Anionic polymerization isomeric species

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

Bestian and Clauss proposed that the polymerization occured with isomerization on a cationic alkyltitanium species or one of its associated forms. Propagation by anionic and cationic species accounts for their results more easily. Most of the oligomer low molecular weight product was from anionic type propagation (Equation 8). However, the 7.8% of the dimer and the 30% of the trimer fractions were produced by cationic propagation of the n-butyl group (Equation 9). 

Anionic polymerization. For some heterocyclic monomers the unique chemical structure of the growing species follows unequivocally from the monomer structure. However, in many cases isomeric structures have to be taken into account. For instance, for symmetrical monomers, like thietane, the carbanion but not the thiolate anion was proposed (4). Unsymmetrically substituted monomers can provide active species by a- or B- ring scission. Unusual structure of activated monomer was proposed for NCA and lactams. These structures can not be distinguished by spectrophotometric methods, and application of H- or 13C-NMR looks more promising. 

The chemistry of cationic polymerizations is usually significantly more complicated than that of anionic polymerizations. In many cases, the initiator added to start the polymerization does not, itself, start the polymerization, but first forms the actual active species in the polymerization mixture, itself, by reacting with itself, a cocatalyst, or the monomer. In addition, the initiating and growing species can participate in a whole series of side reactions such as transfer, isomerization, or termination reactions. 

Other monomers which were found to respond positively to anionic polymerization were the 2-furfurylidene ketones, which gave phantom polymers following a propagation mechanism involving the isomerization of the active species before each addition step [4d, 4e]. 

Whereas the cationic polymerization of furfurylidene acetone 3a engenders crosslinked structures (25), the use of anionic initiators results in linear structures (26). However, the propagation is preceded by an isomerization of the active species which eliminates the steric hindrance to propagation arising from the 1,2-disubstitution in the monomer structure. A proton shift from the 4- to the 2-position places the negative charge at the extremity of the monomer unit and the incoming monomer can add onto this anion without major restrictions. The polymer structure thus obtained is ... 

In addition to covalent species and carbenium ions, the equilibria may involve onium ions, which are formed by reaction of carbenium ions with noncharged nucleophiles [Eq. (46a)]. This decreases the carbenium ions lifetime, and therefore the time available for isomerization to more stable and less reactive carbenium ions via hydride and alkyl anion shifts [Eq. (46b)]. Decreasing the probability of rearrangements by decreasing the carbenium ions lifetime is especially useful because such rearrangements can not be prevented by decreasing the polymerization temperature. 

With isoprene in THF the situation is more complicated. While with Li as counterion the system is chemically stable up to 0 °C, with Na" considerable instability arises even at 40 °C. In the former situation reasonable kinetic behaviour is exhibited and u.v. absorption spectra show the presence of three types of anionic centre with absorption maxima at 287, 305, and - 335 nm. The first is assigned to a cis species, the second to the corresponding trans isomer, and the third to some irreversibly isomerized entity whose structure is still by no means certain. At low temperatures — —40 °C) in the absence of common ion salts, as with butadiene polymerizations, fast reactions occur owing to the presence of free anions, predominantly in the trans form, and addition of common ion species depresses the rate to very low values. Above —20 °C under ion pair conditions polymerization via the cis isomer of the active centre appears to be abundant, but, unlike butadiene, there is the added complication in the possibility of 3,4- and 1,2-terminal units arising (20) and (21). In THF the... 

Correlations of structures and reactivities for anionic and cationic ring-opening polymerization are reviewed. The following topics are discussed chemical structure of active species and their isomerism, determination of active centers concentration, covalent vs ionic growth and correlations between structures of active centers or monomers and their reactivities. 

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