Anionic polymerization propagating species

Monomeric alkyl lithium polymerizes isoprene through an anionic type propagating species. The transition between cis 1,4 and trans 1,4 polymerization is not clear since mono-ene polymerization also occurs in this region. Increased dielectric constant of the media, the addition of ethers, or the use of high lithium alkyl concentrations increased the character from that weakly anionic for the cis-diene polymerization to the slightly more anionic requirements for 3.4-monoolefinic polymerizations. 

Like in anionic systems, living, cationically growing polymers facilitated kinetic studies of these polymerizations. For example, Stan Penczek, who spent one year with us, demonstrated after his return to Poland that free ions and ion pairs participate in the cationic polymerization of tetrahydrofuran and succeeded to determine the individual rate constants of these species. However, in this system still another species, not yet encountered in anionic polymerization, propagates the polymerization, namely, the covalently bonded ester. 

As indicated, the growing end is associated with an MX -type counteranion, SbFg . These polymerizations were carried out at low temperature (—70 and —90 °C) in a polar solvent (CH2CI2). In view of the low nucleophilicity of the SbF anion, the propagating species under these conditions is most likely to be highly dissociated. 

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Anionic polymerization is a powerful method for the synthesis of polymers with a well defined structure [222]. By careful exclusion of oxygen, water and other impurities, Szwarc and coworkers were able to demonstrate the living nature of anionic polymerization [223,224]. This discovery has found a wide range of applications in the synthesis of model macromolecules over the last 40 years [225-227]. Anionic polymerization is known to be limited to monomers with electron-withdrawing substituents, such as nitrile, carboxyl, phenyl, vinyl etc. These substituents facilitate the attack of anionic species by decreasing the electron density at the double bond and stabilizing the propagating anionic chains by resonance. 

What are the propagation species in anionic chain polymerizations ... 

The situation is quite different in chain polymerization where an initiator is used to produce an initiator species R with a reactive center. The reactive center may be either a free radical, cation, or anion. Polymerization occurs by the propagation of the reactive center by the successive additions of large numbers of monomer molecules in a chain reaction. The distinguishing characteristic of chain polymerization is that polymer growth takes place by monomer reacting only with the reactive center. Monomer does not react with monomer and the different-sized species such as dimer, trimer, tetramer, and n-trier do not react with each other. By far the most common example of chain polymerization is that of vinyl monomers. The process can be depicted as... 

Contrary to the high selectivity shown in cationic and anionic polymerization, radical initiators bring about the polymerization of almost any carbon-carbon double bond. Radical species are neutral and do not have stringent requirements for attacking the re-bond or for the stabilization of the propagating radical species. Resonance stabilization of the propagating radical occurs with almost all substituents, for example... 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

In the relatively few anionic polymerizations initiated by neutral nucleophiles such as tertiary amines or phosphines the proposed propagating species is a zwitterion [Cronin... 

The radical-anion proceeds to propagate in the same manner as discussed above for initiation by sodium naphthalene. (Polymerizations in liquid ammonia are very different from those in organic solvents in that free ions probably constitute the major portion of propagating species.)... 

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 ) ... 

The need for solvation in anionic polymerization manifests itself in some instances by other deviations from the normal reaction rate expressions. Thus the butyllithium polymerization of methyl methacrylate in toluene at — 60°C shows a second-order dependence of Rp on monomer concentration [L Abbe and Smets, 1967]. In the nonpolar toulene, monomer is involved in solvating the propagating species [Busson and Van Beylen, 1978]. When polymerization is carried out in the mixed solvent dioxane-toluene (a more polar solvent than toluene), the normal first-order dependence of Rp on [M] is observed. The lithium diethylamide, LiN(C2H5)2, polymerization of styrene at 25°C in THF-benzene similarly shows an increased order of dependence of Rp on [M] as the amount of tetrahydrofuran is decreased [Hurley and Tait, 1976]. 

---