Anionic propagation

The mechanism of the polycondensation reaction remains unclear. A vanety of possible reactive intermediates have been suggested, including sdyl radicals and sdyl anions. An anionic propagation mechanism (100,101,103) has been strongly suggested, although the case is by no means setded (104). Other Synthetic Methods. 

The effect of penultimate units on the rate constants of anionic propagation is observed also in other systems. For example, the addition of styrene to the lithium salt of 1-phenyl-n-hexyl anion is 4 times faster than to polystyryl lithium 51). Similarly, the addition of monomer to the lithium salt of 1,1-diphenyl-n-hexyl lithium is faster than the addition to 1,1,3-triphenyl-n-octyl lithium or 2-poly-sty ry 1-1,1-diphenyl ethyl lithium, the latter two salts having comparable reactivities52 . See also Ref.53)... 

That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The dimerization occurs to form the styryl dicarbanion instead of - CH2CH4>CH4>CH2 -, since the former is much more stable. The styryl dianions so-formed are colored red (the same as styryl monocarbanions formed via initiators such as n-butyl-lithium). Anionic propagation occurs at both carbanion ends of the styryl dianion... 

The mechanisms for nucleophilic GTP and electrophilic GTP are not the same. Electrophilic GTP proceeds by an associative or concerted mechanism that does not involve anionic propagating centers. Initiation involves a concerted addition of methyl trimethylsilyl dimethyl ketene acetal to monomer to form species XXV. The overall effect is to transfer... 

Nucleophilic GTP proceeds by a dissociative mechanism in which the propagating centers are ionic—similar to other anionic polymerizations [Hertler, 1994, 1996 Quirk and Bidinger, 1989 Quirk et al., 1993 Webster, 1992, 2000]. The anionic propagating species XXVII is generated in low concentrations by nucleophilic displacement of the trimethylsilyl group by the nucleophilic catalyst (W+Nu ) ... 

Control of the side reactions is achieved through two factors (1) reversible complexation of the anionic propagating species XXVII by the silyl ketene acetal polymer chain ends XXVI maintains the concentration of the anionic propagating species at a low concentration and (2) the bulky counterion W+ (e.g., tetra-ra-huty I ammonium, tris(dimethylamino)sulfonium) decreases the reactivity of the anionic propagating centers toward the terminating side reactions. 

Anionic propagating centers are transformed into radical centers in the presence of a second monomer that undergoes radical propagation, by reaction with trimethyl lead chloride... 

In Eq. 7-5a Z represents a functional group, such as O, NH, Si—O, CO—O, and CO—NH in ethers, amines, siloxanes, esters, and amides, respectively. In Eq. 7-6 Z represents an anionic propagating center, such as alkoxide or carboxylate, derived from the cyclic monomer. 

In all cases reported, except In the least polar solvents like dloxane, both Ion pairs M, Cat+ and free Ions M- were found to contribute to the propagation. In some cases even triple Ions were shown to participate In the anionic propagation process... 

If cation-radicals are in fact formed, there should be the same number of anion-radicals as counterparts. This stoichiometry is not established. Neither anion-radical nor anionic propagation has been detected. Since anionic propagation would form homopoly AN, the total polymer was extracted with saturated zinc chloride solution. There was, however, no extractable fraction. 

Bhattacharyya, D. N., C. L. Lee, J. Smid, and M. Szwarc The absolute rate constants of anionic propagation by free ions and ion pairs of living polystyrene. Polymer 5, 54 (1964). 

It was shown from the study of styrene and MMA in a KNOa-DMF system using tracer techniques for the composition analysis that the free-radical contribution is apparently confined to the first few percent of polymerization, whereas at later stages the reaction is anionic (34). This is consistent with the inhibitor studies. On the same monomer pair with a variety of solvents with ammonium salts, only with tetrahydrofuran did a possible free-radical reaction accompany the anionic propagation, with the others, DMF and dimethylacetamide, as well as without solvent, an anionic reaction accompanying the free radical one was assumed (35). 

Reaction 4 is favored by the strong electron affinity of nitroethylene (30). The carbanion may be formed by ion-molecule reaction between the anion radicals and the nitroethlene molecules (reaction 5), to which the latter add successively, and polymerization proceeds by anionic propagation (reaction 6)... 

In THF or diglyme, 9-fluorenyllithium, which is dissociated extensively into ion pairs, catalysed through a strong anionic propagation to produce syndiotactic polymethylmethacrylate. The effectiveness of ethers to help dissociate the ion pairs was found to be related to the Lewis base strength of the ethers. 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

Shima, Smid and Szwarc (56) studied the effect of the methyl substitution in the polymerization of butadiene, isoprene and dimethyl-butadiene. They showed that the electron-donating methyl group decreased the rate of polymerization catalysed by polystyrylsodium. This same electron releasing effect of the methyl is seen, since the 3.4-structure, not 1.2-structure, is produced predominantly from isoprene. This results from the anionic propagation mechanism of the alkali metal alkyl catalysed polymerization of dienes which produced 1.2 and 3.4-structures. 

Another type of ionic species was proposed by Uelzmann (92). Uelzmann suggested that from trialkylaluminum and titanium tetrachloride, TiClg)+ and (R3A1C1) caused cationic initiation on the titanium followed by an anionic propagation on the aluminum ion. Bestian and co-workers (70) proposed similar cationic intermediates which propagate by anionic shifts. These steps are the opposite of the anionic initiation and cationic propagation proposed in this review for the butene-1 polymerization. 

J pKu of ihe conjugate acid of the anionic propagating intermediate hpA , value in DMSO. 

Anionic polymerization of s-caprolactam is used to make cast or RIM polyamide-6. Using a premade lactam chain end and a metal catalyst, it proceeds rapidly at 100-160°C, well below the melting temperature of the polymer, Tm 220°C. The propagation differs from anionic propagation of most unsaturated monomers because the growth center at the chain end is not represented by an anionically activated group but by a neutral N-acy-lated lactam, and the anionically activated species is the incoming monomer (Table 2.26). 

From the literature developing, it does not appear that anionic propagation produces random copolymers, but rather in those cases where two monomers polymerize a block polymer is produced. In some cases, in which the initiator is very active, the less reactive monomer will be initiated to a small extent, but then the other monomer takes over... 

These initiators produce anionic propagating species by attack on the C=C double bond of vinyl and diene monomers. A common example of this type is n-butyl lithium. The C—Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. The unaggregated form is more active for initiation. Butyl lithium is usually available as a solution in hexane. Addition of tetrahydrofuran to this solvent increases the concentration of the unaggregated initiator by forming a 1 1 complex with this compound. This accelerates the rate of initiation of styrene ... 

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