Anionic Homopolymerization

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

Anionic homopolymerization of poly-THF macromonomers bearing terminal styryl, a-methylstyryl or methacryloyl groups has not been reported so far. 

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AlkyUithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. / -ButyUithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched stmctures. Because of the high degree of association (hexameric), -butyIUthium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

Monoisocyanates undergo anionic homopolymerization at subambient temperatures to yield nylon-1 polymers (polyamides) (63). 

This equation permits the calculation of equilibrium constants for polymerization-depolymerization from copolymer composition data extrapolated to zero Mi feed. The agreement between equilibrium constants calculated in this manner from free radical copolymerizations and those obtained from anionic homopolymerizations is shown in Table II, and again emphasizes the thermodynamic character of this work. 

Studies of such systems are reported in the literature. Worsfold and Bywater (28) determined kp for the anionic homopolymerization of a-methylstyrene in tetrahydrofuran solution and Allen, Gee, and Stretch (I, 2) studied the polymerization of styrene in dioxane. Both groups utilized the dilatometric technique to follow the reaction and show the absence of termination. 

Since kp for the anionic homopolymerization of styrene in tetrayhdrofuran solution is / 600 liters per mole second and the respective activation energy is only 1 to 2 kcal. per mole, the entropy of activation is substantially more negative (by about 14 eu.) than AS for a radical polymerization of styrene. It is likely that the additional decrease in the entropy of activation is due to immobilization of the counterion in the transition state in the middle between the last unit of the growing end and the new unit being added, i.e. 

Table II. Effect of Counterion on the Rate of Anionic Homopolymerization of Styrene in Tetrahydrofuran at 25 C.

Several papers57"59 were devoted to investigating a complex process such as the cationic copolymerization of monomeric formaldehyde with dioxolane in the gas, liquid, and gas-liquid phases. It is known that polyacetal resins are industrially produced by copolymerizing cyclic acetals (trioxane, 1,3,5,7-tetraoxane), or by anionic homopolymerization of monomeric formaldehyde with subsequent modification of end groups. 

Oxetanes, 4-membered cyclic ethers, polymerize exclusively by cationic mechanism 1 3), although coordinative anionic homopolymerization and copolymerization with C02 was claimed 4 5) for the unsubstituted oxetane. 

Figure 28.1 Initiation and propagation steps of the anionic homopolymerization of epoxy groups.

For example, methyl methacrylate block copolymers are much less studied than those of styrene. Anion chain transfer occurs at the pendent ester group, drastically reducing the yield of block copolymers. Poly(methyl methacrylate-b-isoprene) has been prepared, however, by using an ingenious chain cap of l,l -diphenylethyl-ene(27,28). i l diphenylethylene will not anionically homopolymerize, therefore it adds only one mer to the macroanion. This anion is more stable in the presence of methyl methacrylate, but will initiate further polymerization. Other workers have reported the preparation of isoprene-methyl methacrylate block copolymers by sequential addition to "living" polyisoprene anions(29,30),... 

The homopol5unerization of diisocyanates is only useful for specialty diisocyanates, such as aliphatic 1,2- or 1,3-diisocyanates (3) and aromatic o-diisocyanates (4), which polymerize via cycloaddition processes. Anionic homopolymerization of monoisocyanates takes place by addition across the 0=N bond to form nylon-1 polymers. Polyamides are also obtained fi"om diisocyanates and enamines or ketenaminals. This reaction proceeds by a [2 -i- 2] cycloaddition reaction with subsequent ring opening to form polyamides. [2 - - 4] cycloaddition polymerization to form heterocyclic polymers is observed with carbonyl diisocyanate (5). Ring-opening polymerization occurs in the reaction of bis-epoxides... 

The anionic homopolymerization of monoisocyanates proceeds by addition across their C=N double bond to give nylon-1 pol3rmers (27). The reaction is generally conducted in dimethyl formamide (DMF) at —50°C, using sodium cyanide as the initiator. The infrared spectra of the nylon-1 homopolymers show a strong absorption band at 1700 cm (C=0) and a band from 1280 to 1390 cm , indicating a disubstituted amide structure. The nylon-1 polymers obtained in the anionic polymerization reaction are hsted in Table 4. 

When the nature of R and R is such that the basicity of the tertiary amine is enhanced, an anionic homopolymerization of epoxides takes place through the following mechanism [71] ... 

Therefore, substituted ureas may be used as latent initiators for anionic homopolymerization of epoxy resins. Pearce and Morris [73] reported the use of 1,1-pentamethylene-3-phenyl urea, prepared by the reaction of phenyl isocyanate with piperidine in dry benzene, to cure a tetraglycidyl methylenedianiline resin toughened with carboxyl-terminated butyl rubber. These formulations showed an excellent stability at 23°C and led to a high Tg product when cured at 170°C. 

The formation of aromatic isocyanate trimers is of economic importance, because rigid insulation foams, having isocyanurate structures built into their network structure, are produced from aromatic diisocyanates. Triphenyl isocyanurates with hydroxyl or carboxyl groups in their p-positions can be obtained on hydrolysis of McsSiO- and McsSiOCO-groups, respectively, with hydrochloric acid °. Such trifunctional compounds are of use in the construction of network polymers. The mechanism of the phenyl isocyanate trimer-ization, using Pd(o) diimide catalysts was elucidated recently. The initial steps of this trimerization reaction involve a chain growth process as encountered in the anionic homopolymerization of isocyanates. 

Quite recently,the anionic homopolymerization of a few substituted p-lactams and the copolymerization of some of the above pairs have been smdied in order to prepare polyamide 3-derived polypeptides displaying biological properties. The solution polymerization or copolymerization, initiated by li amide disubstituted with trrmethylsilyl groups and activated with 4-tert-butylbenzoyl chloride, does not have living character. From that study, some insights emerged into the reactivity of the above p-lactams in terms of their acidities, as well as electrophUidty of the imide end groups. 

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