Chain polymerization by anionic

CHAIN POLYMERIZATION BY ANIONIC MECHANISM 2.8.1 Mechanism and Kinetics... 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

What monomer may be polymerized by anionic, cationic, and free radical chain techniques Which chain polymerization technique would you select to polymerize (a) isobutylene, (b) acrylonitrile, and (c) propylene ... 

Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by anionic catalysts. The key chain-carrying step is nucleophilic addition of an anion to the unsaturated monomer by a Michael reaction (Section 23.11). 

According to these experimental results, the proposed reaction mechanism for the formation of poly(styrene oxide) with a regular chain structure by anionic polymerization involves the oxirane ring opening exclusively at the 3 position. However, two kinds of active centers, A and B in the reactions above, occur in the initiation step. The active center A, formed by a-ring opening, adds to a monomer molecule in the next step, but in the second step the oxirane ring is opened at the 3 position. 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

I Table 29.2 Alkanes Polymerized by Anionic and Cationic Chain-Growth Mechanisms... 

PS and PEO chains—obtained by anionic polymerization—have been coupled to aryl ether dendrons that carried a benzyl bromide function at their focal point [46-48]. to-Hydroxy PEO chains have also been linked to aryl ether dendrons carrying a methyl ester function at their periphery [49]. With mono-... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

C. Formation of MAIs By Anionic Chain Polymerization-Anion Radical Transfer... 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Values of CP measured in the presence of added PMMA (for example) will depend on how the PMMA was prepared and its molecular weight (i.e. on the concentration of unsaturated ends). PMMA formed by radical polymerization in the presence of a good H-donor transfer agent (or by anionic polymerization) would have only saturated chain ends. These PMMA chains should have a different transfer constant to those formed by normal radical polymerization where termination occurs by a mixture of combination and disproportionation. This could account for some of the variation in the values of CP for this polymer... 

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