Monomers, vinyl anionic polymerization

Chain polymerization in which the active center is an anion, usually carbanion. The method is mostly used to polymerize vinyl monomers carrying electron-withdrawing substituents (e.g., CN, —COOR, —COR, —aryl). The polymerization is frequently initiated by n-butyllithium. 

Protected vinyl based glycomonomers are also suitable for ionic polymerization. Isopropylidene protected monomers are often applied in anionic polymerization, while monomers for cationic polymerizations are often based on vinyl-ojqr functional groups. Figure 2.7 provides examples of glycomonomers suitable for ionic polymerization. 

After all this discussion about radical polymerization and new methods to develop processes to obtain better control of the polymerization, the question remains Why Why should one use these novel methods to polymerize vinyl monomers The answer that first comes to mind is supplementation of anionic and cationic polymerization as the primary means of obtaining well-defined (co)polymers, in these cases by radical polymerization processes which are more tolerant of impurities, functional groups and are applicable to a wider range of monomers. This increased level of control over radical polymerization will allow industry to tailor a material to the requirements of a specific application using the most robust polymerization process available, ensuring the polymers have the optimal balance of physical and chemical properties for a given application. 

A cationic or anionic hydrophilic vinyl monomer is polymerized with a hydrophobic vinyl monomer. This polymer is thinly coated onto a substrate using an appropriate solvent to make a tvidely used sensor as shown in Fig. 2. For example,a copolymer made of a vinyl monomer and reactive cationic monomer is coated on a substrate and has been made into... 

The Poisson distribntion finds common use in the study of many physical phenomena. For example, consider the case of anionic living polymerization. Using an initiator snch as n-butyl lithium that forms a carbanion, we can polymerize vinyl monomers (Fignre 7.10). Initiation is rapid, so that we start growing each chain at the same time. In a very small time interval At, the probability that we add a monomer unit to a specific chain during this interval is p[M](At), where is a propagation rate constant and [M] is the monomer concentration. To accoimt for the changing monomer concentration, we define a scaled time... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

The kinds of vinyl monomers which undergo anionic polymerization are those with electron-withdrawing substituents such as the nitrile, carboxyl, and phenyl groups. We represent the catalysts as AB in this discussion these are substances which break into a cation (A ) and an anion (B ) under the conditions of the reaction. In anionic polymerization it is the basic anion which adds across the double bond of the monomer to form the active center for 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. 

Radical copolymerization is used in the manufacturing of random copolymers of acrylamide with vinyl monomers. Anionic copolymers are obtained by copolymerization of acrylamide with acrylic, methacrylic, maleic, fu-maric, styrenesulfonic, 2-acrylamide-2-methylpro-panesulfonic acids and its salts, etc., as well as by hydrolysis and sulfomethylation of polyacrylamide Cationic copolymers are obtained by copolymerization of acrylamide with jV-dialkylaminoalkyl acrylates and methacrylates, l,2-dimethyl-5-vinylpyridinum sulfate, etc. or by postreactions of polyacrylamide (the Mannich reaction and Hofmann degradation). Nonionic copolymers are obtained by copolymerization of acrylamide with acrylates, methacrylates, styrene derivatives, acrylonitrile, etc. Copolymerization methods are the same as the polymerization of acrylamide. 

Photoinitiators provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end groups. Under suitable conditions, and in the presence of a vinyl monomer, a block AB or ABA copolymer can be produced which would otherwise be difficult or impossible to produce by another polymerization method. Moreover, synthesis of block copolymers by this route is much more versatile than those based on anionic polymerization, since a wider range of a monomers can be incorporated into the blocks. 

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. 

Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by basic (anionic) catalysts. The chain-carrying step is conjugate nucleophilic addition of an anion to the unsaturated monomer (Section 19.13). 

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. 

When undiluted 2-vinylfuran was added to metallic sodium (mirror or particles) an orange colour developed and some resinous material was deposited on the metal surface. On prolonged contact much of the monomer was converted into a partly-insoluble reddish resin with spectra unrelated to those of standard poly(2-vinyl-furan). Reaction of diluted monomer with sodium gave a milder interaction, but no evidence of living anionic polymerization. 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

The striking simplicity of the methacrylate polymerization caused by the introduction of a t-butyl group was noted also in anionic polymerization of related monomers such as t-butyl acrylate57), t-butyl crotonate 58) and t-butyl vinyl ketone. 

To be eligible to living anionic polymerization a vinylic monomer should carry an electron attracting substituent to induce polarization of the unsaturation. But it should contain neither acidic hydrogen, nor strongly electrophilic function which could induce deactivation or side reactions. Typical examples of such monomers are p-aminostyrene, acrylic esters, chloroprene, hydroxyethyl methacrylate (HEMA), phenylacetylene, and many others. 

Our investigations agree with arguments in earlier articles by other authors, namely that empirical reactivity indices provide the best correlation with the goal values of the cationic polymerization (lg krel, DPn, molecular weight). On the other hand, the quantum chemical parameters are often based on such simplified models that quantitative correlations with experimental goal values remain unsatisfactory 84,85>. But HMO calculations for vinyl monomers show, that it is possible to determine intervals of values for quantum chemical parameters which reflect the anionic and cationic polymerizability 72,74) (see part 4.1.1) as well as grades of the reactivity (see part 3.2). 

Table 9. Anionic polymerization ability of vinyl monomers limits of quantum chemical reactivity indices from HMO calculations...

Reactive radical ions, cations and anions are frequent intermediates in organic electrode reactions and they can serve as polymerization initiators, e.g. for vinylic polymerization. The idea of electrochemically induced polymerization of monomers has been occasionally pursued and the principle has in fact been demonstrated for a number of polymers But it appears that apart from special cases with anionic initiation the heterogeneous initiation is unfavorable and thus not competitive for the production of bulk polymers A further adverse effect is the coating of electrodes... 

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. 

Applying these methodologies monomers such as isobutylene, vinyl ethers, styrene and styrenic derivatives, oxazolines, N-vinyl carbazole, etc. can be efficiently polymerized leading to well-defined structures. Compared to anionic polymerization cationic polymerization requires less demanding experimental conditions and can be applied at room temperature or higher in many cases, and a wide variety of monomers with pendant functional groups can be used. Despite the recent developments in cationic polymerization the method cannot be used with the same success for the synthesis of well-defined complex copolymeric architectures. 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

Almost linear polymers with pendant vinyl groups are formed as intermediates in the anionic polymerization of 1,4-DVB due to the different reactivities of monomers and pendant vinyl groups. 1,4-DVB microgels are formed towards the end of monomer conversion. In the anionic polymerization of EGDM or 1,3-DVB, reactive microgels are formed already at the beginning of the polymerization. 

---