Methyl methacrylate, anionic polymerization

There are some indications that the situation described above has been realized, at least partially, in the system styrene-methyl methacrylate polymerized by metallic lithium.29 29b It is known51 that in a 50-50 mixture of styrene and methyl methacrylate radical polymerization yields a product of approximately the same composition as the feed. On the other hand, a product containing only a few per cent of styrene is formed in a polymerization proceeding by an anionic mechanism. Since the polymer obtained in the 50-50 mixture of styrene and methyl methacrylate polymerized with metallic lithium had apparently an intermediate composition, it has been suggested that this is a block polymer obtained in a reaction discussed above. Further evidence favoring this mechanism is provided by the fact that under identical conditions only pure poly-methyl methacrylate is formed if the polymerization is initiated by butyl lithium and not by lithium dispersion. This proves that incorporation of styrene is due to a different initiation and not propagation. 

In a similar manner polyisoprene-polyethylene oxide block copolymers can prepared301. It is surprising that the poly(methyl methacrylate) anion can be successfully used for the polymerization of ethylene oxide without chain transfer302. Graft copolymers are also prepared by successive addition of ethylene oxide to the poly-... 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

The main limitation to the method is that the anion of one monomer must be able to initiate the polymerization of a second monomer, and this may not always be the case. Thus, polystyryl lithium can initiate the polymerization of methyl methacrylate to give an (A - B) diblock but, because of its relatively low nucleo-philicity, the methyl methacrylate anion cannot initiate styrene propagation. Best results are achieved when two monomers of high electrophilicity are used, e.g., styrene (St) with butadiene (Bd) or isoprene and (A - B - A) triblocks can be formed as shown in Equation 5.17a and Equation 5.17b. 

Thus, one should expect similar behavior for transition metal enolates where there is significant covalent character to the M-O (or M-G) bond. This section will focus on polymerization of (meth)acrylate esters by group 4 metallocene (or the related group 3 and lanthanocene ") initiators where the mechanism of this process is analogous to the classical GTP process. Of course, the polymerization of (meth)acrylates by other transition metal complexes has been reported frequently in the literature however, in many cases the mechanisms of these processes are less well understood or involve free radical or other forms of initiation. Recent examples of other transition metal-mediated methyl methacrylate (MMA) polymerization processes that may proceed via a GTP or anionic mechanism are given. " "- " ... 

Some abnormalities were reported in the initiations of methyl methacrylate polymerizations in toluene by butyllithium. Their nature is such that they suggest the possibility of more than one reaction taking place simultaneously. One, which must be the major one, is that of the oiganomet-allic compound reacting with the carbon-to-carbon double bond as shown above. The other, minor one, may be with the carbon-to-oxygen double bond. The major reaction produces methyl methacrylate anions. The minor reaction, however, yields butyl isopropenyl ketone with an accompanying formation of lithium methoxide ... 

Lithium methoxide does not initiate polymerizations of methyl methacrylate. The ketone molecules, however, react with carbanions on the growing chain. The resultant anions are less reactive than methyl methacrylate anions and can only add new methyl methacrylate monomers slowly. Once added, however, the reaction proceeds at a normal rate. Polymerizations of methyl methacrylate in polar solvents, on the other hand, proceed in what might be described as an ideal manner with formations of only one kind of ion pair. ... 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Living polymerizations do not have either transfer or termination reactions, that is, the active chain carrier remains bound to an individual polymer chain up to the yield determined by the monomer-polymer equilibrium. The ionic ends of the living polymer can thus be used to produce block polymers of defined structure. This ability, however, depends on the polarity of the growing macroanion and the monomer to be added on. To a first approximation, the polarity can be described in terms of what is known as the e values of the two monomers. Electron-poor monomers have high e values and electron-rich monomers have strongly negative e values (see also Section 22.2.5). For example, the poly(methyl methacrylic anion) (monomer e = 0.40) starts the polymerization of acrylonitrile e = 1.20), but not that of styrene e = —O.SO). Conversely, however, the poly(styryl anion) can start the polymerization of methyl methacrylate. 

Lithium methoxide does not initiate polymerizations of methyl methacrylate. The methoxide molecules, however, react with carbanions on the growing chain. The resultant anions are less reactive than methyl methacrylate anions and can only add new methyl methacrylate monomers slowly. 

The preparations by anionic mechanism of A——A type block copolymers of styrene and butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of c/s-1,4 microstructure [346]. This is discussed in Chap. 4. It is more difficult, however, to form block copolymers from methyl methacrylate and styrene, because living methyl methacrylate polymers fail to initiate polymerizations of styrene [347]. The poly(methyl methacrylate) anions may not be sufficiently basic to initiate styrene polymerizations [345]. 

Bezuglyi and co-workers have published information on the reactivity of anion radicals and dianions of some a- substituted 9,10-anthraquinones generated electrochemically. The ions from the substituted anthraquinones will polymerize styrene whereas methyl methacrylate will polymerize only in the presence of ions from the unsubstituted anthraquinone. This idea has been used by Miertus et al. for the polymerization of acrylonitrile initiated by electrolytically prepared radical anions of benzephenone and 2,2 -bipyridyl. Funt and Hsu have extended this... 

Acrylonitrile (H2C — CHCN), methyl methacrylate [H2C=C(CH3)C02CH3J, and styrene (H2C=CHC6H5) can all be polymerized anionically. The polystyrene... 

Anionic Polymerization of Methyl Methacrylate 3.1 Basic Observations... 

Our understanding of the intricacies of anionic polymerization of methyl methacrylate was greatly improved during the last 15years by the meticulous and persistent work of the Mainz group. To appreciate fully the progress made in this field it is advisable to summarize the older studies and the ideas developed in those days. 

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. 

Fig. 2. Arrhenius plots of the rate constants of the anionic polymerization of methyl methacrylate in THF as the solvent and with Na+ orCs+ as the counterion. (R. Kraft, A. H. E. Muller, V. Warzelhan, H. Hocker, G. V. Schulz, Ref.35>)...

Methyl methacrylate Free radical polymerization similar to the above. Also susceptible to rapid anionic polymerizationinduced by RMgX or Na in liquid NHs CH, —CH,—C— 1 COOCH3 Tg 90 Amorphous, even when stretched. Hard. Soluble in aromatic hydrocarbons, esters, dioxane, etc. 

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