settling rate, percent solid settled, supernatant clarity, sediment volume and slurry viscosity. The polymer concentration and polymer anionicity required for maximum flocculation were seen clearly to depend on the response studied. Both the settling rate and the supernatant clarity with the nonionic polyacrylamide flocculent showed at pH 4.5 a marked increase to a maximum at about 25 mg/kg, whereas with the anionic polymer settling rate and supernatant clarity showed maxima at 10-25 mg/kg, but the system was totally... 

Ethylene oxide (ETO), 20 555, 596, 632-673 12 62 24 268. See also Ethylene oxide polymers anionic or cationic reactions of,... 

The ejected electron may become trapped on a suitable site in the matrix of the polymer, thus forming a polymer anion radical. 

Alternatively, the electron, or the polymer anion, may react with an existing cation radical producing an excited state of the polymer molecule, P. For example. 

Polymer anion radicals are usually less reactive than the cation radicals, and are often stable at 77K, but they are usually unstable at room temperature. Excited state species can undergo decomposition by a variety of routes including (i) homolytic cleavage to form two neutral radicals, (ii) heterolytic cleavage to form an anion and a cation, or (iii) bond rupture with the formation of two neutral molecules. 

Charge and ionization of the polymer. Anionic polymers provide better efficiency than cationic or uncharged polymers with respect to both adhesiveness and toxicity [35]. Furthermore, polymeric adhesives with carboxyl groups are preferred over those with sulfate groups [36]. 

Apart from salts with homologous polymer anions, Graham s glass always contains trimeta- (6-10%), tetrameta- (up to 4%.) and very small amounts of pentameta- and hexametaphosphate (see Section IV,D) with cyclic anions and also, to a small extent, cross-linked phosphates with tertiary P04 tetrahedra (see Section V) (284, 85, 364) ... 

Butyllithium initiation of methylmethacrylate has been studied by Korotkov (55) and by Wiles and Bywater (118). Korotkov s scheme involves four reactions 1) attack of butyllithium on the vinyl double bond to produce an active centre, 2) attack of butyllithium at the ester group of the monomer to give inactive products, 3) chain propagation, and 4) chain termination by attack of the polymer anion on the monomer ester function. On the basis of this reaction scheme an expression could be derived for the rate of monomer consumption which is unfortunately too complex for use directly and requires drastic simplification. The final expression derived is therefore only valid for low conversions and slow termination, and if propagation is rapid compared to initiation. The mechanism does not explain the initial rapid uptake of monomer observed, nor the period of anomalous propagation often observed with this initiator. The assumption that kv > kt is hardly likely to be true even after allowance is made for the fact that the concentration of active species is much smaller than that of the added initiator. Butyllithium disappears almost instantaneously but propagation proceeds over periods from tens to hundreds of minutes. The rate constants finally derived therefore cannot be taken seriously (the estimated A is 2 x 105 that of k ) nor can the mechanism be regarded as confirmed. 

Primary processes induced by ionizing radiation in the solution are excitation and ionization of the solvent molecules. Subsequent electron attachment to solute polymers leads to the formation of polymer anions. So far the radical anions of poly(methyl methacrylate) (PMMA) [46, 47], substituted PMMA [46, 47], poly(4-vinylbiphenyl) (PVB) [47-50], poly(l-vinylpyrene) (PVP) [50], organopolysilane [51] and substituted polyacetylene [52] have been studied. 

The es reacts with PVB to give a polymer anion with a high efficiency [47]. The rate constant was evaluated as 4.7 x 109 mol -1 dm3 s-1 in hexamethylphos-phorictriamide. The absorption specra of the radical anions of PVB [47] and PVP [48] are similar to those of biphenyl anion and pyrene anion, respectively, to mean that the excess electrons trapped by the polymers are essentially localized on the side groups. 

Tanaka et al. studied the decay reactions of PVB radical anions produced by electron pulses in MTHF [47]. At low concentration ( < 0.05 base-mol dm - 3) of polymers the decay reaction followed a simple second-order kinetics. The charge neutralization reaction is responsible for the decay curve as is the case of biphenyl radical anions. However, the rate constant of the polymer anions was only a half or one-third of that of the biphenyl anion, because of the small diffusion coefficient of the polymer ion in solution. At high concentration of the polymer, a spike was observed in the time-profile of the PVB anion this was attributed to the retarded geminate recombinations within micro-domains where the polymers were entangled with each other. 

The electron transfer from aromatic radical anions to various electron acceptors takes place efficiently in solution. Likewise, when a second solute, pyrene, is added to the MTHF solution of PVB, the electrons transfer from polymer anions to pyrene occurs [50]. The rate constant determined by pulse radiolysis is approximately a third of that of the electron transfer from biphenyl anion to pyrene. 

Defined in this way, anionic polymerizations can only be expected when the cation is derived from one of the most electropositive metals, the cation is strongly solvated, and the polymer anion is highly stabilized by resonance. These conditions are frequently met with sodium (206—208) or potassium (209, 210) catalysts in basic solvents with polar monomers or dienes. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

When lithium alkyl catalysts are used in non-solvating media such as aliphatic hydrocarbons, the polymer-lithium bond is not sufficiently ionic to initiate anionic polymerization so that the monomer must first complex with vacant orbitals in the lithium. A partial positive charge is induced on the monomer in the complex, and this facilitates migration of the polymer anion to the most electrophilic carbon of the complexed monomer. This type of polymerization is more appropriately termed coordinated anionic and will be discussed in the next section. There does not appear to be any evidence that alkyl derivatives of metals which are less electropositive than lithium and magnesium can initiate simple anionic polymerization. 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

Natta et al. (167,188,287,298,312) have built a strong case in favor of a coordinated anionic mechanism in which an electropositive metal complexes and polarizes the monomer and a polymer anion adds to the positively polarized carbon of the monomer. One of the points which was used to support the anionic mechanism was that the order of reactivity for ethylene, propylene and butene is opposite to that of cationic catalysts. The lower reactivity of propylene and butene versus ethylene was attributed to the electron releasing alkyl groups (287), but steric hindrance is believed to be a better explanation. Support for the steric effect is indicated by the influence of bulk placed at some distance from the double bond (116). For example, reactivity decreases sharply in the order pentene-1 > 4-methylpentene-l > 4,4-dimethylpentene-l, although basicity of the double bonds must change only very slightly. 

In order to maintain good monodispersity, generation to generation, it is very desirable to use living end group polymers (anionic and cationic) [33, 187] for... 

Anionic polymerization of masked disilenes has opened up a novel route to polysilanes (95). I-Phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes can be used as masked disilenes. n-BuLi works as an initiator. The polymerization may involve the attack of the polysilanyl anions on a silicon atom of the monomer, resulting in the formation of the new propagating polymer anion and biphenyl. This method is applicable to aminopolysilane synthesis (Scheme 28). 

In poly(methylphenylphosphazene), [Ph(Me)P=N] 58, both the phenyl and methyl substituents are potential sites for formation of derivatives [65]. Deprotonation of ca. half of the methyl substituents on this polymer was carried out in THF at — 78 °C using n-BuLi. On treatment of the intermediate polymer anion with ferrocenyl ketones and subsequent quenching with a mild proton source, phos-phazenes 59 containing the OH functional group were prepared. The amount of substitution, determined by NMR and elemental analysis, was found to be 45 and 36%, respectively, for polymers 59a and 59b (Scheme 10-27). For these substituted polymers was 187000 and 154000, respectively, and no degradation of the parent polymer occurred. 

Once chain initiation is complete, the monomer consumption rate is determined only by the chain propagation step. With the less efficient lithium alkyl initiators in hexane or cyclohexane, rather large amounts of monomer are needed to complete chain initiation. The appearance of a first order decay in monomer concentration, invariably obtained in these experiments, is not a very sensitive indication of the complete absence of initiator. Analysis of trial samples for hydrolysis products of lithium alkyls or spectroscopic determination that the polymer anion concentration has reached a plateau are preferable. A seeding technique is often used [32, 59] where the real initiator is a pre-formed active polymer... 

---