Anionic chain polymerization carbonyl monomers

The anionic chain polymerizations of polar monomers, such as methyl methacrylate, methyl vinyl ketone and acrylonitrile, often yield complex polymer structures due to nucleophilic reactions of the carbonyl and nitrile groups. 

In the previous communication(7a) we postulated the initiation mechanism involving the attack of BuLi on the vinyl double bond of the butyl lsopropenyl ketone formed. However, from the results mentioned above we will propose the following mechanism for the polymerization of MMA with BuLi in toluene, which is slightly different from the previous one. On mixing the monomer with BuLi the initiator reacts with both olefinic and carbonyl double bonds of the monomer. The attack on the olefinic double bond produces the MMA anions, which add the monomer to form the growing chains(E). 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Major commercial synthetic specialty polymers are made by chain-growth polymerization of functionalized vinyl monomers, carbonyl monomers, or strained ring compounds. Depending on monomer structure, the polymerization may be initiated free radically, anionically, or cationically. Copolymers or terpolymers with random, alternating, block, or graft sequences can be prepared under appropriate reaction conditions. There are numerous mediods used to prepare specialty polymers in the research laboratory. However, only a few are of commercial interest. Of particular commercial interest is synthesis of specialty polymers in solutions, dispersions, suspensions, or emulsions. 

Substituent groups on the double bond must stabilize the negative charge that develops in the transition state for the monomer addition step. They must also be stable to reactive anionic chain ends [13, p. 94]. Monomers that can be polymerized anionically include vinyl, diene and some carbonyl-type and cyclic monomers. We note that because of its lack of any substituent group, polyethylene cannot be polymerized anionically. We describe in a later section how to make a living polymer that is very similar to polyethylene. 

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. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Super glue is a polymer of methyl cyanoacrylate. Because both the cyano and carbonyl groups of the monomer help stabilize carbanions, this compound is sensitive to polymerization by the anionic mechanism. The tube of glue contains very pure monomer, which does not polymerize until it contacts an initiator. However, contact with any nucleophile causes rapid polymerization. Therefore, when the tube is opened, polymerization is initiated by water in the air, by SiOH groups on a glass surface, by FeOH groups on an iron surface, or by various nucleophiles that are part of the proteins in skin. The adhesion between the polymer and the surface to which it is applied is very strong because the polymer chains are covalently linked to the nucleophiles that are part of the surface ... 

These reactions are similar to those describing the initiation and polymerization reactions in the anionic polymerization. It follows from this scheme, that only reactions involving the protonized lactam molecule, (90) and (99), can contribute to chain growth. Reaction (98) results in the incorporation of one monomer unit, only when followed by aminolysis (93b) at the cyclic carbonyl of the acyllactam. 

Polar Vinyl Monomers. The anionic polymerization of polar vinyl monomers is often complicated by side reactions of the monomer with both anionic initiators and growing carbanionic chain ends, as well as chain-termination and chain-transfer reactions. However, s5mthesis of polymers with well-defined structures can be effected under carefully controlled conditions. The anionic polymerizations of alkyl methacrylates and 2-vinylpyridine exhibit the characteristics of living polymerizations under carefully controlled reaction conditions and low polymerization temperatures to minimize or eliminate chain-termination and chain-transfer reactions. The proper choice of initiator for anionic polymerization of polar vinyl monomers is of critical importance to obtain polymers with predictable, well-defined structures. As an example of an initiator that is too reactive, the reaction of methyl methacrylate (MMA) with w-butyllithium in toluene at -78°C produces approximately 51% of lithium methoxide by attack at the carbonyl carbon (85). 

On the other hand, in the purely anionic polymerization of five- or higher-membered cyclic esters, the carbonyl carbon of the monomer is attacked with subsequent acyl-oxygen bond scission and reformation of the alkoxide anion. In the coordination polymerization, this is also the carbonyl carbon that is now first coordinated with alkoxide species and then the acyl-oxygen bond is broken with reforming of the covalent alkoxide chain end. In the already formed macromo-lecular chains, the same ester bonds are present as those being the site of the nucleophilic attack in the monomer molecules. These processes are illustrated in Scheme 12, where the active centers are shown as. ..-OMt, for both anionic and covalent centers. 

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