Vinyl chloride anionic polymerization

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

The polymerization of olefinic compounds like acrylonitrile, vinyl chloride, styrene, methylmethacrylate can be initiated by anion. The mechanism, in general, can be given as... 

Anionic polymerization of vinyl monomers can be effected with a variety of organomciallic compounds alkyllithium compounds are the most useful class. A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds ate soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. 

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). 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

The color change and the sensitivity of conversion to order of addition of monomers and peroxide indicate that in order to obtain an AFR polymer the polar monomers must first be complexed or allowed to react with the active or living end of the anionic polymer chain, or otherwise solvate it before the polymer chain is attacked by the peroxide. Success or failure of the subsequent free radical block polymerization depends on the nature of the complex or reaction product formed. The resultant species are no longer active for propylene polymerization. The necessity of complex formation has also been observed by Milovskaya and coworkers (4). They have shown that vinyl chloride, a weak complexing agent, can be polymerized effectively with triethylaluminum peroxide only when it is present with a more active complexing compound such as an ester or an ether. 

The similarity in the reactivities of free ions and corresponding ion pairs derived from the same cyclic monomer is more intriguing. Whereas ion pair reactivities are about 10 times smaller than corresponding free ion values in the anionic polymerization of vinyl monomers [39], and probably of the same relative proportions in cationic systems, the difference in cationic ring opening polymerizations is considerably less. For polymerization of THF in methylene chloride the factor is only 7, and for polymerization of 3,3-dimethylthietan, 40 in methylene chloride and 1 in nitrobenzene. Because the overall reactivity in cyclic monomer reactions is lower than for olefinic polymerizations, it might be expected that difference between free ion and ion pair reactivities, within one system, would also be less. However, this does not seem to be the whole answer. Plesch [44] has pointed out that in the polymerization of cyclic ethers and thietans (and presumably, therefore, other cyclic monomers)... 

Recent investigations [259] have indicated that the polymerization is not conventional free radical in character but is likely to be coordinated anionic. In support of this view are the reactivity ratio coefficients in copolymerization of vinyl chloride with vinyl acetate and methyl methacrylate, which are different from those found with free radical initiators. 

The initiator can be a radical, as in ethylene polymerization (Section 7.10), an acid, as in isobutylene polymerization (Section 7.10), or an anion. Radical polymerization is the most common and can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, however, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 C, using BFy and a small amount of water to generate BF3OH" catalyst. 

The applicability of various types of initiation mechanisms to the polymerization of common olefin monomers is summarized in Table 8.1. We see that isobutene can be polymerized only by cationic initiation, whereas monomers, such as vinyl chloride, methyl methacrylate or acrylonitrile with their electronegative substituents will not yield at all to cationic initiation. Vinyl chloride, however, does not respond to anionic initiation as well. Though halogens can withdraw electrons inductively and push electrons by resonance, both effects are relatively weak. Vinyl chloride thus does not undergo either anionic or cationic polymerization. 

The above examples show the complexity of the systems involving radical-anions derived from compounds of higher electron-affinity. It is not surprising, therefore, that benzophenone ketyl and other similar compounds do not initiate styrene polymerization, although they initiate polymerization of acrylonitrile or methyl-methacrylate. On the other hand, the monomeric dianions of benzophenone initiate polymerization of styrene as well as of other monomers, but not of vinyl chloride or acetate. Mechanisms of these initations were not investigated and presumably are complex. 

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