Anionic polymerization carbanion stability

Electron-withdrawing substituents adjacent to an olefinic bond tend to stabilize carbanion formation and thus activate the compound toward anionic polymerization [19],... 

Although the living nature of anionic polymerization of styrene testifies to the relative stability of the carbanion at the chain end, observations indicate that it may undergo spontaneous transformations.182 Polystyryl sodium in THF was shown to undergo termination involving hydride elimination followed by proton transfer ... 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

To broaden our understanding of the chemical behavior of these novel monomers, it would be appropriate to try anionic polymerizations of captodative monomers. Inasmuch as sulfur is able to stabilize adjacent carbanions, a-alkylthioacrylates and -acrylonitriles should respond well to anionic initiators. Cationic polymerization of certain captodative monomers may also be of interest as alkylthio- and cyano-substituents can stabilize a cationic propagating center. 

Anionic polymerization occurs through carbanion intermediates. Effective anionic polymerization requires a monomer that gives a stabilized carbanion when it reacts with the anionic end of the growing chain. A good monomer for anionic polymerization... 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

Polymerization Catalysed by Acids and Bases. Carbonium ions and carbanions respectively are carriers of the chain transfer in cationic and anionic polymerizations respectively. Ionic polymerization mechanism was exploited for the synthesis of polymeric stabilizers in comparison with the free-radical polymerization only exceptionally. The cationic process was used for the synthesis of copolymers of 2,6-di-tert-butyl-4-vinylphenol with cyclopentadiene and/or for terpolymers with cyclopentadiene and isobutylene [109]. System SnCWEtsAlCla was used as an initiator. Poly(lO-vinylphenothiazin) was prepared by means of catalysis with titanium chlorides [110]. Polymers of 4-[a-(2-hydroxy-3,5-dimethylphenyl)ethyl]-vinylbenzene [111] and 3-allyl-2-hydroxyacetophenone [112] were also prepared under conditions of cationic polymerization. 

Anionic poiymerization. Anionic polymerization is an addition polymerization in which the growing chain end bears a negative charge. The monomers suitable for anionic polymerization are those that have substituent groups capable of stabilizing a carbanion through resonance or induction. TVpical monomers that can be polymerized by ionic mechanisms include styrene, acrylonitrile, and methyl methacrylate (Table 14.20). 

The crystal structure of a single stabilized nitronate carbanion derived fn n phenylnitromethane is reported as the polymeric ethanol solvate (185). The same geometry found in the nitronate anion (185) was also found for the r-butyldimethylsilyl ester derived fiom quenching this nitronate. References to other structures containing carbanions stabilized by a nitro group are given in Table 3 however, it is to be noted that these are derived fiom highly acidic carbon acids. 

Poly(methyl methacrylate) cannot be prepared by cationic polymerization because the carbonyl group destabilizes the intermediate carbocation. It can be produced by anionic polymerization because the carbonyl group stabilizes the carbanion intermediate by resonance. 

Section 27.9 Anionic polymerization of alkenes that bear a carbanion-stabilizing substituent (X) can be initiated by strong bases such as alkyllithium reagents. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Chain polymerization of alkenes can also be catalyzed by anionic reagents. Anionic polymerization occurs most readily when the alkene carries a carbanion stabilizing substituent. Even ethylene can be polymerized anionically, however, if a... 

Draw the carbanion intermediate for anionic polymerization of acrylonitrile (CH2=CHCN), and show how it is stabilized by resonance. 

For monomers that undergo anionic polymerization, the process continues until all of the monomers have been consumed. But even after all of the monomers have been exhausted, the process is not actually terminated until a suitable acid (such as water) or electrophile (such as CO2) is added to the reaction mixmre. In the absence of a suitable acid or electrophile, the end of each polymer chain will possess a stabilized carbanion site, and the polymerization process can continue if more monomers are added to the reaction mixture. For this reason polymers generated through this process are often called living polymers. 

Diphenylmethane is the conjugate acid of the diphenylmethyl carbanion, and the equilibrium acidity constants (Ka) have been measured both directly and indirectly in the gas phase and in solution [3]. The most extensive investigations of the effect of structure on acidity for carbon acids have been carried out in DMSO using a carbon indicator method to determine relative acidities and this scale was anchored with potentiometric measurements to provide an absolute scale of acidities [3, 43]. A summary of relevant pKg values for various carbon acids is shown in Table 2. The data in Table 2 are especially relevant for considering the reactivity of 1,1-diphenylmethyl carbanionic species as initiators in anionic polymerization. In general, an appropriate initiator for a given monomer is an anionic species that has a reactivity (stability) similar... 

The substituent on styrene determines whether cationic or anionic polymerization is preferred. When the substituent stabilizes a carbocation, cationic polymerization will occur. When the substituent stabilizes a carbanion, anionic polymerization will occur. 

---