Anionic polymerizability

Another important consequence of the limitations concerning cross-addition is that anionic polymerization is not suited for the synthesis of random copolymers. If a mixture of two anionically polymerizable monomers is reacted with an initiator, the most electrophilic monomer will polymerize while the other is left almost untouched 30). In other words, a general feature of anionic binary copolymerization is that one of the reactivity ratios is extremely high while the other is close to zero. 

The ability to ionically polymerize apparently correlates in many cases with the capacity of the substituents to act as electron acceptors (anionic polymerizability) or as electron donors (cationic polymerizability) on the rt-bond of the vinyl group. These relationships should be visible in carefully chosen quantum chemical parameters. 

In the case of the anionic polymerizability it is possible to find limits of the quantum chemical parameters which separate those monomers capable of polymerization from those not (see Table 9). 

The polydispersity of polymers prepared in this way is usually very low for example, a value MJM of 1.05 was found for a sample of poly(a-methylsty-rene). Living polymers can also be used for the preparation of block copolymers after the consumption of the first monomer, a second anionically polymerizable monomer is added which then grows onto both ends of the initially formed block. By termination of the living polymer with electrophilic compounds the polymer chains can be provided with specific end groups for example, living polystyrene reacts with carbon dioxide to give polystyrene with carboxylic end groups. 

Some anionically polymerizable monomers are listed below, roughly in the order of increasing electroaffinity, i.e. in the order of increasing tendency to polymerize anionically. 

Anionically Polymerizable Monomers Listed in the Order of Increasing Electroaffinity... 

The metallocene catalyst with cationic nature and spatially opened active site provides favorable condition for the incorporation of p-alkylstyrene (p-ms) to polyolefins. The p-ms groups can be easily metallated to produce "stable" polymeric anions for graft-from polymerization. With the coexist of anion-polymerizable monomers, we have prepared many graft copolymers, such as PE-g-PS, PE-g-PMMA, PE-g-PAN, PP-g-PS, PP-g-PB, PP-g-PI and PP-g-PMMA. 

The lithiated polyethylene copolymer was then suspended in hexane or THF solvent. The graft-from reactions were carried out in slurry solution by reacting the lithiated polyethylene copolymer with anionic polymerizable monomers, such as styrene and p-methylstyrene. After certain reaction time, 10 ml of isopropanol was added to terminate the graft-from reaction. The precipitated polymer was filtered and then subjected to fractionation. Good solvents for backbone and side chain polymers were used during the fractionization, using a Soxhlet apparatus under N2 for 24 hours. The soluble fractions were isolated by vacuum-removal of solvent. Usually, the total soluble fractions were less than 5 % of the product. The major insoluble fraction was PE graft copolymer, which was completely soluble in xylene or trichlorobenzene at elevated temperatures. 

The anionic polymerizability of 67 is much lower than that of 58. This difference is due not only to the lower ring-strain of 67 and but also to a greater steric hindrance that the lactam anion of 67 receives when it comes closer to the carbonyl... 

The lack of anionic polymerizability of 69 can be ascribed to the instability of 71 due to a stereoelectronic effect. [9] One of the lone pair orbitals on the nitrogen atom of 71 is perfectly antiperiplanar to the C(l) —0(2) bond, which causes this bond to cleave fadly. Thus, as soon as 71 is formed, it is transformed to 72, so that anionic polymerization by the activated monomer mechanism does not take place. 

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Schoonbrood, H.A.S., Unzue, MJ., Beck, O. and Asua, J.M. (1997) Reactive surfactants in heterophase polymerization. 7. Emulsion copolymerization mechanism involving three anionic polymerizable surfactants (surfmers) with styrene-butyl acrylate acrylic acid. Macromolecules, 30, 6024-33. 

There are exceptions to this guideline, however. Thus, a-methylstyryl initiation of styrene or styryl initiation of isoprene are both faster than the reverse initiations. If inductive effects only are considered, then positive e numbers favor anionic polymerizability. 

A list of some anionically polymerizable unsaturated monomers is given in Table 5. They can be listed in four classes styrenic monomers, hydrocarbons monomers, halogenated monomers, and polar monomers. As a general principle, a good match between the nucleophilicity of the initiator and the electron affinity of the double bond is essential to obtain good initiation efficiency without side reactions. 

The polymerization of isocyanates to 1-nylons is thought to be a new example of anionically polymerizable systems requiring basic catalysis and low temperatures. Thus, the initiation step can be considered as the attack of the anion (X ) at the isocyanate group as shown below. 

Woods, J.G. Rooney, J.M. Imaging Method for Vapor Deposited Photoresists of Anionically Polymerizable Monomer U.S. Patent 4,675,270, June 23, 1987. 

Precipitation chromatography, as proposed by Baker and Williams and improved in our laboratory, has a systematic error caused by the nonuniformity of the fractions. This error can be eliminated if the fraction nonuniformity is determined by a special kind of mixing experiment. Another interfering effect which is produced in anionic polymerizates by traces of impurities can be eliminated by testing the BW fractions by GPC. Exactly determined molecular weight distribution functions of samples which are synthesized under well defined kinetic conditions provide much information about elementary reactions of the polymerization process. This is demonstrated by some examples of the anionic polymerization of styrene and methyl methacrylate. 

A Combination of the BW and the GPC Methods for Controlling the Fractions of an Anionic Polymerizate... 

Figure 7. Distribution of an anionic polymerizate with two side maxima...

Figure 8. Correction of the BW fractions of an anionic polymerizate of polystyrene by gel permeation chromatography (10)...

J.4.2.3 Heterocyclic Monomers A variety of heterocyclic monomers can be polymerized by anionic ring-opening polymerizations. The types of anionically polymerizable heterocyclic monomers include oxiranes (epoxides), thiacyclopropanes, thiacyclobutanes, lactones, lactides, lactams, anhydrides, carbonates, and silicones... 

Figure 5.1 Cyclic monomers capable of anionic polymerization. Table 5.1 Anionic polymerizability of vinyl monomers.

Both, 2- and 4-vinylpyridines show a higher anionic polymerizability than those of styrene and 1,3-diene monomers, because the electron-deficient pyridine ring... 

Phenyl vinyl sulfoxide (8) shows a high anionic polymerizability, similar to that of (meth)acrylates and N,N-dialkylacrylamides [158-160]. Hogen-Esch first reported that 8 underwent anionic polymerization in TH F at — 78 °C to afford polymers with relatively narrow molecular weight distributions (Mw/M = 1.1-1.5) [158]. AnAB diblock copolymer, PS-b-poly(8), was also synthesized by the sequential addition of styrene, followed by 8. Subsequently, the anionic polymerization was improved by using 4-methylphenyl vinyl sulfoxide (9) with l,l-diphenyl-3-methylpentyllithium in the presence of a 20-fold excess of liCl in THF at —78°C [161]. In this way. 

Various substituted styrenes also anionically polymerize readily. These include methyl, methoxy, dimethylamino, t- butyl and other groups which are electron donating to the benzene ring and do not themselves react with carbanion centers. Substituents such as chloro or nitro can be anionically polymerized only at imder very carefolly controlled conditions. 2,3, or 4 vinylpyridine can also be anionically polymerized. Any aromatic or condensed aromatic compounds with a vinyl substituent is potentially anionically polymerizable, or co polymerizable with any other anionically polymerizable monomer. 

Reaction 17 shows that carboxylate containing polymers have been used to initiate graft polymerizations of anionically polymerizable monomers such as lactones (54). 

Although, there are many more cationicaUy polymerizable monomers than anionically polymerizable ones, relatively few cationic polymerizations (e.g., isobutene polymerization to produce polyisobutene and butyl rubber - copolymer of isobutene with small fractions of isoprene [18]) are performed industrially because macrocations are highly reactive and prone to suffer termination and chain transfer reactions. 

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